Problems focusing at infinity

[CalebSimpson]

Okay, not sure if this is a lens or censor issue so I decided to place it here.

I have been having problems for quite a while now with my camera (Canon 1d Mark IIn) when focusing at infinity. If I shoot wide open at f/2.8 on my 70-200 or f/4 on my 17-4omm distant subjects (at infinity) appear out of focus, but if I stop way down, say f/8 they become in focus. When focusing on a subject close up shooting wide open all is good.

Any idea why this would be happening when the subject is at infinity? I was shooting climbers this past weekend from an adjacent cliff. I wanted to have a shallow depth of field on the climber, but due to this issue had to shoot f/9 to get a sharp enough image.

Canon recently did a repair that fixed soft images. Before the fix pretty much half of my images came out soft, now it's only happening at infinity.

So, is this likely a lens calibration issue or something to do with my sensor?

and the diopter is set fine, images appear in focus through the screen, I have tried both manual and auto focus.

[toverman]

I have both lenses, a 70-200 f/2.8 and the 17-40 f/4.

I am probably not completely understanding your question, but at least with your 70-200, shooting wide open is the opposite of what you want to do when you want infinite depth of field. Wide-angle lenses inherently have a deeper depth of field at all apertures unless you are focused on a subject that is very close to you. I'm guessing this is stuff you already know well, and that something else isn't working correctly.

[JLMC]

Wow, I just love this forum. You answered my question and I didn't even have to ask...how cool is that!!

Thanks for posting!

Originally Posted by toverman

I have both lenses, a 70-200 f/2.8 and the 17-40 f/4.

I am probably not completely understanding your question, but at least with your 70-200, shooting wide open is the opposite of what you want to do when you want infinite depth of field. Wide-angle lenses inherently have a deeper depth of field at all apertures unless you are focused on a subject that is very close to you. I'm guessing this is stuff you already know well, and that something else isn't working correctly.

[Patti Edens]

Check this site for DOF info: http://www.dofmaster.com/dofjs.html

I'm not too clear on what you are doing. How are you focusing at infinity? And why? The newer lenses do not have a very clear marking for infinity. Why not just autofocus on the climber or on the wall behind him?

I think I'm missing something?

[jeffkohn]

Depth of field aside, it should be possible to focus at inifinity even with wide apertures and long focal lengths; things closer than infinity wouldn't be in focus of course, but infinity should be in focus.

It's possible that the lens is out of alignment and won't focus to infinity, but I have to wonder how are you focusing at infinity? Are you doing it manually by turning the focus ring all the way to infinity? That won't work with many modern lenses. Due to the complexity of their designs, it's actually possible to focus them past infinity.

[dminton]

Lens mount on the body is f'd up. I had this problem a few years ago. My body had gotten knocked really hard while it was mounted on a 400 and it bent the mount on the body.


1. It's not focusing correctly for two different lenses. Gotta be the camera.

2. It's not focusing at infinity because the lens flange distance isn't right and its shifting the point of focus forward. The lenses are fine at everything but infinity because there is room to adjust the focus forwards and backwards, but when the camera tries to rack the lens down to infinity, there just isn't anymore room to go, so it stops at the end and that's it. It tries to keep going to make the image come into focus, but there just isn't anymore room for the focus groups to move, so it stops.

Unscrew the lens mount from the body and check it out.

[CalebSimpson]

Toverman, what you are saying makes total sense, but based on the DOF calculater Patti posted I should have an in focus subject at f 2.8 with how far away they are.

These images shot at 195mm, please check them out and let me know what your thoughts are (keep in mind they are sized down from the RAW, unprocessed image)

The first at f/2.8 http://calebsimpson.com/Climber_f2_8.jpg
The Second at f/5 http://calebsimpson.com/Climber_f5.jpg

The climber is clearly sharper at f/5 and was even sharper when I shot at f/9

This whole thing baffles me. The camera just recently got back from Canon and the issues with close up images being soft was solved but until now I had not shot distant subjects.

Dave, you may be right, but that is going to suck if you are because it's now out of warranty. It was first fixed under warranty.

Oh, and on another not now that I Think of it, I was in AF mode focused as far as it would go. So I guess that is right at, or right before infinity. I didn't shift it over manually, but when I did, just to test to see if the AF was slightly off the image became totally blurry. (happened in both directions).

[Patti Edens]

Originally Posted by CalebSimpson

Oh, and on another not now that I Think of it, I was in AF mode focused as far as it would go. So I guess that is right at, or right before infinity. I didn't shift it over manually, but when I did, just to test to see if the AF was slightly off the image became totally blurry. (happened in both directions).

Here's what I've read about focusing on infinity:

Autofocus on the moon or on something way in the distance (horizon) and see where the mark is on your lens. That is about as close to infinity as you will get and that's where you should manually put the lens if you want to be focused on infinity.