Tamron 24-70 f/2.8 - Aperture Test

Started Dec 14, 2012 | Discussions thread
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Re: Tamron 24-70 f/2.8 - Aperture Test
In reply to pipee, Dec 15, 2012

Going from 2.8 to f4 doesn't necessary means that the real exposure is half in a zoom lens.

What is guaranteed is that DOF is doubled, otherwise the lens cheats.

Except f-factor there is the T-factor in a lens, which is the transmission of light.

Usually in zooms, T factor for a 2.8 lens may by T=3.5 (or even worse).

When you stop down, the T reaches or matches the f.

For instance in the Tamron it may be:

f=2.8 --> T=3.5 (half stop difference)

f=4 ---> T=5 ( 2/3 stop difference)

f=5.6 ---> T=6.3 (1/3 stop difference)

f=8 --> T=8 ( no difference)

Thats why you see this effect.

To see if your aperture works OK, you must set the camera on a tripod,

shoot xmas tree lights out of focus at 1m at all apertures and measure the

area of the circles. it must be halfed at each aperture increase.

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