Best way to check out proper focus on lens?

Quicklabs

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Sometimes, I think my Sigma 30 1.4 is spot on; at other times, I'm not so sure. This morning, I ran a focus test chart trial. It seemed that the left side had some mild back focus issues at the widest apertures. Everything looked good after about 5.6 or so. Obviously I bought this lens for shooting wide open.
What other ways can I check this out to differentiate between lens issues and operator error?
Thanks in advance!
Laura
 
How did you do the focus test? I have used this LINK before - about 2/3 of the way down the page are step by step instructions which should rule out any type of operator error / variance.

If it's back / front focusing after following these, you should send it in for calibration (include the test shots from the focus tests), or return it as defective if it's still new enough and get a different copy of the lens.

Good luck!
 
Everything looked good after about 5.6 or so.

Good observation! You just learned a fundamental property of lenses. In fact, almost *all* lenses achieve maximum sharpness at around f/5.8 - f/8.

Shooting at wide aperature will always involve some trade-off in sharpness. Wide aperature also have a very shallow depth of field, so your focus need to be extremely accurate.

Sorry, but those are the rules of physics that cannot be changed.


-Paul
 

Good observation! You just learned a fundamental property of lenses. In fact, almost *all* lenses achieve maximum sharpness at around f/5.8 - f/8.

Shooting at wide aperature will always involve some trade-off in sharpness. Wide aperature also have a very shallow depth of field, so your focus need to be extremely accurate.
I'm going to guess that it was a DoF issue, not a sharpness issue. If the AF was working properly, even F1.4 should lock right on to the correct focus area.

From what I've seen, DSLRs having the focus slightly off is not terribly uncommon, I've seen reports of examples of just about every DSLR having to sent back for focus adjustment. My first two seem to be spot-on but my newest is just a touch off and is currently away getting tweaked by the manufacturer (I had it fixed via the camera's AF adjustment but wanted it "perfect" while it was still under warranty.) For my initial tests, I used one of the standard focus charts you can find, but someone recently made a set of them that you can get here. They're marked as being for the K20D but of course, they should work equally well with any DSLR. They even include a page that you cut and fold to give you the proper 45' angle, and include three different charts for wide, normal, and telephoto lenses.

However, if you're seeing issues with only one corner of the lens being off, than that may not be a focus issue at all. Only the actual focus point is "guaranteed" to be in focus, and with only F1.4, you'd need to make sure that the camera and whatever you're shooting are perfectly straight on. That particular lens has been noted a few times as being heavily biased towards center sharpness at the expense of corner sharpness - so you may just being seeing more or less normal behavior for the lens. But if it's under warranty and you can live without it for however long it takes to get checked out, it's probably worth sending it to them.
 
Thanks for your thoughtful response, Groucho. I'm going to do some more work on this issue this weekend. I've never tested any of my other lenses, so it could be my DSLR. The focus issue isn't really at the corners (although CA is pretty bad there), it's at the left center area. I've written to Sigma, and they've asked that I send the lens in with the camera, as well as with the test shots that most clearly demonstrate the problem. I want to get out this weekend as well and take some "real"pictures to see what I'm getting. I've wondered if I had the camera set right at 45, so the site you suggested will help a lot.
Thank you.
 















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