Whoops, half this negative is overexposed

[laura_vryhof]

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

[john_hicks]

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.

[j.l._kennedy]

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.

[laura_vryhof]

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

[tony_brent]

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.

[thomas_wollstein]

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:

 

<p>

 

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.)

 

<p>

 

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.

 

<p>

 

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.