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Old Jan 24, 2009, 9:48 PM   #1
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which do you think is better low light, no flash, only availibe light



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Old Jan 25, 2009, 6:49 AM   #2
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Depends more on the focal length that you need as both with have very similar low light qualities with the same aperture. If you want more of a portrait lens then probably the 50 is the way to go if you need wider then the 30 is the better option.
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Old Jan 25, 2009, 9:11 AM   #3
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They are both f1.4 so both will be the same.

However, the 30mm with the 1.5x crop factor, becomes much more of a standard lens, whilst the 50mm becomes a short telephoto.

Really does depend on what you are shooting and how much room you will have.



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Old Feb 3, 2009, 8:02 AM   #4
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A late post for this: I think if your question is which lens is better for low light, I'd say based on Physics and Math,that the Pentax 50 f/1.4 would be a faster lens than theSigma 30 f/1.4. Since aperture is measured as a function of focal length (i.e. focal length divided by a number (i.e. f/1.4), a larger numerator (i.e. focal length) divided by a constant 1.4 would give you a larger aperture.
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Old Feb 3, 2009, 9:54 AM   #5
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jelpee wrote:
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A late post for this: I think if your question is which lens is better for low light, I'd say based on Physics and Math,that the Pentax 50 f/1.4 would be a faster lens than theSigma 30 f/1.4. Since aperture is measured as a function of focal length (i.e. focal length divided by a number (i.e. f/1.4), a larger numerator (i.e. focal length) divided by a constant 1.4 would give you a larger aperture.
Yes, but the expression of aperture (the ratio of the pupil diameter to the focal length) is what determines exposure. And since they both have a maximum aperture of f/1.4, they are the same with respect to how they would affect exposure.

So, to answer the OP's question, neither is better.

But optically, the 50/1.4 is better. See PhotoZone.com's review of both lenses:

Pentax SMC-FA 50mm f/1.4

Sigma AF 30mm f/1.4 EX DC

So if the angle of view doesn't matter, I'd go with the Pentax.
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Old Feb 3, 2009, 11:39 AM   #6
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TCav wrote:
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jelpee wrote:
Yes, but the expression of aperture (the ratio of the pupil diameter to the focal length) is what determines exposure. And since they both have a maximum aperture of f/1.4, they are the same with respect to how they would affect exposure.

So, to answer the OP's question, neither is better.

But optically, the 50/1.4 is better. See PhotoZone.com's review of both lenses:

Pentax SMC-FA 50mm f/1.4

Sigma AF 30mm f/1.4 EX DC

So if the angle of view doesn't matter, I'd go with the Pentax.
True...that makes sense. I guess then the question to askmay be what is the outer diameter of each lens. I imagine that a lens with a larger OD (or filter size) would be the faster lens since it would gather more light at the 1.4 f-stop!
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Old Feb 3, 2009, 11:53 AM   #7
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jelpee wrote:
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True...that makes sense. I guess then the question to askmay be what is the outer diameter of each lens. I imagine that a lens with a larger OD (or filter size) would be the faster lens since it would gather more light at the 1.4 f-stop!
But the filter diameter doesn't matter. What matters is the pupil diameter in relation to the focal length.

And that relationship is referred to as the f-stop.If theshutter speed and ISO setting remain constant, both the 30mm f/1.4 and the 50mm f/1.4 will produce the same exposure value at their maximum apertures, just as a 28mm f/2.8 lens and a 200mm f/2.8 lens would produce the same exposure value. The only difference is the angle of view.

The maximum pupil diameter of the 50mm lens would be 50/1.4, or about 35.7mm, while the maximum pupil diameter of the 30mm lens would be 30/1.4, or about 21.4mm. So the 50mm lens has the larger pupil diamter. But because the angle of view of the 50mm lens is narrower than the 30mm lens, less light per square millimeter would come through the pupil. But because they both have the same maximum aperture, they both admit the same amount of light through to the image sensor.
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Old Feb 3, 2009, 1:50 PM   #8
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I think I just blew a circuit in my brain:?!
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Old Feb 3, 2009, 2:26 PM   #9
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An interesting discussion about aperture. It got me thinking about a couple of lenses I have, so I did some checking. To give a real-world example to support TCav's point, I have two 50mm lenses, one a 1.7 and one a 1.4. Both have 49mm filter diameter, so their front elements appear to be the same, but one is faster than the other.The difference in speed comes from the aperture blade design - the M 1.4 is listed as 7 elements, 6 groups (not sure what that means) while the M1.7 has 6 elements, 5 groups. What doesn't quite make sense to me is that the 1.4 lens is longer than the 1.7. My source was http://kmp.bdimitrov.de/lenses/primes/normal/index.html.
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Old Feb 3, 2009, 2:58 PM   #10
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mtngal wrote:
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... the M 1.4 is listed as 7 elements, 6 groups (not sure what that means) while the M1.7 has 6 elements, 5 groups.
Elements are the actual pieces of glass in a photographic lens. Groups are the way those pieces of glass are assembled. If there was an air gap between each pair of elements, then the number of elements and groups would be the same, but if two elements were in contact over their entire polished surface, then those two elements would be in a single group. This is important because when light travels from one medium (say, for instance, glass) to another (say, for instance, air) it refracts, or bends. So when light travels through a lens that has 7 elements in 7 groups, it is refracted 14 times, but if it travels through a lens that has 7 elements in6 groups, it is refracted only 13 times.

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What doesn't quite make sense to me is that the 1.4 lens is longer than the 1.7.
It doesn't matter. At a focal length of 50mm, theSigma50-500mm f/4.0-6.3 is much longer than both of them (put together!) and the reason has nothing to do with the maximum aperture.
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