# Thread: Basic Optics Questions - Exit pupil....

1. ## Re: Basic Optics Questions - Exit pupil....

Rodenstock stated in one of the Imagon brochures that combining the area of all of the smaller holes (wide open), plus the central opening, does not yield the same opening as the computed f stop for the focal length... thus the term "H" (Helligket or lightness) instead of "f". The difference according to Rodenstock is slight (not quantified) but a difference nonetheless. I hope I am not confusing folks needlessly and I can't add more because that is all the information I was able to obtain from a lot of really old German technical notes on the Imagon. Someone with better math skills and understanding of diffraction/refraction can probably calculate the difference between "H" and "f" values.
Joel

2. ## Re: Basic Optics Questions - Exit pupil....

Originally Posted by Bob Salomon
Not that easy. Each Imagon has more then a single focal length, the peripheral holes control how much of the outer part of the lens is projected onto the central holes effect.
Kinda, but not really, but sort of, but no, but yeah. The Imagon, and all soft focus lenses, have a spread depth of field. That's how soft focus works. But you also focus soft focus lenses at the taking aperture, and pick the effect and focus you want from that range. With h/stops, the central hole is dominant, and it will be pretty consistently one focal length. It's the outside holes that are a bit shorter in focal length, giving that diffused glow...

It's a learning curve thing...

3. ## Re: Basic Optics Questions - Exit pupil....

f/# = focal length divided by aperture makes an assumption about the shape of the aperture, which is why the “H” value deviates from an f/#. f/# is really just a measure of light throughput, so with oddly-shaped apertures the mathematics are more complex and, in a practical sense, you’re better off measuring relative throughput similar to how cinematographers have their lens speed characterized by T-stop.

That said, with no lenses in front of the aperture, it’s a straightforward f/# = focal length divided by aperture cutout diameter. Without the funky aperture of the Imagon, it’s straightforward as well. So forget about the H factor if you’re making your own aperture. f/7.7 = 300mm / 38.9mm. Focal length stays constant... a 300mm lens is a 300mm lens regardless of the aperture diameter or shape (for the purposes of this discussion).

You can verify your focal length by measuring the distance from aperture to image plane when focused at infinity.

To your original question about f/# vs relative size of exit and entrance pupils... I usually just eyeball them..if exit and entrance pupil (the physical aperture in your case) look the same diameter then I don’t worry too much about the differences in actual vs working f/#.

5. ## Re: Basic Optics Questions - Exit pupil....

That speaks to the issue of using a symmetrical lens which was the subject of a previous post.thanks

6. ## Re: Basic Optics Questions - Exit pupil....

Just as general nit-picky food for thought...

Originally Posted by Nodda Duma
f/# = focal length divided by aperture makes an assumption about the shape of the aperture, which is why the “H” value deviates from an f/#. f/# is really just a measure of light throughput, so with oddly-shaped apertures the mathematics are more complex and, in a practical sense, you’re better off measuring relative throughput similar to how cinematographers have their lens speed characterized by T-stop...
F/stops are simple mathematical ratios. T-stops are "Transmission stops" with the value derived from actual bench-testing of lenses with high-end equipment not available or practical for most photographers. But being a simple coated doublet, the Imagon's f/stops are almost certainly closer to t/stop values than most lenses.

Originally Posted by Nodda Duma
You can verify your focal length by measuring the distance from aperture to image plane when focused at infinity.
With a soft lens that has a spread "zone of focus", that's a judgement call at best, and one that will change as different apertures change which areas of the lens are dominant, (the dreaded "focus shift").

Originally Posted by Nodda Duma
To your original question about f/# vs relative size of exit and entrance pupils... I usually just eyeball them..if exit and entrance pupil (the physical aperture in your case) look the same diameter then I don’t worry too much about the differences in actual vs working f/#.
Just for fun, I dug out my 300mm Imagon and the h/9.5-11.5 disk. It measured 23mm (f/13) through the front, and 27mm (f/11) through the rear, (presuming 300mm.) That's a half-stop difference. Most shutters are off by that much, so whether the OP wants to worry about it is up to him...

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