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...

[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...

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.