To help solidify our understanding of how aperture and shutter speed work together, I'd like to suggest that you take some time to do a quick experiment. For this experiment I need you to identify a subject to photograph. It should be something fixed (doesn't move) so it isn't a variable in the equation. It should also have good lighting. I also recommend that you either use a tripod, or place your camera on something solid, like a table. Once again, we want to eliminate extra variables from our shots.
On your camera, please select an ISO value other than AUTO. I know we haven't really discussed ISO, so my recommendation for this is to select an ISO of 200 if you are in a VERY bright room or an ISO of 1600 if you are in a room that doesn't have a ton of light.
Next, set your camera to Aperture Priority. On my Canon is the "Av" on the mode dial. On my Nikon it's the "A"on the mode dial. You may need to look at your camera's manual if you're unsure how to set your camera to Aperture Priority. (Side note: I use this setting probably 80% of the time for my photography)
Now pick the widest aperture setting that your lens will shoot. Most kit lenses start at f/4. Some of you may be able to go even lower than that, like f/2.8 or if you're really lucky f/1.8 or f/1.4. If you're one of the lucky ones to have a very fast lens like a 50mm f/1.8, set your aperture at f/2.0. (fast lens? what is he talking about? Sorry, I'll get to that later.)
Ok, you should now have a "still life" subject to shoot, using a stable platform (tripod or table.) Your camera should be on Aperture Priority, using a fixed ISO of 200 or 1600 depending on how well lit your room is. And finally you should have your aperture set to it's lowest setting possible based on the lens you're using (but not lower than f/2.0).
Summary of our experiment:
Subject - Fixed
Aperture - Variable; we will change this manually starting with the lowest value we can based on lens
Shutter Speed - Variable; your camera's light meter will adjust this (light meter??)
ISO - Fixed at either 200 or 1600
A light meter is a sophisticated measuring device in your camera which will assist you to get the proper exposure. We'll probably have a full blog post dedicated to this topic. In this experiment, the light meter is going to help make sure the shutter speed is properly set to ensure a correctly exposed image is captured.
My setup:
Subject: My daughter's unicorns set up on a fireplace mantle
Aperture: I'm using a 50mm f/1.4 lens so I will start at f/2
Camera set to Av
ISO: 1600.... I'm shooting in a basement with two overhead bulbs (normal 60w light bulbs)
Ok... Let's start shooting:
50mm f/2 ISO 1600 1/200 exposure
This first image is set at f/2 which is a very large aperture opening. Since I started with such a large aperture the DoF is very thin. I'm focused on the larger unicorn which is only a couple inches behind the other two. You can see that the front two unicorns are blurry due to the shallow Depth of Field.
Given these settings the light meter used a shutter speed of 1/200 seconds to get a properly exposed image.
50mm f/2.8 ISO 1600 1/100 exposure
In this second image, I have closed down the aperture to f/2.8. Remember that f/2.8 allows in half as much light as f/2.0. Since that's the case, the light meter chose a 1/100 second shutter speed to get the right exposure. This is what we talked about in the last post... 1/2 the light coming in due to the aperture means we have to double (2x) the time the shutter is open.
The DoF has changed too. The front two unicorns are slightly sharper due to the smaller aperture. They're not sharp yet since f/2.8 still produces a pretty shallow DoF.
50mm f/4 ISO 1600 1/50 exposure
To continue the experiment, I adjusted the aperture to f/4. Remember this is the only thing we are changing on our own. Once again f/4 allows 1/2 the light in as f/2.8 did therefore the meter adjusted the shutter speed down to 1/50 second. This keep the shutter open 2x as long as it did when we had the aperture set to f/2.8.
As a result of the smaller aperture, the DoF continues to get deeper (slightly.)
50mm f/5.6 ISO 1600 1/25 exposure
In this final image I've stopped down (a phase used when decreasing the aperture) to f/5.6. Since this is the next full stop (f/4 to f/5.6 is one stop), the aperture is letting in 1/2 the light. Our brilliant light meter has adjusted the shutter speed to keep it open 2x as long at 1/25 seconds. Pretty cool. The math is easy when the light meter is doing the work for us.
Depth of field has once again gotten a little deeper and those front two unicorns are a little sharper. To have them fully sharp (given how close they are to the lens) I would have needed to go to an aperture of f/11.
Hopefully this is starting to make a little sense. In all honesty, it will likely take time and practice before you feel really comfortable with these concepts. The key is... don't give up!!
I threw in a few new terms/phrases that I want to discuss in a little more detail: Stop & Stopped Down
Everything we've talked about with regard to aperture have been related to full stops or f-stops. Although our cameras are capable of fractional stops, I'm going to avoid using anything but your standard stops during our discussions. These are full f-stops:
f/2 - f/2.8 - f/4 - f/5.6 - f/8 - f/11 - f/16 - f/22
Each movement to the right (f/2 to f/2.8 for example) is a full stop and means that the aperture lets in 1/2 the amount of light. Each movement to the left (f/11 to f/8 for example) is a full stop and means that the aperture lets in 2x the amount of light. You'll hear photographers say Stop-Up or Stop-Down and that means they are adjusting their aperture.