Yet again, I have bitten off a huge chunk of photographic goodness and I will chew it up and hand it back to you in nice, pre-masticated, understandable bite-sized pieces.
How does exposure affect image quality? That is the question of the day...or the week.
Exposure, simply put, is the amount of light that the digital sensor (or the film) absorbs. It is determined by three things:
1. The length of time it is exposed to the light. Measured as Shutter Speed
2. The amount of light that comes in at once. Measured as Aperture
3. The sensitivity of the sensor (or film.) Measured as ISO
The Analogy:
Imagine your sensor (I'll stop saying film every time, you know who you are.) is a bucket. Like this one that my wife Lori uses to put flowers in before she builds the most beautiful wedding bouquets on the planet. (How was that for a shameless plug? You really should go to her website, though.)
Your light meter/camera meter gives you the proper exposure, visualized by a line on the side of the bucket. We'll say that it is 2 liters. That is the correct amount of light/water for the scene. You have two factors that go into filling the bucket. How much water you pour in at once, and how long it takes you to pour in the water.
In the end, it really doesn't matter whether you dump the entire 2 liters in at once or whether you run a hose and fill it up slowly. In the end, you still have 2 liters of water in the bucket.
Think of the amount you dump in at one time as the aperture and the length of time it takes you as the shutter speed. Keep the analogy in mind as we go over the parts.
How do you measure light?
We do it in "stops". Stops are not nearly as difficult as most people make them out to be. One stop up is double the amount of light and one stop down is half. Pretty simple.
1. Shutter speed is measured as a fraction of a second, 1/8, 1/60, 1/1000. Half the amount of time, is one stop different. It's one stop brighter (double the light) when you move from 1/125 of a second to 1/60 of a second. The standard stop measurements go from 1/60 up and down, ex 1/30, 1/15, 1/8. 1/4 and 1/125, 1/250, 1/500, 1/1000, etc.You can see how each is either double or half the amount of light of the one above or below it. Modern cameras allow either one or two increments in between each full stop--it can be more confusing, many photographers don't even know what the official stops are.
There are no absolutes with shutter speed because how much light you get in that amount of time is based on how much is available and how much is coming in at once.
Shutter speed affects Image Quality mostly by controlling movement. If you shake the camera or your subject moves while the shutter is open, the image comes out blurry. Both the camera and the subject will move much less in 1/500 of a second than they will in 1/8 of a second. Typically, faster speeds will yield crisper, sharper images. With image stabilization on lenses photographers often just make a blanket statement that now that camera shake is minimized they can shoot at very slow shutter speeds. True, but you have to be aware that your subjects, be it people, animals, or even trees, move.
2. Aperture is measured in f-stops. It's a complicated equation (and a whole different blog post) to get the numbers but it boils down to each number being double the size of the one below it--therefore letting in twice as much light. (starting to get the pattern?) The traditional standard stops are 2.8, 5.6, 8, 11, 16, 22 etc. Each is one stop different from the one next to it. Again, modern cameras allow you to choose whether you want one or two options for apertures in between these standard ones.
The difference between aperture and shutter speed is that an aperture is always the same amount of light. So 2.8 on my 7D is the exact same amount of light hitting the sensor at one time as 2.8 on my G11 point and shoot. Remember that, it makes using flashes MUCH easier later on.
The main effect on Image Quality from aperture is depth of field. It's an amazing artistic tool but if you want a 3-foot-deep object entirely in focus and you're shooting at 1.8 and 50mm, you're looking at about one inch that will be in focus. Typically it takes 5.6-8 to focus an entire group of people 3 lines deep.
3. ISO is not really reflected in the bucket analogy. If you have ideas on how to use it, let me know. I toyed with using it but it just complicated things.
ISO is basically the measurement of the sensitivity of the sensor or how quickly it absorbs light. In the good old days of film, ISO was determined by the film you used. It was difficult to change film in the middle if you were outside using 100 iso film then went inside where you would need 800 to get any light at all. Digital is awesome. Now all you have to do is switch the sensitivity to wherever you want it.
ISO effects Image Quality with noise, mostly. The higher your ISO, the faster shutter-speeds you'll get, but with that extra speed you pay the price in noise. Not all noise is bad. I actually like the texture you get from a little noise, the question is what kind of noise. Vast improvements have been made in the look and feel of noise at higher isos. For example, I hated anything above 800 iso on my 40d but even though there is still quite a bit of it at 6400 on my 7D it is more pleasing and I actually mind it less. It doesn't interfere with the shapes and lines in the image like the ugly digital-looking noise did before.
Frankly, images will almost always look better at higher isos and correct exposure than they will under-exposed at lower isos.
That's basic exposure. The simplicity is maddening. I will expand on all of these as time goes by.
Next week: White Balance...my arch nemesis.
May 13, 2010
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I'm enjoying this helpful series, Jon, but I think you mixed this one up: "It's one stop brighter (double the light) when you move from 1/60 of a second to 1/125 of a second." -JP
ReplyDeleteYou're right. I fixed it.
ReplyDeleteJon