by Mike Rosen
Hi <<First Name>>,
I hope this finds you in good health and spirits. Maybe I can help in that regard by showing you how to determine the proper exposure for your photos.
Bet you didn't realize that could make you a happier person! But, think of it: The only alternative is abdicating this critical creative task to your camera's design engineers by using the automatic point and shoot mode ("P" on most cameras) or one of the preset scene modes we discussed last month. Wow!
OK, here we go. Warning: This is a little long. I tried keeping it shorter, but couldn't. Read it. It's worth it.
What is a "correct exposure?"
The two primary exposure variables are aperture (lens opening) and shutter speed. A great way to understand these is included in Jim Miotke's book, "Better Photo Basics" (I'm paraphrasing): The camera is like a dark house with only one window (the lens in this analogy). The larger the window (lens opening or aperture), the more light we allow to get in, or reach the camera's sensor (film in the old days). How quickly we close the window's shutters (shutter speed) determines how long a time period that amount of light gets in.
There are many combinations of aperture and shutter speed that will provide "correct" exposures, meaning images that are not too dark (underexposed), too light (overexposed), or blurry (too slow a shutter speed--or not holding the camera steady).
Today most mid-range and all upper-range cameras, whether point and shoot or DSLR, allow the photographer to manually control aperture and shutter speed settings rather than relying solely on auto or preset modes. Once you've learned how, you've effectively seized control back from those engineers (however good-intentioned) and opened the way to creating your own more compelling photos.
How do you do it?
First, read your camera's instruction manual (where have you heard that before?) to learn how to operate in the aperture priority mode (usually "A" on a dial on the top of the camera) and shutter speed priority ("S") mode. Once you choose aperture or shutter speed priority, the camera will automatically set the other, allowing the correct amount of light to hit the sensor and generating a correct exposure.
Shutter speed is easy to understand as it is measured logically in fractions of a second (the smaller the fraction the faster the shutter speed). You'll want a relatively fast shutter speed to avoid blur in an action shot, like say 1/250th of a second or faster, and a slow shutter speed in low light situations, like perhaps 1/10th of a second (holding the camera very steady, of course).
Aperture can initially be confusing to grasp because it is measured by something called f-stops. And, perversely, a large f-stop number (like f/22) means a small aperture or lens opening, and a small f-stop number (like f/4) indicates a large aperture.
What is "depth of field?"
Aperture is directly related to "depth of field," or how much of your image is in sharp focus, from foreground to background. The larger the f-stop number, meaning the smaller the aperture or lens opening, the greater (deeper) is the depth of field. The smaller the f-stop, or the larger the aperture, the shallower the depth of field.
To illustrate, here are two photos I made of exactly the same scene (near Naples, Florida) on the same day at the same time.
The exposure of the first image was 1/15th of a second at f/32. The extremely small aperture resulted in a deep depth of field with lots of extraneous background detail. It's too busy for my taste.
The second was captured at 1/400th of a second at f/5. The background is blurred out (shallow depth of field), which tends to isolate the main subject, lessening the chance of the viewer's eye roving.
Note that both exposures are "correct." Which one is "better" depends on what you, the photographer, are trying to create. I like the second version better. But the point is you are in control--not the camera.
A couple of other examples
For this recent shot near Florida's Lake Okeechobee, I wanted to "stop the action" so the cows and herd dogs would be in sharp focus. I set the camera to shutter priority (S), and dialed up the shutter speed to 1/640th of a second. The camera simultaneously increased the lens opening up to its maximum (f/5.6 in this case). I was pleased that the fast running dogs registered fairly sharp. The relatively large aperture resulted in a shallow depth of field, so the background is blurred. Again, I like this because it isolates the primary subject and prevents the viewer's eye from being diverted to other stuff.
Finally, for this scene at a flower farm near Immokalee, Florida, I wanted extreme depth of field so the entire image would be in focus, from the flowers and laborers in the foreground to the pots at the rear. So I chose aperture priority (A) and picked a quite high f-stop number (f/19). The camera automatically adjusted the shutter speed to 1/45th of a second. Any slower might have resulted in the guys becoming blurry.
There. No doubt you feel happier and in better spirits than just a few moments ago. Still, I know this whole subject can seem confusing, so if you have questions, email me and I'll try to address them next month.
All the best,
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If you feel confused or uncertain about camera basics like aperture, shutter speed and depth of field, I suggest you pick up a good book on the subject. There are scores of them, but one I like is Jim Miotke's "Better Photo Basics--The Absolute Beginner's Guide to Taking Photos Like the Pros." You can order it on-line at Amazon for $14.95.
HEADS UP: I will be teaching a course on how to make better pictures this April-May, as part of the OLLI senior education program (Osher Lifelong Learning Institute of the University of Minnesota). Check out OLLI's spring 2011 course guide by clicking HERE. I'll have more details next month.
SECOND HEADS UP: I will be exhibiting some of my nature and landscape images at the Minnesota Valley National Wildlife Refuge (Bloomington near the airport) in June and July. More info coming.
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