Do I have a focusing issue?

I have recently picked up a second hand Sigma 17-70. It seems really sharp for usual shots but when trying out it close up performance it seems not to focus quite where I was expecting. Below is an example from the garden, 70mm F 6.3 1/400s, I was focusing pretty much in the centre of the flower but it seems to have focused further into the image. I have also attached a typical newspaper shot, 53mm F4.0 1/40s, this time focusing on the seventh entry down from the header 'Movie Greats' in the dead centre of the shot.

Am I doing the right tests or is there a better way to check?

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

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

Either it is back focusing or you are trying to focus closer than the lens's minimum focus length, looks like.

Are there any tests that are better than the ones I have done (or just better execution) that I can do or should I take it in and get a store to have a look?

For one thing..... put it on a tripod when that close. It takes very little movement to end up focusing on something different than you initially thought.

You can stop down a bit more than f6.3. When you are focusing so close like that the DOF is VERY small even with a fairly small aperture.

It looks like your lens might be back focusing, but as the others said it could be you are hitting the minimum focus distance as well, so make sure you are not too close.

http://www.focustestch​art.com/

The second shot was on a tripod with remote release and about 80cm from the paper. I will have a look at the focus test site and see how it goes.

I was in -->exactly this position<-- and it was a major PITA.
You could save yourself a lot of delay and hassle by doing what I did.
Sell it and buy an 18-55 IS.

xarqi wrote in post #7787355
I was in -->exactly this position<-- and it was a major PITA.
You could save yourself a lot of delay and hassle by doing what I did.
Sell it and buy an 18-55 IS.

Did Sigma ever repair the lens or just send it back?

red snapper wrote in post #7787500
Did Sigma ever repair the lens or just send it back?

To be honest, I have no idea. Everything I'd been told by Kennedy's indicated that it had just been sitting on a shelf in Australia for two months. When I finally gave up in disgust and recalled it, it arrived within a week, but there was some documentation saying that a part had been replaced.

I'd already decided that I didn't ever want to have to deal with Kennedy's again, and that meant no more Sigma products for me, and so I was resolved to selling the lens anyway. I could see no point in exhaustively testing it once I got it back on the off chance that they had actually done something about it.

I listed the lens on a NZ online auction site with full disclosure of its chequered history. The buyer opted to take the risk that Sigma had repaired it based on the documentation. As far as I know, both parties were happy with the trade.

I have been doing a bit more testing this morning using the battery test. Interesting results. In each shot I focused on the second battery. I did notice however that the square around the focus point in the view finder was slightly outside the right edge of the battery. By adjusting the camera slightly to bring this inside the battery,I was able to get the expected result, the last shot. Also I took about 4 or 5 shots each time refocusing from infinity. 1 was not quite right (photo 3) but the rest were good. So....is this now user error and now having a sharper and much better lens is now highlighting my need to be far more accurate with my focusing? Or is this still something that shuold get looked at?

F4.5 1/40 70mm

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

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

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

Focusing is not always a slam dunk. I have a tall church steeple over a mile away that none of my lenses can reliably focus on. It looks like a thick hair in the center focus point. With the distance and the fact that the target is relatively small, the focusing error is just too great. The AF misses but just slightly. Easily noticeable when pixel peeping at 100%.

I also have a building right accross the street. If I focus on the concrete balconies (they have verticle expansion joints), the same thing. All my lenses are a bit off with that target.

After awhile, when you get the odd OOF image, you learn what works and, more importantly, what may give trouble to the AF system.

bohdank wrote in post #7791223
Focusing is not always a slam dunk. I have a tall church steeple over a mile away that none of my lenses can reliably focus on. It looks like a thick hair in the center focus point. With the distance and the fact that the target is relatively small, the focusing error is just too great. The AF misses but just slightly. Easily noticeable when pixel peeping at 100%.

I also have a building right accross the street. If I focus on the concrete balconies (they have verticle expansion joints), the same thing. All my lenses are a bit off with that target.

After awhile, when you get the odd OOF image, you learn what works and, more importantly, what may give trouble to the AF system.

Cheers, I'll keep playing around and see what results I get, there's nothing like a bit of buyers remorse to make you paranoid!

Don't know what's going on with the first shot. Your depth of field with the 700mm @ f/4 3-4 feet away is about 2.5 inches, which is about what you've got with the middle two batteries. That says to me your AF is probably in the ballpark.

On the flower shot, if you weren't using a tripod and were not closer than the lens's minimum focusing distance, is it possible you were swaying a bit forward and backward as you made the shot? The possibility is that if your camera was set on One Shot and you pressed the shutter button halfway, fixed AF and then swayed forward as you fired the shot, you pushed the point of focus foward to where it landed. Try the same situation using AI Focus.

AI Focus sucks.... try AI Servo although that is not the type of situation AI Servo necessarily works that well either since it really needs something moving in a predictable (it is predicitve focus) way in relation the camera.