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I'm hoping I can get some better insight into how my camera works and how I can apply it in the future.

I have a custom built camera with a Canon EF 200 lens mounted to it. The camera allows me to hook a frame grabber to it and read out raw data into a file that can then be opened by excel. I need to correct the vignetting, so I took dark data (with the lens cap on) and light data (by pointing the camera into an integrating sphere). I then used that data to calculate the flat field gain table and applied it to one of the integrating sphere images. It worked as I expected and the imagery looks perfect - completely white, no rings or drop off at the corners. The entire time I was doing this, the aperture was set to f2.8 (fully open).

Now, when I went to apply my flat field gain table to the actual imagery I wanted to correct, I noticed that the images had been taken with different aperture settings of either 5.6, 8.0 or 11.3.

My question is, does this completely invalidate the flat field gain table I already calculated? Or is the change so minor it wouldn't be noticeable? Ideally I would just take another round of integrating sphere images, but I don't have access to that equipment anymore so I'm kind of stuck.

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2 Answers 2

up vote 2 down vote accepted

Vignetting depends highly on both aperture and focused distance.

For example, see http://www.dpreview.com/lensreviews/nikon_35_1p8g_n15/3 which has an example on how wildly the vignetting varies by shooting a 35mm f1.8 for cropped sensors on a full frame at different focus and apertures.

So, not only you should worry about apertures, but also the focused distance is a big factor as it moves glass elements (and in most lenses changes the effective focal length slightly).

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I have to admit, I'm not sure I see how this really answers the question. You've established that it does effect it, but not how it effects it and, to be fair, the level of impact is going to be very lens dependent. The vignetting behaviour of the Nikon 35mm f/1.8 is not the same as a Canon 200mm lens. –  John Cavan Sep 14 '13 at 3:25
    
I suppose if you are going to do a measure for comparison it is because you're aiming at almost 100% exact compensation of the vignetting for whatever purpose the op has - likely not to show the picture in the living room. Otherwise he would drag the vignetting slider in whatever software he's using instead of worrying about all this convoluted measurements. In fact, he mentions he needs it for data analysis in excel. As such the differences will not be negligible for aperture and focus changes whatever any review of the EF200 says, because the reviews target a "pleasurable picture" purpose. –  Marco Mp Sep 14 '13 at 8:57
    
Yes, I can appreciate that. I suspect it lies in that the title asks how and the body of the question does not... –  John Cavan Sep 14 '13 at 12:24
    
Vignetting shouldn't depend on focus distance for a telephoto lens as the focussing group is behind the aperture. –  Matt Grum Sep 14 '13 at 14:53

Vignetting in telephoto lenses is due to the entirety of the entrance pupil not being visible from certain angles.

The entrance pupil is the image of the aperture stop as seen from the front of the lens. Detach the lens and look through it at a bright light. You should be able to see the circular wide open aperture. Now look at it from the size and you'll only be able to see a vertical slit. Light is being blocked, hence less light from the edges reaches the sensor causing vignetting. If the aperture was smaller, more of it would be visible from these extreme angles. It's exactly the same mechanism that causes "cat eye" bokeh with large aperture lenses:

If you work out the entrance pupil distance you could probably calculate aperture dependence of vignetting geometrically, but that still wouldn't be 100% accurate, it might be easier just to calibrate at each whole stop and use the closest value.

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