20 1.7 not much better in low light than 14 2.5

Started Jul 6, 2011 | Discussions thread
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Loga
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20 1.7 not much better in low light than 14 2.5
Jul 6, 2011

Look. People always talk about the 1.1 EV advantage that 20 1.7 has over the 14 2.5, but in fact, it is much less.

There is a rule that you should keep the shutter speed at least twice as the (35mm eqv) focal length to get pictures without blurring. Based on that, with 20 1.7 which is 40mm eqv you need 1/80 to take pictures safety. 14 2.5 which is 28mm eqv you need 1/56. So because of the focal length the 14mm has about 1.4x advantage.

Yes, the 20mm has 1.7 max aperture, so it is about 1 EV faster. That means, that 20mm has 2x advantage in shutter speed.

So, the real advantage of the 20 1.7 over the 14 2.5 is just (2 / 1.4) which equals to 1.4. If you take a picture with 20 1.7 at ISO400, then you can do it with 14 2.5 at ISO500 - ISO640.

Note, that the rule I referred above is not significant in this deduction. Some people have better technique to hold the camera without shaking, some have worse. But for a certain person the real difference will always be about 1,4X not 2,0X.

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