Sigma 15-30/3.5-4.5 EX DG IF for Canon exposure problem

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SQLGuy Veteran Member • Posts: 6,447
Sigma 15-30/3.5-4.5 EX DG IF for Canon exposure problem

Hi folks,

I just got a new-old-stock copy of this lens. It mostly works pretty well. AF is noisy but accurate, and it's quite sharp in the center, and decent at the corners.

However, I noticed an exposure problem with it at the wider focal lengths. As far as I can tell, the lens is overexposing when stopped down by about the amount of difference in its variable aperture from the long end. What I mean by that is that, if I stop down to, say, F5.6 or F8 at 30mm, where "wide open" is F4.5, the stopped down exposure is still correct. But if I stop down to 5.6 or 8 or whatever at 15mm, where wide open if F3.5, then the stopped down exposure is about 0.7EV over exposed. In the middle of the range, stopped down exposure is about 0.3EV over. Wide open is always fine.

I'm using the lens on an MC-11 on an A7R2, but I tried several other lenses, including one other one with a variable aperture and did not see this behavior with them, so I'm pretty sure this is the lens and not the adapter that's responsible. Actually, I also tried it on a Viltrox III adapter, and with the MC-11 on an A7R, and saw the same behavior with those combinations as well, so I'm really fairly confident it's the lens that's causing it.

My suspicion is that the lens is not correctly setting its aperture when the variable aperture would make it brighter. E.g. at 30mm, F8 is F8, but at 15mm F8 is really more like F6.3.

Anyone seen this type of problem before? I can work around it with EV compensation, but that's something I'm likely to forget to do from time-to-time. If it's something that Sigma could correct by reprogramming the lens or something, that would be great.

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A7R2 with SEL2470Z and a number of adapted lenses (Canon FD, Minolta AF, Canon EF, Leica, Nikon...); NEX-7 converted to IR.

 SQLGuy's gear list:SQLGuy's gear list
Canon PowerShot G9 Nikon D200 Sony Alpha DSLR-A900 Sony Alpha NEX-7 NEX-5T +15 more
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