Using the histogram

[terrijo]

Hi all. I'm a new EOS 10D user..well, within the the last 6 months. I'm also a very amature photograher with a lot to learn. I have learned enough about the camera to now understand the histogram and what it is telling me about the photo (at least somewhat). My question is, if the historgram tells me the picure is too dark, or overexposed in areas, how do I know what adjustments to make on the camera so I can reshoot the shot? I've read the owner's manual, but still need to ask here :-)

[JoeTampa]

That depends. The histogram will only tell you if the shot is too light or too dark if the majority of the image is average grey. If you are shooting a night scene, your exposure may be dead on but the histogram will show it waaaaay on the left side. Same with snow - but waaaay on the right. Therefore, the histogram is not an "exposure meter", but a graph of black to white pixel ranges. You have to account for what you are shooting when evaluating what the histogram is telling you.

[G3]

OK, forget the histogram for a minute. That's way beyond the question you are asking. Let's assume that the photo is, in fact, either over or under exposed.

If a photo is over exposed it means too much light was allowed to strike the image sensor. If it is underexposed it means that not enough light was allowed to strike the image sensor.

There are two ways to control how much light strikes the image sensor. One way is by changing the size of the opening in the lens (aperture). The other way is by controlling how long the shutter remains open. Which one you use depends on what your subject is, and what effect you want to achieve with the photo.

If you want to control how much of the subject matter of the photo is in focus from front to back, you will use the aperture. This is called controlling the depth of field. If you want to have just the main subject in focus and you want the background to be blurry, you will use a large opening in the lens (small f-number). If you want the main subject and the background to all be in focus, you use a small opening (large f-number). The f-number is a ratio of the size of the lens opening to the focal length of the lens. It is actually expressed like this:
f2.8 is expressed as 2.8:1, meaning that the focal length of the lens is 2.8 times the size of the lens opening (measuring the diameter of the opening). f4 is expressed as 4:1, meaning that the aperture is 1/4 the size of the focal length of the lens. So, if you have a 100mm lens, the aperture at f4 would be 25mm across.

If you want to stop action in your subject, you would control how long the shutter is open. In that case, you would use a fast shutter speed, such as 1/500th of a second or 1/1000 of a second. If you wanted to get a blurring effect from motion of the subject, you would use a slow shutter speed. Say for instance, you want to photograph a waterfall and get that blurred water effect you see so often in those photos, you would choose maybe 1 second or so.

Keep in mind that there is a correct exposure for the ISO you have chosen (the ISO sets the sensitivity of the image sensor), which is going to be a combination of shutter speed and aperture settings. For instance, the correct exposure of a particular scene might be f16 at 1/125th of a second with the ISO set at 100. If you decide to use a faster shutter speed, then you will also need to change the aperture to a larger opening (smaller f-number) to compensate for the shorter time the shutter will be open in order to still get a correct exposure.

If you use the Tv (time value) setting on your camera, then you choose the shutter speed and the camera picks the appropriate aperture. This is called Shutter Priority Mode. If you use the Av setting (aperture value), then you pick the aperture setting and the camera chooses the appropriate shutter speed. This is called Aperture Priority Mode. In Program Mode and in Full Automatic mode, the camera chooses both. The difference is that in Program Mode, you can "shift" either the shutter speed or the aperture and the camera will do the rest. In Full Automatic mode the camera chooses both and you can't control it. In Manual Mode, you will do all of the work, but...you still have the meter in the camera for a guide to let you know if the combination you have chosen is correct.

If the exposure the camera chooses in any of these mode is off a little one way or another, then you have another function you can use called Exposure Compensation, where you are telling the camera to either under or over expose by a certain amount from the exposure that the camera has calculated. There is also another function called Automatic Exposure Bracketing, where the camera will automatically take 3 shots, one shot properly exposed, one shot under exposed, and one shot over exposed.

Hope that helps a little.

[terrijo]

Yes, it does help. I just need to digest it and play :-) Another excuse to shoot more pictures. Thank you so much!

[chris.bailey]

Nice one G3. Also remember that not every picture will show a perfectly distributed histo, a lot will depend on the subject. Ski pics, for instance would not be expected to show too much at the black end and portraits against a black background would. Its a matter more of matching the histo to the subject and trying to avoid blow outs.

[rick barclay]

Thanks for that explanation.