Focus Issue with 50D and 35L 1.4

I just bought a used 35L 1.4. After taking about 500 shoots, I have found out that I have a big problems with AF. It is very inconsistent. It focuses all over the place, sometime front sometime back, and about 70% are dead on. This issue happens from f1.4 to f4.0 (I have not tested anything higher) and a very high percent of the out of focused shoots happened while focusing over 10ft or longer.

I have also notice that it happens quite a bit with my 50 1.4 but not as much as the 35L, however I have not noticed this AF issue at all with my zoom lenses (16-36 II, 24-105 is, 70-200 f4 is, 28-105, 70-300 is). I don't think it is the lens, am I wrong? Does the 50D not working well with prime lenses? Does any body have the same issue that can help me out. I am think about getting 85 1.2 and 135 f2 in a near future, but now I might have to upgrade to 5d II first. Thanks

I can't comment on the 35L, but I have the 24L II and that can sometimes be erratic. I did read somewhere where a Canon representative said that fast wide angle lenses can be prone to inaccurate focus due to 2 things.

1). A relatively thin depth of field when focussing wide open (as all lenses do) at targets that are quit indistinct or small. For example, landscapes etc.

2). Fast wide angle lenses are very hard to design and wide open they have more optical errors that can confuse the AF sensors.

My findings are the same, large expanses can be inaccurate, but if I have a large target to focus on it's fine. As for landscapes, you'll be 99% stopping it down so that should cover any error. But I tend to manual focus landscapes anyway so I get exactly what I want in focus.

I have an 85L II as well, and that's far more accurate, but I'm usually focussing on quite large targets when I use that.

All your zooms have smaller wide open apertures and relatively fewer optical errors in comparison to f/1.4 lenses. Any errors are immediately masked by a greater DOF as well hence you don't see it. Over the years I've come to find that auto focus is just an aid and not a 100% accurate solution to imaging.

I hope this helps.

So I should not send the lens or the body in. There is nothing wrong with them? So 85 1.2 and 135 f2 is a go for me.

The 50D has micro adjust (I think), so you could try that first and see if things improve. My 24L has +12 on my 5D2 and +10 on 1D4. I use the Canon recommended technique which is posted below.

I don't think sending them to Canon will improve matters much, if at all. Like I said, it's the nature of the beast with fast aperture W/A lenses.

You'll find the 85L and 135L may need little or no MA adjustment, as in my case with the 85L. Buy them, you'll not regret it.

This is Canons recommended method. It's the one I use and I find it's pretty reliable;

1. Mount the camera and lens on a tripod. If the lens has IS, shut it off.

2. Use Live View to manually focus on a stationary, flat, high-contrast object that is at the centre of the viewfinder and parallel to the plane of focus. The camera-to-subject distance should be no less than 50 times the focal length of the lens. For a 50mm lens this would be at least 2.5 meters, or approximately 8.2 feet.

3. Focus the lens at its maximum aperture. Use Live View magnification if necessary to assure that the image is as sharp as possible.

4. Without touching the focusing ring or moving the tripod, turn off Live View, and return the camera to One-Shot AF, using only the centre AF point.

5. Gently press the shutter button down halfway (or the AF button if using back-button AF) while observing the focusing ring or scale on the lens. It should not move. If it does, take note of whether AF moves the plane of focus closer (front-focus) or further away (back-focus). If there is no shift in focus your lens is well-calibrated and requires no adjustment.

6. To determine the correct amount of adjustment necessary, take three sets of images at micro-adjustment settings of -10 , 0 and +10; in other words, three consecutive images at -10, three consecutive images at 0, and three consecutive images at +10.

7. Examine the resulting images on your computer monitor at 100% pixel magnification.

8. Take additional sets of test images at different micro-adjustment settings if necessary until you can determine which setting produces the sharpest image.

9. Register the corresponding micro-adjustment setting in the camera.

I don't think MA would do any good because it just missed focus to the front and to the back. I took pics of my son was standing in the middle of a bush and a playground. The first shot was dead on. The second shot was focusing on the playground 5 ft away. The third was dead on. The fourth shot, the bush in the front 1ft was in focus. Neither of was was moving. When it focuses right the images were supper sharp even at f1.4

FWIW, I haven't had a focus issue with my 35 L yet. It has been MA'd on my 7D.

Op, don't mean to be sarcastic or anything but pure question, what focusing point were you using? Is it single or automatic? Because if its automatic, then that's not a lens error. Try usiong single focus point first.

had focus issue on mine, got it calibrated! its perfect now .. have heard the 35L can have some front/backfocus issue.

strobe monkey wrote in post #12169759
Op, don't mean to be sarcastic or anything but pure question, what focusing point were you using? Is it single or automatic? Because if its automatic, then that's not a lens error. Try usiong single focus point first.

I only used single center point focus. I never let the camera picks for me. I might have to send it in

tomme wrote in post #12170352
had focus issue on mine, got it calibrated! its perfect now .. have heard the 35L can have some front/backfocus issue.

tomme, did you send in the lens or the body?

wrxrocks wrote in post #12170820
tomme, did you send in the lens or the body?

i sent only the lens in.

i might do just that, but I called canon yesterday and they say I should send in both. I need the body for the next three weeks. There are many events coming up for me.