Constant aperature DSLR zooms ???

ROCKMORE

ROCKMORE

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I had a shoot with the 7D and a 24-105mm f4 L zoom yesterday. Although it is a nice lens there is no way the exposure is a constant f4 from one end to the other. I had to correct about one stop from 24 to 105. I don't care what the numbers say I know what I'm looking at. Not even close.

Is this the same case with 16-35mm f2.8 L / 17-40mm f4 L and all the other claimed constant aperture DSLR zooms out there?
 
the newer the lens, the more constant it will be.

something like the newer 70-200 2.8 should be pretty constant through the range.

24 to 105 is a pretty big gap, one stop(or less) is not really unexpected.

you can check lens reviews before buying?
 
I have the 24-105 and it is constant f4 all the way through.

For the money, the Canon 17-55 f2.8 is USM is awesome.
 
The f-stop is the same. The t-stop is not. You're looking at diffraction, the length of the lens bouncing around some light that doesn't make it through the aperture. F-stop is simply the size of the opening, and would only be perfectly accurate as exposure if the lens perfectly transmitted all light hitting it, which isn't possible. However, it should be close.

Keep in mind these are not built for video. So you can't be too bothered that it doesn't work just like you want as a video lens. This doesn't really come up in stills.
 
Cassius said:
The f-stop is the same. The t-stop is not. You're looking at diffraction, the length of the lens bouncing around some light that doesn't make it through the aperture. F-stop is simply the size of the opening, and would only be perfectly accurate as exposure if the lens perfectly transmitted all light hitting it, which isn't possible. However, it should be close.

Keep in mind these are not built for video. So you can't be too bothered that it doesn't work just like you want as a video lens. This doesn't really come up in stills.
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Bingo. Great post, Cassius.
 
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