Why aren't photos overexposed in the middle?

[Meysha]

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?

 

[Chase]

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!

 

[Meysha]

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?

 

[LittleMan]

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*

 

[Chase]

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

 

[Chase]

This is a pretty cool little explanation: http://www.answers.com/topic/focal-plane-shutter

 

[LittleMan]

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.....

 

[Hertz van Rental]

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).

 

[LittleMan]

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:

 

[Meysha]

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.

 

[Meysha]

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 van Rental]

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.

 

[Meysha]

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.

 

[Christie Photo]

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.

 

[Hertz van Rental]

it's concievable to have 1/2 stop or more difference in exposure with a leaf shutter on 4x5 film from center to edges.

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.....