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General photography questions and answers: Discuss Apertures....Just as a point of interest. Given that the size of an aperture is designated by its fraction of the ...
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Old 21-01-2009, 11:07   #1 (permalink)
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Apertures.

Just as a point of interest. Given that the size of an aperture is designated by its fraction of the focal length of a lens,is,say,an f 2.8 105mm faster than an f2.8 50mm: or is an appropriate amount of light lost during its journey through the lens barrel?
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Old 21-01-2009, 11:10   #2 (permalink)
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Re: Apertures.

The EV is (theoretically!) the same for a given aperture and shutter speed, whatever the focal length, so the camera should meter the same at f/2.8 whether it's a 50mm or 500mm lens. The 500mm one will be a bit bigger though.
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Old 21-01-2009, 11:29   #3 (permalink)
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Re: Apertures.

You could always test it on the two lenses you mention, then report back to us your findings Just set up a shot using a tripod (to maintain an equal distance for the test shots), shoot with each lens, and note the EXIF on both shots Probably not the most scientific way of doing it, but it's only for us chickens

I have a feeling that if they are equidistant, then there will indeed be a difference in EV (because more of the BG will be affecting exposure value), but that if you move closer to the subject for the wider lens - to make the subject of equal size in the frame per the tele lens - then that would be the real test for the lenses themselves (rather than shooting conditions) affecting the EV.

Let the debate begin Yeah we could simply look it up on Luminous Landscape (etc.) but where's the fun in that
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Old 21-01-2009, 11:37   #4 (permalink)
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Re: Apertures.

Yes, the luminosity of the subject would have to be the same for both lenses. So for a scientific comparison you'd have to fill the frame with something like a grey card under identical lighting in both cases.
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Old 21-01-2009, 16:55   #5 (permalink)
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Re: Apertures.

Quote:
Originally Posted by silkstone View Post
The EV is (theoretically!) the same for a given aperture and shutter speed, whatever the focal length, so the camera should meter the same at f/2.8 whether it's a 50mm or 500mm lens. The 500mm one will be a bit bigger though.
Still having trouble getting my head around this (it must be an age thing). The aperture Dia at f2.8 on a 50mm is 17.8mm ; the Dia at f2.8 on a 105mm is 37.5mm, thats twice the size. Hence,more light!
Now,what about compacts. The aperture size at f2.8 at the equivalent focal length of 35mm on my G9 is a mere 2.64mm Dia,thats a lot less light-even allowing for the reduced sensor size. Yet what you are saying is, that the EV is the same for all! Its time like this my head starts hurting!
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Old 21-01-2009, 17:33   #6 (permalink)
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Re: Apertures.

The light transmitted as a percentage of the field of view is more relevant than the actual dimension. Both of the sizes you quote might represent (say) 50% of the available light.

I think you need to forget physical dimentions and think of the f number as a function
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Old 21-01-2009, 17:40   #7 (permalink)
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Re: Apertures.

Aperture (f-number) = focal length / iris diameter. If you think about light-gathering properties, a wide angle lens is gathering light from a wide area, whereas a telephoto lens is gathering light from a much smaller area. Hence to gather the same amount of light, the telephoto iris (aperture diameter) has to be proportionally larger.
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Old 21-01-2009, 17:41   #8 (permalink)
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Re: Apertures.

Quote:
Originally Posted by Dabhand16 View Post
The light transmitted as a percentage of the field of view is more relevant than the actual dimension. Both of the sizes you quote might represent (say) 50% of the available light.

I think you need to forget physical dimentions and think of the f number as a function
Thanks for that ,Dabhand. That makes it all crystal clear.......
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