Controlling the depth of field in photography is a very important creative tool. Whether you use a big depth of field so that all the picture appears to be in sharp focus, or a shallow depth of field to isolate the main subject, knowing how to control the depth of field of an image is obviously very useful.
There are many variables that affect depth of field. Where you focus within the image is one of them and in this blog that’s what I’ll concentrate on. It’s important to give this due consideration. The further away you focus, the greater the depth of field, although if you focus too far into the image, elements within the foreground (front of the image) may start to lose focus. It is a good idea to experiment with different points of focus from the same viewpoint and assess how they affect the image’s depth of field. You can do this by either assessing the image on your camera’s monitor (if digital) or by using the depth of field preview button on your camera. For maximum depth of field for a given aperture, focus a third of the way into the scene. This will keep the foreground in focus as well as giving maximum sharpness to the background. (The amount of the image in apparent focus extends a third of the way in front of the focus point and two thirds behind it.)
Look at Fig 1 (red arrow). You can clearly see that the railings are dropping out of focus. The point of focus for this image was the ball on the top of the first railing. (See blue arrow) Which has resulted is a fairly narrow depth of field.
Now look at Fig 2 (red arrow). You can now see that the railings are still in focus. The point of focus for this image is the ball on the second railing. (See blue arrow).
All other camera settings are the same for both images. Same aperture, same focal length, same camera position. The only thing that has changed is what part of the image I have focused on, which, as you can see has had a massive effect on the depth of field on the photograph.
Gaining maximum depth of field by focusing a third of the way into a scene is a good starting technique. It will serve you well for the majority of situations. However when depth of field is really critical (ie when you have a foreground object that’s really close to the camera that you want to keep in sharp focus as well as some far away background detail) a more accurate technique may be required. Using a depth of field scale and hyper focal focusing is a very precise way of ensuring maximum depth of field at any given aperture. I will cover this technique in a future blog.
In order to focus on the right part of the scene or subject you will need to either manually focus your lens or make sure that the auto focus mode you use is set to single AF. By setting single AF it is then possible to pick which focus point the camera uses, therefore enabling the photographer to focus exactly where he or she wants to. In this case a third of the way into the scene.
NOTE : APPARENT FOCUS
There is only one point of the image that is in complete focus. Apparent focus is the area of the image that also appears to be in perfect focus. In reality this area is not in perfect focus but it is so slightly out-of-focus that the human eye cannot distinguish between the two.