Sigma 85mm f/1.4 underpose issue

Read this before on different forums that the Sigma 85mm f/1.4 underexposes 1/3 to 2/3 stop compared to Canon 85L, even at manual mode.

A couple of weeks ago, there was a photographic show here. I brought my Canon 85 f/1.8 and did some comparison shot with the Siggy 85. Shoot at manual mode, with same ISO, different shutter speed and aperture combination. Also shot a few video clips to compare.

The siggy under the same ISO, f/1.4 , 1/160 sec shutter, the exposure is almost exactly the same as f/1.8, 1/100 sec shutter with Canon 85. Also tried f/1.4 and 1/200 sec shutter, it's slightly ( a little) darker than f/1.8, 1/100 sec with Canon 85. The video clips confirm the same thing.

I'm sure that the Sigma 85 is indeed 2/3 stop faster than the Canon 85 f/1.8. At least in manual mode, which is what matters to me most.

Hopefully this is helpful to people who are thinking about upgrading to siggy 85, but concerned about the real speed of the sigma.

I will post pictures later after I upload them.

That sounds about right..

genjurok wrote in post #12526357
The siggy under the same ISO, f/1.4 , 1/160 sec shutter, the exposure is almost exactly the same as f/1.8, 1/100 sec shutter with Canon 85.

So let me understand you...

If you use an aperture not quite a stop faster than F1.8 you get get not quite a doubling of shutter speed on the sigma with equal exposure?

That sounds exactly right...I'm not seeing the issue here.

Maybe you should have tried the sigma at f/1.8 as well to see whether the shutter speed was different to canon's?

Thanks. That's a good info to know.

Sdiver2489 wrote in post #12527107
So let me understand you...

If you use an aperture not quite a stop faster than F1.8 you get get not quite a doubling of shutter speed on the sigma with equal exposure?

That sounds exactly right...I'm not seeing the issue here.

I don't think he's reporting an issue
I think he's saying he's read/heard about an issue, did some limited testing, and doesn't believe there's an issue

Yapo wrote in post #12527178
Maybe you should have tried the sigma at f/1.8 as well to see whether the shutter speed was different to canon's?

I think I did that, and they looked the same exposure to me.
But keep in mind that I always shoot at manual mode, because I want to test the speed of lens, not metering.

Alrite, here are some samples. I couldn't find the pictures, but did find the video clips I took with both lens at different aperture , shutter , but the same ISO. Exposure was manual mode. I extracted the frames from the clips and uploaded them.

ISO 320, aperture 1.8 and 1.4, shutter varies between 1/60sec, 1/100sec and 1/125 second.
Lighting was consistent. Here are four pictures. The Canon 1.8 , 1/125 sec is obviously the darkest one. But the Siggy 1.4, 1/100 sec and Canon 1.8, 1/60 sec are very very close in terms of exposure,
although color look a bit different. Histograms look almost identical.

Canon 1.8, 1/125 sec

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HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

Canon 1.8, 1/60 sec

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I don't ETTR (I know!) but going off just the meter in 90% of the situations I shot in with it, I had to compensate for one stop in order to get correct exposure. This hasn't happened on any other lens I've owned, just that. This is from hundreds of shots testing out my Sigma 85. It's not a huge deal but it's noticeable.

I don't see the problem.
Your first set of shots, you're using same shutter speed but wider aperture on the sigma, and the sigma shot is brighter.
Your second set of shots, you closed the canon down, which lets in less light, but you've slowed the shutter speed, which lets in more light. With the sigma, you've opened it wider, which lets in more light, and increased shutter speed, which lets in less light, but you haven't exactly balanced the exposures out, so its normal for one to be brighter, and the sigma is, so where's the problem?

Also, to make it simple and less confusing, why don't you just set them both to 1.8 or 1.4, and use the same shutter speed, then you can see if one over or under exposes...

Sirrith wrote in post #12528474
I don't see the problem.
Your first set of shots, you're using same shutter speed but wider aperture on the sigma, and the sigma shot is brighter.
Your second set of shots, you closed the canon down, which lets in less light, but you've slowed the shutter speed, which lets in more light. With the sigma, you've opened it wider, which lets in more light, and increased shutter speed, which lets in less light, but you haven't exactly balanced the exposures out, so its normal for one to be brighter, and the sigma is, so where's the problem?

Also, to make it simple and less confusing, why don't you just set them both to 1.8 or 1.4, and use the same shutter speed, then you can see if one over or under exposes...

This is why I said this.

Sirrith wrote in post #12528474
I don't see the problem.
Also, to make it simple and less confusing, why don't you just set them both to 1.8 or 1.4, and use the same shutter speed, then you can see if one over or under exposes...

As I said in a previous post, I did some comparison shots at same aperture and shutter, and they look identical.

But that's not my main point. The most important thing for me is to know the real speed of siggy at f/1.4. Is the siggy indeed 2/3 stop at f/1.4 faster than canon at f/1.8? That will decide if it's worthing upgrading for me.

That's why I did the test and compare both at
f/1.4 , 1/100sec vs canon f/1.8 , 1/60sec

and the result to me looks good. The Siggy at f/1.4 is indeed 2/3 faster.

Basically I'm not reporting an issue, as someone said before, rather I'm reporting it's a non-issue (to me)

I was concerned about this issue when I bought my copy. I compared it with a 85L and when shooting both at 1.4 the L is slightly brighter than the Sigma. The difference disappears by 1.6
If you look at transmission data from fast lenses, they are not quite as fast as promised, but in this case the difference is subtle (for MY purposes).

Yes,, I see your point..

But if you shoot with the Sigma at say f/1.8 at 100th second at 100 ISO and the Canon at the exact same exposure settings, the images should have the same brightness, unless there is a problem with one of the lenses.