"Off-brand" lens or less than professional body?

[CortrightCreations]

I'm doing a job for the Army right now, and they want us to use their equipment. Unfortunately the Mark 4 the have is generally taken, and I'm stuck with a Canon 40D and it's 18-200 Sigma (the Mark tends to travel with the high quality lenses, which I don't think would fit the 40 anyway). I've been having trouble with the AF in my shots. I'll frame and set up the shot, set the focus point for a soldier's face, and click the shutter, but then the image result has the slightest out of focus blur on the face of the soldier. It's not the AP that's the problem - it even happens at 7.1 AP. It happens when I zoom all the way in, focus on the eye, and zoom back out to click the shutter. Is it the lens, the body? Or am I doing something else wrong?

 

[radiorickm]

#1, I assume you are referring to a 1D-Mark IV(Canon). That being the case, the lenses (probably L's) you refer to will fit on the 40D no problem. I shoot a 40D, and I really love the camera, don't doubt it's capabilities.
I have not used that Sigma lens, but from your description, here's what I see.


When you zoom all the way in, then focus, then move back out to shoot, you are probably taking it out of focus during this procedure. Any thing you change on the lens after focusing is going to have some effect. Just curious, are you focus locking THEN, to hold the settings, (holding the shutter half way down) or just letting it refocus after zooming out. Just try focusing without zooming and see if that makes any difference. I certainly don't have a lens that will maintain absolute focus after zooming.

 

[Overread]

Most DSLR zoom lenses shift the point of focus when the focal length is changed. This is normally not a problem for stills photography because you just let the AF re-focus after adjusting the focal length and most of the time you won't be fast zooming and focusing at the same time.

Another point to remember is to think of focusing like moving a sheet of paper parallel to the front of the lens. The sheet of paper represents the plane of focus, with the thickness of the paper being the aperture (ie depth of field). Smaller apertures (bigger f numbers) mean a thicker sheet of paper (greater amount of depth of field) whilst wider apertures (smaller f numbers) mean a thinner sheet of paper (thinner depth of field).
Now this is important because if you were to, for example, use the middle AF point only and focus that upon the subjects eye and then adjust the frame of the shot you can end up moving the plane of focus off the original point you focused with.
Don't be afraid to use the outer AF points, the 40D is more than capable of using them to get a good lock on focus.

The 40D is older, but its still a very capable camera body and will fit all Canon EF and EFS lenses on the market (doesn't matter how big or expensive; if its EF or EFs the 40D will fit it).

 

[CortrightCreations]

I hadn't heard the zoom in to focus and zoom back out theory until I got here - when it was suggested to another photog who was having trouble with the AF on his camera as well. Since then I've been using a combo of my method - a careful framing and AF-lock using either the half-down shutter or the button depending on the framing. I haven't found it to be any more effective. Perhaps it is the Sigma lens, then. Good to know I can use some of the faster lenses on the 40D. Maybe it will improve the clarity/focus.

 

[Derrel]

On a lens with a HUGE focal length range, like an 18-200mm, focus shifting during zooming is not at all unheard of. MANY zoom lenses are not 100% parfocal, meaning that as they are zoomed, the focus point shifts. I never expect a lens like an 18-200 or 28-200 or 28-300 to have the same exact focus point at 28mm as it will at 300mm...that's just not realistic.

The best thing to do, I think, is to focus with AF at the shooting focal length, and to allow the camera to re-focus as the lens is zoomed.

One thing to note too is that at short focal lengths, the AF brackets cover a BIG area, which can easily lead to the AF point settling in on something that's not EXACTLY the desired focusing target. Also, the AF brackets are not typically "exactly" the same thing as what the AF system uses; typically, the exact AF area used is a little off to one side, or off to the top or bottom, of the bracket's area. At wide-angle settings, this discrepancy can EASILY allow the AF bracket to appear to be, say, on a person's face or head, but can actually be a little off to one side, so that the AF bracket's actual, active, working AF point if a little bit different from what a casual user might expect from looking at the bracket.