XTi Focus Problem

I am helping a friend with an XTi problem. We've tried using autofocus and manual focus and it will not focus on the subject. At first I thought it was a problem with the kit lens but I put my 24-70L on it and got the same results. I took the lens off and nothing seems to be wrong with the mirror or the action of the mirror. My only guess is that something is wrong in the prism. Anyone know anything about this?

Check the diopter.

jobe1492 wrote in post #8377297
Check the diopter.

Did you try to set it in MF mode, set the distance to say 10 ft using the scale, and shoot something exactly 10 ft away ?

OK, it is focusing somewhere, so do the inclined ruler trick to see where. You can't depend on the printed scale on the lens since it is just a general guideline. Check to see if Custom Function 4 has been set to anything other than the default value of 0. While it is possible for things to break, my money is on something else like a custom function and not knowing to use the * button when custom function 4 is set to 1 or 3. It is not necessary for the camera to be in manual mode to manually focus, just the mode switch on the lens. Switch the lens to M and then Look through the viewfnder to focus on a subject. As previously mentioned, it may be necessary to adjust the diopter wheel. Finally, change the focusing system to use only the center point so that you know for certain where the camera is focusing.

Thanks everyone for the assistance. Custom function 4 is set to use the * button. That's the only way both of us shoot. And you're right Bill, it IS focusing somewhere. When the mirror is up and the shutter is open, focus is purely a function of the position of the lens optics as long as the sensor is aligned correctly to the centerline of the lens elements. There is a focus point in the pictures, just not where it should be. This rules out any problem with the lens. When I look through the viewfinder at a high contrast subject and turn the focus ring from one extreme to another, the subject never appears in focus. That's why I suspect the prism area of the camera. Oh, the diopter could be the problem in this case, except that all the images are also OOF (when using AF). It seems to not be able to judge the distance correctly because like I said above, on a high contrast subject, you can adjust the focus ring all you want, you can't achieve focus in the viewfinder.

I forgot to ask, does it try to find focus? For instance, if you set the focus to infinity manually and then tried to focus on something close, does it hunt around for focus? Maybe something inside the camera moved such as the secondary mirror or focus sensor.

Bill Boehme wrote in post #8379424
I forgot to ask, does it try to find focus? For instance, if you set the focus to infinity manually and then tried to focus on something close, does it hunt around for focus? Maybe something inside the camera moved such as the secondary mirror or focus sensor.

Yes, it does try to find focus. And it seems to "lock in" as quickly as you might think it should.

I am now starting to second guess myself and revisit the diopter suggestion. I only had the camera in my posession for about 10 minutes. At that time, the diopter idea didn't cross my mind - sometimes the most obvious culprit is the last one I think of. I took a few pictures of different things that had high contrast and sharp edges. Here's where I may have gone wrong. I looked at the image on the screen on the back of the camera and zoomed in to 100% and the images still appeared to be out of focus. I'm not that familiar with an XTi. It could be that the camera screen is sub-par for such things. (On my MarkIII that's usually a pretty good way to determine if the focus was good or not) After the suggestions here I called the owner and asked him to check the diopter. He did and it brought the image into focus in the viewfinder. I'm thinking now that maybe it was the diopter and I sent everyone on a wild goose chase, or adjusting the diopter masked the problem and we'll have to wait until he can download the pictures he took to see if they all are really out of focus. I'll post more when I get more information.

The diopter has nothing to do with the camera focus, and in fact, you can focus manually with a focus screen (not a split prism) pretty much as accurately WHETHER THE DIOPTER IS SET OR NOT. The way to see if the thing is "in focus" or not is to take some shots at a high contrast target ON A TRIPOD, using either a remote or the timer. and then looking at them on your computer. Make sure you can display the photos "at 100 %"

The focus screen/ prism/ viewfinder has nothing to to with the camera's AF focus system, in fact the camera WILL AF with the focus screen out of the camera.

This is starting to make me think maybe it's been dropped and the mirror knocked asckew.

Phil Light wrote in post #8379987
Yes, it does try to find focus. And it seems to "lock in" as quickly as you might think it should.

I am now starting to second guess myself and revisit the diopter suggestion. I only had the camera in my posession for about 10 minutes. At that time, the diopter idea didn't cross my mind - sometimes the most obvious culprit is the last one I think of. I took a few pictures of different things that had high contrast and sharp edges. Here's where I may have gone wrong. I looked at the image on the screen on the back of the camera and zoomed in to 100% and the images still appeared to be out of focus. I'm not that familiar with an XTi. It could be that the camera screen is sub-par for such things. (On my MarkIII that's usually a pretty good way to determine if the focus was good or not) After the suggestions here I called the owner and asked him to check the diopter. He did and it brought the image into focus in the viewfinder. I'm thinking now that maybe it was the diopter and I sent everyone on a wild goose chase, or adjusting the diopter masked the problem and we'll have to wait until he can download the pictures he took to see if they all are really out of focus. I'll post more when I get more information.

Unfortunately, you can't depend on the XTi's LCD image at 10X zoom to tell if something is sharp. Even something that is in razor sharp focus when viewed on your computer monitor will, at best, be "fuzzy-sharp" on the camera's LCD. I have become used to this on my XTi and can actually use it to judge sharpness reasonably well, but not as well as I would like. For things like bird images and astro images, I would like a better display. I doubt that it is a hardware problem -- just a limitation in the resolution of the internal JPG.

Let me get this straight ~ in neither AF or MF can you get a sharply focused image in the viewfinder or the 'taken image' si that correct???

Right, AF AFAIK relies on the secondary mirror to deflect the image to the focus detection unit in the base of the camera ~ if the secondary mirror is dislodged/misaligned then the camera will not achieve "focus", it may think it has but not so.

Having said that focusing with the lens set to MF has nothing to do with the AF mechanism i.e. if the secondary mirror is misaligned then MF usage should be fine ~ are you saying that using MF you can see no change at all in the viewfinder!!! Well firstly on the presumption that the diaptor adjjuster may be 'out' at the very least set it to midway point and try the MF again, if still unable to focus that is bizarre to say the least.

Hmmm! another thought? If you are looking throught the viewfinder at brightly lit subject is there any vignetting in the viewfinder...forgot all (most) of the above as I have.......just re read one of your replies and your friend has in part solved the question by adjusting the diopter.

If he can see & take well focused images using MF but when set to AF the point of focus is to the fore or backwards of that chosen point then surely (if all lens are doing the same) it is body induced back or front focusing problem and needs the TLC of the Canon techs.

Rambled enough

ttfn

jobe1492 wrote in post #8377297
Check the diopter.

We have a winner.

And a loser (me) . It just goes to show you, start with the easy, obvious stuff.

Thanks Bill and everyone for the help.

You might also try using Live View to focus on the rear screen with highest multiplication. However, as has been said, it should work anyway with AF on and a reasonable subject. I'm a bit suspicious of your really high contrast subject. If the light was too bright at a knife edge, it might cause a problem.

I feel like I should apologize for how drawn out this was. I only had the camera for a few minutes at first, then lots of discussion, then I got to check out the camera again and download the image to my computer to see for sure. It definitely was a diopter adjustment problem. I suppose the reason it didn't cross my mind is because I typically set mine when I get a new camera then I never touch it again. I just plain forgot about it.

It may just be me, but at least this has been an interesting discussion.

(jobe1492 is probably saying, "dumb@ss!!!")

Gary McDuffie wrote in post #8382712
You might also try using Live View to focus on the rear screen with highest multiplication.......

After two and a half years, I still can't seem to find Live View on my XTi -- well actually, Live View is when I look through the viewfinder -- it doesn't get any more "live" than that.

make sure the AF mode is on "AI focus".
and what mode are you shooting in? put it in auto and set it on a tripod and see what u get