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DPChallenge Forums >> Hardware and Software >> considering L glass
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AuthorThread
 03/17/2010 07:59:38 PM · #1
I have a question that maybe someone here can help me with.
I want to compare L glass and kit glass. L glass has a 77mm objective lens and my kit lens has a 57mm objective. If I use both lenses to take the same picture will the L glass let me have a faster shutter speed if all the other parameters are the same? It seems to me that it should, I want to bounce the idea around some more experienced heads.

TIA
Gene

03/17/2010 08:03:41 PM · #2
Assuming the ISO and aperture value (Av) settings are the same, the shutter speed will be exactly the same.
 03/17/2010 08:10:36 PM · #3

Thanks for the quick reply.
Doesn't the larger objective get more light to the sensor?

Thanks again
Gene
 03/17/2010 08:10:55 PM · #4
F/4 is f/4 regardless of lens. 1/125 is 1/25 regardless of lens. The only information you need to know regarding how much light a lens can deliver to the focal plane is its maximum f/stop. An f/2.8 lens delivers double the light, at its maximum aperture, that an f/4 lens does. But if you stop the f/2.8 lens down to f/4, the light delivered is the same.

R.

Message edited by author 2010-03-18 00:15:19.
 03/17/2010 08:14:29 PM · #5
An f/stop is the ratio of the diameter of the aperture to the focal length of the lens. So a 25mm aperture on a 50mm lens would be f/2. A 25mm aperture on a 100mm lens would be f/4. On a 200mm lens it would be f/8. and so forth.

So, the longer the lens is, the larger its diameter is going to have to be to engineer the equivalent f/stop. The kit glass, by and large, is f/3.5 to f/4 maximum aperture, whereas L glass tends to have more f/2.8 and f/2 apertures, generally. A noticeable exception is the wonderful 70-200mm f/4L, which is noticeably narrower and lighter than its 70-200mm f/2.8L sibling.

R.

ETA: Doing the first para in reverse, a 200mm lens would have to have a 100mm aperture to deliver f/2, whilst a 50mm lens needs only that 25mm aperture. This means, obviously, that a much larger diameter lens barrel is required to deliver the same amount of light.

Message edited by author 2010-03-18 00:17:18.
 03/17/2010 08:17:22 PM · #6
Originally posted by Gene243:
Doesn't the larger objective get more light to the sensor?

Sure, but objective diameter isn't a reliable indicator of lens speed (it changes with focal length). A Canon 50mm f/1.8 lens has a 52mm filter size. A Canon 24-70mm f/2.8 is 77mm while the similar Tamron 28-75mm f/2.8 is 67mm. To determine speed, you go by the f-number, not objective diameter.

Message edited by author 2010-03-18 00:20:01.
 03/17/2010 08:20:51 PM · #7
Bear thank you Now I understand my error. I thought the Fstop ratio was Objective over appature.
My last photography class was in 1981. I'm on the list for the next class at the local trade school.

Thanks
Gene
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