# Lens angle of coverage/image circle

• I
• Nil
In summary, a lens designer can scale up or down any linear dimension to change the size of the image circle. However, this does not mean that any ol' scaled design can be constructed. There is no simple answer, as optical systems can be extremely complex.
Nil
Hello,

I was wondering what parameters determine the angle of coverage/size of the image circle of a lens.

For example, for a fixed focal length, aperture and flange focal distance of a camera, what can a lens designer do to change the size of the image circle.

I'd also like to know how the following elements influence the image circle and aperture of a lens for a given focal length:

1. Diameter of the lens
2. Length of the lens
3. Size of the front and rear element
4. Size of the other elements
5. Number of elements
6. Number of groups
Thanks!

Fantastic question- the image circle size is usually not discussed as part of lens design, I had to dig around to find any information. This question is very relevant for lenses that support movements: tilt-shift lenses, view camera lenses, etc.

The quick answer is that any optical design can be scaled up (or down) to increase (decrease) the image circle:

https://sunex.com/2019/09/12/scaling-as-a-lens-design-tool/

But that doesn't mean any ol' scaled design can be constructed...

vanhees71
There's no simple answer, as optical systems can be extremely complex and changing one parameter in one system may not achieve the same results if you change that same parameter in another system. For example, increasing the diameter of the lens in a single-lens refractor (while holding all other parameters the same) will do nothing to the size of the light cone. It just changes the focal ratio of the system. However, if that same lens was the field stop of another optical system, increasing its diameter would increase the FoV of your system and thus increase the size of the image circle in both physical and angular dimensions.

The same holds for even a single lens if you allow for different sizes and shapes of the front and back elements, and account for the mounting brackets or whatever is going to hold your lens in place.

I was asking for a fixed focal length. In the article you linked, the following is mentioned: "All linear dimensions such as effective focal length, back focal length, total track length, or lens diameter scale linearly." which means as the image circle increases, so does the focal length.

For example, Canon makes the EF-S 24mm f/2.8 for APS-C and the EF 24mm f/2.8 IS USM for full frame. Nikon makes the AF-S 35mm f/1.8G DX for APS-C and the AF-S 35mm f/1.8G ED for full frame. I've attached the lens construction images.

How can the lens designer keep the same focal length and aperture but modify the image circle?

#### Attachments

• Canon 24mm APSC.jpg
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• Canon 24mm FF.gif
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• Nikon 35mm APSC.jpg
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• Nikon 35mm FF.jpg
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Nil said:
How can the lens designer keep the same focal length and aperture but modify the image circle?
Increase/decrease the diameter of the field stop, if possible.

Nil said:
How can the lens designer keep the same focal length and aperture but modify the image circle?

The field of view and image circle diameter are related through the focal length, so keeping the focal length constant gives a trade-off between image circle diameter and angular field of view. I think the only option is the scale the diameter of the optic. Consider this lens (an extreme example):

The image circle is 1 meter in diameter, the field of view is 45 degrees: for large-format photography, this lens is a 'normal lens'. If we keep the same focal length (1100mm) and look for a 35mm lens with those same specs, the field of view is much smaller (a couple of degrees or so). The 35mm lens also has a much smaller diameter (at f-stops identical to the XXL lens).

I suggest examining the design differences between a tilt-shift lens designed for 35mm format photography and a fixed lens with the same focal length as the tilt-shift lens. Tilt-shift lenses have image circles larger than fixed lenses to accommodate the movements, so you can see what is required.

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## What is lens angle of coverage?

Lens angle of coverage refers to the area of the subject that can be captured by a lens. It is determined by the focal length of the lens and the size of the camera's sensor or film.

## How is lens angle of coverage measured?

Lens angle of coverage is typically measured in degrees. It can also be expressed as the diameter of the circle that the lens can capture, known as the image circle.

## What is the relationship between lens angle of coverage and focal length?

The angle of coverage increases as the focal length of the lens decreases. This means that a wide-angle lens will have a larger angle of coverage than a telephoto lens.

## Why is lens angle of coverage important?

Lens angle of coverage is important because it determines how much of the subject can be captured in a photograph. A wider angle of coverage allows for more of the scene to be included in the image, while a narrower angle of coverage can be used to isolate a specific part of the subject.

## How does sensor size affect lens angle of coverage?

The size of the camera's sensor or film affects the angle of coverage by determining how much of the image circle is captured. A larger sensor or film will capture a larger portion of the image circle, resulting in a wider angle of coverage.

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