"Extra" Aperture on Some Shutters (Copal/Schneider Symmar 135mm convertible)

[alt.kafka]

I've noticed that on more than a few of my shutters, there's a decent amount of "extra" aperture that is wider than the widest marking. What's the deal with this extra little bit of aperture? Is it generally assumed that totally wide open isn't part of the lens specification, and that *some* amount of stopping is required to meet specs? I get that some were manufactured/scaled for a specific lens which may have had a smaller exit diameter. But in the case where the exit diameter is really about the same size as the maximum aperture on the shutter, is there ever a part of the lens that is, according to the manufacturer's spec's, "unused", for lack of a better word?

Specifically, I have a 135mm Symmar convertible. f/5.6 on the scale is decently smaller than the widest that that shutter can go. Probably less than a full stop, but it's there. Now, the Copal 0 that it's in definitely isn't factory original to the lens. However, the scale on it has convertible markings, and they match up with the correct specs for that lens. So, either it's a.) just wrong, and I'll need to work out my own scale, or b.) correct, and the Symmar is designed to be operated a little stopped.

I guess my specific question is most easily answered if someone who has an original shutter on their 135mm Symmar convertible. At f/5.6, is it stopped down a little, or is it dead nuts wide open?

[Tin Can]

I have seen that on a few shutters and lenses

Normal

[abruzzi]

if you're looking through the lens from the front with the shutter open, do you see the aperture blades once you're wider than the widest indicated aperture?

On the couple lenses I have where there is space for the aperture to open wider than the indicated max aperture, the aperture blades disappear once I'm past the max aperture for the glass. My assumption was always that in that space, the lens mechanically obscured the rest of the travel so it had no actual affect on the actual aperture. (My Angulon 90/6.8 has a huge amount of travel after it passes 6.8--more than the distance between 6.8 and 32, but I don't see it open wider after that.)

[alt.kafka]

if you're looking through the lens from the front with the shutter open, do you see the aperture blades once you're wider than the widest indicated aperture?

When the aperture is set to f/5.6, the iris is partially closed. It will open wider than that, and I can retract the iris all the way, so that it's not obstructing anything.

On the couple lenses I have where there is space for the aperture to open wider than the indicated max aperture, the aperture blades disappear once I'm past the max aperture for the glass. My assumption was always that in that space, the lens mechanically obscured the rest of the travel so it had no actual affect on the actual aperture. (My Angulon 90/6.8 has a huge amount of travel after it passes 6.8--more than the distance between 6.8 and 32, but I don't see it open wider after that.)

No, that's a situation I understand. I have at least one shutter where the iris is blocked, because it would seem the lens it was originally on had a smaller exit diameter. I probably have a lens around here where the exit diameter is smaller than whatever the maximum is that can fit on a standard shutter, as well.

I'm referring to a situation where the stated aperture and the shutter max aperture are the same, but there iris isn't quite open all the way, and it would seem that the lens could actually have a wider aperture than stated. Basically, I'm trying to figure out whether I should trust this f/5.6 marking.

[abruzzi]

I think the only way to figure out what you want for certain is to actually measure the opening and see if it amounts to what ƒ5.6 should be on a 135mm (i.e. 24.1mm), then try a couple of other apertures to see if they scale correctly (i.e. ƒ22=6.1mm, ƒ45=3mm). FYI, that is measured as seen through the front elements of the lens. I've found it a bit difficult to do those measurements accurately, but the small fudge factor, probably isn't enough to affect exposure much.

[alt.kafka]

I think the only way to figure out what you want for certain is to actually measure the opening and see if it amounts to what ƒ5.6 should be on a 135mm (i.e. 24.1mm), then try a couple of other apertures to see if they scale correctly (i.e. ƒ22=6.1mm, ƒ45=3mm). FYI, that is measured as seen through the front elements of the lens. I've found it a bit difficult to do those measurements accurately, but the small fudge factor, probably isn't enough to affect exposure much.

OK, that makes total sense. So the implication of this is that, for a given aperture, say f/5.6, the diameter of the iris will necessarily be larger or smaller depending on the focal length of the lens. I understand that the actual diameter is a little difficult to measure, due to the variety of configurations of front lenses. But, in general, it should be necessary that some lenses will have a different iris size, and consequently, 'extra' aperture.

So, for the 135mm Symmar and Copal 0, I can accurately measure that the rear diameter is actually 19mm, and my actual iris opening is around 15mm. So, that looks like it's around 2/3 of a stop difference. When I unaccurately measure how these look through the front lens, it comes out to ~24mm, as you said, and the maximum iris opening is around 19mm. i.e. the same ~2/3 stop difference, as one would expect.

So maybe I just need to put a little more aperture on this compared to the scale.

[Bob Salomon]

I think the only way to figure out what you want for certain is to actually measure the opening and see if it amounts to what ƒ5.6 should be on a 135mm (i.e. 24.1mm), then try a couple of other apertures to see if they scale correctly (i.e. ƒ22=6.1mm, ƒ45=3mm). FYI, that is measured as seen through the front elements of the lens. I've found it a bit difficult to do those measurements accurately, but the small fudge factor, probably isn't enough to affect exposure much.

Or calibrate and use a light meter on the gg while the camera is focused at infinity on the sky.

[abruzzi]

OK, that makes total sense. So the implication of this is that, for a given aperture, say f/5.6, the diameter of the iris will necessarily be larger or smaller depending on the focal length of the lens. I understand that the actual diameter is a little difficult to measure, due to the variety of configurations of front lenses. But, in general, it should be necessary that some lenses will have a different iris size, and consequently, 'extra' aperture.

So, for the 135mm Symmar and Copal 0, I can accurately measure that the rear diameter is actually 19mm, and my actual iris opening is around 15mm. So, that looks like it's around 2/3 of a stop difference. When I unaccurately measure how these look through the front lens, it comes out to ~24mm, as you said, and the maximum iris opening is around 19mm. i.e. the same ~2/3 stop difference, as one would expect.

So maybe I just need to put a little more aperture on this compared to the scale.

there are more knowlegable people that me on this, but as I understand it, its not the actual opening that matters, but how large the opening looks through the front elements of the lens. The size you should see is the focal length of the lens divided by the ƒ number. So 135/5.6=24.1mm. If you have a Harbor Freight near you, buy the cheapest digital calipers they sell (less than $10, but for these puposes it should be fine). Rather than measurig the actual opening, open the calipers so the readout shows 24.1mm, then apply a little tape so it doesn't move. Open the shutter, and set the aperture to 5.6. Lightly rest the tips of the calipers on the front of the glass (the cheap calipers have plastic tips, so if you're careful, they shouldn't scratch anything) and see if the measurement is pretty close. Its probably easiest to put the lens on a light table facing up so you can see through it easily, and don't have to worry too much about it moving.

You can use this technique to create a new aperture scale if you find the one you have is way off.

EDIT: Bob's technique will work as well as is measures the actual amount of light that passes. I'm not 100% sure how you would actually do it.

[Dugan]

I just sold a Wollensak 90mm f12.5 wideangle lens...it was actually an f6.8 lens.
The shutter aperture opens all the way to f6.8 for focusing, but the aperture marks start at f12.5 because that's where full coverage at infinity focus starts for 4x5.
So, there's a bit of empty space on the aperture scale.

[reddesert]

In an original shutter, the aperture will often (but not always) be mechanically stopped so it only opens to the max rated aperture, like f/5.6 in a f/5.6 lens. Even though the shutter might be capable of a wider opening. There are times where this is not true, like Dugan's 90mm/12.5 and abruzzi's Angulon 90mm/6.8, where the wider opening is only for focusing.

Once a lens is remounted in a different shutter, there are at least two issues:
- was a correct aperture scale applied to the shutter, so that f/5.6 is actually f/5.6?
- the mechanical stop may be absent or in the wrong place, so that even if the scale is correct, you can open the iris past f/5.6.

You can determine the first issue by measuring the diameter of the iris as viewed through the front element as described above. The second issue is common and not anything to worry about. But the "extra" opening isn't useful, optically.