Monday, July 12, 2010

Understanding Depth-of-Field (DOF)

What is it?
DOF is the portion of the image that is in sharp focus.  

What are the factors that influence DOF?
1.            Sensor’s size (all else equal, the smaller the sensor’s size, the greater the DOF). e.g. point-and-shoot cameras tend to produce images with greater DOF than DX DSLRs (D3000, D5000, D90, D300s), while DX DSLRs produce images with greater DOF than FX DSLRs (D700, D3s, D3x).
2.            Aperture (all else equal, the smaller the aperture, the greater the DOF).
3.            Focal length (all else equal, the smaller the focal length, the greater the DOF) e.g. a wide-angle24mm lens tends to produce images with greater DOF than a telephoto 200mm lens.
4.            Focusing distance (all else equal, the greater the focusing distance, the greater the DOF).

How to use it
By controlling DOF we can add interest and creativity to our photos.
Portraits: Usually when shooting portraits we want to achieve small DOF in order to isolate our subject from the background:
·           Use a wide aperture (e.g. f2.8, or even f1.4).
·           Use the biggest focal length possible (e.g. 200mm is better that 18mm).
·           Get as close as possible to your subject.

Landscapes: Usually when shooting landscapes we want to achieve the opposite; great DOF so that almost everything is in focus:
·           Use a small aperture (e.g. f11, or even f22).
·           Use the smallest focal length possible (e.g. 18mm is better that 200mm).
·           Get as far as possible from your subject.
(! Note: the above guidelines concern things you can do in order to achieve the desired DOF. Composition rules, depending on the situation, may instruct something else!)   

Extra tips
·           DOF in not equally “distributed” between the foreground and the background of your subject. In fact there is a ratio of 2:1 between foreground and background DOF. In other words, DOF is twice as much in front of the subject you are focusing at, than it is behind the subject. If you are shooting landscapes with big DOF, focus your lens at the 1/3th of the distance covered by the frame.
·           When using shallow DOF to isolate a number of similar subjects from the background, sometimes you are wondering on which subject you should focus your lens. Rule of thumb states that you should focus on the nearest subject. You should have a very good reason to violate this rule!
·           Glossary: you may have heart the term “bokeh”. It’s the area of the image that is out of sharp focus, created by using small DOF. Good lenses tend to produce sweet bokeh.          

Your living room is a good place to start practicing DOF. Take 2 objects and place them on a table so that they cover different distances from your lens. Try to isolate the front subject from the background subject (set your camera to Aperture Priority Mode, use your lens’ wider aperture, if it’s a zoom lens, use the biggest focal length and get as close as possible to your subject). You see it’s very easy! Then try various scenarios; change the distance between the objects, change your lens distance from your main subject, change your focal length, aperture and composition. Try to alter one variable at a time in order to see how each variable affects the final image. In one hour time you’ll be using DOF like a pro! Then get out there and do some landscape photography. Set your camera on a tripod and take some shots by altering aperture, lenses, focusing distance and composition. Review the results and see what works best in certain situations. Experimentation has always been part of photography, not just for practicing, but to make unique pictures as well. Have fun!  

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