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Fast lens question.

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Old Monday 1st February 2010, 16:25   #1
Shaggy2070
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Fast lens question.

I'm pretty new to DSLR photography and was wondering about fast lenses.

If you had two cameras of the same model and settings (Aperture Priority, Iso 200 for eg.) and one camera had a 400mm f2.8 and the other had a 400mm f5.6 lens. The f2.8 is classed as a fast lens so if it was stopped down to f5.6 would the shutter speed be faster than the standard f5.6 or would they get the same shutter speed???
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Old Monday 1st February 2010, 16:41   #2
erisian.pope
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stopped down to f/5.6 they would have the same shutter speed. The f/2.8 lens is called fast because you can open it up to 2.8 and achieve a faster shutter speed (assuming all other settings are equivalent).

Basically exposure, for any given scene with whatever level of light is there, is affected by the ISO, the shutter speed, and the aperture. Adjusting any one of these will affect one or both of the other. Assume ISO 100, shutter speed 1/100 sec and aperture f/8. Changing the aperture to f/5.6 (opening it up one stop) would allow you to either halve the ISO (ISO 50) OR double the shutter speed (1/200). Changing the aperture from f/8 to f/4 (opening it up two stops) allows you to either quarter the ISO (ISO 25) OR quadruple the shutter speed (1/400) OR alter both (halve ISO = ISO 50 AND double shutter = 1/200).

Does this make sense?

But my point with these details is to show that, other than changing the amount of light in a scene (sun comes out from clouds, or use a flash) there are no other factors that will affect the exposure. The only way a lens affects shutter speed is with the aperture.

Here's a wikipedia writeup on this: http://en.wikipedia.org/wiki/Exposure_%28photography%29
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Old Monday 1st February 2010, 17:21   #3
Roy C
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Another advantage of a 'fast' lens is better/quicker auto focussing. No matter what f stop you dial-in the camera will always open the lens right up while focussing, only closing to your chosen aperture when the shutter fires.
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Old Monday 1st February 2010, 17:47   #4
Shaggy2070
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Thanks guys.
I knew "Fast Lens" referred to being able to open up to f2.8, I just wasn't sure if it would be faster at f5.6.
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Old Tuesday 2nd February 2010, 01:15   #5
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Well assuming all settings are equal there may still be differences in the exposure. Me and a mate were shooting Barn Owls once and we took a very similar shot with identical settings. He was using a 500mm with a TC giving him f5.6 and I was shooting with a 100-400mm at f5.6 but his picture was noticeably brighter which I would atrribute to a better light transmission through his lens. Also his front element is obviously bigger maybe giving him another advantage.
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Old Tuesday 2nd February 2010, 01:34   #6
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Quote:
Originally Posted by Jaff View Post
Well assuming all settings are equal there may still be differences in the exposure. Me and a mate were shooting Barn Owls once and we took a very similar shot with identical settings. He was using a 500mm with a TC giving him f5.6 and I was shooting with a 100-400mm at f5.6 but his picture was noticeably brighter which I would atrribute to a better light transmission through his lens. Also his front element is obviously bigger maybe giving him another advantage.
The TC adds glass, which is why the fast lens becomes a slower lens. Brightness can be achieved with exposure control as well.
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Old Tuesday 2nd February 2010, 04:31   #7
Jaff
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The TC adds glass, which is why the fast lens becomes a slower lens. Brightness can be achieved with exposure control as well.
Obviously. But my point was that with identical settings (shutter speed. ISO and aperture) his shot was a little bit brighter than mine although we were both shooting manually. So two different lenses at the same aperture+ISO in aperture priority may not give quite the same shutter speed owing to what I said before.

So in theory a 400mmf5.6 prime may well get a slightly better shutter speed than a 100-400mm in identical circumstances because it is a lens with excellent light transmission which is down to the very few elements it has.
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Old Tuesday 2nd February 2010, 16:35   #8
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Adam , I think these in the field comparisons are pretty unreliable. I have gone through similar exercises with others and got varying results. The trouble is having a similar shot is not the same as having an identicle one. That is not dismissing your thoughts. I think different camera types can give variations in exposure readings and even cameras of the same type too.The histogram on canon and nikon cameras according to AM will give different readings. Have you tried switching cameras to the lenses and see what happens when you know the scene is exactly the same?
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Old Tuesday 2nd February 2010, 19:54   #9
Jaff
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Originally Posted by a.dancy View Post
Adam , I think these in the field comparisons are pretty unreliable. I have gone through similar exercises with others and got varying results. The trouble is having a similar shot is not the same as having an identicle one. That is not dismissing your thoughts. I think different camera types can give variations in exposure readings and even cameras of the same type too.The histogram on canon and nikon cameras according to AM will give different readings. Have you tried switching cameras to the lenses and see what happens when you know the scene is exactly the same?
Well it's funny you mention that because while I've still got Paul's 30D I want to do some identical shots with my 40D to compare things like noise so can also see if there are any differences in exposure. You're right though my friend was using a 20D so that may be again another factor accounting for the slight difference.

If I get the energy to do the test shots I'll maybe post the results for people to see what they think, might be able to include a 50D in it as well.
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