how do you calibrate f stops on a lens???

[Frank_E]

this can be considered a followup to my thread here:

http://www.largeformatphotography.info/forum/showthread.php?t=54669

where I sought input on how I might mount this lens
if you want to find out about the outcome of this exercise I have posted two pictures below. What I did, is make a homemade lens board using thin plywood (the camera is a Nagaoka 4x5) and simply did a "press fit" into the hole which I made to very tight tolerances. I could run a bead of glue around the flange, but it really doesn't need it, because the lens is in there so tightly already.

Anyways to my question of the day
(by the way I find this a very knowledgeable and ready to share group.....)

I have heard previously that if you mount a lens into an alternate shutter you need to recalibrate/remark the appropriate f stops to get correct exposure.

I didn't know if the lens and illex shutter were originally mated to gether, but am suspicious because this 6.5 inch lens is rated at f4.5 but the iris opens up, according to the scale, up to more than f1.9. After 1.9 it can still open a little more.

So this leads me to various questions:

1. Both the lens and the shutter have "Illex Paragon" written on them, causing me to think the were originally mated. But why the above discrepancy?

2. How can I do a measurement to ascertain what the correct f stop measurements of the lens might be? I do have a Gossen Digsix lightmeter.

thanks again, in advance, for your very valuable and generous advice

[Denis Pleic]

1. Both the lens and the shutter have "Illex Paragon" written on them, causing me to think the were originally mated. But why the above discrepancy?

2. How can I do a measurement to ascertain what the correct f stop measurements of the lens might be? I do have a Gossen Digsix lightmeter.



1. Your lens and shutter were definitely NOT mated together. Ilex made many things labeled "Paragon" :) Your shutter originally housed a 75/1.9 lens intended for taking photos of oscilloscope CRT screens. Info about "Ilex Oscilo Paragon" is easily found via Google.

2. You might use your light meter (preferably with some kind of optic probe) to measure light falling on the ground glass. Make sure you point the camera towards a uniformly lit surface (use artificial light, i.e. lightbulb or something, as long as it gives uniform illumination). Make sure the lens is wide open, and find the combo on the light meter that gives you f4.5. Then start closing the iris - without changing the intensity of light... and find the positions of other f-stops by reading the light meter as you close the iris.

Just a hint: the formula for calculating the f-stop is: lens focal length / iris opening = f-stop. This formula will give you another way to find the f-stops.

Shift it around, and you get: lens focal length / f-stop = iris opening

Applying the formula, the opening of F1.9 on the original 75mm lens that your shutter was designed for yields iris opening of approx. 39.47 mm. This opening (i.e. leaving the iris on F1.9) with your new lens (6.5 in = 165mm) would yield approximately F4.2 (but it won't be exactly that because the lenses are not the same). Measure the iris opening from the front of the lens - it will probably be less than 39 mm...

So, you have two approaches - a practical one (use the light meter and measure light on the ground glass) and one which needs some calculation, but might be a bit problematic, since you need to measure the iris opening very precisely.

I'd go for the hands-on approach (using the light meter).... or the combination of both techniques.

Denis

[aduncanson]

Actually, measuring the aperture is further complicated since the value given by the formula above is the Entrance Pupil, not the physical size of the aperture.

Some here have suggested measuring the entrance pupil by placing a point source (not easy to make practically) at the focal point of the lens (not easy to locate precisely) and measure the diameter of the pencil of light exiting the front of the lens.

Part of the problem is that due to the magnifying power of the front cell, the apparent size of the aperture varies with the distance from which you observe it. The entrance pupil is the apparent size from an infinite distance. Therefore, I have suggested placing a well illuminated white panel behind the lens and measure the diameter of the aperture with binoculars from across the room.


This is why you may want to find a way to use your light meter to calibrate the aperture scale.

[Peter K]

Some here have suggested measuring the entrance pupil by placing a point source (not easy to make practically) at the focal point of the lens (not easy to locate precisely) and measure the diameter of the pencil of light exiting the front of the lens.
This suggestion was made originaly by H. A. Steinheil (1832 - 1893), of course not in this forum. But Emmanuel Bigler has it described here (http://www.largeformatphotography.info/forum/showthread.php?t=32332).

I've made a black plastic plate from a DVD-box in the same size as my 4x5" fresnel-lens. So the plate with the white LED can easily exchanged with. And on this plate is plenty of room for a switch and batteries. The thickness of the groundglass has a neglegtable influence on the measuring result of the diaphragm diameter in the entrance pupil. But the clear spot in the middle of the groundglass helps a lot.

Peter

[Denis Pleic]

I know it's not as simple as my explanation above... :)
Personally, I used the light meter approach when I was facing the same problem...

Denis

[percepts]

Note that the lens extension must be set exactly at infinity when doing above suggestions.

But I have a question. If you were to place a piece of white paper in a film holder and use the same make of white paper as your illuminated subject, then if you meter the illuminated subject, should metering the paper in the film holder and calculating the difference from the subject give the current fstop of the lens? If that is correct then it will acount for light absorption by the lens and frees you from making any measurements of actual aperture size or worrying about where the entrance pupil is located. Lens must be set at infinity.
But you have to setup a darkroom environment, remove bellows and ensure no light from the subject floods onto the paper in the film holder. You really need to measure both subject and paper in film holder from same angle, ideally perpendicular.
I think that will give 1/10 stop accuracy is if your light meter is that accurate.

[rdenney]

By the way, my Ilex Paragon 8-1/2" f/4.5 lens is mounted in an Ilex No. 4 Universal shutter. It might have also been mounted in an Acme No. 4, or one of the other Ilex general-purpose shutters.

Were I dealing with this, I would take a known-good lens, set it to a known aperture (say, f/5.6), aim it at an evenly illuminated surface, and then with a spot meter under my focus cloth measure the center of the ground glass, making sure to be aligned with the middle of the bright spot on the focus screen.

Then, I would put the mystery lens in the camera, aim it at the same surface, and rotate the aperture control until the meter read the same thing. I would mark that spot f/5.6. I would then keep turning it until the meter read one EV down, and mark that f/8, and so on. I would not trust a mis-mated shutter to provide the same maximum opening as the original shutter, so the maximum aperture on the lens might not be the place to start.

My Pentax spot meter has a tripod mount, and I think I would use that to fix the position of the meter as a matter of convenience.

Rick "who has a 65/8 SA in an uncalibrated Prontor Press shutter that needs this same treatment" Denney

[Peter De Smidt]

I agree with Rick. If you have access to a friend with a Sinar 4x5 metering probe and meter, this will allow you to match illumination at the film plane to 1/10th of a stop.

[Mark Sawyer]

how do you calibrate f stops on a lens???

1.) measure the bellows extension.

2.) measure the aperture through the front element (entrance pupil).

3.) divide #1 by #2. This is your f/stop.

For the "absolute" value, do step 1 at infinity.

[aduncanson]

1.) measure the bellows extension.

2.) measure the aperture through the front element (entrance pupil).

3.) divide #1 by #2. This is your f/stop.

For the "absolute" value, do step 1 at infinity.

Mark, Does this imply that the entrance pupil for a telephoto lens of a given f/stop would be smaller than the entrance pupil for a conventional lens? That would be a surprise. After all the image size, or magnification, is the same for a tele design. It would need the same entrance pupil (light gathering ability) to form an equally bright image.

I guess that what I am trying to say (as gently as possible) is that the measure of bellows extension required is from the focusing screen to the rear nodal point of the lens, or when focused at infinity, the focal length. So why measure it? (Unless the focal length is unknown, in which case we have a whole 'nother contentious kettle of fish.)

[Tim Meisburger]

I think some of these explanations are over-complicated. I have a rapid rectilinear lens that is so old that it uses an old system of f stop numbering that bears no relationship to the current system. This lens has a front element and a rear element, and by removing one or the other the lens can be converted into about a 270mm and a 340mm. So, I was faced with determining the f stop for three different lenses. All I did was focus at infinity with each set up, measure the focal length from the film plane to the middle of the shutter (i figured that would be close enough), then set the aperture to each of the old f stops and measured it with a ruler, then performed the calculation (focal length divided by diameter of the aperture) to get the modern f stop and recorded that is a table. Later I made a paper label listing f stops for all three focal lengths that I taped over the old scale. For three focal lengths the whole operation took about 45 minutes. You could probably do one in 15.

[aduncanson]

...I have suggested placing a well illuminated white panel behind the lens and measure the diameter of the aperture with binoculars from across the room.

Perhaps "over-complicated" should be my middle name. I know it sounds ridiculous, but I just performed the experiment of trying to measure the entrance pupil on my 210/5.6 Caltar-S II, first in my hand using my bifocals to read the scale. I measured 31mm. Then I tried from across the room (16 ft.) with my hand held 10x binoculars, propping the scale up against the filter ring of the lens. That time I measured between 35mm and 37mm. The calculated entrance pupil for this lens is 37.5mm. I conclude that measuring the entrance pupil in a lens of this design (a 6/4 plasmat) from close up can induce an error of greater than 1/2 stop.

I measured the entrance pupil of a 127mm Ysarex in my hand and got 24.5mm rather than the calculated value of 27mm. This smaller error (still .28 stops) is probably due to the weak negative front cell of the Tessar derived design. Using the binoculars from 16 ft, I measured between 26 and 28mm.

Tripod mounted binoculars or a spotting scope would help reduce the uncertainty in measuring the entrance pupil from a distance in this way. Photographing the entrance pupil along with a scale and analyzing an enlarged image seems to be a promising technique. It might be easily accomplished with a tripod mounted digital camera with good resolution and a long lens. My attempts to do this hand held using a digicam without those features dramatically illustrated the error introduced by measuring from close up. From about 8 inches, the Ysarex entrance pupil measured just 22mm.

Also illustrated in this data is that you really do not have to be at a very great distance to make an accurate entrance pupil measurement. Across a reasonably sized room is sufficient.

Afterthought: Some of the error in the measurement from my digicam close up is due to parallax arising from the depth of the entrance pupil behind the scale. Measuring by eye with a scale, one instinctively tries to compensate for this. Subtracting a calculated estimate of the contribution of parallax still leaves an entrance pupil measurement of approximately 24mm.

[rdenney]

I was also unable to resist experimentation. I measured three apertures on my 8-1/2" Ilex Paragon, which is a Tessar design, using calipers laid across the front of the lens filter ring and looking at it from maybe a foot away. The f-stops I measured were very nearly exactly the marked f-stops on the lens. (example: f/5.6 marked = f/5.54 measured).

Then, I tried it with a 65/8 Super Angulon, and ran into a couple of immediate problems. The first is that the small apertures on a 65 are small indeed, and the precision of the measurement method not really good enough. The second is that the Prontor Press shutter has five blades and it requires some creativity to know where to measure to get the effective diameter of a similar circle. But at f/8 I was able to get pretty close, not that that helps.

Rick "back to using a meter" Denney

[aduncanson]

Rick, I have been thinking about this, particularly the implications of my afterthought on parallax. It occurred to me that one could accurately measure the entrance pupil from close at hand by moving your eye back and forth to determine each edge of the aperture to be where it appears when your eye looks straight into the lens, that is looking parallel to the lens' axis. A mirror perpendicular to the lens axis could be used to line up the aperture edge, the eye, and the reflection of the eye. Is it possible that your method using calipers achieved this same alignment?

As for the pentagonal aperture, measure from one point directly across to the opposite flat side. Multiply that measurement by .962 to get the diameter of a circle with same area as the pentagon. (Somebody may want to check that factor.) Of course if the aperture blades are not straight but are curved, the problem is more complicated, but I would consider making some intermediate assumption (say .980) and calling it good enough.

- Alan

[Jason Greenberg Motamedi]

Somewhere on this forum is a description on how to measure the entrance pupil using a laser light pointer. As I recall, the basic points were to focus the camera at infinity. Bring the camera inside in a dim room and face the lens towards a smooth white surface, within a foot or so. Shine a laser pointer (the smaller and rounder the point the better) into the ground glass. An image will form on the "smooth white surface" which is identical to the entrance pupil plus or minus the size of the laser point. At that point to get the fstop you need to measure the size of the laser light coming out of the lens and divide it by the focal length of the lens.

This is not my idea and I can't recall whose it was, but I have used this many times in the past with success.