Focus is simply a distatance (measured from the sensor plane. There's a mark on the top of your camera body which looks like an "O" with horizontal line passing through the center of it -- that marks the position of the image sensor inside your camera body.)
If you enable all focus points then it doesn't mean the camera will focus on them all... it means the camera will evaluate them all and then choose one. The algorithm typically uses the point which is able to achieve focus at the nearest focusing distance. This is because if you were taking a photo of a person and there is a background... you probably want the camera to focus on the person and not the background.
But there are two techniques that can help you achieve the results you want.
First... there's a term called "depth of field" (or simply "DoF" for short). For any given exposure, there is a range of distances at which a subject will appear to be more-or-less in "acceptable" focus (even if not located exactly at the precise focus distance.) If I focus a camera on a subject which is located 10' away, there's a good chance that anything 9' away or even 11' away will also be in acceptable focus. I say "chance" because the exposure settings determine if the "DoF" is narrow (subjects probably wont appear in focus if they are not at the proper distance or extremely close to it) or broad DoF (subjects might be several feet closer or farther and still be in acceptable focus.)
The lens opening (or "aperture") size is controlled by a set of diaphragm blades which can slide in or out to make a larger or smaller diameter opening in the lens for the light to pass through. When it's larger, more light can pass through, but the depth of field becomes narrower. When it's smaller, less light passes through (meaning you will need to take a longer exposure) but the depth of field becomes much broader.
In other words, if you put your camera into Aperture value mode (the "Av" setting on your mode dial) and then dial it up to a higher number... say f/16 or maybe even f/22) then you'll notice the range of distances which appear to be in acceptable focus are much broader then when it's set to a lower value (such as f/2.8). The one trade-off is that using very high aperture values creates an issue called "diffraction" which ever so slightly softens the focus. Usually it is not noticeable without close inspection. For this reason, some photographers prefer to avoid shooting at high aperture values. (they might try to stick to values which are lower than f/16... such as f/11 or lower.)
One point of common confusion is that the number you see in the Aperture value is a ratio. It's actually 1/___ your number and sometimes written as f/__.__. That means that higher numbers actually mean the aperture size is physically smaller (people will commonly assume that a larger number means a larger aperture but in this case it's the opposite.)
When you use a small aperture (large "Av" or "f-stop" value) the amount of light that can pass through the lens is reduced and that means the camera has to collect light longer (slower shutter speed) to gather enough light for a proper exposure. This means you may need a tripod unless you have a lot of light.
Some camera models have a mode called "A-DEP". A-DEP is an automatic mode... but with a special feature in that it's designed to try to make sure you get the depth of field you want in a shot. If your camera has the mode, you'll need to read the manual to learn how it works.
There is an alternative which will work, but it requires software. You can use a technique called "focus stacking".
Focus stacking means you switch the lens to manual focus mode and you take many exposures... each single exposure changes the focus by perhaps just a few millimeters and you shoot enough of these exposures so that everything from the nearest point to the farthest point is in focus. All of these seperate images are fed into a focus stacking program and will be merged to create a single image which shows everything in sharp focus.
Photoshop has the feature built-in (I do not think Lightroom has this feature built-in). There are also a number of applications or plug-ins written just to handle this feature.
I've used it really only experimentally, but to use it, I put the camera on a tripod (using a macro lens). I wrap a piece of ordinary masking tape around the focus ring on the camera lens (becaue I intend to write on it and I don't want to write directly on the lens.) I then look through the lens and focus the lens manually on the nearest point of focus that I care about -- then use a marker to put a mark on the masking tape indicating the position of the focus ring. I then manually focus to the farthert point in my subject -- and make another mark on the tape.
Having the tape with the two marks... I now manually rotate the focus ring to the first mark, take a shot, and slightly turn the focus ring (just a tiny amount), take another shot, adjust focus again, take another shot... and just keep going until I get to the second mark on the tape (indicate the other end of the focus.) At this point I probably have a dozen or so images. Those images will all be sent to Photoshop's "Photo Merge" feature.
Tim Campbell
5D III, 5D IV, 60Da
