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09/10/2007 08:27:24 AM · #1 |
i have a stupid question..
f-stop indicates focal length/ aperture size
so a 100 f/2.8 is much wider than a 50/f2.8 ..
in same light conditions the 100mm would allow faster shutter speeds..?
is that correct ?..
in that context wouldnt the f stop be miss leading.. being not the proper indication of the real diameter of the aperture ?
Message edited by author 2007-09-10 09:12:48. |
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09/10/2007 09:17:45 AM · #2 |
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09/10/2007 09:23:00 AM · #3 |
F/stop is defined as the ratio of the physical diameter of the aperture to the focal length of the lens. The reason these are stated as ratios instead of actual physical aperture size is because light intensity diminishes by the square of the distance traveled. Any lens set to any given aperture delivers the same intensity of light to the sensor plane, which is the whole point. Otherwise the whole concept of "exposure value" (EV) would be too complex to contemplate.
A 25mm aperture on a 50mm lens is f/2.0. On a 100mm lens you'd need a 50mm aperture to deliver the same amount of light the longer distance, and that would be f/2.0 on that lens.
R.
Message edited by author 2007-09-10 09:23:42.
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09/10/2007 10:06:26 AM · #4 |
| thanks bear_music.. i didnt know that :) |
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09/10/2007 10:31:32 AM · #5 |
| Yeah, what Bear said. Aperture is always aperture, wether that be on 50mm or 100mm or on a P&S camera with a smaller sensor or a medium/large format camera with a vastly bigger film/sensor than the 35mm format allows. |
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09/10/2007 11:33:18 AM · #6 |
And *that* 'splains why you'd have to be Arnold Schwarzenegger to carry an f/2.0 version of a 500mm lens! ;-)
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09/10/2007 11:37:34 AM · #7 |
| Great, I didn't know that either. But now I wonder, why is my 20 mm/1.8 prime so much bigger than my 35/2 and 50/1.8 ? |
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09/10/2007 11:39:06 AM · #8 |
One additional note -- the aperture size in in area, not diameter.
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09/10/2007 11:44:44 AM · #9 |
Originally posted by mark_u_U:
Great, I didn't know that either. But now I wonder, why is my 20 mm/1.8 prime so much bigger than my 35/2 and 50/1.8 ? |
It's because of the size of the front element that's needed. For fast wide angle lenses, the required front element size is much bigger than for slower lenses of the same focal length. |
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09/10/2007 12:02:32 PM · #10 |
Originally posted by HawkeyeLonewolf:
One additional note -- the aperture size in in area, not diameter. |
I don't think so. If it were area, than doubling the f-stop would halve the light. That is, F/4 to F/8 would yield half the light. But it yields 1/4 the light, not 1/2. To get half, you go from F/4->F/5.6.
The only way for F/4 -> F/5.6 to halve the light is for the actual measurement to be squared. That is, the ratio of the f-numbers is 1.4. 1.4x1.4=2. Thus, increasing F-stop by 1.4 cuts the light by 1/2.
Message edited by author 2007-09-10 12:06:44.
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09/10/2007 12:06:01 PM · #11 |
Originally posted by levyj413:
Originally posted by HawkeyeLonewolf:
One additional note -- the aperture size in in area, not diameter. |
I don't think so. If it were area, than doubling the f-stop would halve the light. That is, F/4 to F/8 would yield half the light. But it yields 1/4 the light, not 1/2. To get half, you go from F/4->F/5.6.
The only way for F/4 -> F/5.6 to halve the light is for the actual measurement to be squared, which is what happens - you square the change in diameter to get the change in area. |
Correct. The f/stop ratio uses diameter. The AREA of the aperture opening is a squared function, which explains why it works with the inverse square law of light falloff.
R.
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09/10/2007 12:10:21 PM · #12 |
Originally posted by Bear_Music:
Originally posted by levyj413:
Originally posted by HawkeyeLonewolf:
One additional note -- the aperture size in in area, not diameter. |
I don't think so. If it were area, than doubling the f-stop would halve the light. That is, F/4 to F/8 would yield half the light. But it yields 1/4 the light, not 1/2. To get half, you go from F/4->F/5.6.
The only way for F/4 -> F/5.6 to halve the light is for the actual measurement to be squared, which is what happens - you square the change in diameter to get the change in area. |
Correct. The f/stop ratio uses diameter. The AREA of the aperture opening is a squared function, which explains why it works with the inverse square law of light falloff.
R. |
Yes, I was remembering the wrong fact... and spreading disinformation all over the site!
My bad.
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