Optics question

Let say you have a large format lens (or any format for that matter) with a blemish on the rear element that is placed towards the outer edge of the glass. It is only visible when looking thru the front element at f/8 and larger aperatures.
Is the glass at the outer edge of the rear element of the lens performing a function when (for an extreme) the lens is stopped down to f/64? In other words will the blemish have less of an effect on the recorded image the further the lens is stopped down?

If the blemish is out of the image path after stopping down, it shouldn't be a problem. Further stopping down should have little effect. Some blemishes, such as Schneideritis, chips, and scratches, can scatter light onto the film from outside the image cone. An efficient lens hood reduces this.

There are two things that would concern me about a blemish on a lens:

1) If the blemish were in the light path, it would have a large effect - one could assume that light hitting this blemish would be scattered and would end up on the film in a place other than where it is desired. Depending on the nature of the blemish and the light, this might result in bright spots appearing, or if more diffuse, then a general loss of contrast. If the blemish is on the edge of the rear element, then one could expect these effects to get smaller the smaller the aperture. Note that even with a small aperture, with some movements you may be placing the blemish on the edge of the rear element in the light path.

2) Even if the blemish does not fall in the light path, light from outside the image area is liable to hit the blemish and be scattered onto the film, resulting in a loss of contrast or a flare like anomaly.

The usual tip is to use India ink or similar to blacken the blemish, preventing (or at least reducing) the defect from scattering light.

Cheers, Bob.

blemish

I would test the lens and see if you can see anything in the negative or print. You will probably be very surprised that the lens will work out just fine. I have a lens that is very hazy on the front element due to cleaning marks. Perhaps only a slight loss in contrast I can adjust for in the exposure / darkroom.