Sigma 50mm on D800 focus issue?

Killerbob

I have the Sigma 50mm, f/1.4, and thus far haven't really had the time to play with it. Yesterday I took some pictures and I noticed some focus issues on the pics. I tested it out this morning, and these are the results:

Test using LiveView:


Test using ViewFinder:


I think I see a problem. Does anyone agree, not agree? And, if there is a problem, how do I proceed? I tested my Nikon 14-24mm, Nikon 24-70mm, Nikon 80-400mm, and Nikon 105mm lenses as well, and though I do see some differences when magnifying to say 400%, it is nowhere as obvious as with the Sigma 50mm. Hence I think it is just the Sigma lens / Nikn D800 combination. Should I talk to Nikon or to Sigma?

Killerbob

I haven't played with the AF at all. I was hoping I wouldn't have to, but I guess the results show me I need to.

I choose the Sigma because of the good reviews I read on the web. I guess if/when I go for a 85mm, I'll stick to Nikon.

TaoTeJared

Just use AF-S and use the center spot for the "accurate" AF spot - per the body. Saying that, it diffidently looks like the lens is quite a bit out of alignment. 1.4 will be the softest of the apertures, may try steps up to 2.8 and then use a 5.6 setting to see the sharpest it could be.

Looks like you may have to send the lens in to be re-calibrated. Sigma is notorious for being out of focus on the 50mm.

Killerbob

So I'll go through the AF finetuning to get my Sigma tack-sharp. How sharp should I expect it to be at f/1.4? And, how much out of alignment need it be before I need to send it back

And, what is the coloration called; it is practically redish compared to a black LV shoot...

FlowtographyBerlin

@Killerbob: Fine-tuning the AF is perfectly normal, especially on a hires camera like the D800. Since I went digital, I never had to not use AF fine tune, and for AF lense I've always used brand stuff only.

You can use the downloadable Focus Test Chart, it does the job perfectly. For many of my photographer friends, the first thing to do when they get a new lens is adjusting AF fine tune – for me too meanwhile.

BTW: When you're adjusting the AF fine tune, mind the focus shift! Most lenses shift focus quite a bit when stopped down. Never used the Sigma, but it would be very strange if it was an exception. As a compromise, you can calibrate at an aperture of 2.8 or so.

FlowtographyBerlin

Do you really think it's a good idea to compensate for focus shift?

I guess it depends on how you use the lens. If you only use it wide open anyway, then not, of course. But if you calibrate to wide open and use your lens at f/8, you lose quite some sharpness where you're focussing. How much this matters is a matter of the subject, of course, but i.e. for portrait work, it's quite a bit.

You can find a logic for arguing for one way or the other. The thing is, at f/1.4, even the Sigma won't be tack sharp even if calibrated, while at f/8 it will. In my experience, f/2.8 is a good compromise.

BTW, you only need the last page of that PDF, and do read the brief instructions. Getting the (easy) setup right does make a difference for a stress-free calibration.

Killerbob

Thanks, I'll try that on the weekend.

I can appreciate that wide open even the Sigma won't be tack-sharp, but at least it will be better than right now (see my pics above). I think I will try to calibrate first at f/1.4, or perhaps f/1.8. After all, it was because of this capability I got the lens in the first place.

TaoTeJared

So I'll go through the AF finetuning to get my Sigma tack-sharp. How sharp should I expect it to be at f/1.4? And, how much out of alignment need it be before I need to send it back

And, what is the coloration called; it is practically redish compared to a black LV shoot...

Shoot an image at f/5.6 or f/8 - that is the sharpest the lens will be. Trying to put into words how sharp it will look at f1.4 is about like listening to a 3-yr old describing quantum mechanics.

If you really want to be messing with focus shift and fine tune - I would suggest picking up Lens Align. That is what I use to fine tune lenses and it makes sure everything is lined up correctly. At 1.4 it is really easy to miss-tune it if you have stuff off just ever so slightly and make things worse.

TaoTeJared

The 50 was not designed to be at it's sharpest wide open, it was designed for better bokeh. Most all lenses that are faster than 2.8 are designed for bokeh first, sharpness second and are at their sharpest at 5.6ish. There is a bit of physics but most all lenses are at their sharpest at 2-stops down from wide open.
I looked it up, looks like the conciseness was at f/5.6 - f/7.
http://www.dpreview.com/lensreviews/sigma_50_1p4_c16/5 gives the easiest view to see it.

Of Sigma's lenses, only the 35mm 1.4 hit it's sharpest at f/4. Diffraction entered in by f/8 which is lower than most.

Killerbob

Does this mean I should run "FoCal Pro" on another f/stop than the widest? I.e. at what f/stop is it most important to have the MA set correctly?

FlowtographyBerlin

@Killerbob: You can easily (and investing nothing but something like 30 mins) check out yourself what the effect of the focus shifting is:

Take that Focus Test Chart, fine-tune at f/1.4, and then stop down to f/5.6 or so and see where the focus is. To check the practical downside, just leave your camera on the tripod and shoot a non-moving subject at f/5.6 or something at a typical distance that you want to use the lens for, one time use the AF (that you fine tuned to f/1.4) and one time use Live View. Compare the images and see how much sharpness you miss at 5.6 and if it matters to you.

You can repeat the same thing with calibrating at 2.8 and comparing the results at 1.4 and 5.6 and 8.

It's a little flddling around, but no calibration system will get rid of the focus shift on your lens – you will have to compromise and just have to decide where.

Ironheart

At this level of adjustment, wouldn't it really be about personal preference and at what aperture you tend/want to shoot at? If you want to use wide open, calibrate there, if you like to be stopped down a bit, calibrate there? In fact you could have your cake and eat it too, if you write the calibration setting down for each f/stop you could reprogram in the field as needed (this seems pretty anal).

FlowtographyBerlin

At this level of adjustment, wouldn't it really be about personal preference and at what aperture you tend/want to shoot at? If you want to use wide open, calibrate there, if you like to be stopped down a bit, calibrate there?

Exactly, that's what I meant with my last sentence ;-)