Shutter speed and aperture work together to properly expose a picture. If you put your camera in Shutter Speed priority mode and set it to 1/500 the aperture the camera chooses might be f2.8. Then, if you go down one setting to 1/250 you'll see the aperture come up one setting to f4. The same thing happens in Aperture priority mode. So, it's just a sliding scale. It's helpful to play with this yourself to get a feel for it. This site has a manual camera simulator that shows the relationship well: http://www.photonhead.com/simcam/shutteraperture.php
Now, if you set your camera to Manual mode and press the shutter halfway down, it should give you a light meter reading (see your manual for details). This is the camera's suggestion for proper exposure. There should also be a graph displayed that looks like this: [-2..1..|..1..+2]. This graph makes photography in manual mode so easy it hurts. The indicator should start in the middle, at zero; this means that the exposure is what the camera thinks it should be. If you change the shutter speed or aperture up or down you should start to see the indicator slide right or left towards more positive numbers or more negative numbers. Positive numbers mean that the your picture will be overexposed or lighter than what the camera would choose on “Auto”; negative numbers mean the camera thinks your picture will be underexposed or darker than what it would have chosen.
If you want to pick an exposure that's zero, or the same as the camera would choose in Auto, you could just use Shutter Speed priority or Aperture priority mode -that's what they do. But you may have noticed that your camera doesn't always choose correctly. Cameras choose their exposure setting based on a pre-programed “average scene” (yes, even film cameras). It works well for most pictures containing green grass, trees, flowers, people, etc -but you may have noticed that snow scenes usually come out kindof blue or brown instead of white. Or the opposite will happen and in a dark scene like the night sky with fireworks, the black sky may come out grey. That's because these scenes are far from average, and this is where you'd want to override the camera's exposure selection to make the picture look more like the scene looked in real life.
For the snow scene, to make the snow white instead of blue or brown, you'd take a meter reading of the scene -usually by pressing the shutter button halfway down. The camera will then give you a reading like 1/500 f4.8. But you know that the camera thinks the scene is darker than usual. (The way I think of it is by asking myself “Is the scene darker or lighter than green grass?”. In this case, it's lighter. The camera will try to make the scene look like green grass, so it'll make the snow darker. Therefore, I have to tell the camera to make it lighter. Lighter means overexposed or a positive number on that sliding scale.) So, decide what you want to change: the shutter speed or the aperture or both? In this case the aperture is already a very small number, so I'd change the shutter speed. Make the change(s) and watch that sliding scale. Take a picture at +1 (1/250 f4.8), then take another picture at +2 (1/125 f4.8). That's called “bracketing” or taking several pictures at different exposure levels to make sure you get the shot you want.
Homework: Pick a scene any scene. Put your camera on Manual mode and take a picture at the settings that your camera recommends. Write down the picture # and the settings. Next, take a picture of the same scene at one stop down (-1 on that sliding scale), make your notes, then do it again at -2. Go 3 steps down if you want, then do +1, +2, etc. When you're done, look at the difference the changes made in your photos. What's great about that sliding scale is it'll always be consistant. Your aperture and shutter speed combinations will change wildly from scene to scene, but -1, -2, +1, +2 exposure will always be consistantly lighter or darker.
(INSERT EXAMPLES OF -2, -1, 0, +1, +2 HERE)
So, to summerize:
- If you have a picture and all you want to change is the depth of field, use Aperture priority mode.
- If you have a picture and all you want to change is removing or adding blur to a moving subject, use Shutter Speed priority mode.
- If the scene is ligher or darker than an average scene (green grass), use manual mode and the sliding scale to correct the exposure.
There you go! You know how to use the manual settings on your camera. Go forth and take great pictures!
Advanced topics:
The Tricky Part:
If you want to learn more, I'll warn you of one thing. Other webpages and books will refer to “large apertures” as apertures with small numbers like f2.8. I find this extremely confusing when you're just starting out, but you'll see it out there, so let me explain it here. The reason they say f2.8 is “large” is because aperture is really a measure of how wide the shutter opens when you take a picture. An aperture of f2.8 makes the shutter open very wide, while a larger number like f16 only opens the shutter a little bit.
This makes more sense when you think of the way shutter speed and aperture act together (that sliding scale thing, when one goes up, the other comes down). When the shutter opens wide (2.8) it lets a lot of light in, which is why you need a fast shutter speed to counter it. If the shutter opens just a little (f16) the shutter speed has to be slower to let the same amount of light to fall on the film (or sensor on a digital camera).
References and Further Reading:
Here's some links to webpages I like to use as references or for further reading on various photography subjects:
- This site is like a photography book online. Good in-depth articles on a broad range of topics: http://www.camera.canon.com.my/archive/photography/art.htm
- This one has a lot of info on digital photography in general including file formats, etc: http://www.earthboundlight.com/phototip-archives.html
- Here's a site with info on composition, using a flash, as well as covering exposure topics: http://www.dcmag.co.uk/Vital_Skills.aN_XJz4.html
- This site covers in-depth photo-geek topics: http://www.tpub.com/content/photography/14209/
- Want to learn to figure out your exposure settings without relying on your camera's light meter? (Or do you just want to learn how exposure works in better detail?) This site breaks down exposure very well: http://www.fredparker.com/ultexp1.htm
I hope this site has been of help to you. If you'd like to meet other aspiring photographers to ask questions, get critique on your work, etc join the Photography_Beginners email group on YahooGroups: http://groups.yahoo.com/group/photography_beginners/
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