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Old Jun 28, 2012, 5:53 PM   #1
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Default Question about F stops

I'm considering adding a 50m prime lens to my Sony. Just based on f stops, how much more light does a 1.4 offer vs. a 1.8? I know there is a formula, but I am just curious how much more light the 14 offers.
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Old Jun 28, 2012, 6:20 PM   #2
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I could be wrong but I think its 2/3 fstop difference between the 1.8 and 1.4 with the additional benefits of shallower depth of field of the 1.4
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Old Jun 28, 2012, 6:22 PM   #3
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http://digital-photography-school.co...-for-beginners

http://en.wikipedia.org/wiki/F-stop
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Old Jun 28, 2012, 6:58 PM   #4
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An f/1.4 lens (wide open) pulls in quite a bit more light than an f/1.8 lens (wide open).
Something of the region of 50-60% more light- which is quite a bit..!
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Old Jun 28, 2012, 8:10 PM   #5
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An aperture of f/1.4 lets in twice as much light as an aperture of f/2.0 (which lets in twice as much light as an aperture of f/2.8, etc., etc., etc.) An aperture of f/1.8 is about 1/3 stop larger than f/2.0, and an aperture of f/1.6 is about 1/3 stop larger than that.

These are all approximations, of course. The actual values are irrational numbers. For instance, f/1.4 is actually f/1.414213562373095... (or the square root of 2.)

On the other hand, f/2.0 is really 2.0.
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Last edited by TCav; Jun 29, 2012 at 6:26 AM.
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Old Jun 29, 2012, 9:48 AM   #6
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The f/1.8 will let 1.65306122448979.. times as much light through than f/1.4. (The ratio of 1.8 squared to 1.4 squared.)
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Old Jun 29, 2012, 10:53 AM   #7
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Another way to think of it is to do the math, and you come up with an effective aperture of around 28mm for the f/1.8 lens, and about 35mm for the f/1.4. Then do the rest of the math on your checkbook and decide if the difference in light gathering is worth the difference in price.

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Old Jun 29, 2012, 11:15 AM   #8
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You should also be looking at other considerations. For example, Canon has a 1.8 and 1.4 as well. The 1.4 is built better, focuses faster and has better bokeh.

The bokeh is probably the biggest thing you want to look into if you're considering the lens for shallow DOF and not just low light. The aperture blade design may likely be different as well.
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Old Jun 29, 2012, 12:19 PM   #9
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Well, I have Sony and I am pondering how much better their 1.4 is vs. their 1.8. I suspect enough to warrant the price difference
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Old Jun 29, 2012, 1:04 PM   #10
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Quote:
Originally Posted by BillDrew View Post
The f/1.8 will let 1.65306122448979.. times as much light through than f/1.4. (The ratio of 1.8 squared to 1.4 squared.)
Right idea, but you reversed the lenses. And, as TCav noted, f/1.8 and f/1.4 are rounded off numbers, so it makes little sense to carry the math out any further than two digits. FWIW
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