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Photo Forum / Film Photography / Medium format / July 2009Q: Exposure correction for leaf shutters
I have read that for leaf shutter speeds used at the faster speeds for smaller apertures such as f11, f16 and f22 then there is an exposure correction that should be used. It is supposedly in the publication "Kodak Professional Photoguide" but I don't have a copy of this. Does anyone know what this exposure compensation table contains? > I have read that for leaf shutter speeds used at the faster speeds for > smaller apertures such as f11, f16 and f22 then there is an exposure > correction that should be used. It is supposedly in the publication > "Kodak Professional Photoguide" but I don't have a copy of this. Does > anyone know what this exposure compensation table contains? I found mention of this table and partial values in this post: Hey Todd, I remember asking this same question about a year ago after coming across a chart on Exposure Correction for Leaf Shutters in the KODAK Professional Photoguide. Basically, the chart notes that at 1/125 and f16, you need to reduce the lens opening by 1/3 stop. 1/250 @ f11 = -1/3 1/250 @ f16 = -2/3 1/500 @ f8 = -1/3 1/500 @ f11 = -2/3 1/500 @ f16 = -1 Basically, look at it as a sort of right angle triangle with the right angle at the south-east and you can picture the chart. I hope that this helps. Kent > > I have read that for leaf shutter speeds used at the faster speeds for > > smaller apertures such as f11, f16 and f22 then there is an exposure [quoted text clipped - 22 lines] > > Kent I am wondering if some kind person who has this guide can send me a scan. I am wondering if some kind person who has this guide can send me a scan. <<<<<<< Faster to type it f stop | 1/125 | 1/250 | 1/500 f/8 | None | None | -1/3 f/11 | None | -1/3 | -2/3 f/16 | -1/3 | -2/3 | -1 Remember, _reduce_ exposure by amount indicated. You might want to test it, though. I don't recall ever having exposure problems with modern leaf shutters (Fuji 645, Mamiya 7, even post war Rolleiflexes).  SignatureDavid J. Littleboy Tokyo, Japan > I am wondering if some kind person who has this guide can send me a > scan. [quoted text clipped - 14 lines] > David J. Littleboy > Tokyo, Japan Thanks for that. I'm seeing this problem on my old folders when I use f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge of shutter speed. The shots I take are overexposed by between a third stop and a half stop which really shows up on slide film. > Thanks for that. I'm seeing this problem on my old folders when I use > f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge > of shutter speed. The shots I take are overexposed by between a third > stop and a half stop which really shows up on slide film. It may also be that the shutter is slower than the marked speed due to age, i.e. lack of lubrication or being out of adjustment, or exaguration (I don't think I spelled that correctly) by the manufacturer. Many 35mm SLR's from Japan in the 1950's and 1960's 1/500th was really 1/300th, and I believe many cheaper leaf shutters where similar. The main exception would be Kodak leaf shutters, by now they are all slow to unusable without being taken apart, the old grease removed and re-oiled. If they were done properly, even in the 1960's they won't need it now. If you have a CRT Television set you can photograph the screen and count the scan lines, but those are on their way out. Geoff. SignatureGeoffrey S. Mendelson, Jerusalem, Israel gsm@mendelson.com N3OWJ/4X1GM Geoffrey S. Mendelson wrote,on my timestamp of 1/07/2009 7:39 PM: > If you have a CRT Television set you can photograph the screen and count the > scan lines, but those are on their way out. Turn off progressive scanning and it works the same with a LCD screen. At least mine does. But I don't count the lines, I just watch through the back while working the shutter: each successively higher speed should produce a band of lighted screen roughly half the width of the previous. >> Thanks for that. I'm seeing this problem on my old folders when I use >> f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge [quoted text clipped - 10 lines] > 1/300th, > and I believe many cheaper leaf shutters where similar. Good point. Interestingly, the Fujiblad leaf shutters only go up to 1/400. > If you have a CRT Television set you can photograph the screen and count > the > scan lines, but those are on their way out. Science is always making such wonderful improvements on things. Sigh. SignatureDavid J. Littleboy Tokyo, Japan > >> Thanks for that. I'm seeing this problem on my old folders when I use > >> f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge [quoted text clipped - 22 lines] > David J. Littleboy > Tokyo, Japan The number of lines will surely give you the total open time of the shutter - not its "exposure time" which is based on the "amount" of light let in and not the total open time of the shutter. Part of the total open time of the shutter will be the blades moving to the open position and the blades moving back amd during those blade movement times then only about half the light will be allowed through, at full lens aperture, because, on average, the lens is half closed during these blade movement periods. To give a simplified example then if the blades took 2 ms to open and then immediately, when fully open, closed again, also taking 2 ms, then the shutter is open for 4 ms but it only supplied 2 ms worth of full light at full aperture and so the exposure time is 1/500th sec. If the aperture of the lens is just a small part in the middle then it will be allowing full light through, in this case, for most of the 4 ms and so the effective exposure time is 1/250th sec. Hence, in this simplified example, if you were using an f16 stop with a shutter speed of 1/500th sec then you would have to stop down by a full stop to give you the correct exposure for that aperture. > The number of lines will surely give you the total open time of the > shutter - not its "exposure time" which is based on the "amount" of [quoted text clipped - 14 lines] > stop down by a full stop to give you the correct exposure for that > aperture. That's exactly it: shutter efficiency is the problem at fast speeds and small apertures. Not something you measure by simple trying to count how many scan lines you can see (a giant leap in science and technology indeed!) through the lens. Though too slow shutters, due to old grease, add to the problem. And different models/makes may be different too. So the best thing would be to do exposure tests, and try to determine the required correction from exposed and processed film. > > The number of lines will surely give you the total open time of the > > shutter - not its "exposure time" which is based on the "amount" of [quoted text clipped - 27 lines] > > - Show quoted text - Or just guessing and hope you get lucky first time and so don't waste the cost of a roll and processing. Underexposed slide film is better than overexposed so at f11 and 1/250th sec then half a stop underexposed might be a good guess. > Underexposed slide film is better than overexposed so at f11 and > 1/250th sec then half a stop underexposed might be a good guess. I used to shoot slide film a little over exp for scanning, so bracketed for nominal and over. OTOH, my 80 f/2.8 is a bit sluggish so a nominal setting yielded slightly underexposed Velvia: http://photo.net/photodb/photo?photo_id=6046198&size=lg ... which the Nikon 9000ED scanner had little trouble with though the shadows are completely dead. >>> Thanks for that. I'm seeing this problem on my old folders when I use >>> f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge [quoted text clipped - 9 lines] >> 1/300th, >> and I believe many cheaper leaf shutters where similar. I've measured my 1970's Sonnars to just shy of 1/500. (1/450 ish) > Good point. Interestingly, the Fujiblad leaf shutters only go up to 1/400. > [quoted text clipped - 3 lines] > > Science is always making such wonderful improvements on things. Sigh. That's consumer technology. Science always leaves room for other measurements. I've used a photocell and oscilloscope to verify leaf shutters, for example. >>>> Thanks for that. I'm seeing this problem on my old >>>> folders when I use [quoted text clipped - 34 lines] > for other measurements. I've used a photocell and > oscilloscope to verify leaf shutters, for example. This will give you the actual graph of the shutter opening. The correction for efficiency depends partly on the size (mass) of the shutter blades and on some other factors but in Compur and similar shutters can be figured by assuming the opening and closing times combine to about 1/1000 second. The marked shutter speeds are based on the equivalent exposure at maximum aperture. Since the shutter is not fully open for part of the time the effective exposure at small stops will be greater than the marked time. Opening and closing times are constant in most shutters to this error becomes greater as the shutter speed increases. On small and medium Compur shutters, with marked top speed of 1/500, the effective exposure time at smaller stops is around 1/400th providing the shutter is clean and working properly. This difference is of no consequence for B&W or color negative but might be for reversal films. Note that focal plane shutters also suffer from efficiency errors but these go the other way, that is, the effective shutter speed is higher (exposure less) than the marked speeds. The correction depends on the amount of diffraction at the edges of the slit and vary with the distance from the slit to the film and the angle of the light beam from the lens. Hence it varies with stop, focal length, and distance of the lens from the shutter. The difference caused by loss of efficiency can be significant in large FP cameras like the Speed Graphic at the top speeds but can also be significant in some 35mm cameras. For this reason measuring the shutter speed using a simple shutter tester requires some care. One often finds Speed Graphics with the shutter tension adjustments cranked way up because someone was trying to get a 1/1000th second reading with a simple tester. Unless the light source is adjusted correctly the top speed may read as low as 1/800th on the tester. Similarly a measurement on a leaf shutter at its top speed will read low even though the shutter is operating correctly. Typical total open time (speed measured at the center of the aperture) will be about 80% of the marked speed at the top speed. -- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com > I am wondering if some kind person who has this guide can send me a > scan. [quoted text clipped - 14 lines] > David J. Littleboy > Tokyo, Japan Some kind person did send me a scan and it is clear that these exposure compensations are examples only. What would be really helpful is knowing the opening and closing time of various shutters. |
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