How to Fill Out a Lens Specification Sheet for Drone Imaging Applications
Drone imaging spans a wide range of use cases, from low-altitude agricultural surveys to high-altitude environmental monitoring, from wide-area mapping to narrow-field object identification. The performance demands on the lens differ significantly across these use cases, and completing the specification sheet properly ensures that your optical designer understands what matters most for your system. Whether you are a systems integrator, sensor specialist, or project engineer, this guide is designed to demystify the terminology and guide you through the decision-making process.
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Before going into the details of the spec sheet, this page will give you the technical background required to understand the optical terms in it.
Part I: Technical Background
F-number (F/#)
Let’s start with the fundamentals and define F# (pronounced “eff number”) as this term will be used frequently. If this concept is already familiar to you, please proceed to the next section, otherwise click the drop down below for more information.
F-number
The F-number, also known as the f-stop or focal ratio, is a measure of the aperture of a lens in photography, defined as the ratio of the lens’s focal length to its diameter (F/# = F/D). For a simple lens, the diameter is obvious, but the diameter, D, and the focal length, f, are shown in the picture above for clarity. More complicated lenses, such as the two shown below, require more detail. In both figures, the diameter is labelled ‘D’ and the focal length ‘F’. And in both cases, the diameter of the beam of light (blue rays) is the same as D. This is by definition. The rays that bound the cone of light that is focused onto the center of the image are called marginal rays. If you extend the marginal rays back and the marginal rays from the front of the lens forward, they will intersect one focal length away from the image.
The aperture of a lens determines the amount of light that gets to the image sensor. This amount is measured in terms of the “speed” of the lens, which is another term for F/#. The speed of a lens is a term that dates back to the early days of photography when the speed of a lens determined how fast the image built up on the photographic plate. Lower numbers correspond to shorter exposures.
An important thing to understand about F/# is that it relates to the diameter of the aperture, while exposure time relates to the area of the aperture. Doubling the diameter of the aperture quadruples the area, so it reduces the exposure time by a factor of four. That is why it is common for F/# to increase in increments of 1.4 (the square root of 2). Common F/numbers for lenses are F/0.7, F/1, F/1.4, F/2, F/2.8, F/4, F/5.6, F/8, F/11, F/16 and F/22.
So why don’t all lenses have low F/#s? Because as the F/# decreases, aberrations increase very rapidly. This will be discussed below.
For more information on F/#, see this page: https://www.eckop.com/resources/optics/numerical-aperture-and-f-number/
Ground Sampling Distance
Ground Sampling Distance (GSD), Ground Resolution and Spatial Resolution are terms used when specifying remote sensing system resolution. They are all used to define the smallest feature a remote sensing system can resolve on the ground. If your system is not looking at the ground, resolution defines the smallest feature on whatever the system is imaging. Either way, we’ll refer to the thing your imaging system is looking at as “ground”.
The two factors that affect resolution in a system are image sensor pixel size and lens resolution. We will discuss pixel size first.
Image Sensor Pixel Size
The simplest definition of resolution is the size of one camera pixel on the ground. This is dependent on the altitude of the system and focal length of the lens being used. The following figure helps define the terms.
By similar triangles, we see that
\( \frac{GSD}{H} = \frac{w}{F} \),
where GSD is the width of a pixel on the ground, H is the altitude, w is the width of a camera pixel and F is the focal length of the lens. By assuming an ideal lens, we can use simple geometry to define the system resolution. The system can perform no better than this.
In general, small pixels and/or a long focal length lens give the best possible resolution. However, lens resolution may become the limiting factor and will be discussed next.
Lens Resolution
Resolution is a topic that affects all types of lenses. It has three components: the diffraction limit, aberrations and manufacturing defects.
Diffraction is a phenomenon that relates to light traveling as a wave. It is inherent in basic physics and no amount of effort can work around it except in very special circumstances.
Aberrations, on the other hand, are rooted in the details of a lens design; reducing them is the chief job of a lens designer.
Although some manufacturing processes approach perfection, none attain it. The remaining imperfections degrade lens resolution, so they must be considered.
How is resolution determined? The most common definition was developed by Lord Rayleigh in 1879 and concurrently by Ernst Abbe. It states that two objects are just resolved when their blurred images are separated by sufficient distance that there is a noticeable gap. Here’s a picture of that:
The individual curves are shown in blue and orange while the sum is shown in dark teal. The dip between the two peaks is about 27% below the maxima. We’ll return to this when we discuss PSF and MTF.
Aberrations
The image formed by a simple lens is blurry because it has aberrations. The underlying reason for this is that the basic law for how light interacts with matter, Snell’s law of refraction, is nonlinear. It states that the refractive index of the material in which the light starts multiplied by the sine of the angle at which it strikes an interface is equal to the refractive index on the other side of the interface multiplied by the sine of the angle at which the light leaves the interface. The sine function is nonlinear, so the angles change nonlinearly. Ideally, the changes would be linear, and the difference between the sine function and a linear function is the source of aberrations.
Spherical aberration, coma, astigmatism, Petzval curvature and distortion are the basic aberrations. There’s a lot more information on each of these on our website, but here’s a brief description. Spherical aberration causes a rotationally symmetric blur that is the same everywhere in the image; it increases as the fourth power of the aperture, where aperture is the inverse of F/#. Coma causes an ice cream cone shaped blur that increases linearly in size with the distance from the center of the image; it increases as the cube of the aperture.
Astigmatism causes points to appear as lines; the size of the lines increases as the square of the distance from the center of the image and the square of the aperture. Petzval curvature causes the best image to lie on a curved surface rather than a plane. Finally, distortion causes the image of a rectangle to look like something other than a rectangle. Neither Petzval curvature nor distortion depend of the F/#. More on distortion below.
It may be helpful to discuss how optical engineers quantify aberrations. First, remember that in a perfect lens, light from a single point on the object is focused onto a single point on the image. One can imagine spherical waves of light expanding from the object and later converging onto the image point.
One way to measure aberrations is to measure the difference between the actual wavefront produced by a lens and the sphere that best matches it. This measurement is denominated in the number of wavelengths of deviation and is known as wavefront error. For example, if a lens has two waves of aberration, the actual wavefront formed by the lens differs from the nominal sphere by at most two waves (roughly 1 µm). Now let’s turn to diffraction.
Diffraction Due to Apertures
Even if a lens is designed to have no aberrations, the image of a point is not an infinitesimal point. Here’s why. Light travels as a wave. This is true even for individual photons. And any time a wave interacts with a boundary there is diffraction. For example, think of light from a star entering the first lens (or mirror) of a telescope. Most of the light is refracted (or reflected) and continues on toward the focus. However, light that enters close to the edge interacts with it and is redirected as if it were emitted from the edge.
This is the definition of diffraction. This diffracted (redirected) light interacts with the undiffracted light near the focus of the system and spreads the light into a distinct pattern. The shape of the pattern depends on the shape of the aperture. For a circular aperture, the result is an Airy disc while for a square aperture it is a “sinc²” pattern. You’ll see a picture of an Airy disc in the section on PSF below.
Manufacturing Defects
After a lens is designed, it must be produced. A good designer will attempt to minimize the sensitivity of the lens to manufacturing imperfections, but there will always be some level of performance degradation. Thankfully, this degradation affects MTF in roughly the same way as aberrations; there are no new wicked problems introduced.
However, for best performance, it is wise to keep the combination of aberrations and manufacturing imperfections to less than some error budget. The budget has two main components: residual aberrations in the design and aberrations due to manufacturing errors. A reasonable budget for drone lenses is to limit the sum of these components to less than 0.5 waves.
PSF & MTF
Now that you’re familiar with the concepts behind imperfect images, the next question to ask is how to visualize these imperfections. One answer used by optical engineers is the Point Spread Function (PSF). A simple way to understand the PSF is that it is a plot of the amount of light hitting the image as a function of position, assuming the light originated at an infinitesimally small point on the object (like a star).
The shape of the PSF can be used by an experienced optical engineer to determine what aberrations are present in the image, but it has a more widely used relative, the Modulation Transfer Function (MTF). To calculate an MTF, one takes a Fourier transform of the PSF. Great, but so what? The valuable thing about the MTF is that it gives us a detailed description of the resolution of a lens, which is what we’re looking for. But first, let’s dig a bit deeper into the PSF.
Quick Recap of GSD and Lens Resolution
- GSD defines the size of a pixel projected onto the ground, and depends on the geometry of sensor pixel size, altitude, and focal length:
- \( GSD =\frac{w*H}{F} \)
- Resolution may be limited by lens performance or geometry. Aberrations and diffraction may degrade ideal GSD.
- Lens resolution is governed by three factors: diffraction (physics-limited), aberrations (design-limited), and manufacturing defects (process-limited).
- Aberrations stem from the nonlinearity of refraction and include spherical aberration, coma, astigmatism, field curvature, and distortion.
- A system’s effective resolution is best quantified by PSF and MTF, which integrate all error sources into spatial performance metrics.
Point Spread Function (PSF)
As mentioned above, the PSF is a plot of the amount of light from a point on the object hitting the image as a function of position. To begin to get a feel for the PSF, let’s start with the PSF of a perfect lens with a circular aperture. A circular aperture diffracts light into a PSF known as an Airy disc. Here’s a picture of one:
And here’s a cross-section:
This is what an observer sees when looking through a really well made telescope at a star. The radius of an Airy disc is defined as the radius from the central peak to the first minimum. It can be calculated by:
RA = 1.22*λ*(F/#),
Where RA is the radius of the Airy disc, λ is the wavelength of light in microns (0.54µm for green light), and F/# is the F-number of the lens. Quick reminder: F/# is the focal length of a lens divided by the diameter of its entrance aperture. Below is a table showing the Airy disc radius as a function of F/#.
| F/# and Airy Disc Radius | |
|---|---|
| F/# | RA (µm) |
| 16 | 10.5 |
| 8 | 5.3 |
| 4 | 2.6 |
| 2 | 1.3 |
It is a good idea to ensure that the F/# of the lens is low enough to produce an Airy disc radius smaller than the pixel size of the camera. In the case of the GPixel GMAX3265 sensor with 3.2µm pixels, a recommended F/# would be F/4.9 or faster (lower F/# is “faster”). It is possible to use a lens with a slower F/# and still resolve features close to the GSD, but it will be more difficult and may require computational image processing to give acceptable images.
If a lens has aberrations, they will spread out the PSF and reduce its peak amplitude. To make this more concrete, compare this PSF with the Airy disc picture.
In this example, which is one of many possible forms for the PSF, the peak intensity is reduced to 0.416 and there is a secondary peak. Clearly, a lens with aberrations like this will not put the same amount of energy on a pixel as the above diffraction limited lens.
Although PSF helps visualize how lens aberrations inhibit sharp imaging, they don’t quantify the resolution of the image. To do that, we switch from PSF to MTF. The two are mathematically convertible but show different aspects of lens performance in the presence of aberrations. PSF shows the form of the aberration while MTF shows the resulting resolution.
Modulation Transfer Function (MTF)
MTF is the gold standard in the optics industry for evaluating lens performance. It gives a complete representation of how sharp an image is, expressed in terms of “modulation” and “spatial frequency”. Before getting into the technical details, let’s use an analogy to clarify the concept.
An Analogy for MTF
At some point or another we have all been introduced to the eye chart, officially the Snellen chart. When our vision is tested, almost everyone can read the E at the top of the chart and people with 20/20 vision can read the eighth line down. But as you go down the chart, each line gets progressively more difficult to read. Going down the chart, to smaller and smaller letters, corresponds to “spatial frequency” increasing. Letters getting more difficult to read corresponds to the “contrast” or “modulation” decreasing. A typical MTF chart shows a line that starts at 1 (perfect modulation) and slopes down to the right, which means that modulation or contrast is decreasing as spatial frequency increases.
The Definition of "Modulation"
Modulation is essentially equivalent to contrast. High modulation means that the image of a black object looks black and white looks white. As modulation decreases, blacks begin to look dark gray and whites like light gray. When modulation drops to zero, it is impossible to distinguish black and white; both are mid-gray. Here’s a plot of a sine wave with two different modulations:
Assume the above sine wave plot is the amount of light in an image, and the peaks represent bright portions, and the valleys represent dark portions. Let’s call the amount of light in the peaks “max” and the amount in the valleys “min”. Then the modulation, or contrast, is given by:
\( modulation = \frac{max-min}{max+min} \)
From this equation, you can see that if min = 0, modulation will be 1.0, and if max = min, the contrast will be 0.0. When the modulation is near one, the contrast is very high, and the image looks sharp. Conversely, when the modulation is near zero, it is so blurred that it is difficult or impossible to see details.
The image below correspond to the high contrast (left) and low contrast (right) curves.
Spatial Frequency
Spatial frequency measures the resolution or fineness of detail. Mathematically, it is the period of the sine wave in the above plot. Here’s a picture that includes multiple spatial frequencies that increase from left to right:
Increasing spatial frequencies correspond to increasing resolution. The units of spatial frequency are cycles per mm. A cycle consists of a pair of lines, one black and one white, or one complete cycle of a sine wave, from 0 to 2π. It is important to remember that spatial frequency increases as the period gets smaller. For example, if we had 1 black line and 1 white line fit into 1mm the period would be 1 mm, and the spatial frequency would be 1 cycle per mm. But when one black and one white line fit into 0.5 mm, the period is 0.5 mm and the spatial frequency is 2 cycles per mm. Line pairs per millimeter (lp/mm) and cycles per millimeter (cy/mm) are roughly the same thing.
If the resolution we want is 1 pixel and pixels are 3.2 µm wide, (3.2 µm = 0.0032 mm) we get a spatial frequency of 156 cy/mm. This is called pixel frequency or Nyquist frequency.
Another way to get a picture of spatial frequency is to think in terms of dots per inch (dpi). Printers went through a revolution when Hewlett Packard introduced its first laser printer that produced 300 dpi. A few years later 600 dpi became the norm and now printers put out so many dpi that we don’t care anymore. The same thing is happening in cell phones. My first cell phone had a 1.75″ display with 320 x 240 resolution. That works out to 230 dpi. Now you can get a cell phone with over 400 dpi, so the picture is much sharper.
So how does that relate to spatial frequency? Actually, dpi is spatial frequency! Let’s change the units to make this clear. To make one cycle, we need two pixels so one can be on and the other off. This means that 400 dpi = 200 cycles per inch. Since optics is done in metric, we still need to convert to millimeters. There are roughly 25 mm in 1 inch, so we just divide cycles per inch by 25. Thus 400 dpi = 200 cy/in = 8 cy/mm.
Diffraction Limit
Just as diffraction keeps a lens from forming an image of a star that is exactly a point, it keeps modulation from being 1 at all spatial frequencies. Modulation must decrease as spatial frequency increases.
Mathematically, the MTF curve is just the Fourier transform of the PSF. For example, the Airy disc corresponds to the diffraction limited MTF plot below.
Note that this curve is different for every wavelength and F/# (or NA). However, the only thing that changes is the scale of the abscissa (x-axis) that is normalized to one in the plot above. The cutoff frequency is given by:
\( \lambda_c = \frac{1}{\lambda (F/\#)} \)
Referring back to the eye chart, smaller details are harder to see. In terms of MTF, this is expressed by saying that the modulation decreases as spatial frequency increases.
Perfect lenses are called “diffraction limited” because diffraction keeps them from resolving infinitesimal detail. Some types of lenses, such as telescopes and microscopes, come very close to reaching the diffraction limit. Others, for example camera lenses, rarely approach it. In general, the wider the field of view a lens has, the harder it is to approach the diffraction limit.
Information Transfer and MTF
Another way to look at the MTF of a lens is in terms of information transfer. At very low spatial frequencies, nearly all of the information gets transferred by the lens from the object being viewed to the camera pixels – the MTF is 1 (100% transfer). At high spatial frequencies (very fine detail), the MTF drops to 0 (0% transfer), which means that no information is being transferred to the pixels by the lens.
In addition to the amount of information transferred by the lens, the receiver of that information must be considered. Although it is rarely discussed, the Aerial Image Modulation (AIM) curve can be used to how much information is lost. The AIM curve for square pixels is given by a sinc (sin x / x) curve, where x = π/2 at the Nyquist frequency of the image sensor. As always, no information is lost at very low spatial frequencies. At the Nyquist frequency (500 divided by the pixel width in microns), 36% [1 – sin(π/2) / (π/2)] of the information is lost, and at twice the Nyquist frequency, all information is lost. Beyond that, there is aliasing, but let’s ignore that for now.
Aberrations (MTF)
A simple case of aberrations is defocus. This case is useful because in small quantities all aberrations affect the MTF the same way. Let’s define “small” as roughly a quarter of a wavelength of light, although the approximation degrades gracefully. Here’s a plot of how various amounts of defocus affect MTF:
Note that even a quarter-wave of aberration has a noticeable effect on MTF. The definition of a quarter-wave is that the wavefront from a single point on the object (e.g. a star) that is converging on a point on the image deviates from a perfect sphere by up to 0.135 µm, assuming a wavelength of light of 0.54 µm (green). For a lens with a wide field of view, it can be quite difficult to achieve this level of performance.
A camera lens might have an MTF plot like the one below:
Note that there are five curves on the plot and also that the maximum spatial frequency is 100 cy/mm. The blue curve (labeled TS 0.00 deg) is for the center of the image. Before discussing the green and red curves, we need two more definitions.
Tangential: related to resolution for periodic patterns with the period tangent to a circle centered on the optical axis.
Sagittal: related to resolution for periodic patterns with the period parallel to a line through the center of the image.
As you may have surmised by now, periodic patterns away from the center of the image have an MTF that depends on their orientation. If you orient a periodic pattern of parallel lines so one of the lines passes through the center of the image, the MTF of that pattern will be determined by the Sagittal MTF curve, which is the one labeled “S”. MTF also varies with the distance from the axis of symmetry (optical axis) of the system. That’s why there are separate curves for 0.00 deg (the optical axis or center of the image), 14 deg and 20 deg. This particular lens was analyzed by tracing rays through the lens that made a 14 degree angle with the optical axis and another set at 20 degrees. In this lens, things don’t get much worse between 14 and 20 degrees, but they are different. Other lenses will behave differently, depending on the aberrations of the optical system. But that’s another topic.
Using MTF, we can compare the performance of different lenses with similar specifications. Below is an MTF plot for a 60mm F/4 lens. Note that the top (black) curve is the diffraction limit.
The minimum acceptable MTF is 20% (0.2 on the plots) for the Nyquist frequency of your image sensor. If you were to use the GMAX3265 image sensor, Nyquist is 156 cy/mm. On the chart above, a few curves fall below 20% at < 156 cy/mm, so this lens would not be acceptable.
Here is another MTF plot for a 60mm F/4 lens, but this one is designed for higher resolution.
All fields are better than 20% at 156 cy/mm, so this lens is well designed for the GMAX3265 image sensor. It is not at the diffraction limit, but it is a good compromise between cost and performance.
Quick Recap
MTF quantifies how well a lens preserves contrast at different spatial frequencies, making it the industry standard for evaluating image sharpness.
Modulation measures contrast; it drops as spatial frequency increases—similar to how smaller letters on an eye chart become harder to read.
Spatial frequency is measured in cycles/mm and relates directly to lens resolution; the Nyquist frequency is when adjacent pixels are white or black.
Diffraction, aberrations, manufacturing errors, and sensor limitations all reduce MTF, especially at high spatial frequencies; even quarter-wave errors cause significant degradation.
A well-matched lens-sensor pair should maintain ≥20% MTF at the sensor’s Nyquist frequency—anything less wastes pixels.
For Further Reading on Modulation Transfer Function (MTF):
Modulation Transfer Function (MTF) – SPIE Optipedia
A technical overview from SPIE’s Optipedia with diagrams and formulas.What is Modulation Transfer Function? – Synopsys
A glossary-style explanation with context for imaging systems.
Distortion
If you’re designing a drone camera system, how much distortion is acceptable? Too much can limit your ability to stitch images together but too tight a specification will drive up the price of a lens.
Let’s take a real-world example: a camera with a Sony IMX540 24.5 MP sensor. With 2.74 µm pixels, the distance from the center to the corner of the sensor is 9.63 mm. A mere 1% distortion at that distance means 96 µm—roughly 35 pixels of error. For mapping applications, that’s a big deal.
Sure, you can correct distortion in software, but that means more image processing. Optically correcting it to less than one pixel (0.03% distortion) eliminates the processing—but it’s also challenging. And even a lens designed to be free from distortion can be affected by slight manufacturing variations.
That’s why it’s critical to work with experts who can help you balance optical design, manufacturing tolerances, and software correction. We’ve designed lenses for drone mapping and other demanding applications, and we can help you find the right balance for your needs.
Curious about the different types of distortion and how they impact your design?
- Learn more about distortion here.
- Learn more about aberrations here.
Part II: Filling Out the Specification
If you understand the Technical Background, you are ready to fill out a specification sheet for your drone lens! You can download an PDF file specification sheet using this button.
Note that all of your specifications should be entered in the “Specification” column. Some of the cells in this column are pre-filled with example data. Please erase this and fill in your own data if it differs. In addition, your comments are welcome in the field at the bottom of the second page.
Description
Project Title: This cell should give a name to the lens.
Date and Revision: The date and revision number must be revised any time this specification sheet is changed.
Application
Application: Describe how the lens will be used in four words or less.
Example Entries:
- Agricultural mapping
- Infrared surveillance
- Hyper-spectral imaging of vegetation
Lens Type: Specify either fixed-focus or zoom. Zoom lenses cost more and weigh more than fixed-focus lenses, but they add flexibility. If you choose a zoom lens, you’ll have to also decide whether it should be motorized or not. Motorized zoom lenses can be adjusted in-flight given the required electronics and software, while non-motorized zoom lenses must be adjusted pre-flight.
Object Description
Object Full Field of View (FFOV)
The FOV of a lens can be specified as the width of the swath that is being imaged at a given fly-height or as an angle. It may be specified in terms of horizontal, vertical, or diagonal, but horizontal is the most common because it is the same as your swath width. Specifying the full width or full angle is most common, but lens designers rely on the rotational symmetry of a lens, so they always work with the half-field or half-angle. This specification is critical for planning coverage swath width and image overlap.
A reasonable FOV for a telescope is roughly 2°, while a really wide fisheye can image as wide as 240°. (Yes, it can see behind the camera.) The FOV of lenses for drones typically lies between these extremes; anything between 20° and 60° for a horizontal FOV is common.
Object Distance
Enter the altitude at which the drone will be flying. If there is a known range, please also enter the minimum and maximum altitude.
Ground Sampling Distance
Enter the minimum distance between two points on the ground that must be resolved. See the Technical Background for details.
Object Space Components
Describe anything between the object and the first surface of the lens, including dimensions and materials. Common components include a dome, window, or filter.
Aperture
F/# or Entrance Pupil Diameter
To minimize exposure time, you want the fastest practical lens. Lenses can be made as fast as F/0.7, but lenses that fast are heavy and expensive. Lenses slower than F/8 are inexpensive and lightweight but require much longer exposure times. An F/# in the range of F/2 to F/4 is reasonable.
Iris
An iris is the part of a lens that controls the F/#. Lenses can be designed with either a fixed or adjustable iris. Fixed-iris lenses are simpler, lighter and less expensive than lenses with an adjustable iris. Most image sensors have a wide range of exposure and gain settings, so adjustable irises are rarely required. If you do need the aperture to be adjustable, this can be done either manually or by motor.
Entrance Pupil Position
In most cases, this is Not Applicable (N/A). However, if your drone lens is on a gimbal and it is important to have the apparent aperture of the lens in a fixed position as the gimbal is actuated, the entrance pupil position must be specified.
Image
Image Circle Diameter
This is typically the diagonal of the image sensor in millimeters. For critical applications, it is necessary to specify the image circle diameter slightly larger than the image sensor diagonal to allow for lens centration and the exact position of the image sensor on its PCB.
Image Space Components
Describe anything between the last surface of the lens and the image sensor. Filters are the most common image space components. This is also the place to mention if a minimum or maximum distance between the lens and the image sensor is required.
Image Sensor Cover Glass
If you know which image sensor you want to use and have the datasheet that describes the thickness and position of the cover glass, enter that information here. If the refractive index of the cover glass material is given, that is also helpful to the lens designer.
Image Sensor
Image Sensor
If you know which image sensor you want to use, enter the manufacturer’s part number here. Otherwise provide the approximate diagonal size (e.g., 1/2.3″) or active area dimensions. This determines the required image circle. We are happy to assist you in choosing an economical and readily available image sensor.
Image Sensor Resolution
Again, if you have chosen a specific image sensor, enter the number of pixels (width x height). Otherwise, enter the approximate number of megapixels.
Pixel Size
Enter the size of the pixels for your chosen image sensor in microns. This number will be used to calculate the Nyquist frequency, so don’t enter any units
Nyquist Frequency
The size of the pixels on the image sensor sets the Nyquist limit of your system. To avoid aliasing and underutilized resolution, the lens MTF will be designed to match this frequency. To calculate it, divide 500 by the size of the pixels in microns.
Spectral Range, Lens Transmission
Spectrum Type
Drone lenses can be used to produce monochrome, color, multi-spectral or hyperspectral images. Or they can be used to provide the input for an imaging spectrometer. Please specify which here.
Wavelength Range
Enter the minimum and maximum wavelength of interest. Examples: Visible (430–680 nm), NIR (700–1000 nm), SWIR (1000–1700 nm). Hyperspectral imaging gives additional information about the scene. For example, it can provide a better understanding of plant health for precision agriculture.
Peak Wavelength or Wavelengths
Color image sensors typically have filters with peaks at 450 nm, 540 nm and 650 nm, so specify these if you are recording images with a normal color image sensor.
Spectral Transmission
Optical glasses have high transmission from about 420 nm through 2000 nm, so transmission can be greater than 90%. However, if a wide range of wavelengths is used, antireflection coatings become less effective. For visible wavelengths, a 3 or 4-layer coating is sufficient to reduce reflectance below 0.5% per surface, but covering a range from 400 to 1000nm requires a 10-layer coating which can only reduce reflectance to 1%. Covering the full 400 – 1700 nm range of an InGaAs image sensor makes it difficult to reduce reflectance below 1.5% per surface.
Image Quality, Design Performance Targets
MTF Requirements
As noted in the Technical Background, MTF defines spatial frequency response. It’s simplest to specify the minimum MTF at the Nyquist frequency for the image sensor at the center of the image (on axis), 70% of the distance to the corners of the image, and at the corners. A typical specification would be >0.3 across the field.
Type of Distortion
Most drone lenses are used to map a rectangle on the ground to a rectangle on the image sensor. This is called “f-tan(Θ)” distortion. (That’s a theta in the parentheses.) If you need something else, here’s the place to specify it.
Distortion
Even small distortions can accumulate into large spatial errors in mapping. Specify the maximum distortion as % of image height, e.g. <2%. If software correction is planned, we can provide a table of the distortion in the design as a function of distance from image center.
Relative Illumination and Vignetting
Images with dark corners are the result of two major design errors. The first is vignetting (pronounced vin-YET-ing). This happens when the lens designer under-sizes the first or last element of the lens, either intentionally, to reduce blur, or unintentionally. For old-fashioned film cameras, vignetting was acceptable, but for remote sensing, it isn’t. Modern image sensors have a highly linear response to exposure, and data analysis is much easier when an image is uniformly bright.
Relative illumination describes how evenly light is distributed across the image sensor, from center to corner. In drone applications, especially those involving wide-angle lenses, falloff in illumination can degrade image quality and affect calibration for radiometric or photometric measurements. Falloff can stem from both natural cosine-fourth law effects and mechanical obscuration (vignetting), e.g., aperture edge shading or internal barrel structures. If your application requires uniform brightness for image stitching, vegetation indices, or thermal mapping, specify a minimum relative illumination at the field edge—typically >70% for wide-angle lenses.
Chief Ray Angle
By chief ray angle (CRA), we are referring to the angle between the normal to the image plane and the chief ray. Some image sensors are sensitive to this angle. If CRA is too large, the corners of the image get dark. Sensors with microlens arrays are particularly sensitive. Provide your sensor’s CRA tolerance, if available.
Mechanical and Environmental
Barrel Length and Diameter
If there are constraints on the physical size of the lens, enter them here. Irregular form factors should be described or sketched.
Lens Mount
Mounts affect not just compatibility but also rigidity, alignment and thermal expansion. Most mounts have a standard flange-focal-distance, which is the distance between the surface where the lens seats and the focal plane. Common mounts include M12, C-mount, CS-mount, Nikon F, and Canon EF. Custom mounts are quite common as well. Add a note if your mount has custom back flange depth tolerance requirements.
Center of Mass
While it’s impossible to predict the location of the center of mass of a lens before it’s designed, we can guarantee that the location will be within a certain distance of nominal once the design is complete. Please let us know if this is required for your application.
Total Mass
Drones often have payload weight requirements. Let us know if this affects the lens.
Athermalization
It can be quite expensive, but it is possible to design a lens that will not go out of focus over a wide range of temperatures. Please specify “N/A” or give a temperature range. In the comments, please tell us whether focus must be maintained across the range. If there is no comment, we’ll assume that you will refocus the lens any time the temperature changes.
Storage and Operating Temperature Range
Enter the operating and storage temperature ranges (e.g., −10°C to 50°C). If the operating temperature range is large and focus must be maintained over the range, please specify that the lens must be athermalized.
Ingress Protection
Specify if the lens must meet a given IP rating (e.g., IP65 for dust and splash). This will incur additional expenses for design and testing, so make sure you need what you specify. A note regarding cleaning methods (e.g., must withstand isopropanol wipe) and lens coating durability is appreciated.