vignetting

[Dennis Smith]

I need a way to check for vignetting when I am reaching the max of my lens's covering power. An image will look fine on the ground glass and I cannot detect any light falloff due to camera movements, but when I develop the negative the vignetting appears. I feel like I am making some dumb rookie mistake that I should know the answer to.

I am currently using a 14" Kodak Commercial Ektar lens on an 8x10 camera. A chart I consulted indicated the lens has an image circle of 444mm. Any help will be greatly appreciated.

Thank you

[Joanna Carter]

One method is, assuming the ground glass has clipped corners, to look through the corners towards the stopped down lens. If you don't see a complete circle, you have vignetting.

[Toyon]

Maybe you should go with the flow? I see many crapulous "Fine Art photography" images that have edge vignetting photoshopped in.

On a more practical note: You may want to test individual lenses to see how much movement you can use before vignetting shows up. I know for example that my 120 will only accept 5mm in each direction before vignetting.

[Emmanuel BIGLER]

You should combine both methods as Joanna and Toyon mentioned
If you don't see a complete circle...
.. the iris shape is like a cat's eye.

A simple computation yields the following results for maximum allowable shifts:
data :
- image circle = 444 mm
- image size, say 245 mm L horizontal by 195 mm V vertical (a 8x10" minus about 6 mm of hidden edges)

result:
maximum shift horizontal = + - 77 mm / about 3"
maximum shift vertical = + - 88 mm / about 3.5"

the formulae are :
image size L by V image circle of diameter D
maximum shift Horizontal = + - SQRT(D^2-V^2)/2 - L/2
maximum shift Vertical = + - SQRT(D^2-L^2)/2 - V/2

data for different formats
4x5" L = 120 mm V = 94 mm
5x7" = (not sure) = L = 170 mm V = 120 mm
8x10" L = 245 mm V = 195 mm

[Dennis Smith]

Thanks to all for the suggestions

Dennis