Attention Tamron 24-70 VC owners

Started Jul 28, 2012 | Discussions thread
OP Horshack Veteran Member • Posts: 6,155
Problem #2: Aperture lever miscalibration

Some more testing and I quickly realized that there is a 0 stop EV difference between f/2.8 and f/4.0 on my copy, whereas obviously there should be a 1 stop EV difference (or somewhere close to 1EV after accounting for T-stop variability). This EV delta was measured in the center (discounts any vignetting) and was measured on two separate bodies (discounts bent/misaligned aperture lever on the body). It's clear the aperture lever on the lens is miscalibrated; I confirmed this by measuring not only EV but also DOF. I see hardly no DOF difference between f/2.8 and f/4.0, whereas I see the expected difference between f/4.0 and f/5.6.

I took the lens off and visually compared the lever-to-aperture-blade action between the Tamron and my Nikon 24-70 f/2.8. It's hard to compare visually but I can see that the Tamron has some slop near the wide-open lever position whereas I don't observe that slop on the Nikon (ie, the aperture blade is slower to close on the Tamron in response to lever movement). This would explain the EV+DOF miscalibration on the Tamron.

Here's an EV comparison of the Tamron @ f/2.8 vs f/4.0, with the shutter speed halved for f/4.0. The two images should have equivalent brightness but they're almost exactly 1EV apart:

And here's a DOF comparison demonstrating that DOF does not change appreciably between f/2.8 and f/4.0, whereas it does between f/4.0 and f/5.6. Compared the DOF changes between the middle and right batteries:

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