90D + Sigma 17-70 apparently underexposing - wonder why?

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totleytom Contributing Member • Posts: 554
90D + Sigma 17-70 apparently underexposing - wonder why?

I received my 90D yesterday and as you'd expect I've been firing off a number of test shots. Early this afternoon I walked around my neighbourhood and took a number of images. As I was reviewing them I became aware that many of them seem to be under-exposed, so I dialled in +1 stop compensation. As I went on I noticed that it seemed to be only the shots taken with my normal walk-around lens, a Sigma 17-70 f2.8-4 'C' zoom, which were showing this. Shots taken with a Canon lens were exposing normally.

Below are 3 images, all taken within a few seconds of each other, of the same scene. The first is with the Sigma at 17mm, no compensation; the second is also with the Sigma, +1 stop; and the third is with the Canon 10-18, with no compensation. I've resized them but they've had no other processing. The Sigma was set to 17mm, and the Canon to 18mm. Although these were taken deliberately to highlight this issue, if it existed, other shots from the walk show the same issue. So these are 'typical'. Obviously there's little photographic merit in the shots - they are test shots, nothing more.

I've been using the Sigma for a couple of years or more, on a 750D, and I don't recall seeing this behaviour on that camera. Unfortunately I don't have the 750D any more! - it was p/x'd for the 90D. I never had an 80D.

Does anyone have any thoughts?

Sigma 17mm no exp. compensation

Sigma 17mm plus 1 stop exp. compensation

Canon 18mm no exp. compensation.

 totleytom's gear list:totleytom's gear list
Canon EOS 90D Canon EF 70-200mm F4L IS USM Canon EF 40mm f/2.8 STM Sigma 17-70mm F2.8-4 DC Macro OS HSM | C Canon EF-S 24mm F2.8 STM
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Canon EOS 80D Canon EOS Rebel T6i (EOS 750D / Kiss X8i)
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