Basic Optics Questions - Exit pupil....

[Peter De Smidt]

I'm not very good with mathematical formula, nor am I very good with the physics of lenses. I'm trying to figure out what size to make Water-house stops for Imagons.

If I'm not totally off-base, which could easily be the case, the relevant formula is:

Focal length/relative aperture = diameter of entrance pupil of lens, noting the that diameter of the Water-house stop does not equal the diameter of the entrance pupil.

One issue is that Imagons don't have optics in front of the stop. Does exit pupil diameter tend to equal entrance pupil diameter? That's more of a general interest question. I didn't figure this out to make my stops.

In looking at my 300mm Imagon, I noticed that a hole size of 31mm gives an "H" value of 7.7.

Because I'm an idiot, I plugged those values into the formula to give an effective focal length of FL = 7.7 x 31 = 238.7, thinking that I could use the 238.7 figure with actual hole diameters to give f-stop equivalents, hoping this took care of the entrance pupil/actual stop diameter situation. In practice, this worked fine....but I usually expose negatives a bit exuberantly, i.e. when in doubt give more exposure. So my method is good enough for me, but some people have inquired about me making stops for them, and I'd like to make sure I'm not leading them astray as to the f-stop equivalents.

[Tin Can]

I would approach this empirically.

Get another 300mm lens and shoot them both in the same light with varying apertures.

Compare.

Maybe even just use a light meter on the gg...

[Peter De Smidt]

I'll go this way if the mathematical way doesn't work.

[Corran]

Peter, doesn't the Imagon have "sieve" aperture disks? Meaning that the aperture given is a combination of all the holes present?

I've only seen them in pictures, I don't own one.

Anyway, apparent aperture size is complicated since a front element generally enlarges the actual aperture size. However if the Imagon has no front element, it seems to me that you would simply divide the focal length by the wanted aperture to get the size hole you need for your Waterhouse stop. In other words, if you wanted f/32 on your 300mm, you would simply use a stop of size 300/32 or 9.375mm. If I were you, I'd confirm this as Randy mentions, except with a DSLR behind the camera and compare exposures at infinity and a given f/stop.

[Peter De Smidt]

Yep, they have sieve-style apertures, but I don't want to use them. I dislike them. The outer holes can be closed, and Rodenstock gives an H value for peripheral holes open or closed. The idea is to have circular apertures that are bigger than the largest single aperture in the supplied disks. This gives more glow/SF effect than the supplied disks with the peripheral holes closed, but it doesn't give the nasty out of focus highlight rendering of the sieves. I've tested this on my own lens, and it works well. I"m not going to have the lenses on hand. Someone has 8 Imagons or so, all of different focal lengths, and asked if I could help. I definitely don't want to be sent the lenses.

[Tin Can]

That’s a big ask

[Mark Sawyer]

One issue is that Imagons don't have optics in front of the stop. Does exit pupil diameter tend to equal entrance pupil diameter?

No. The entrance pupil is what you go by; in the Imagon's case, there's no front element to magnify the aperture's diameter, so the entrance pupil will be the same as the physical aperture. If you measure from behind (the exit pupil), the element will magnify the aperture, and give you a different measurement.

The H-stop value will be different because you have all the tiny outer apertures in addition to the main central aperture. You'd have to figure the area of one of those little apertures, multiply it by the number of them, add that area to the area of the central aperture, and figure what size aperture would have that area in order to find the equivalent f/stop.

And someone will probably criticize you because those little apertures are so small they're diffraction limited... :rolleyes:

[Peter De Smidt]

Thanks, Mark. What confuses me is that the measurement of the included disk, just the large center hole, doesn't give the expected result when plugging it into the formula. (Rodenstock gives an H value both for the peripheral holes open and when they're closed.)

Measured diameter of aperture of 31mm gives an "H" value of 7.7, according to Rodenstock.

In the formula, that gives: FL = 7.7 x 31mm = 238.7mm (But the FL is 300mm)

[Bob Salomon]

No. The entrance pupil is what you go by; in the Imagon's case, there's no front element to magnify the aperture's diameter, so the entrance pupil will be the same as the physical aperture. If you measure from behind (the exit pupil), the element will magnify the aperture, and give you a different measurement.

The H-stop value will be different because you have all the tiny outer apertures in addition to the main central aperture. You'd have to figure the area of one of those little apertures, multiply it by the number of them, add that area to the area of the central aperture, and figure what size aperture would have that area in order to find the equivalent f/stop.

And someone will probably criticize you because those little apertures are so small they're diffraction limited... :rolleyes:

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.

So it is probable that a single larger hole will create the effect of the central hole Plus the peripheral holes and prevent the spectral that bother the OP.

Why doesn’t he just use the aperture in a shutter which is continuously variable rather the try computing single disks?. With the 300 in a Copal 3 shutter the 5.8 to 7.7 h stops do not exist as the shutter threat is too narrow so that lens in shutter is 7.7 wide open.

[Peter De Smidt]

Because I've already made disks for myself, and they work great. I don't have a spare shutter that the cell would go in. Currently, I just use a Sinar Shutter. Making the disks is easy on a CNC.

[AJ Edmondson]

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

[Mark Sawyer]

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...

[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.

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/#.

[Peter De Smidt]

Thanks, Nodda. That's very helpful.

[cowanw]

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

[Mark Sawyer]

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


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.


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").


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...