|Lesson number 2 - How your camera thinks
||[Aug. 30th, 2006|08:00 am]
Enjoy a monthly photo assignment with critiques an
Unless you're using a disposable camera the chances are good that your camera has some sort of light meter built into it to help the camera decide how long to leave the shutter open for and which aperture setting to use. And the chances are that this is the meter that you are going to use to help you take your pictures. I'm willing to bet that most people just let the meter do whatever it wants to do and everyone usually gets pretty good pictures... usually...
This lesson is to help you understand what the camera is doing when it is taking a picture. Let's start by looking at a picture.
I know that the picture may be a bit distracting but it is a good example of how a camera looks at the world. To start with, your camera is only interested in levels of light so it is looking at the world in levels of brightness... or black and white if you'd rather call it that. Then it takes a reading of the average light level and adjusts the exposure using that information to get a grey image.
There is a very particular shade of grey that your camera is trying to reproduce. Long before I was born a group of scientists compared all the whites, blacks and greys of a lot of pictures and decided that the average value of all photographs works out to be 18% grey. The 18% refers to the fact that it would reflect 18% of all light. Just for comparison, something that reflects 0% of all light would be perfect black and something that reflects 100% of all light would be a perfect white.
This metering method works wonderfully as long as the picture you are trying to take has the average blend of highlights and shadows but what if you're trying to take a picture of something white or black? Your camera is going to try to make the picture grey without having any idea that the subject is something other than average.
It's a good thing that most cameras (including my cheap point and shoot camera) have some sort of exposure compensation so you can tell your camera when you are going to be taking a picture of something darker or lighter than usual. If you decide you want to take a picture of some nice black leatherwork then you will need to tell your camera that you want to make the picture darker than average.
For the image above, to keep the texture of the leather, I underexposed the image by 2 stops. It is possible to bring up a fair amount of detail using a program on the computer but you are more likely to have a better image if you expose it properly to start with. I could show an example of taking a picture of something white but I think that you probably have the idea already.
There is another situation that can really confuse a camera. That is when the light on the subject you want to take a picture of is different from the light on the background. A very common example of this is when you have a performer on stage and the spotlight is on them. Your camera will typically take in the entire scene and decide that the whole thing is mostly black, requiring a long exposure, even though the person you want to take a picture of has a bright light on them and a short exposure would probably be adequate.
Something that is kind of ironic about this is that a basic disposable camera doesn't get fooled by the difference in lighting because it only has the one setting which is usually pretty good for taking a picture of someone in a spotlight. I have to laugh when I see pictures taken by someone with a disposable camera that come out great and they were sitting next to someone with an expensive SLR who can't figure out why his pictures look like a white silhouette on stage.
There are a few remedies for the difference in lighting. On expensive cameras you can usually select spot metering which will just meter the amount of light reflecting from the subject in the very centre of the lens. From there it's still a good idea to adjust the exposure compensation as described above for subjects wearing primarily white or black. For cameras that don't have different metering options it's usually a good idea to force the flash to turn on. Making this adjustment basically disables most cameras meters and sets the shutter speed to 1/60 of a second (this number can vary and you can find out for certain by reading your owners manual).
There are only two ways to be absolutely certain of the correct exposure in a scene. The easiest (and more expensive) method is to use a handheld meter right at the location of your subject and measure the amount of light there at that moment. The other method requires the purchase of a card that reflects 18% of light (they are called grey cards in the photo industry) and aim your camera at it while holding it up at the location of your subject to get a reading of the best exposure for the grey card. Even though I own a meter I tend to guess and adjust my exposure compensation on the fly. One of the benefits of having a digital camera is that I can see the results of my picture almost instantly after releasing the shutter.
I have simplified camera metering a bit here for the sake of brevity. Through the lens metering has evolved immensely since those first scientists decided that 18% reflectivity was the ideal. The first adjustment was to make the central 70% of the image more important in the metering than the outer 30%. Then the bias was adjusted for the lower part of an image to compensate for the fact that the sky is usually brighter than the subject material. Some modern cameras are smart enough to know what parts of an image are in focus and meter just the in focus areas. No matter how fancy these metering systems get they are still trying to make your picture grey.
Now for the assignment part of this lesson. Without adjusting anything using your computer and without using any flash I want to see photographs of subject material that is primarily black or primarily white. I am going to be looking for detail in the shadows and highlights and making sure that the image doesn't appear to be washed out or muddy. If people are really stuck for subject material I do have a Darth Vader helmet that I am willing to let people borrow. I also have spare tripods if needed and am willing to meet up with people to show how a handheld meter works and compare it with your in camera meter against a variety of subjects.
Lets say the deadline is September 16 and see how many photos I get before then. Any pictures submitted before then will be reviewed as soon as I get a chance. From now on I am going to take your raw image and make some adjustments (kind of like I did for the last assignment) just to show how I would change things if it were my image. If you have a different vision of how your image should look then please follow up my suggestion. Photographs can be very subjective and people can have extremely different visions.
Commentary on any lesson is also welcome from anyone who wants to say anything.