large rear optical elements question

[John Jay]

The P+S adaptors make the folowing qualification....

''The unit is optimized for 35mm lenses with large rear
optical elements, such as Cooke S4 Primes, Zeiss Ultra Primes and Zeiss HS as well as Zeiss Distagons with focal lengths over 85mm.''

My question is - what is the reason behind this and what is the breakpoint (expressed in mm diameter of the rear element) when the rear element would no longer be considered large.

[Guy Genin]

Hi John,

The reason behind this is the ground glass. It turns out that to obtain the desired control over the depth of field, we need to create a physical image in the film plane of the film lens, so we need a ground glass. The drawback of the ground glass is not all the light can be captured by the relay system of the adapter. With modern film lenses designed to cover more than the Academy format there is enough light distributed across the image. With older film lenses, just designed to cover the Academy format, there is a significant loss of light in the corners that becomes unacceptable though the adapter. The diameter of the rear element is a good indicator of the type of lenses you are looking at, but this is not a hard rule as it will be different depending on the focal length.
This is why I cannot give you a hard number in mm. The lenses listed in your quote are the proven lenses.
Now this is relevant only to the film lenses, if for example you are considering still lenses like the Nikon. They are designed to cover a considerably larger format so they would not have the problem of uneven light distribution. However they are not designed for moving pictures so they are not likely to be as smooth and strong mechanically.

Guy Genin.

[John Jay]

Guy

Superb answer thanks.

I infer from this that, in respect of SLR lenses, the issue you have described would not exist for portrait and longer lenses but may be problematic for standard to wide lenses - in which case a rear element of say 27mm would be safe

for example a Nikkor 28mm F2.0 (~14mm rear) would not be a wise choice

[Guy Genin]

Jack,

The answer I gave you is very relevant to the motion picture film lenses. In the case of the still lenses the problem is not as important as the Mini 35 digital transmits only the center of the original frame. So yes, the longer lenses with element as big as 27mm should not have any noticeable vignetting. But the final answer for any lens and the Nikkor 28mm F2.0 in particular, is to test it.
If you or somebody out there, has a chance to setup the adapter with this lens in front a uniformly lit background (It is critical to have even light every where in the frame) then the image captured by the camera will tell us immediately if the vignetting is acceptable.