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09-05-2009, 07:25 PM   #1
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What Makes a Great Low Light Lense?

What Makes a Great Low Light Lense?

Is ther a metrics which lenses can be measured and compared?

09-05-2009, 07:35 PM   #2
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f-number. The smaller the better. eg. f1.4 or f2 lenses are better than an f4 lens in low light.
09-05-2009, 07:56 PM   #3
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Let me ask the question another way.

Do all lenses at f3.5 allow the same amount of ligth? For example would a 40mm and 70mm allow the same amount of light at f3.5? Or does the 40mm allow more light because of the bigger angle of view?
09-05-2009, 08:01 PM   #4
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Same......

09-05-2009, 08:55 PM   #5
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QuoteOriginally posted by Firebird Quote
Let me ask the question another way.

Do all lenses at f3.5 allow the same amount of ligth? For example would a 40mm and 70mm allow the same amount of light at f3.5? Or does the 40mm allow more light because of the bigger angle of view?
The same in theory, in practice, it can differ. A simple Tessar design with 4 elements will probably pass more light than a complex zoom with 15 or more elements even if the aperture is identical.
A good low light lens will have a fast maximum aperture and will be reasonably sharp wide open.
There is no such thing as a good low light zoom lens.
09-06-2009, 02:55 AM   #6
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With binoculars and telescopes the size - ie diameter - of the front element is also significant in terms of their ability to gather light. I assume the same would apply to camera lenses, though I'm not 100% sure about that. Wheatfield?
09-06-2009, 04:54 AM   #7
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QuoteOriginally posted by Wombat Quote
With binoculars and telescopes the size - ie diameter - of the front element is also significant in terms of their ability to gather light. I assume the same would apply to camera lenses, though I'm not 100% sure about that. Wheatfield?
Same principle, except that binoculars are always "wide open".....you don't have aperture blades to stop down and constrict the ability to pass light.

Remember, the f-stop is a ratio of the diameter of the tube passing light and the focal length. Doesn't matter if the constricting factor is aperture blades or a small front element....either way it is cutting down that ratio and making the maximum aperture smaller (making the lens "slower"). Think about they the fast prime long telephoto lenses have such honking huge front elements....they have to be huge in order not to slow the lens down.
09-06-2009, 10:08 AM   #8
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Is there software which can measure the amount of light which created the exposure?

I would like test some of my lenses at the same exposure, same lighting, same subject, etc. The only variable will be the four different lenses which I have.

09-06-2009, 11:46 AM   #9
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You could use pretty much any graphics program to do that. Take a picture with each lens with the same settings and the just use the dropper tool to sample the pictures and find out the brightness of a selected area. You can then compare the values.
09-07-2009, 08:38 AM   #10
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I completed some simple low light tests and found the following:

1) the wider the field of view the more light captured
- I took a number of pictures at f5.6/400 with 18-55mm kit at differnt focal lengths
- I took a number of pictures at f5.6/400 with 50-200mm at different focal lengths

2) of my three lenses (M 50mm/f2; DA 18-55 II; DA 50-200 WR) they all performed differently at 50mm/f5.6/400
- the 50-200mm captured more light followed by 18-55mm and lastly the M 50mm

3) the higher the quaility of the lense the more light captured
- DA 70mm at f2.8/400 captured more light than M-50mm at f2.8/400
09-07-2009, 09:15 AM   #11
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I'm assuming, though, you're talking about differences of a small fraction of a stop? That's normal for there to be slight variations from lens to lens due to the amount of glass the light has to pass through. Also, testing any zoom lens wide open is unlikely to yield a completely accurate result, as the stated maximum f-stop is usually just an approximation that varies continuously as you change focal lengths, not in discrete chunks as the camera reports.

In general, though, to get back to the original question, a given f-stop is the same amount of light regardless of focal length or lens; there just might be slight variations in practice. The very minor differences between lenses are not normally what one considers when evaulating lenses - all f/2 lenses let in more light than all f/2.8 lenses, which in turn let in more light than all f/3.5 lenses, etc. And the small difference in light let in by different f/2.8 lenses is seldom anywhere near as significant as the oher things that would influence one's buying decision - focal length, sharpness, size, price, etc.
09-08-2009, 08:42 AM   #12
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wont the front diameter of the lens also affect the amount of light entering the camera?
09-08-2009, 10:05 AM   #13
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QuoteOriginally posted by Marc Sabatella Quote
I'm assuming, though, you're talking about differences of a small fraction of a stop? That's normal for there to be slight variations from lens to lens due to the amount of glass the light has to pass through. Also, testing any zoom lens wide open is unlikely to yield a completely accurate result, as the stated maximum f-stop is usually just an approximation that varies continuously as you change focal lengths, not in discrete chunks as the camera reports.

In general, though, to get back to the original question, a given f-stop is the same amount of light regardless of focal length or lens; there just might be slight variations in practice. The very minor differences between lenses are not normally what one considers when evaulating lenses - all f/2 lenses let in more light than all f/2.8 lenses, which in turn let in more light than all f/3.5 lenses, etc. And the small difference in light let in by different f/2.8 lenses is seldom anywhere near as significant as the oher things that would influence one's buying decision - focal length, sharpness, size, price, etc.
What he said. Before most lenses had coatings and cameras had TTL metering people doing reviews of lenses use to give the T/stop variants from the f/stop. This was important if you were doing some things and using a non TTL meter as you could see between and a a stop (more on a bad lens) but now unless you have a special need or use it just is not that relevant.

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09-08-2009, 11:56 AM   #14
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QuoteOriginally posted by manishved Quote
wont the front diameter of the lens also affect the amount of light entering the camera?
Well, sure, but that's already accounted for when they compute the maximum aperture. That is, it is the fact that a lens has a large diameter front element that allows it to have a large maximum aperture in the first place. So that's not something additional you need to consider.
09-08-2009, 08:49 PM   #15
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I did some research after reading DAZ's post.

If my understanding is correct, a specific f-number allows the same amount of light regardless of lense with the subject located at infinity.

However if the subject is not located at infinity, there is a working f-number. Which may explain my variations in low light tests as the subject was less than 5 feet away.
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