Taking charge of your digital camera Pt. 2
September 8, 2011 7:28 AM - denton
Last installment we covered the basic operation of our camera in ‘P’ aka ‘idiot mode’. Now we’re going to get really adventurous and move that mode dial from ‘P’ to ‘A’. ‘A’ stands for 'Aperture', or 'Aperture Value', or more commonly, ‘Aperture Control’. If you recall, the lens aperture is the size of the opening in the lens diaphragm, which in turn controls how much light strikes the sensor. I would argue that of all the non-P modes, this is by far the most important to understand.

Aperture Control means just that... when we turn the mode dial to ‘A’, the camera will be expecting us to tell it what aperture to use for the next photo. And why would we want to control, or choose the aperture? Because the aperture that the camera uses can have HUGE effects on the image, especially at the extremes (wide open, like f/2.8, f/4.0, or, stopped down, like f/11, f/22). If you control the aperture, the camera will select a shutter speed to match that will, given enough light, provide a proper overall exposure.

The size of the aperture controls what is called ‘depth of field’, or, in simpler terms, it controls what will be in focus in the image, and, conversely, what will be out of focus. Easier to understand by seeing some images.

Here is a portrait of my niece. This is an excellent example of an image shot in ‘A’ mode with the lens aperture wide open. You would call this ‘shallow depth of field’. What is in focus here? Basically, the eyes and everything that is in the same plane as the eyes. The tip of her nose is slightly out of focus, the ears are considerably out of focus, and of course the forest background has receded into a wonderful melange of shades of green. This image, in a nutshell, is why you want to be able to force that aperture as wide open as you can for portraits like this.

Since we all love cats, here’s a similar cat portrait. You can possibly make the argument that the nose should be in focus as well. However, in any portrait, human or animal, we must get the eyes in focus first and foremost. This image was taken at an aperture of f/2.0. Probably closing down another f stop, to 2.8, would have brought the nose into better focus.

You’ve all seen those great sports photos shot in ‘A’ mode. You can shoot ‘wide open’ to isolate a subject from a crowd. Here’s a NYC Marathon shot that shows this:

Or, to isolate a subject from a confusing background.

Obviously, ‘A’ mode works best at the extremes. We’ve seen now what happens when we use Aperture Control to force the lens aperture wide open. Would we ever want to force it to its ‘stopped down’ limit? Of course. If we want to make sure that everything in our image is sharp from front to back, we will choose an aperture like f/11 or f/16.
Here’s a classic example of an interesting old wooden fence. We want this fence to be in focus from top to botttom. So we’re going to pick a ‘small’ aperture, like f/11.

Here’s another, taken at a farm in the Schoharie Valley region of the Catskill Mountains, (that I hope still exists), of an array of pumpkins.

Note that these images are both acceptably sharp front to back.

Obviously, things are not quite this simple. Going back to the ‘honey’ example, if we are in a romantic candle-lit restaurant with our honey, we can probably do a nice portrait in ‘A’ mode with the lens wide open. Want to get the honey with the chef in the kitchen behind, both nice and sharp? Sure you can ‘stop down’ the lens in ‘A’ mode, but there might not be enough light to pull it off (to give you a fast enough shutter speed that will still result in a sharp photo without blurring from camera shake).

There are further interesting and scientific ways to use Aperture Control... use your favorite search engine to find ‘hyperfocal distance’ or use this link as a start.

September 8, 2011 8:16 AM - InsertSnappyNameHere
Great part two! I have a question...when you force the aperture wide open, how can you tell what part of the image will come into sharp focus? Is it as simple as what you aim the center of the camera on?
Edited at September 8, 2011 8:16 AM
September 8, 2011 7:23 PM - denton
You have the right idea snappy... altho it's pretty camera-dependent. DSLRs will have a little box in the center of the viewfinder, you place that box on what you want sharp (an eye for example) and shoot. If the eye is not in the center of the image, after focusing using the box you press the shutter release half down. On most cameras this will lock exposure and focus, then recompose and finish depressing the shutter (many DSLRs have a focus lock button to accomplish the same thing). This is actually a lot faster and intuitive than it sounds.

On pocket cameras you may want to delve into the menus and see what kind of focusing options there are. Often there is a 'wide field' focus and a regular center focus. Set it to center focus and do as you suggest.

I keep forgetting to say this... experiment widely cuz unlike film, digital is essentially free once you have the camera and computer.
September 9, 2011 11:37 AM - Arkady
You really make things wonderfully clear, denton.