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Photo Forum / Film Photography / Medium format / August 2007About central (diaphragm) shutters at high speeds/large apertures
Hello, I have a old 6x6 TLR (Zeiss Ikon Ikoflex), it has a mechanical central shutter (Compur Rapid). I remember that there is an issue with the exposure at high speeds and large apertures with this kind of shutter, because of the time needed for the shutter to fully open (and to fully close). This raises two questions: -- when we select a speed of 1/125 (for instance), how does the shutter is supposed to behave ? If it starts opening à t=0, does it starts closing at t=1/125s ? Or does it actually start closing *before* that, so that it is fully closed at t=1/125 ? -- typically, how fast were these shutters to go from full closure to full aperture ? And finally: are there some tables around that give the exposure corrections for that ? Thanks, -- pehache > Hello, > [quoted text clipped - 22 lines] > -- > pehache There's a good discussion and some hand-drawn diagrams showing how a Compur shutter works here: http://rick_oleson.tripod.com/index-88.html He doesn't give numbers, except for shutter speeds, but shows the cam mechanism that controls the shutter blades' opening and closing speeds. Also that there are usually several mechanisms governing different speed ranges. --Nicco On 16 août, 18:11, Niccolo Machiavelli <Machiave...@communo.firenze.it> wrote: > There's a good discussion and some hand-drawn diagrams showing how a > Compur shutter works here: http://rick_oleson.tripod.com/index-88.html > He doesn't give numbers, except for shutter speeds, but shows the cam > mechanism that controls the shutter blades' opening and closing speeds. > Also that there are usually several mechanisms governing different speed > ranges. Thanks, that's pretty interesting. Indeed, I have to select the 1/500 speed *before* cocking if I want it. Now I better understand why (this enables the use of the booster spring). -- pehache > I have a old 6x6 TLR (Zeiss Ikon Ikoflex), it has a mechanical central > shutter (Compur Rapid). > I remember that there is an issue with the exposure at high speeds and > large apertures Small aperture. > with this kind of shutter, because of the time needed > for the shutter to fully open (and to fully close). [quoted text clipped - 5 lines] > starts closing at t=1/125s ? Or does it actually start closing > *before* that, so that it is fully closed at t=1/125 ? The shutter speed is roughly the time between the shutter being half open and half closed again. > -- typically, how fast were these shutters to go from full closure to > full aperture ? That varies a bit per brand and type, and shutter size. But perhaps as much as 1 ms. > And finally: are there some tables around that give the exposure > corrections for that ? Nope. The effect is small. And variable with aperture. So no tables needed, nor really possible. > > I remember that there is an issue with the exposure at high speeds and > > large apertures > > Small aperture. Yep, that's what I see now from the next answer, that the shutter is calibrated to give correct exposure at large apertures... > > with this kind of shutter, because of the time needed > > for the shutter to fully open (and to fully close). [quoted text clipped - 8 lines] > The shutter speed is roughly the time between the shutter being half open > and half closed again. OK. Close to my first hypothesis, then... > > And finally: are there some tables around that give the exposure > > corrections for that ? > > Nope. > The effect is small. Not that small, more than 1 IL in the worst cases... > And variable with aperture. That's why a table may help :-) -- pehache > Hello, > > I have a old 6x6 TLR (Zeiss Ikon Ikoflex), it has a mechanical central > shutter (Compur Rapid). > And finally: are there some tables around that give the exposure > corrections for that ? The 1958 Ilford Manual of Photography gives the following table. It simply calls it the performance of a "good between the lens shutter," but it is very likrly from a Compur-Rapid because the 250th speed is almost as inefficient as the 1/500th speed - a characteristic of the old Compur-Rapid shutters which used an extra spring for top speed. The Synchro-Compur shutters which were contemporary to the 1958 Ilford Manual were actually quite a lot better and had the modern speed sequence. Marked Effective Exposure time in milliseconds Speed f/3.5 f/5.6 f/8 f/11 f/22 1/10 100 103 105 108 110 1/25 40 43 45 48 50 1/50 20 22 24 26 27 1/100 10 12 13 14 15 1/250 4 5 6 6.5 7 1/500 2 2.5 3 3.5 3.8 If you are shooting negative film, there is no need to worry; an extra bit of exposure does no harm at all. If you are shooting slide film, the difference is serious at 1/500 and 1/250. It is arguably significant at lower speeds. On a more modern shutter the problem is only likely to be serious at 1/500th of a second. Peter. -- pirwin@ktb.net > The 1958 Ilford Manual of Photography gives the > following table. It simply calls it the performance [quoted text clipped - 7 lines] > to the 1958 Ilford Manual were actually quite > a lot better and had the modern speed sequence. OK. So as a first approximation, I think I can use the following table for the Compur-Rapid, just assuming that the 1/250 is as bad as the 1/500 of the table, the 1/100 is as bad as the 1/50 of the table, and so on... > Marked Effective Exposure time in milliseconds > Speed [quoted text clipped - 6 lines] > 1/250 4 5 6 6.5 7 > 1/500 2 2.5 3 3.5 3.8 So the shutter is calibrated to give the right effective exposure at large aperture, which makes sense. Small apertures are used mostly when the body is on a pod and when we have time to think about a correction :-) > If you are shooting negative film, there is > no need to worry; an extra bit of exposure > does no harm at all. Sure... > If you are shooting slide film, I do shoot slide film (B&W slide film), that's why I worry about that. > the difference > is serious at 1/500 and 1/250. and probably already at 1/100 with a Compur-Rapid... > It is arguably > significant at lower speeds. On a more modern > shutter the problem is only likely to be > serious at 1/500th of a second. I guess that on modern electronically-controlled shutters, the corrections are even handled by the system to keep the effective exposure always correct ? -- pehache > > The 1958 Ilford Manual of Photography gives the > > following table. It simply calls it the performance [quoted text clipped - 12 lines] > 1/500 of the table, the 1/100 is as bad as the 1/50 of the table, and > so on... Sorry, I think I missed your point. What you actually say is that this table IS likely for a Compur-Rapidv (and not for a better shutter). (my english is not perfect and I read it too quickly at first). So I can use it as is... -- pehache > So the shutter is calibrated to give the right effective exposure at > large aperture, which makes sense. Small apertures are used mostly > when the body is on a pod and when we have time to think about a > correction :-) The problem can only arise if you are using fast film (Say EI 100 or faster) in full daylight. The shutter makers didn't think you would normally be doing that, and certainly not with reversal film. If you were shooting 32 speed Ektachrome, you would never run in to the problem - at 1/500 second you would be using f/4 under full sun. Nowadays EI 100 is a normal speed for reversal film, and if you shoot under full sunlight, you may have to keep shutter efficiency in mind. > I do shoot slide film (B&W slide film), that's why I worry about that. I've never shot B&W reversal in a still camera. What film and processing are you using? Peter. -- pirwin@ktb.net > The problem can only arise if you are using fast film (Say EI 100 > or faster) in full daylight. The shutter makers didn't think you [quoted text clipped - 6 lines] > shoot under full sunlight, you may have to keep shutter efficiency > in mind. Definitely... I take the opportunity of this thread for another question: some time ago I read in an old manual that transmission losses in lenses without multicoating were not negligible... I remember something like 30% losses for a 4-element lens (Tessar). Can someone confirm that ? > I've never shot B&W reversal in a still camera. What film and > processing are you using? Actually I used to shot with Agfa Scala, which is now discontinued. I still have some rolls, however, and some stores still have some also. I do not process it on my own, but rather send them to a lab (a single one in France is still processing this film). -- pehache On Aug 20, 12:28 pm, pehache-tolai <pehach...@gmail.com> wrote:> > I take the opportunity of this thread for another question: some time > ago I read in an old manual that transmission losses in lenses without > multicoating were not negligible... I remember something like 30% > losses for a 4-element lens (Tessar). Can someone confirm that ? For an uncoated lens with six air-glass surfaces (as in a Tessar or other lens with three groups) you get around 74% transmission (0.95^6), of image forming light so your figure is about right. A single coated Tessar will pass more than 90% of the light (say 0.985^6). But, some of the reflected light hits the film in the form of flare, so in practice your exposure hardly changes at all. The exposure recommendations for Kodachrome did not change with the advent of coated lenses. The case may be different in complex lenses with more than 3 groups, but with Triplets, Tessars and Sonnars it is close enough not to matter. Even the difference between 74% (for uncoated) and 90% (for single coated) is less than a third-stop, and since at least some of the reflected light does appear as flare, one shouldn't expect very much exposure difference. > Actually I used to shot with Agfa Scala, which is now discontinued. It is too late for me to try Scala now. I have been planning to try my own reversal processing, but I have never got around to it. Peter. -- pirwin@ktb.net > This raises two questions: > [quoted text clipped - 8 lines] > And finally: are there some tables around that give the exposure > corrections for that ? >From the Kodak Professional Photoguide, 1981: <quote> Corrections for faster blade shutter speeds and small apertures Blade shutters at their faster shutter speeds and small lens apertures tend to give more exposure than is indicated by the shutter speed, f- number combination. This is not a manufacturer defect but is due to the inertia of moving parts combined with the geometry of the lens- shutter-diaphragm structure. It will usually occur in daylight photography with fast films and will cause overexposure of up to 1 stop. You can find the correction factor to apply in these situations by running an exposure series on a fast transparency film such as Kodak Ektachrome 200 (Daylight). This effect does not occur with focal-plane shutters. The following table gives an example of the type of correction you may find necessary. (cut and paste the following to notepad so you get a monospaced font) Lens aperture Speed 1/125 1/250 1/500 f1.4-5.6 None None None f8 None None 1/2stop f11 None 1/2stop 2/3stop f16 and smaller 1/2stop 2/3stop 1stop </quote> Aren't you glad some of us never throw away old manuals? ;-) On Aug 17, 1:32 am, pehache-tolai <pehach...@gmail.com> wrote: > This raises two questions: > [quoted text clipped - 8 lines] > And finally: are there some tables around that give the exposure > corrections for that ? >From the Kodak Professional Photoguide, 1981: <quote> Corrections for faster blade shutter speeds and small apertures Blade shutters at their faster shutter speeds and small lens apertures tend to give more exposure than is indicated by the shutter speed, f- number combination. This is not a manufacturer defect but is due to the inertia of moving parts combined with the geometry of the lens- shutter-diaphragm structure. It will usually occur in daylight photography with fast films and will cause overexposure of up to 1 stop. You can find the correction factor to apply in these situations by running an exposure series on a fast transparency film such as Kodak Ektachrome 200 (Daylight). This effect does not occur with focal-plane shutters. The following table gives an example of the type of correction you may find necessary. (cut and paste the following to notepad so you get a monospaced font) Lens aperture Speed 1/125 1/250 1/500 f1.4-5.6 None None None f8 None None 1/2stop f11 None 1/2stop 2/3stop f16 and smaller 1/2stop 2/3stop 1stop </quote> Aren't you glad some of us never throw away old manuals? ;-) An interesting thread. But how important is all this given that the sutter speeds of mechanical shutters, especially older ones, are probably not precisely accurate to begin with. Remember reading somewhere a report that found many 1/500 speeeds on several mechanical shutters to be closer to 1/300- 1/400. So it's probably not worthwhile tryng to make very minor exposure adjustments anyway. Recently, Norm Fleming <nfleming@ms.umanitoba.ca> posted: > An interesting thread. But how important is all this given that the > sutter speeds of mechanical shutters, especially older ones, are > probably not precisely accurate to begin with. Remember reading > somewhere a report that found many 1/500 speeeds on several > mechanical shutters to be closer to 1/300- 1/400. So it's probably > not worthwhile tryng to make very minor exposure adjustments anyway. Hmmm. A couple of things come to mind. The variance in exposure times in this thread were *worst* at 1/500th, so it is possible that what you recall was reporting the same phenomenon. Also, 1/350 (the intermediate of your estimates) would be about 1/2 stop off. Given a choice between being 1/2 stop or 1 stop off, I'll take the lesser of the two, thank you! ;-) The bottom line is that being a mechanical issue the variance will be repeatable, therefore it can be tested, measured and compensated for in use. I see that as a Good Thing compared to some other issues. Neil > (cut and paste the following to notepad so you get a monospaced font) > Lens aperture [quoted text clipped - 7 lines] > Aren't you glad some of us never throw away old manuals? > ;-) Sure ;-) !! -- pehache > Hello, > [quoted text clipped - 22 lines] > -- > pehache There is a common misconception about this. The shutter speed is really a measure of the amount of light it allows to fall on the film. The true time it takes will be longer as the shutter has to speed up to open and speed up to close. > There is a common misconception about this. The shutter speed is > really a measure of the amount of light it allows to fall on the film. Yes... but at which aperture ? It appears from the tables posted by other contributors that the shutter speed is usually calibrated for the full aperture and that corrections are needed for small apertures. -- pehache |
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