Disclaimer: Before I get started, I should preface the article by saying that this is somewhat of an advanced topic. If you aren't comfortable with terms like aperture, shutter speed, ISO and metering and how they work together to create an image, then this article may confuse you at times. Don't be afraid, just break out your camera manual and give it a shot!
The Brilliance of Today's Digital SLR's
Digital cameras are incredibly intelligent and powerful tools. They can detect and read light, decide exposure values, adjust for white balance, detect contrast to achieve focus, and a whole list of other things that go on behind the scenes that we never even notice. DSLR's are so smart in fact, that some new photographers make the argument that there is no reason to ever get out of automatic! While I could write an entire article on why that would be a mistake, I will tuck that away for later.
Manual shooting is something that many photogs (even pro's) are afraid of. When you shoot manual, you are basically turning most of your camera's systems off. You still have a light meter, but you have to decide if that meter is going to be correct or not. To do this, you have to adjust both aperture and shutter speed to get the correct exposure.
While all photographers are different, I often put aperture before shutter speed. What I mean is that I rarely use shutter speed as a creative function (there are certainly times that I do, but I can easily switch settings if needed). Because of my shooting style, when I shoot in manual I'm usually only adjusting my shutter speed to make the meter show the correct exposure. I've already decided my aperture and I want it to stay where I put it.
Exposure compensation essentially lets the photographer shoot in Aperture Value with a manual mindset. Because our cameras are so smart these days my exposures are usually spot on. In Av mode, I simply set my aperture (or depth of field) and my camera decides what my shutter speed will be. When I shoot in Av, I'm constantly looking at my the shutter speed my camera is deciding on and making sure it's quick enough to get sharp images. If it's too slow, I first adjust my ISO up to get it quicker. If I have to push the ISO too far I'll then consider changing my aperture if possible.
Knowing When Your Camera Will Fail
Sometimes the camera screws up. After all, it's only a computer! Fortunately, with a little experience you can pretty much predict when and how your camera will miss the mark on exposure. You see; while it seems a bit odd a camera is quite freaked out by the colors black and white. If left to it's own devices a camera will try to take black or white and turn it to gray. In other words, if you're taking a picture of a person in the middle of snow covered field your camera will try to underexpose the image to get away from having any white in the frame. To a camera, white means one thing: Blown highlights. On the flip side, if you take a picture of that same person against a black background the camera will want to overexpose the image to get away from having clipped shadows. Once you learn how a camera works in this area, it becomes easy to know when you need to override what your camera thinks.
Here's an example:
I was out doing some yard work one day and found a black widow spider with a nest underneath a brick wall. Being the typical male that I am, my father-in-law and I proceeded to find a stick and play with the spider and eventually got it crawling around on the stick. I ran inside and grabbed the camera while my father-in-law held the stick in the air.
I had just gotten back from a photo shoot and hadn't changed my camera setting back to normal yet. I had spot metering on, which means instead of using the entire image to decide what the exposure should be, it will only use the single focus point that you have selected in your viewfinder. When I placed that single focus point over the spider, all the camera saw to meter from was black. Since all it saw was black, here's what my camera produced…
Let's be honest, that isn't a very good image. My camera was spot metering on the black widow, so I ended up with a washed out, overexposed image that just looks sloppy. Seeing the mistake on the back of my camera, I quickly dialed in -1 stop of exposure compensation. This means I'm telling my camera to come up with an exposure as it sees fit, but whatever it decides, I'm putting in an override to underexpose it by 1 stop of light. Lucky for me, when I dialed in the correct exposure, everything came together; the position of the spider, the focus, and the beautiful shiny black color of the black widow.
Here's the next image I took, along with a bit of post processing to add a bit of detail.
Note: One black widow spider was eventually harmed in order to bring you this image.
Great, so how do I compensate exposure!?
It's really easy! On a Canon xxD DSLR, just use the dial wheel on the back of the camera when you're in Av or Tv mode. If you're in Av, spinning the wheel in either direction will adjust the exposure compensation up or down using shutter speed to let in more or less light. If you're in Tv mode, spinning the wheel will adjust aperture to let in more and less light. If you are at 0 exposure compensation, you will see a mark in the middle of your exposure meter on your cameras LCD or in the viewfinder. If you spin the wheel in either direction, you will see the mark move either up or down.
If you have a Canon Rebel series camera or any Nikon camera, simply refer to your camera's user manual and refer to the section on Aperture Value (Canon) or Aperture Priority (Nikon).
Putting It All Together
So, why shoot in Aperture Value with a mindset of shooting in Manual, when you could just shoot in Manual? Well, it basically comes down to one thing: Time. Knowing that my camera will get the shutter speed part of the equation right about 9 times out of 10, that means that I can simply set my aperture and take pictures at will. I don't have to constantly be adjusting shutter speed up or down to get the meter in the middle, I just let me camera decide that part and keep shooting away. If I notice that my camera is starting to get it wrong, I look at my environment and try to figure out why, then adjust my exposure compensation accordingly.
So Does This Mean Manual is Useless?
Absolutely not! There are still plenty of times when manual is the clear winner. I like to use manual any time my lighting conditions are going to be somewhat constant throughout a series of photographs. If I'm in a room with constant lighting and my subject isn't moving around much, I'll always shoot manual because I can simply dial in the exposure for the subject and shoot away. If I'm shooting into the sun, I'll use manual. Any time I need complete control over the camera, I go manual. It just depends on the situation.
Feel free to let me know if you have any questions or comments. I always do my best to answer them in the comments below. You can also follow me on twitter (@jamesdbrandon) and shoot me a question there as well. Have fun!