Ok this question has been bugging me for a few months and I can't figure it out.
Why aren't photos over exposed in the middle of the picture?
From what I understand the shutter opens from the middle and goes toware the edge (Like our pupils when we go from a bright room to a dark room) so therefore the middle of the aperture sees light first.
As it opens further the light enters into these 'newly opened' areas of the aperture. (But, remember light is also still hitting where it first hit - the centre) So I would expect the edges of a photo to be darker than the middle if this is true, because well, the middle gets more light.
Can someone please help me understand why this just doesn't happen?
it's concievable to have 1/2 stop or more difference in exposure with a leaf shutter on 4x5 film from center to edges.
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I doubt that very much. The design of the shutter blades, the speed the blades move, the way in which they move and the limiting of blade shutter speeds on 5x4/10x8 means that the exposure differences are negligible.
If this were not the case then leaf shutters would be unuseable for top end studio work - 1/2 stop would be far too noticeable. So as even the most expensive LF lenses still use them.....
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."
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.
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look better with your subject smack bang in the middle of the photo go for it. that your fingers and camera strap arent obstructing the view finder.
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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.
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.
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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.
No,Chase is right.
Large format lenses and some medium format lenses have leaf shutters that work a bit like the aperture - but modern 35mm all have focal plane shutters (but what's the betting KSMatt knows of an obscure one that doesn't!)
The first shutter moves across the film very quickly - the second shutter follows after. The shutter speed determines what the time lag is. The exposure time is determined by the gap between the shutter blades so at fast speeds the gap is quite narrow (this gives a practical upper limit to the fastest shutter speed).
The net result is that the whole neg receives the same exposure so there is no density difference.
You can get odd effects now and then with moving objects, though.
With leaf blade shutters the centre of the neg does indeed get more exposure than the edges. But the blades move so fast that the actual difference is so small as to be undetectable (something like 1/20th of a stop).
As it moves across, each portion of the frame receives light for the same amount of time. The part that is exposed first is also covered first as the shutter closes. Hopefully that makes sense
I don't understand....
If it all folds down to one side...... say the bottom.... doesn't it all have to come back from the bottom? then when it comes back up it leaves the top of the frame open longer..... :confused:
Ahh in my wrong version I imagined it going like this:
Open -> -> -> -> (Then change direction and...)
<- <- <- <- Close
But there's actually two bits that go in the same direction and follow each other. (Duh!) It's so obvious now.
Thanks.
Oops,, yeah. I meant the way the shutter moves.
Well even if it does go from one side to the other - why isn't one side brighter than the other?
I think you're confusing the aperture with the shutter motion. I believe most of the modern cameras use a focal plane shutter that actually moves across the frame either vertically or horizontally. Having said that, I'm probably wrong! :lol:
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.
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:)
This is a pretty cool little explanation: http://www.answers.com/topic/focal-plane-shutter
with my canon EOS rebel 2000 it doesn't work like that... it works like my other post I just posted.... :meh:
As it moves across, each portion of the frame receives light for the same amount of time. The part that is exposed first is also covered first as the shutter closes. Hopefully that makes sense
I always understood that this is exactly why focal plane shutters came into existance. In earlier days when film was larger and shutters (film too) were getting faster, it could be a problem. Think in percentages. If you're making a 15 second exposure, you can get acceptable results just using a lens cap as a shutter. But when faster films allowed for 1/200 second and faster exposures, it's concievable to have 1/2 stop or more difference in exposure with a leaf shutter on 4x5 film from center to edges.
This is a pretty cool little explanation: http://www.answers.com/topic/focal-plane-shutter
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.
You can get odd effects now and then with moving objects, though.
Oh thanks. I was actually typing my reply when you guys all posted so it's nice to see I get it.
What sort of effects?
I could understand them being dim (because they're moving out of the open bit of the shutter into the bit that's yet to be open)... or could you actually have them appearing a few times in the frame without blur between if they're moving fast enough ??
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.
hmm...... good question.... I have no idea but maybe it's because it's such a short ammount of time between when it starts opening and when it's finished being open.... it's not like it's slow... so maybe that's it.
*That's my guess*
Hmm. ok, here's what I've come up with.
If the object is:
- moving faster than shutter = blurred
- moving same speed as shutter = long elongated stretch of object.
- moving slower than the shutter = sharp image
- not moving = sharp image
Is that right?
I thought that to get the elongated look you'd have to be going slightly faster than the shutter - but then it would have to be blurred. But you didn't mention anything about blurring in your post, so I'm not sure about this.
If an object is moving past the camera in the same direction that the shutter is travelling in then it can get distorted.
Imagine if something was moving at the same speed as the shutter - one little 'slice' of it would be stretched across the frame.
In the early days of focal plane shutters it was noticed that racing cars sometimes got elongated and their wheels went oval. Try working out why.
 Nobody understands...any help please?
 50 points for someone willing to do this!!!!!!!!!!?
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Why aren't photos overexposed in the middle?