Rear focus limit?

[Gofer]

I bought a Sigma 17-70mm last week for my Canon 400D and today decided to check for it's focus accuracy. Below is the result. It obviously has a rear focus tendency but I'm not sure what would be consider as acceptable or within 'limits'. Should I send this lens back for a replacement or is this as good as it gets given the price. The other I did notice from several shots I took today is that it doesn't seem to suffer from any purple fringing whatsoever not even at the extreme edge of the frame. I have another Sigma 17-70mm fitted to my Pentax K10D and that focuses smack bang in the centre of the test sheet but that lens does seem to suffer from a modicum of purple fringing.

 

[theMusicMan]

Sorry, but it looks to me as though you have focussed on the 6mm point behind the intended focus point... or is this what you are suggesting is wrong with the lens?

 

[DiddyDave]

Hmmm... I'm guessing that you're not normally using a 17-70mm for macro work? Or even close-up work? Or even anything closer than a few feet away?

So if there's no fringing and it's sharp enough - IF this bit is wrong in your shot above, what on earth does it matter?

 

[Gofer]

Sorry, but it looks to me as though you have focussed on the 6mm point behind the intended focus point... or is this what you are suggesting is wrong with the lens?

Exactly TMM. The camera was set up to use the centre focus point only and that was positioned over the black 'focus here' line, but as you've correctly identified the optimum focus point is about 6mm behind that line. I've checked all my other cameras and other lens and they all focus directly over that line.

 

[Gofer]

Hmmm... I'm guessing that you're not normally using a 17-70mm for macro work? Or even close-up work? Or even anything closer than a few feet away?

So if there's no fringing and it's sharp enough - IF this bit is wrong in your shot above, what on earth does it matter?

I'm not sure Dave. Yes the lens is sharp - at the focus point and yes it doesn't suffer from fringing, but if I wanted to take a shot with the lens wide open to blur the back ground the sharp focus point would be approx 6 - 8 mm behind where it should be. If I was trying to get the eyes in sharp focus of a portrait for instance I'm not sure this lens wouldn't let me down. I've probably answered my own question now!

 

[DiddyDave]

the sharp focus point would be approx 6 - 8 mm behind where it should be. If I was trying to get the eyes in sharp focus of a portrait for instance I'm not sure this lens wouldn't let me down. I've probably answered my own question now!

No you haven't - first let us know how far away from your lens was the test sheet?

I suspect, only inches

If you ever take a portrait that close, then yes there could be a problem - but I take lots of portraits, and with an 18-70mm lens, and I'm never closer than a couple of feet - so even if the focus point was 8mm off, I'd never notice! And nor I suspect would you

Added to which, the front of an eyeball to the outside corner of the eye could easily be 6-8mm - so focus on that, lock the focus point and you're sorted

 

[Gofer]

You're absolutely right of course Dave inasmuch I doubt I would notice any slight out of focus. Just for the record, when I took the shot of the test sheet I suppose the camera was approximately 18 inches or so away, at full zoom, wide open (f4.5) and at an angle of 45 degrees. I did take some close ups this afternoon of various flowers in the local park and that did seem to highlight the problem but then again I'm sure I was probably looking for it. Normally I'd be quite happy to manually focus but these lenses have such a short focus path it's practically impossible and I certainly can't achieve a better a focus result than the auto focus. I'll just have to avoid taking close ups of flat surfaces square on! Thanks for your thoughts.