Photo above courtesy of Sentera
Are the lenses you’re using for your drone camera meeting your needs, or are they falling short for some reason? Are the corners of the image dark or blurry? Are your images blurry due to too long an exposure? Are you getting the areal coverage you need? Or are the mechanics not working for you?
If any of that sounds familiar, a custom lens for your drone may be the answer. Designing around your exact sensor, optics, and mounting requirements has given our clients solutions when catalog optics weren’t close enough. Below, we cover what drives that decision optically and mechanically, and what it costs.
Why choose a custom lens for your drone?
Optical Considerations
As an optical engineering team, image quality is an important consideration for us. We spare no effort in making sure any lens we design and manufacture for you meets your needs for cost and performance. For example, it’s common for images from off-the-shelf lenses to have dark or blurry corners. There are several possible reasons for this.
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.
If light strikes an image sensor at too high an angle, it does not contribute to the formation of the image. One way to quantify this is the chief ray angle (CRA) of a lens. The chief ray is the center of the cone of light exiting a lens toward a pixel on the image sensor. If the CRA is too large, some of the light that should illuminate a pixel is lost. At Eckhardt Optics, we are careful to match the CRA of a custom lens to the acceptance angle of the intended image sensor. Incidentally, if the CRA is nearly zero, the lens is considered “telecentric”.
A lens can produce a uniformly bright image and still have blurry corners. This is again a design error. No reasonably priced lens can produce images free from blur, so the job of a lens designer is to strike the balance between cost and image quality. If the MTF (Modulation Transfer Function) of a lens provides all the sharpness the image sensor can utilize at an acceptable cost, the job is done. This is always our target.
A sharp, uniformly bright image is useless if it is too dim. This makes the “speed” of a lens important. The speed of a lens is related to how long it takes to build up an image on the image sensor. It is measured as F/# (pronounced eff-number). The lower the F/#, the less time it takes to generate an image. Practically speaking, an F/# between 1.4 and 5.6 is a reasonable expectation. Every doubling of the F/# results in a 4X increase in the time it takes to generate an image, so F/# frequently increases by steps of √2 which gives a doubling of the exposure time.
The field of view (FOV) of a lens is the last optical consideration we’ll deal with. If a drone flies at a given height, the FOV determines how large an area on the ground will be captured on the image sensor. This is covered in detail on our Ground Sampling Distance (GSD) page.
It is important to note that our GSD page assumes that the lens has what is known as “F-tan(theta)” distortion, and that it is well corrected. For more information on the types of distortion, click here, and for more on the basics of distortion, click here. Drone lenses used for mapping or “mosaiced images” can have stringent distortion requirements.
For a specific example of distortion, take a camera with a Sony IMX540 24.5 MP sensor with 2.74 µm pixels. The physical distance from the center of the image sensor to the corner is 9.63 mm, so 1% distortion would be 96 µm, or 35 pixels. This can be corrected in software, but that requires additional image processing. Optically correcting the distortion to less than one pixel tightens the specification to 0.03%, which can be difficult even in design.
It is also possible that manufacturing errors can introduce variability between lenses at this level. Correcting distortion for wide-angle lenses (FOV > 60°) can be a challenge, so it is best to work with experts in the field of lens design and manufacturing. We can help you determine the best specification for your application.
Mechanical Considerations
Everyone wants a lens that is as small and light as possible; that’s a given. But what about center of mass, ingress protection and insensitivity to vibration? Let’s look at those in more detail.
The center of mass (CoM) of a lens is rarely important if the lens is fixed, but it can become critical if the lens is being scanned. At Eckhardt Optics, we can provide the location of the CoM for the lenses we design and hold it to within a tolerance in manufacturing. You may never find this necessary, but some of our customers do.
Ingress protection (IP rating) is also an issue for some of our customers. If your drone encapsulates the lens in a housing, IP rating is unlikely to be a concern. However, if you need a lens that is impervious to the elements, IP rating becomes essential. There are two numbers in an IP rating.
The first is the resistance to penetration by dust. A rating of 6 is quite common, which means that the lens is impervious to dust. This results in an IP rating of IP6x, where ‘x’ is the value for water penetration. A value of 5 for ‘x’ (IP65) means that the lens is impervious to penetration by moderate jets of water from any direction but cannot withstand immersion. IP67 means the lens can be submersed in up to 1m of water without ingress. Higher ratings are nice to have, but they carry a cost, so it is best not to over-specify a lens’ IP rating.
Drones vibrate! This is not usually a problem for lenses, but there are cases where it can be. If you have accelerometer data for your drone and want to make sure it will not impact the performance of one of our lenses, we will be happy to run the numbers for you as part of the design process.
It’s also worth noting that off-the-shelf lenses may not have an appropriate mechanism for connecting to your camera or image sensor PCB. Mounts like an M12x0.5 or C-mount thread or a bayonet mount for Nikon, Canon or Sony are readily available, but they don’t work for all situations. We can design a mounting system that will accommodate your needs. We’ve done dozens, so we can easily reuse one that we’ve already done or modify it to suit.
How much does a custom drone lens cost?
The best way to get a rough idea of what a drone lens would cost is to see what’s available off the shelf. A great place to start is Edmund Optics. In fact, if you only need one lens and Edmund has what you need, you may not need to look anywhere else. However, if you have optical or mechanical constraints that are not met by an off-the-shelf lens, we’re here to help. Our customers tell us that we’re easy to work with and our lenses work much better than off-the-shelf.
When you are ready for custom lenses, the first consideration is that we’ll have to create a new design. It will be based on our large library of existing designs, but every piece of glass or metal will be optimized for your application. A ballpark cost for this non-recurring engineering (NRE) work is $10k. If your lens is simple, it could be less, and if it’s complicated, it will be more.
We’ll work with you to create a detailed specification for your lens, and when it is finalized we’ll give you a firm quote for the NRE and a rough quote for samples. Creating the specification can take a couple days to a couple weeks. The design will take no more than a few weeks.
When the design is complete, we’ll be ready to manufacture prototype lenses. We typically make 5-10. The prototypes will cost much more than off-the-shelf lenses, perhaps 5X as much per lens. After you approve the prototypes, we can start volume manufacturing. Production quantities will cost roughly the same as a similar off-the-shelf lens when quantities are in the range of 25-50, but the price will drop significantly as the quantity increases.
As always, we’re interested in finding creative ways to solve complicated (or expensive) problems. If we can help you with your next project, let us know!