I noticed a while back that a lot of the landscape shots I was taking had problems with background or foreground objects being out of focus, in spite of shooting at 18mm with a fairly small aperture (f/11, typically). Worse, the effect wasn’t consistent, which made it doubly hard to figure out what was going on. Sometimes a picture shot at f/8 would be just fine, other times I’d have problems even though I was using a smaller aperture than that. It was getting pretty frustrating.
I think I’ve got it figured out now, though, along with a possible solution.
I’m guessing the problem is that, at 18mm, the hyperfocal zone of the lens is so broad and gradual that it’s impossible for the camera to distinguish the sharpest distance from distances which are substantially in front or behind—from the camera’s viewpoint, all of these distances will appear to be equally “in focus,” so the camera will autofocus on a distance that is technically not optimal. This has been borne out by experimentation: When I autofocus on a moderately close object, and then re-autofocus on a moderately distant object, often the lens will not move very much at all, even though it should have, based on comparing the distances of the objects to the distance scale markings on the lens. In the viewfinder there is no visible difference in focus between the two objects, and apparently the camera isn’t able to tell the difference either.
So, if the camera just happens to autofocus the lens in a way where the “most in-focus” object is too far away, then the foreground will end up blurred in the final picture. Likewise, if that object is too close, then there will likely be some softness in the background. In either case, I’m pointing the focus sensor at an object at roughly the correct distance (I hope), but since the depth of field is so wide, the camera isn’t able to precisely center the focal zone on that object, it merely adjusts the lens until the result is “good enough.” Except that it’s not.
(Part of the problem is admittedly that I am using a fairly slow lens—its widest aperture is f/3.5 at 18mm, which is apparently small enough to cause this problem. I wonder how much better things would be if I had an f/2.8 lens? Or an f/1.8 lens?)
Clearly what needs to happen is to get the lens focus set so that the optimal point of the hyperfocal range is resting on an object that’s a little close, but not too close. (Choosing an object that’s exactly halfway between the foreground and the background wouldn’t be the best choice, because at short focal lengths, the hyperfocal range of a lens tends to be asymmetrical, skewed towards the camera.) Since the autofocus mechanism can’t be relied upon to do that by itself, I am left with manually focusing the lens.
But here’s where the second half of the problem crops up: The Nikon D40 is really not at all good for manually focusing on a wide angle lens. In fact, just using the naked eye at 18mm, I would guess I’m even less capable of picking the right setting than the camera is! So how can this problem be solved?
Well, the solution is a bit tedious, but in my initial tests it seems to be working out fairly well. I’ve been using an 18-200mm zoom lens, and while focusing at 18mm is pretty much hit-and-miss, if I zoom the lens out to 100mm or more, it’s quite easy to see if something is in focus or not. At those focal lengths, the hyperfocal range is much more narrow and easy to spot in the viewfinder. It’s also easy to see if the camera is focusing where I need it to, which means I can leave autofocus turned on for the first part of this.
So here’s what the procedure boils down to:
- After making sure my exposure settings are good, I’ll start by composing a rough framing of the scene, then I’ll take note of some close elements in the frame, and some distant elements.
- I’ll then zoom the lens to a telephoto setting, and focus on one of the close elements I noted. Autofocus works fine for this, so that’s what I use.
- I’ll then take a look at the distance scale on the lens, to see where it is. Typically it’s somewhere around the 3 meter mark, but it varies of course.
- Then I’ll repeat this zoom-and-focus procedure with one of the distant elements in the original composition, and again check the focus scale on the lens. Typically it will be closer to the infinity mark, but again there is some variability.
- Then I’ll switch the lens over to manual focus and zoom back to 18mm, or whatever length I was using in the original composition.
- I’ll then manually rotate the focus ring on the lens to a position moderately closer than halfway between the distance from the nearer focus point to the farther one—again, not choosing the exact halfway point is due to the fact that the in-focus zone on a wide angle lens will be skewed closer to the camera rather than being perfectly symmetrical.
- Then I’ll reacquire my original composition in the viewfinder, make sure it’s just the way I like it, and take the shot. Hopefully my original exposure value will still be valid—if it’s not, though, the beauty of a manual focus setting is that it stays where it is, if I need to make any further adjustments (as long as I don’t accidentally bump the focus ring!). It’s also perfectly reasonable to “bracket” the focus a little bit, on multiple shots.
The whole procedure is a bit of a pain in the neck, but far less annoying than getting back to the computer only to discover that my background or foreground is fuzzy.
The one big question in all of this is, which aperture is needed? Since Nikon, in
its infinite wisdom, doesn’t bother putting any sort of depth-of-field markings on lenses anymore, I simply resort to guessing (admittedly, it would be better to utilize an actual depth-of-field chart, so maybe I’ll have to get myself one of those). Depth-of-field is typically pretty wide at 18mm, but it’s not infinite by any means, so I generally won’t bother with apertures wider than f/8. I tend to stick around f/10 or f/11 for the most part, although I honestly don’t have enough practice at this yet to definitely have a feel for it.
The other complication is that the aperture choice becomes more critical at higher focal lengths. Sometimes I’ll zoom the lens all the way up to 35mm or so, for “wide angle” landscapes, and obviously increasing the focal length will decrease the depth of field. But, on the other hand, the original problem becomes less of a problem as depth-of-field decreases, so eventually I can just fall back onto regular autofocus as I zoom in on something.
In the long run, I’m thinking a better solution might be to upgrade my camera body. I’ve heard the D300s has pretty good autofocus—I wonder if D300s owners have these problems? :)