3. Focal length doesn’t affect DoF.
That statement may come as a shock to some readers and some will actually argue the point. The fact is that when a sujbect is viewed at the same magnification, the depth of field won’t change substantially from a wide angle to a super telephoto.
The fact that a longer focal length tends to make background objects appear closer can help enhance the effect of a shallow depth of field, so your choice of lens can make a difference. On the other hand, telephoto lenses tend to have smaller maximum aperture sizes, which limits how shallow the DoF can be. In most cases, how close you can get physically to your subject will determine the best focal length.
4. Choose the right shooting mode.
If you’re comfortable with balancing exposure yourself, set your shooting mode to manual. If not, switch to aperture priority (Av or A) mode and let your camera’s metering system handle the exposure. Note: Don’t count too much on the camera here – there are many situations that can fool its metering system. You may want to learn to use AEB or manually bracket your exposure to be sure you get the best possible results.
5. Focus carefully.
When you’ve got your shot framed properly, take the time to be sure your focus is spot-on. Personally, I prefer to use manual focusing and magnify the subject in my LCD to be positive that I’ve got the sharpness I want, right where I want it. Whatever your preference, be certain you’ve focused properly; a wide aperture means less margin for error.