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Old 10-09-2019   #18
nzeeman
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Join Date: Nov 2005
Location: belgrade
Age: 39
Posts: 1,243
thanks!!! that's actually very useful!! cleared many things that confused me! and thanks to everyone else for helping me understand stuff here... sorry i answer a bit later but was trapped with a baby and baby stuff-you know how kids eat time...
Quote:
Originally Posted by drew.saunders View Post
Every lens design has a maximum aperture, and an angle of coverage. It's rare to get the angle of coverage information from manufacturers except for large format lenses. In LF, the manufacturer doesn't know what film size you're using, so needs to provide the angle of coverage, which will determine the image circle, so you'll know if a lens covers just 4x5" or 5x7" or 8x10" or larger.

The Tessar first came out with a maximum aperture of f/6.3 and an angle of coverage around 50 degrees (I don't have the exact information) and, over time, was re-worked to get f/2.7 and 60-64 degrees. There are no f/2.0 Tessar's, it just can't be done, you'll need a different design (such as the Sonnar) to get there.

Given an angle of coverage and focal length, you can calculate the image circle. See post #4 here: https://www.largeformatphotography.i...angle+coverage
For a LOT of good information.

For 24x36mm film or digital, you need an image circle that's at least equal to the diagonal (43.3mm) and preferably a bit more, so 44mm or larger is best. For APS-C, which is not quite 16mm by not quite 24mm, you need an image circle of about 29mm.

I don't know the angle of coverage of the Sonnar and couldn't find that information, so let's take a theoretical lens design with a 50 degree angle of coverage. With a 50mm focal length, that lens would have an image circle of 46.6mm, and would cover 24x36 and APS-C. With a 35mm focal length, that lens design would have an image circle of 32.6mm, and would not work on 24x36mm, but would work on APS-C.

A lens design with a 64 degree angle of coverage and a focal length of 35mm would produce a 43.7mm image circle, just enough for 24x36 (and more than enough for APS-C). Later Tessar lenses (such as the 35mm f/2.8) just barely covered 24x36mm, so likely had an angle of coverage around 64-65 degrees, but that information is hard to find. I have several large format Tessar design lenses (one Zeiss Tessar, two Fujinon Fujinars, one Schneider Xenar and one Nikkor-M), and most are around 60 degree in coverage, or a bit less.

That's a rather long winded way of saying that a 35mm focal length Sonnar almost certainly doesn't cover the 24x36mm format.

Does that make everything clear as mud?
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