Ever wondered what that button is around the lens mount of your dSLR?
It's not the button you press to take the lens off the camera. On most modern dSLRs there is another button that is usually dedicated to depth-of-field preview.
Depth of field, or DOF, is the range between areas in an image that look sharp. It's easiest to think of it in terms of photos you are already familiar with.
Landscape photos that have most of the image sharp and in focus have deep depth of field. Portraits with the subject in focus but a background blur have shallow depth of field.
The DOF preview button helps you determine what photos will look like before the image has been taken. When the button is pressed, the lens will stop down to the aperture that you have set. Then, you will be able to see a preview of what the final image will look like through the viewfinder.
How do I use it?
Depending on your camera, the button is usually situated near the lens, where your thumb or forefinger can press it without too much effort.
The easiest way to experiment with the button is to be in aperture priority mode. Start at the widest maximum aperture that your lens allows (a small f-stop such as f/3.5) then press and hold down the DOF preview button. Then, close down the aperture by increasing the f-stop in increments such as f/5.6, f/8 and f/11, pressing the button in between each to see what the effect looks like.
If you are looking through the viewfinder, the scene will darken as you close down the aperture. Notice that when you actually take the photo the camera compensates for a more narrow aperture by slowing down the shutter speed to make a correct exposure.
The DOF button works in any of the manual exposure modes (program, shutter priority, aperture priority, manual).
Why is it useful?
Your dSLR's viewfinder shows what the scene looks like when the lens is at its widest maximum aperture. This is great for seeing the scene as bright as possible, but not so good for determining where your depth of field and focus is located. This is where the button steps in.
One of the best ways to use the button is to preview what the background of portraits will look like, so there's nothing distracting going on.
For macro photographers, the DOF preview button can help work out where to place focus for maximum sharpness on the subject, but also providing the minimum amount of depth of field to blur the background.
When is it not so useful?
In anything less than really bright situations, the DOF preview button can make it difficult to actually see anything clearly enough through the viewfinder.
If you find this is an issue for your particular shooting situation, try pressing the button when in live view mode. Depending on how you've set up the camera, the preview onscreen shouldn't darken and you can see where things are in focus.
What else can I do with it?
With dSLRs, the DOF preview button might feel a bit redundant. After all, you can review images after they have been taken using the screen and check focus and the depth of field there. But for situations where you only have time for one shot, and you want to make that shot count, the DOF preview button can be incredibly helpful.
For lenses without a physical aperture ring on the barrel, the DOF preview button will often let you stop down the lens so you can take it off the dSLR body set to that aperture. This makes it useful for techniques like freelensing and reversing the lens for macro photography.
If you find no real use for the button, you can usually reassign it another function within the camera menus.