Monday, March 2, 2009

Exposure in Digital Cameras (Part 3)

In the previous article on exposure we talked about using aperture to control DoF, and produce a correct creatively exposed image.

In this example we will be shooting a kid's soccer game and talk about action and shutter speed.

It's been a sunny day but the sun is low and the light is fading as the afternoon turns into evening. To keep a decent depth of field on our main subject we choose a starting aperture of f8 on our 75-300mm telephoto zoom lens. This gives us a DoF of about 1 foot at 300mm focal length if our subjects are 30 feet away (maybe too close for 300mm) to a DoF of 16 feet at 75mm at 30 feet away. See the Depth of Field calculator. So f8 is not a bad aperture to start with so long as you widen your zoom as the players get closer to you and maintain our desirable DoF.

The problem is the shutter speed. The lower light means we need a longer shutter speed to obtain the correct exposure. In our example the camera determines the correct shutter speed for the f8 aperture is 1/125. However there are 2 problems with this shutter speed. One is it is too long to produce sharp images due to the 300mm focal length of the camera. One can not hold the camera steady enough in our hands to eliminate any body movement during the time the shutter is open. The second reason is 1/125 is too slow for the movement of the soccer players and they will ghost or smear in the images. So at this shutter speed, unless we're really good at panning (which is a viable option), our images will not be sharp and will suffer from severe motion blur.

So to stop the motion we need to increase the shutter speed or reduce the amount of time the shutter is open. This will reduce the blur and again the f-stop can be used to determine the reduction of motion blur. If we double the shutter speed we reduce the amount of motion blur by half. If you are into mathematics and do the calculations here you will see that you can never mathematically eliminate the motion blur, but you can reduce it to the point that the sensor resolution (and subsequently the screen or print resolution) will not show the blur. It's like the old mathematical puzzle, if each step you take gets you half the distance to your destination, you will never reach the destination. But you will get close enough, and that is all we care about in photography.

So where do we need to set the shutter speed to freeze the motion of a kids soccer game?
We could calculate it but that would cause us to have math induced brain freeze, and we wouldn't get to see the results of our experimentation. Well we know at 1/125 it's not enough so lets double it to 1/250 and chimp to find out. Hmm... the motion was reduced but for some reason the photo got darker. What happened there? Oh, we forgot to accomplish the reciprocal change to one of the other camera settings. So we nudge open the aperture by one f-stop to compensate and the exposure is back to where it was before and we do indeed have reduced motion blur. But it's not enough. So one more f-stop on the shutter to halve the motion blur and again a reciprocal change to the aperture to compensate. We should now have a shutter speed of 1/500 and an aperture of f4.

Well after this adjustment we have good news and bad news. The good news is we finally have the motion blur under control but the bad news is our depth of field at f4 is too shallow and focusing has become critical (no room for error) in order to produce sharp images. We have gone from motion blur problems to out of focus (DoF) blur problems.

So what's a photographer to do? We've adjusted all the camera settings and still not getting sharp photos in this dimming light; Or have we? What about ISO? We've never discussed changing the camera's sensitivity or ISO. This 4th variable in our exposure formula, the one I said in an earlier article was the least important under most conditions, has now become quite important in obtaining not only our desired results, but in just obtaining desirable results.

Adjusting the ISO from 100 to 200 and from 200 to 400 and 400 to 800 each will net you 1 f-stop of light each (3 total f-stops from ISO100 to ISO800). So in order to get our aperture back to f8 to maintain our desirable DoF and keep the shutter speed at 1/500 for our desirable stop motion we need to adjust the ISO by 2 stops from 100 to 400.

Now when we capture the image we have our desired DoF, we have stopped our motion to our satisfaction and we have a correctly exposed image. In other words we have a correct creatively exposed image.

Now in the time it's taken to figure that all out for the first time, the sun has set and the camera's ISO won't go up any further. The soccer game's about over anyway but we'll be ready to use all this knowledge at the next game.

In the next article we're going to discuss the histogram and it's uses along with loss of detail in under and over exposed images.

Until then, how about some night photography?

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