Inconsistent focusing with new Canon 50mm and 85mm F1.8 lenses. A distressing problem

[JonHornsby]

My daughter has a Canon Rebel XT and recently purchased a Canon 50mm F1.8 and a Canon 85mm F1.8 lens for this camera. She had two reasons for wanting these lenses: (a) to take pictures when ambient light levels are low without having to use flash and (b) to use the shallow depth of field of these lenses at large apertures to separate the subject from the background. The two lenses are quite sharp but unfortunately, 64% of the images taken at a wide aperture (F2) are out of focus regardless of the focusing mode used. We took a series of photos of a stationary object with the camera/lens mounted on a tripod, using the self timer to eliminate vibration caused by mirror slap. We photographed the subject using Manual Focus override, AI mode, and One-Shot mode.

The overall result was that 64% of the time, the camera focused in front of the subject – far enough in front to create an unacceptably blurry picture of the subject. (To be precise, only 20% of the images were sharply focused in AI mode, only 40% in One-shot mode, and only 50% in the manual “tweaking” mode) This is very annoying. Actually it’s more than annoying, it’s downright unsatisfactory. The problem does not occur with the standard 18-55mm zoom kit lens because the maximum lens opening is only F3.5-F5.6 (greater depth of field) with the kit lens. which takes amazingly sharp pictures.


The problem is apparently caused by the camera's sensor and/or mirror alignment or positioning, and could presumably be corrected by Canon for a "mere" $210. But is it worthwhile to have this camera repaired, or should my daughter get an upgraded Canon camera with more megapixels and the stabilized kit zoom lens? Also, what is the likelihood that Canon would fix the camera free, since this appears (maybe it is, maybe it isn't) similar to the problem that prompted the technical advisory and free repair for EOS-1D cameras which experienced focus tracking problems?

 

[250Gimp]

How old is the Xt?

If it is out of warranty I doubt that Canon will fix it for free.

If you send it in you should probably send the lenses in as well so they calibrated with the body.

 

[JonHornsby]

Thanks, 250Gimp.

A quick follow up. My daughter has sent the new Canon lens back and will sell her Canon XT, which is a couple of years old and out of warranty. She will get a Nikon D300 with 18-135mm lens and a Nikon 85mm F1.8 lens.

 

[Jaszek]

can you post some sample shots? maybe you should've checked where the focusing point was set to.

 

[AlexColeman]

She will be much happier with the Nikon.

 

[JonHornsby]

The vertical type was 1 inch in front of the horizontal type (the thickness of two gum wrapper boxes). Hopefully, the effect will show up at the resolution of the photos in this reply. I'm very new at doing this and had to get help in placing an image in the message.

 

[JonHornsby]

Thanks, Alex. I am certainly not condemning Canon products as a result of this one "glitch." Canon makes fine cameras and lenses. But I think you are right. She will be very happy with the Nikon.

 

[250Gimp]

Were these shots taken with a tripod?

The DOF is very thin at f2, so if you lean into the shot the least little bit after aquiring focus you will get an out of focus shot.

To do this test properly you need to use a tripod, or at least put the camera on something solid so it doesn't move at all.

Cheers

 

[JonHornsby]

Yes, the photos were taken with a tripod, using the self timer to eliminate vibration caused by mirror slap.

 

[Flash Harry]

The money spent on these cameras/lens there should be no such issues and if there is whether in or out of warranty should be corrected free, its time these mega rich companies were brought to book a camera that can't focus is about as much use as a chocolate fireguard.

Canon have had focus issues for years now on some models, I first read of such issues in 2001, my Niks however work with all nikon lens perfectly, the d300 even have a built in calibrator just for this reason, not that I've had to use it yet, so I think another nikon convert will be very happy with the new tool. H

 

[JonHornsby]

Most interesting. I had no idea the D300 had such a feature. My daughter will be glad to hear that.

 

[JerryPH]

D700 and D3 also have this feature... though I am happy to say that none of my lenses need any adjustments, even my 85mm F/1.4 is spot on set at F/1.4 and minimal focus distance.

Before I blame any lens, my first is to blame the person behind the camera. At F/1.8, as mentioned, a tiny lean in or out after teh focus is locked in can cause this effect. Set it on a tripod, and I think we will see that 1 of 2 things will happen:

- Lens will be consistently perfectly focused
- Lens will be consistently out of focus

If it is consistently out of focus, sending it to Canon for adjustment is the answer, if its not out of focus... user technique needs work.

 

[JonHornsby]

JerryPH - I also would have thought that on a tripod the lens would have been consistently in (or consistently out of) focus. However, the tests we conducted showed that it was out of focus a certain percentage of the time, and in focus the remainder of the time. Those percentages were enumerated in my opening post. So given those observations, I could only conclude that focusing was "inconsistent." It couldn't have been user error, because the camera was on a sturdy tripod and the self timer was employed.

 

[tsaraleksi]

If it's always off by the same amount, that is probably the lens or camera. If it's off by varying amounts, in very low light, then chances are pretty good it's just the particulars of the situation or the fact that the XT does not have particularly good low-light focus (it's an older model, entry level camera after all).

Also, just to double check, which focusing point were you using?

What sort of problems was she having with it in the real world that led you to this testing?

 

[JonHornsby]

Although not stated in my opening post, the lighting during our tests was very adequate so the lens did not have to search for a focus in the tests. The camera was programmed to focus on the central square in the focusing screen, and as indicated in the pictures (hopefully they show up on your monitor - one reader indicated they were "X'd" out on his screen), the D in the horizontally printed word "Dork" was where the central square was centered.

In the real world the problem was that with the new lenses, she only got 6 or 8 acceptably sharp pictures out of a "bazillion" exposures. Most of the images were exceedingly out of focus and blurry in bright daylight with the lens at or near maximum aperture (F1.8 or thereabouts). Of course the shutter speeds were quite high so long exposure was not the "culprit."

My daughter has two years of experience using the camera and had never had such a plethora of out of focus images. We decided to use the tripod test to determine if the problem was: (a) "her" (b) the lenses or (c) the camera body. Since the problem of inconsistent focus occurred with both lenses, the conclusion (after consultation with a camera technician on a web site) was that the problem was in the camera, not the lenses. It would have been very unlikely for two brand new lenses to exhibit the same problem.

I certainly acknowledge it is harder to get a tack sharp image with an aperture of F1.8 than it would be at F5.6 due to the shallow depth of field. That was the very reason we conducted those tests - to determine just exactly what was going on. Again, the test procedure is described in my opening post, with additional details given in the pictures. Also, I have tried to clarify what I did in various responses above.

Thanks for your interest and comments.