When you are setting up the exposure for your image (on any setting other than Auto/Program) it’s a game of trade-offs as to what you can do in order to get what you want. Modern cameras have so many built-in tools to do this work for you it’s sometimes difficult to know what’s really going on in the camera. So you get images back and think “this is not what I wanted the picture to look like” and wonder how your camera screwed it up.
Fortunately, there’s a setting on most DSLR’s that enables you to quit blaming the camera and start blaming yourself – Manual. Yes, you get to set everything with respect to exposure settings. And what you set is exactly what you will get. Sometimes that’s exactly what you want….
Not that it’s any less frustrating; it’s just that you can’t beat up on your equipment any more.
Here’s an example.
All I wanted was an image with the log and rock in focus, and an obvious small wave breaking on the shore. Well, with these settings I do have both the rock and log in focus, but the wave is just a jumbled blur. Obviously I need a faster shutter speed.
There – I opened the aperture to let in more light, giving me the ability to have a faster shutter speed to keep the same exposure. And I’ve got my wave.
Wait, though. The log is in focus and the rock isn’t. Obviously my bad – focusing on the wrong place.
Ahh – now the rock is in focus and I’ve still got my wave. It’s looking good.
Hey, now the log is out of focus! What gives?
Oh, I see. I’m fighting a depth-of-field problem. At this aperture (f/2) the DOF is very narrow, narrow enough that focusing on one subject means the other will be out of focus. Obviously I need to get a bigger DOF by closing down the size of the aperture.
Yes – now everything is in focus. But I’ve lost the wave! It looks like the lake is completely still! What’s wrong?
Nothing is actually wrong. All four of these images are correctly exposed; i.e., capturing the same amount of light. The camera is simply, and accurately, doing what I tell it to do. So why can’t I get the composition I want?
I have two camera controls essentially fighting against each other. For the amount of light I’m letting into the camera (exposure), a fast shutter speed that will render the wave I want requires I use a wide open aperture to give me enough light. But a wide aperture results in a very narrow depth of field, meaning the rock and log won’t be in focus together. Close the aperture down to get a wider depth of field and I let less light into the camera; to compensate I have to use a longer shutter speed to let more light in, which will not “stop” the wave as it travels to the beach. I need enough light to use a smaller aperture AND a faster shutter speed. What am I going to do?
There’s one more camera control I can use. I can change my ISO setting to a higher number (notice these are all ISO 100). Using higher ISO numbers basically makes the digital sensor more “sensitive” to light, thereby changing the exposure conditions of the scene (as seen by the camera) and allowing me to use a faster shutter speed and smaller aperture. I imagine changing to ISO1600 will give me what I want.
There’s no picture to prove this works because I didn’t make one at that setting. ISO1600 on this camera results in unacceptable noise in the image, especially in the shadow areas. It will work, though. Going from ISO100 to ISO1600 is an increase of 4 stops of light in the image. I can go back to 1/60 sec. shutter speed (I know that will stop the wave) and close down the aperture from f/2 to f/8 (4 stops), giving me a depth of field sufficient to bring both rock and log into focus. I could probably take care of most of the noise using software.
I could also have made this image earlier in the day when the sun was higher and giving me more light. Possibly I could have used a reflector to concentrate more of the existing light on the scene (hard to do without an assistant). Or I could use a small flash to increase the amount of light (but probably create undesirable shadows).
So, as I said, there are always trade-offs. The master photographers use these to their advantage, crafting exposures that make the settings work for them instead of having to work around the settings. Practice and experience – and paying attention to what you create and how.