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I am calibrating an industrial AVT camera. Is it ok, when I focus on the plane where I will do my measurements with the f/4 aperture then close the aperture to f/16, calibrate the internal parameters of the camera and then open the aperture to f/4 ? Will the calibration change with the changing aperture? I know that none of the parameters (focal length, principal point, lens) should physically change, but is there no effect?

I am not changing the focus (focal length). I need to change the aperture due to bigger depth of field during my calibration and faster camera during my measurements.

missZBH
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2 Answers2

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I think the short answer is: No it doesn't.

The calibration should be the same (within experimental limits) at different apertures. The aperture only affects the depth of field and the amount of light entering the camera. The focal length, principal point, len distortions, etc. don't change - although your ability to measure them accuratley may be affected by the quality of the image you get.

Maybe a larger aperture could in theory capture a better approximation of lens distortion, although reading this article makes me doubt my own words, but if you calibrate at a wide aperture and then capture at a smaller aperture, this should not be a problem. Only if your lens is seriously distorted would it be an issue (IMHO). The article linked says this:

The size of the stop has no effect on the distortion, as the chief ray does not alter its route when the aperture is made smaller or larger.

It would presumably be a simple procedure to perform camera calibration at different aperture settings and see if the results are similar. Certainly I know of no way to infer the aperture setting from a camera calibration matrix, which implies that this information itself is not captured.

Delgan
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Roger Rowland
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It depends on your lens and exactly what values you are calibrating.
If you are just setting the back focus distance, then there is no problem. Or if you are only measuring in the center of the field of view.

Without knowing the focal length, widest aperture, and the general quality of the lens, it's not possible to give a specific answer. For example, if the widest aperture is f/1.4, then the lens should perform well at f/4. But if f/4 is the widest aperture, chances are you will see a lot of aberrations.

Generally speaking, if you start with the widest aperture of a camera lens and stop down, the resolving power in the center of the field of view increases for about 2 stops, and the resolving power in the periphery of the field improves for about 4 stops.

Beyond that, if you continue to stop down, there is no improvement in image quality (only increased depth-of-field and a dimmer image--as a previous answer correctly states). Eventually, as the physical diameter of the f/stop becomes small, resolving power throughout the field will decrease due to diffraction.

For example, on a "full frame" (35 x 24 mm format) digital camera with a good lens, f/22 is noticeably less sharp than say, f/8.

Unfortunately, geometrical (Gaussian) analysis cannot predict the behavior of real lenses, for two reasons: aberrations and distortion.

Aberrations are imperfections in design, materials and/or manufacturing that can be corrected by additional elements, better glass, closer manufacturing tolerances, etc-- but only to a point and only at a price. Ideal lenses exist only in theory; real lenses always perform best (i.e. greatest resolving power) for paraxial rays (traveling near the optical axis).

Not all aberrations are equally affected by stopping down. Most improvement: higher-order spherical Much improvement: spherical, oblique spherical and coma Some improvement: astigmatism, field curvature, axial chromatic Not affected: lateral chromatic

Geometrical (Petzval) distortion (technically not an aberration) also is not affected by stopping down.

Diffraction on the other hand, is a fundamental law of optics--you just have to live with it. Diffraction varies inversely with the physical diameter of the aperture: the smaller the diameter, the bigger the angular size of the Airy disk. As we all know, f-number is focal length divided by diameter--so f/16 is a much smaller hole on f=50 mm lens than on a f=150 mm lens.

Traditional methods of measuring diffraction and confusion by diameters at the projected image (film or sensor) -- rather than by resolving power at the object--tend to understate the performance of longer lenses and depth-of-field of larger formats. But MTF charts tell the real story about the former: the best performing lens in any manufacture's catalog is a long lens or telephoto.

understate the performance of longer focal length lenses.

Diffraction is why pinholes -- which have no aberrations (and no distortions if properly designed) -- are not sharp.

Smaller aperture diameters always have more diffraction (i.e., a larger Airy disk), but diffraction is only significant when the Airy disk is larger than the lens's circle-of-confusion. The better correct the lens, the close it is to being "limited by diffraction" --the technical term for an ideal optical system.

More information: https://www.diyphotography.net/what-actually-happens-when-you-stop-down-a-lens/

M. Ryan
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