Why aren't photos overexposed in the middle?

[Christie Photo]

I doubt that very much.

I knew you'd have some thoughts on this. It is reasonable. I've experienced it first hand. Could it be I was observing something else? I dismissed that it could be vignetting because it wasn't as well defined and the lens coverage was adequate for the format. I was doing flat-art copy with a 5x7 Deardoff. Can't remember the exact lens, but I'm sure it was a Kodak, probably a Copal 1 shutter. Evenly illuminted copy stage.

But I did a bit of homework, and found that the focal-plane shutter pre-dates the leaf shutter by about 25 years. I found the following on the net (so it must be true).

"The first focal-plane shutter, which operated like the guillotine but had an opening of adjustable size, was used by a British photographer named William Engiand as early as 1861. The other main type, the leaf shutter, was first introduced by Edward Bausch in 1887."

 

[Hertz van Rental]

I knew you'd have some thoughts on this. It is reasonable. I've experienced it first hand. Could it be I was observing something else? I dismissed that it could be vignetting because it wasn't as well defined and the lens coverage was adequate for the format. I was doing flat-art copy with a 5x7 Deardoff. Can't remember the exact lens, but I'm sure it was a Kodak, probably a Copal 1 shutter. Evenly illuminted copy stage.
[/I]

It probably was something else, but without knowing the exact lens/shutter (and aperture/shutter speed) combination, and seeing the lighting set-up and the results, I couldn't give you an opinion.
You would get the most middle/edge difference in exposure at the widest aperture - but this should be nowhere near half a stop. As LF lenses are almost always used at small apertures, the actual area that the shutter blades move across is so tiny that exposure variations are virtually non-existent.
It's to do with the exposure variation as a proportion of the total exposure.
And, of course, if you are using flash there is no variation at all.

 

[Hertz van Rental]

I must have been half asleep when I wrote the above comments. I certainly wasn't thinking. So now I apologise because I got it wrong.
There is NO difference between the levels of illumination across a neg when using a leaf blade shutter.
The shutter is almost always situated between the lens elements (at one time you could get shutters that fitted between the lens and the camera body - but the effect is pretty much the same). This means that the opening and closing of the shutter blades affects the light intensity over the whole neg evenly. This is because the shutter acts a little like an iris diaphragm.
The only way that the centre would get more illumination than the edge would be if you had the shutter in front of the film at the focal plane.
If you want me to give you the full optical explanation I will - but trust me, I'm right.
What it does mean is that the shutter efficiency will change with aperture size - that is, the amount of light let in to the camera will not be quite the same as it should be at a given shutter speed. But compensation is usually built in.

 

[mentos_007]

now I get it too... when I read Meysha's first post in this thread I started to think about this and I was very confused about how it really works... now I get it... I hope so thanks

 

[Meysha]

Hertz can you see if my guess at why the car looks elongated is right?
It's up a little higher in the thread. Thanks.

 

[Hertz van Rental]

You only get distortion if the direction of travel of the object is at right angles to the direction of travel of the shutter.
Say the shutter is moving top to bottom and the object is moving from one side to the other.
As the shutter goes down the object moves across. If you plot this on a graph you get a diagonal line. And this is what happens to the image.

I have already said I wasn't thinking straight earlier, so here is the info I missed.
Focal plane shutters consist of two parts. The first shutter moves across, out of the way and letting the light get to the film. The second moves across to shut the light out. They both move at the same speed across the film whatever shutter speed is set. The exposure duration is determined by the size of the gap between the two shutters. The faster the shutter speed the narrower the gap. On long exposures there is a delay between the first opening and the second closing.

 

[Unimaxium]

If you had a leaf shutter in front of the lens, then it would cause uneven illumination. But since the shutter is in the lens, then it doesn't for the same reason that changing the aperture doesn't do it either. It basically has to do with the way the light rays going through the aperture hole / leaf shutter. This is also why pinhole cameras work.