1

For example here is a rear element with a filled in circle in the middle.

enter image description here

What happens to the picture? Is there image quality degradation? Do I lose part of the picture? What about amount off light transmitted?

How do you calculate the effective aperture in terms of light transmitted when something like this happens.

7

It varies based on several factors:

  • Focal length: Same sized obstructions on the rear elements of telephoto lenses show up more than on the rear elements of wide angle lenses.
  • Aperture: Same sized obstructions in the center of the rear element will show up more at smaller apertures than at larger ones.

Roger Cicala, the founder and chief lens guru at lensrentals.com wrote a fascinating blog post that delves into just how much something has to obstruct the optical path before it is noticeable in a photo. There are several examples of what a shot taken with large pieces of paper stuck to the back of the lens looks like.

2

The school photo finishing industry uses cluster lenses to project multiple identical images on photo paper. Try as you might to make all the lenses on a board identical, some pass more light than their neighbors. We resorted to dotting the lenses with opaque spots of paint to equitize. These act to reduce the amount of light that transverse the lens. As rule, no appreciable degrading of image quality resulted. The amount of light reduction is simply the percentage of the surface area opaqued.

On the other hand, once upon a time portrait lenses were favored it they yielded a soft focus. One scheme was to opaque a large circular portion in the center of the lens. This caused the lens to image using only rays from the peripheral which inherently are poorly corrected for aberrations.

  • I fail to see the relevance: a blockage of a complete lens in a cluster of lenslets is completely different from blocking part of the wavefront in phase space. – Carl Witthoft May 25 '16 at 12:21
  • I am taking about a cluster of lenses, each projects a duplicate image. The lens maker fits each lens in the cluster with a fixed size aperture. The diameter of the aperture determines image brightness. The idea is to speed equalize all. A spot of black paint on a lens is often used to fine tune. – Alan Marcus May 25 '16 at 15:06
  • I'm aware of the config - it's basically a Shack-Hartman test assembly. Those lenses are simple lenses (technical term!) so the black dot is near a principal plane & nowhere near a focal plane. Not what the OP is asking. – Carl Witthoft May 25 '16 at 15:33
  • @ Carl Withoft -- Our lenses at Nord Photo Engineering were not simple, they were process lenses of the Apro-Tessar and Raptar Wallensak design. Some were even more complex, somewhat similar to the projection lenses that rotated the image 90 degrees as did the Vista Vision Cine Projection lens. How do you know if the OP is asking about a dot near the focal plane? – Alan Marcus May 25 '16 at 19:59
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Michael Clark's answer should suffice for nearly all commercial camera lenses. The long answer is that the effect of blocking part of an aperture depends on where that aperture is in the lens system. Since the rear element is typically close to the image plane, the blockage occurs primarily in "image space," darkening the geometric area as shown in the link Michael provided. If instead you block in "phase space" , i.e. where the light is not near focus (there can be internal focal planes in complex lens systems), then Fourier optics rules apply. In brief, blocking the center will remove low-spatial-frequency information and give you an image that emphasizes high-frequency regions over low-frequency(i.e. no change in intensity over some distance) regions. If you block ring-shaped regions near the edge of the lens, you'll remove some of the high-spatial frequency information, essentially causing smoothing/blurring of the image.

  • 1
    Since the question includes the qualifier "rear element"... – Michael C May 26 '16 at 2:59

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