My Sigma f1.4 50mm might have focus issues

[danteus88]

Hi everyone!

Ok, so I bought Sigma f1.4 50mm for my Canon 500D, and I suspect (I'm a beginner, keep that in mind please) that it's either having some focus problems, or it's rather soft on anything lower than f3.5

I autofocused on the line.

The test photo I shot at f1.4 is this one:
http://i3.photobucket.com/albums/y78...e/IMG_3336.jpg

and a cropped center because photobucket shrinked the original:
http://i3.photobucket.com/albums/y78...snake/crop.jpg

so what do you think, is my lens (or possibly camera) experiencing some focusing problems?

I also think it's fairly soft, but that might be the result of bad focus, so I'm trying to solve one problem at a time, if there are any.

[inkista]

Your testing methods are questionable and have a greater margin of error than the accuracy you're looking for.

If you are going to test for AF accuracy:

1). Shoot in GOOD light. Low shutter speeds, or high iso can throw off sharpness evaluations. You want to be sure that camera shake and noise are not factors as you judge sharpness/focus, and that the only factors that could be causing the issue is a misfocus.

2). Shoot at a perpendicular target at a given distance. The slanted ruler thing is not the accurate test it's purported to be, because that tiny little square you see in the viewfinder is not accurately depicting the size of the AF sensor. You need a set up more like a LensAlign to seriously test for AF accuracy within half a millimeter. You better also know how to set up the camera to be perfectly perpendicular to your target. High-contrast targets are also ideal. Taping newspaper to a wall can be good, if you know you're perpendicular to the wall in both pitch and yaw. A slight bit of angle shift, and you could be well outside the 0.5mm accuracy you're testing for.

3). You have to put the camera on a tripod. Handholding camera shake can put you well outside the 0.5mm accuracy you're testing for.

4). You need to test at more than a single aperture, as your lens may also exhibit focus shift. And, of course, shooting wide open is going to be the most demanding of the lens, and your test methodology, because your DoF is going to be tiny. Most of us, if accurate focusing with super-thin depth of field is THAT critical, would be using a tripod, manual focus, liveview, and 10x magnification, not autofocus.

But aside from that, the Sigma 50mm f/1.4 is known to sometimes have AF accuracy issues on Canon bodies. If you'd done your homework, you would have known this. Sorry. But them's the breaks.

[autofocus]

From what I can see it looks like it is front focusing...but a few questions
* is your camera set to one shot, and not one of the servo modes?
* did you use the center focus point when shot?
* what was your shutter speed?
* did you use a tripod?
* have you tried manually focusing on the target?
* have you tried testing this with your kit lens set to it's widest aperture?

You have two things working against you...both shooting at your widest aperture and the fact that you were quite close to the target will yield very thin DOF. So, even the slightest change in distance to the target before it was shot could effect your focus.
~or~
You could have a bad copy...it does happen

[ceremus]

Also, it's good if you're aware of some of the unique challenges of shooting with a wide aperture lens, especially one as large as f/1.4: LensRentals.com - How to Shoot With Wide Aperture Lenses

Extremely narrow DoF and Focus Shifting in particular. Having a body with autofocus microadjustment is kind of a big deal when working with these.

[danteus88]

Thank you for answers!

Ok, so I apologize I didn't write answers to some of your questions at first, I'll correct that error now:


* is your camera set to one shot, and not one of the servo modes?
-one shot

* did you use the center focus point when shot?
-yes!

* what was your shutter speed?
1/200 sec (ISO 100)


* did you use a tripod?
yes! a sturdy one, and the head was tilted 45° down. also mirror-lockup and remote-release.

* have you tried manually focusing on the target?
in this test batch, no. but see, that's a good idea. I'll try that tomorrow, thanks.

* have you tried testing this with your kit lens set to it's widest aperture?
actually no. another good idea!

1). Shoot in GOOD light. Low shutter speeds, or high iso can throw off sharpness evaluations. You want to be sure that camera shake and noise are not factors as you judge sharpness/focus, and that the only factors that could be causing the issue is a misfocus.

Yeah, light conditions in my flat seem to be unfavorable... I'll try to shoot again, during the morning with additional light from lamps or something.

tl;dr I'll come back tomorrow with more precise and diverse test results.

Thank all of you very much!

[danteus88]

Hi again.

So I did a new battery of tests, with some of your tips in mind.
The results are interesting and seem to support the theory that my lens have focus issues.

Test details: Canon 500D with Sigma 50mm f1.4 mounted on manfrotto 055xprob, with as much light as I could gather, 100 ISO at all times, and remote control.

Autofocus (center focus point on "N"):
http://i3.photobucket.com/albums/y78/dantesnake/135.jpg
Turned LIVE View on, max magnification, and manual focused until it was sharp(er).
http://i3.photobucket.com/albums/y78/dantesnake/136.jpg

Autofocus (center focus point on 0 yellow number):
http://i3.photobucket.com/albums/y78/dantesnake/130.jpg
Manual Focus, same procedure as above.
http://i3.photobucket.com/albums/y78/dantesnake/131.jpg

Autofocus (center focus point on keypad 5):
http://i3.photobucket.com/albums/y78/dantesnake/128.jpg
Manual Focus, same procedure as above.
http://i3.photobucket.com/albums/y78/dantesnake/129.jpg



sidetopic, here is a slightly cropped picture from above:
http://i3.photobucket.com/albums/y78...nake/136-1.jpg

so you can view it in full-size.
Um, what do you think about the "sharpness" or better yet, lack of, is this normal\acceptable at f1.4? particulary for a prime lens that should be 'very sharp' even 'wide open'?

Thank you in advance ^^ <3

[autofocus]

Looks like manual focus has proven to yield somewhat better results. It could indicate a bad copy of the lens that possibly could have been fixed with the micro adjustment option...unfortunately, your camera model does not have that feature

[autofocus]

I hate to say it, but you may have been better off with either Canon's 1.4, or even the 1.8...and would have saved some money to boot. The sigma is far more impressive and rugged looking though. Here's a piece copied out of dpreview...kind of sums it up:

The Sigma 50/1.4 gives impressively even coverage across the frame on APS-C, and although it's slightly soft wide open, it's very much towards the top of its class. Sharpness increases progressively on stopping down, with optimum results at F4-5.6; at apertures of F8 and smaller, diffraction takes its toll

[ceremus]

Reading the review at TDP, AF microadjust may not even help the problem with this particular lens.

What you also get when shooting with a wide-aperture lens wide open is a narrow DOF (strongly blurred background and foreground - especially with a close focus distance) and a fast, action-stopping shutter speed. Both of these are great attributes, but if you are relying on autofocus for this action it needs to be very accurate to deliver the DOF at the right focus distance. Even a relatively small mis-focus results in a blurred subject. And to cut to the chase, the Sigma 50mm f/1.4 EX DG HSM Lens has proven very inconsistent for me in the focus accuracy department.

I have thrown out as many as 70% or more images from a single shoot of over 100 non-action, wide aperture shots because they were very OOF (Out of Focus). These were not even close-distance shots that are typically most challenging for shallow DOF lenses. The Sigma 50 delivers better results in very good light, but the percentage of salvageable shots is still not as high as expected.

Though a higher percentage of the mis-focused shots were front-focused, a significant number were back-focused as well. Therefore, focus calibration is not going to help this lens. Stopping down the aperture improves the keeper rate significantly - the resulting deeper DOF will include the subject much more frequently. APS-C sensor bodies (1.6x FOVCF) will have less trouble than full-frame bodies - the longer distances required for the 1.6x bodies to frame the scene identically will deliver greater mis-focus-hiding DOF.

The unreliable focus performance is a big disappointment, because otherwise, Sigma has delivered us a very nice lens in the 50mm f/1.4 EX DG HSM.

[Chri5tran]

I've read that this lens is a hit or miss kinda thing. Some people have no problems while others do run into issues.

I was wondering where you bought your lens from and how long ago?

Hopefully my lens, which is being shipped, doesn't have this issue. =/