How does "depth of field" affect my photography?

I have always found this a strange term, why not simply call it Depth of Focus? Anyway, it matters not, what does matter is how it affects your photography and how you can use it to your benefit.

Personally, I normally set my camera to Av or "aperture priority" 95% of the time for more control over the sharpness. For the kind of work I do, the depth of field is more important to me than shutter speed. Although, if I need a fast shutter speed, I can just quickly and easily whack the aperture right open. giving me the fastest speed.

It's all to do with how much light enters your camera and the type of lens that you use. Basically, there are 3 factors that determine the depth of field in your images;

• Focal length of the lens

To put it simply, the shorter the focal length, the greater the DOF (or more of the image will be sharp). I.e., 16mm = More in focus, 400mm = less in focus. When I do a lot of interior work with an ultra wide angle, I generally use f8 or f11 as I know this will suffice for the image quality I need with this lens.

• Distance between camera and subject

If you photograph your subject sitting on a wall about 20-30 meters away or more, and using a wide angle or standard lens, you can almost guarantee that a lot of your image will be in focus whatever the aperture (within reason). However, bring the subject closer to say 2 meters, and the camera will focus on the subject but will more than likely throw the background into blurred oblivion.

• Aperture setting

  The aperture setting has the largest factor in determining the depth of field of your images. Just remember that f4, 3.5 or 2.8 (or bigger) will have shallow or little DOF whereas F8, 11, 16 or smaller, will have greater DOF. This is particularly true if you are doing close up work, a large aperture close up will have very little in focus.

To illustrate these points, have a look at these 2 pictures. I was testing the Sigma 105mm Macro Lens and wanted to check the sharpness at close quarters. The shot on the left was taken by dropping a small amount of milk into a larger container.

The camera was on a tripod and I used 2 studio lights closely positioned, and pre-focussed on the point where I would drop the milk.

Because I was using a telephoto lens and working extremely close, plus the fact that the lights were bright and also close, I needed a very small aperture. In this case, all the way closed at F36! It has to be said that even with this small aperture, parts of the background were blurred. I am told this is due to "diffraction" of the light when you stop down too much, so f16 is normally enough for good, sharp pictures. Also note that a telephoto combined with macro leaves little room for error as the depth of field is at its smallest.

I was pleased with the result but I must point something out to digital camera users at this point.

For the second shot, the leaf was brought inside, rain water and all, and set up in a similar way. This time, however, I used natural low lighting which meant that with an aperture of F29, I needed 30 seconds to make the correct exposure. Remember that when you open or close the aperture, you or the camera has to adjust the shutter speed to compensate. If you close the aperture right down for good depth of field, check the shutter speed as it may become slow and non-hand holdable.

Lastly, this image shows the other extreme. I used a Canon EF 50mm standard lens to get this shot of Max. You can see that the background is completely out of focus with virtually no depth of field. In fact the depth of field is so shallow that even though his head is only slightly turned, one eye is in focus and the other isn't! (You can see a larger and clearer version of this image by clicking on it).

This is because this particular lens has a maximum F-stop of just 1.4 and this was taken at that aperture. I like this effect and use it a lot for portraits. It draws your eye to theirs with no other distractions.

So when you are out and about next, instead of setting the camera to auto, experiment with depth of field and get a bit creative. Used in the right context a large or very shallow DOF can be very effective.

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