Aperture Priority

[daviddewar]

Aperture Priority
by David Dewar on Tue Apr 14, 2009 4:58 pm

So, we are talking about shooting in aperture priority. What does that mean? Well, most cameras whether they are digital or 35mm allow you to shoot in a few different settings. They might be, auto, manual, TV (time value or shutter speed) and aperture. With most digital cameras today we can choose to control one of these settings and let the camera do the work of figuring out what it has to do to the other settings to obtain a good exposure. For example, if we choose TV mode, we tell the camera how fast to shoot and the camera will do whatever it has to do to the aperture to allow enough light to reach the sensor. If we are going to use our cameras in aperture priority, we tell the camera how much depth of field we want and it will make the adjustments to the shutter speed for us. This is simplifying it a little but you get the picture. Get it…….get the picture, I crack me up!

Now we need to understand how our aperture works. Basically, by adjusting the f stop on your camera you are opening or closing the hole in the lens to allow more or less light to reach the sensor. Imagine you are in a dark room; your eyes need to allow more light in so you can see, so your iris opens up. However, this may only allow you to see part way into the scene. Conversely, if you are outside on a bright sunny day, your iris closes down to block out some of the light, and may allow you to see something on the other side of a field. If we are shooting a subject with not much light available we may want to open up the aperture to send more light to the sensor or shut it down to block out some light in a different scenario. Depending on the lens, you may have up to 15 stops, or openings, to choose from. Typically, the numbers start at f/1.4 or 2.8 and go up to f/22 but like I said this depends on the lens. So f/1.4 would be wide open and f/22 would be a tiny little squinty hole. F/1.4=lots of light on the sensor, and f/22= just a little light.



The diagram only shows to f/8 but you get the picture right? Do I have to say it again? I’m hilarious!

So what does depth of field have to do with all this? Again, if you were outside on a bright sunny day and you squint your eyes to see something at the other end of the field, you are expanding your depth of field. At f/22 your lens will focus on more of the scene than it will at f/1.4.



If you look a good portrait, you will probably notice that the subject is well focused and the background is blurred. This focuses the viewers eye to what’s important, the subject. If you look at a landscape of the Rockies you’ll notice that everything is in focus from the front of the scene all the way into as far as your eye can see. Shallow DOF and deep DOF. Simple ain't it! Macro photography is an even more dramatic example of how depth of field affects the outcome of a photo. Take a look at our good friend Dale’s work. His bugs are obviously the center of attention. The surrounding foliage or background is obscured forcing you to pay attention to the subject.

As an experiment, on a bright day go outside and line up 5 or 6 objects spread out over 10 feet ( that’s a little over 3 meters for you younger folks). Leave your camera in one spot focused at infinity looking down the line of objects and start shooting using different apertures. When you look at the photos, you should notice that the DOF changed from having just the front of your line in focus to having the just the middle and finally the whole line focused.

Now there is another concern we need to think about when choosing which aperture we will use. When we shut down the aperture to see the whole scene, f/22, we also shut down the amount of light reaching the sensor right? Unless it is a really bright sunny day, your camera is going to want to keep your lens open for a long time to allow enough light in for a good exposure. Now what do we do. Well if you are on a tripod it’s probably not an issue. You can sit there all day waiting for your shutter to close and it won’t make any difference. If you are doing the shot hand held there is one thing we can do to help the camera out. We can boost up the ISO. By making the sensor more sensitive to the light, the shutter won’t stay open as long. But all of this is subject to what lens and camera you are using. It’s a matter of getting out and experimenting. Remember, digital is cheap.

I hope this starts you off on the right foot in shooting in aperture priority. I've tried to simplify the basics. There is a lot more to talk about when you’re using ETT flash and other special technical situations. Read Dale's post on “Understanding Exposure” he has a very good explanation of apertures there. There is also a lot of information on the internet that can help you out. So get out there and enjoy learning.

David Dewar
Club President

Posts: 766
Joined: Thu Oct 23, 2008 7:15 pm