Whoops, half this negative is overexposed

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Ok, on two seperate occasions, two types of film (one color, one Ilford HP5) two different cameras, and two different students have had negatives that were neatly divided into halves vertically where one half was properly exposed and one half overexposed. I ended up teaching photography at the local high school after a 20 year vacation from the darkroom and I can't come up with a reason for this occurance. Both students have had perfect films from these same cameras. I think it may have to do with improper flash use but can't find a reference and have never had this problem personally. Any input would be appreciated. Thanks

-- Laura Vryhof (vryhof@midmaine.com), October 06, 1999


Sounds like the shutter speed was one or two steps too high for flash usage, such as on a K1000 the synch speed is 1/60 but the shutter speeds was set for 1/250.

-- John Hicks / John's Camera Shop (jbh@magicnet.net), October 06, 1999.

John Hicks is correct. If these were flash pictures it is nearly assured that this is the problem. A focal plane shutter varies shutter speed by changing how closely the first shutter curtain follows the second shutter curtain across the film. The closer the first follows the second, the faster the shutter speed. Most people don't realize that with a focal plane shutter at the faster shutter speeds the film is not all exposed at the same instant. Since the light of a flash unit is many times faster (about 1/10,000 of a second) than the fastest shutter speed, only the sliver of film showing at the time the flash pops will be exposed. For most focal plane shutters 1/60th of a second is the fastest shutter speed at which the film is completely exposed all at the same instant, thus it is used as the "flash sync" speed, although slower shutter speeds work just fine also. Those of us who use LF and MF cameras with leaf shutters don't have this problem (although I never use a flash!) because leaf shutters at any shutter speed have an instant at which the entire film is exposed. Good luck with your class.

-- J.L. Kennedy (jlkennedy@qnet.com), October 07, 1999.

Thanks a bunch, it is always the simplest things we forget to think about. I appreciate the help. I like the sugesstion I got from someone about having them read the manuals and write a report. Unfortunately, the manuals have all been disposed of. The woman who taught the class last year had NO experience with photography and she just chucked stuff out willy nilly. Again, Thanks

-- Laura Vryhof (vryhof@midmaine.com), October 08, 1999.

They can always open the back of the camera and see what happens at different shutter speeds. Point the camera and flash toward a light toned wall. At the proper sync speed and slower, the entire frame will be open when the flash fires. Above that speed only a narrow slit will be open. The flash is quick enough to freeze the motion of the shutter.

-- Tony Brent (ajbrent@mich.com), October 08, 1999.

I do wonder if flash sync was indeed the problem, but this might be because I misunderstood something. Therefore, I would appreciate to read a follow-up. What makes me wonder is this:

As a kid, I made the same mistake myself. I used a flash and had my shutter speed set to 1/125 with the fastest permissible sync time being 1/60. The result was not a negative half of which was over-exposed and the other half being OK, but one that was half unexposed and half OK. This is plausible, too, because as was pointed out, the flash is way too fast for any mechanical element (shutter curtains) to follow it. Thus, when you use an electronic flash, it must fire when the full image window is open. To my knowledge, the fastest sync time (given in your camera manual, and sometimes marked somehow on the camera) is that speed where there is still one instant when the leading shutter curtain has passed the window, and the second one has not started following it yet. At all speeds higher than this, the second curtain starts before the leading one has reached the end its sweep, so there is no such fully open window, and at any instant during the exposure time, only a slit of the negative is exposed. That in mind, I cannot see how part of the film should be *over*-exposed. I could understand *under*-exposure when flash bulbs are used, because these are slow, and there may still be some light before and after the optimum point, but an electronic flash is usually so fast that there should be light when the shutter is fully open and no light at all when the shutter is (partially) closed. (That is why you can use electronic flashes to freeze fast movements. You can't use bulbs for this purpose.)

On second thought, I could think of one way to achieve over-exposure: If the available light was actually sufficient for the correctly exposed part of the negative, the additional light from the flash effective for the other part might cause the over-exposure. In that case, however, using the flash was probably not necessary.

I have read that most focal-plane shutters have vertical slits nowadays, i.e. the shutter curtains run from left to right or vice versa, but not from top to bottom or vice versa. As a first check, find out if the split in negative exposure is parallel to the shutter slit. It should, however, be a shutter problem if it is limited to individual shots on a film that is otherwise OK.

-- Thomas Wollstein (thomas_wollstein@web.de), October 11, 1999.

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