Focus/lens/body problem - help please?

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coyoteboy

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Hi all, I've been invited over here by a fellow I met on another forum, he said you're a nice place to ask questions and know you're stuff so lets see if you can help!

I've recently aquired fairly cheaply a used 20D, it seems to have about 20K shots on it (according to the continuous numbering) and came with a Sigma 18-50 3.5/5.6 lens. Previous owner described it as "perfect". I'm not so sure, or maybe I'm just picky. I got it for a price that meant the lens was effectively free, but I'm trying to nail down a focus problem I'm having. It seems that when I pick the centre point and focus, my focus point is noticably out of whack. I've tried ensuring I had high contrast, planar targets but still if I take a shot of someone portrait style I focus on their nose and actually it focuses on their ears (side-profile shot) miles from the AF point). Likewise, I took a range of shots at the local park, of a castle 500m away, but could not get the castle in focus despite spending about 50 shots on it, ranging in F from 5.6 to 22 and keeping my shutter time faster than 1/125. But strangely MF seems "out" too.

Problem is I've not got any other lenses to test it with to prove if its the body or the lens (I've just bought a prime MF lens to see, waiting on its arrival). I tried it with another body/lens (person who invited me here) and my camera seems to get good focus with his lens most of the time (give or take my odd mistakes) BUT he seems to get decent-ish shots with mine too?! In fact he found that, while I got worse shots at 50mm, he got worse shots at 18mm.

Can anyone advise?

Test shots at http://www.jbuckle.homeip.net/images/photoqs/focus.html and a couple of inclined (under exposed!) ruler shots at
http://www.flickr.com/photos/bucklevision

Much obliged!
 
have you tried manual focus and is it sharp, then if not ,could be a loose element in the lens(been dropped?):shrug:
 
Your shutter count theory is seriously flawed and I doubt its anywhere near right.

the lens is a cheap third party lens so i wouldnt expect too much from it..
 
sorry didnt have my glasses on if you,ve tried manual focus and its off as well would suspect the lens has had a knock get a cheap second hand one or take to a camera store and try one of thier lens on your camera :D:D
 
KIPAX - My shutter count theory is purely going off the folder name, I'm aware its probably not right (any method of retrieving it on these?) but I've no reason to assume it has done more and it looks worn to about that level (in comparison with other cams of similar use). It's been owned solely by one chap for personal use.

I dont expect it to be an L, but I do expect better than you see in those images - I can get better results from my £100 P&S in teh same conditions (I've photos to prove it too lol).

dinorock - yes, I've focused manually through the lens and although it looks crisp to me, and the AF beep confirms it at the same point as I'd have it, it still looks soft. Lens seems undamaged, clean and tidy, if a little cheap in build quality.

Edit:-

I'll grab some shots tonight with the MF prime I've found, this should identify if its a body fault I suppose, or at least narrow down the possibilities. It seems odd that the lens performs differently on someone elses body, yet my body performs fine with his kit lens. A loose element does make sense though, as the OOF focus point seems to shift from one minute to the next, rather than being constant FF or BF.
 
I think it's going to take the use of another lens or body to nail the fault down .. let us know where you are in the world and maybe someone close could help out. :)
 
Is it front or rear focussing? If so you have a problem as it means returning to a service centre for adjustment.
 
Oh yes, I agree it is a cheap lens - definitely not worth getting repaired by a pro, I was just hoping someone might be able to help me identify the fault. I've had it apart once but couldnt get into the focusing optics themselves. In fact when it went back together it actually performs better now than it did at first, but this is not directly due to me I'm sure, as I found nothing out of the ordinary in there. I'm very much a DIY person, I'll take anything apart and fix it if possible, but it's handy to know where to look initially.

Donna - its primarily front focusing, the focal plane could be anywhere between 1" and 6 feet in front of the AF point depending on the direction of the wind and the camera handlers star sign! :) If you check out my first link, you'll see what I mean, all shots AFd on the plank, yet some actually focus 1-2ft in front of the plank and others fail to focus anywhere within frame despite the camera saying it has and despite it looking in focus (or at least looking at best focus) through the viewfinder. The bottom shot was taken while focusing on the sharp edge of the buildings roof, yet it was OOF, even when MF'd. I've tried "walking" through focus points to see if it was just FFing or back focusing and maybe I'd catch it out slightly off where it thought it was meant to be, but I seem to have failed.

To make matters worse I cant really sell the lens as to me it's faulty, even if it seemed to work with someone elses camera - my conscience wouldnt be happy!
 
Your tests are not conclusive but it appears to be front focusing. I would suspect the lens but cheap or not, it should focus fine in those conditions. Send it to Sigma - they are well used to sorting out this kind of thing.
 
I concluded, re the shutter count, that it could fail at any time. I got the body and lens for 200 quid so it was worth the risk. Worst case scenario I'd replace the shutter (either properly or myself) and I'd if I got on with it I'd be moving up to a 40D or more within a year or two. Unfortunately I've not got past this focus problem yet.

I'm willing to try other tests to aid repair if those are not considered exhaustive enough. Not worth sending the lens to Sigma really, their warranty is non-transferrable so I'd have to pay their minimum lens work fee which is 66 quid + postage, which is the same price as the lens is new, give or take a few beers - I think I'd be better buying a standard Canon kit lens as an all-rounder, unless my local camera shop (canon/nikon/sigma etc authorised repairers) will do it for less than the Sigma factory will.

I guess I've stuck myself between a rock and a hard place, ultimately what I'd like is someone to say "try these test, if this happens it's this, you'll need to take this off and bond that back to there" or such like, as my only other options are not financially viable and I may as well just bin it. I guess the number of people willing to strip zoom lenses is small though. What I really wanted to do was factor the body out of the equation so buying another lens is a safe bet, not a waste.

I'm based in Glasgow, by the way, should anyone else be willing to test my bits out with more experienced hands!
 
Chances are the body is fine. Canon 20D body has a reliable reputation; cheap Sigmas less so. To check it, try it with a known good lens and there's your answer.

The lens problem, if it's fairly new, is likely to be a calibration problem. Easily fixed if you know how. I believe some older lenses need rechipping to work with newer cameras.

Either way this tends to be specialist work. I used to take my old manual focus lenses apart, but they were just simple metal rings and focusing helicoids etc. No electronics or fiddly plastic cams, AF mechanisms and stuff.
 
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