Depth of field is the near to far sharpness in your pictures. A picture with shallow depth of field has only the main subject in focus, and everything in front of it and behind it is blurred. The opposite is a picture with deep depth of field, where everything is sharp from right up close to the camera and far into the distance.
Depth of field depends on lots of factors, but the best known is the lens aperture. The wider the lens aperture, the shallower the depth of field. The shallow depth of field look is very popular right now, which is why lenses with a wide maximum aperture are highly sought after.
There are other important factors, including focal length. The longer the focal length of the lens, the shallower the depth of field. Shots taken with a telephoto lens tend to have shallow depth of field regardless of the lens aperture used. Pictures taken with an ultra-wideangle lens tend to show deep depth of field even at wide lens apertures.
Indirectly, sensor size is a factor too. It’s true that the same lens will give the same depth of field regardless of the sensor size, but in reality you use different focal lengths on different sensor sizes. For example, a 50mm lens is ‘standard’ on a full frame camera, but it takes a 35mm lens to give the same angle of view on an APS-C camera. With a smaller sensor you use a shorter focal length lens, which gives you more depth of field. On larger sensor cameras you use longer focal length lenses, which give shallower depth of field.
A third factor is the focus distance. The closer your subject to the camera, the shallower the depth of field. This brings special challenges for close up and macro photography, where the depth of field is increasingly shallow, regardless of the lens aperture used. Expert macro photographers use ‘focus stacking’ techniques to keep tiny subjects sharp from front to back.
Depth of field is not definite
The depth of field in a photo is a zone of sharpness defined by a ‘near limit’ and a ‘far limit’. There are tables and online calculators which will tell you the near and far limits of the depth of field for your current lens, sensor size, focal length, focus distance and lens aperture.
Many prime lenses have depth of field markings either size of the main focus index on the distance scale to show you the near and far depth of field limits at different lens apertures.
But these figures are slightly misleading because the sharpness doesn’t fall away suddenly at these distances. It’s a slow, progressive loss of sharpness and the depth of field is simply the zone in which the image looks acceptably sharp. This is worked out using a chosen ‘circle of confusion’. When a point is out of focus it’s rendered as a disc (circle), but if that circle of confusion is small enough it still looks like a point and the detail looks sharp.
So keep in mind that depth of field figures are just a guide to apparent sharpness, not definite boundaries. Nevertheless, they can be useful, and one measurement in particular – they ‘hyperlocal distance’.
Hyperfocal distance is a very useful concept in landscape photography. What you do is focus the lens so that the far limit of the depth of field falls exactly at infinity. This means you’re making the most of the available depth of field – the near limit is roughly half way between the focus distance and the camera. There are tables that will tell you the hyperlocal distances for different lenses, focal lengths, sensor sizes and lens apertures.
There is another way to estimate depth of field. Cameras will usually leave the lens wide open at its maximum aperture as you compose the picture, which will make the depth of field look very shallow, even though you may take the picture with a smaller lens aperture setting. Many cameras have a depth of field preview feature which stops the lens down to your chosen aperture so that you get a visual impression of the depth of field. This is only approximate and not always helpful, so many photographers don’t bother with it.
Bokeh and blur are not quite the same
Bokeh is a Japanese word that describes the visual quality of out of focus areas in a photo. ‘Good’ bokeh is smooth and soft, ‘bad’ bokeh produces hard edges around out of focus highlights or objects. Bokeh is all about whether the blur looks nice or nasty, not how much blur there is.
Bokeh depends on many factors, including the optical design of the lens and the elements used in its construction. It’s also affected by the lens diaphragm used to control the lens aperture.
This diaphragm is made up of a series of thin, overlapping blades which form the opening through which the light passes. The ideal opening for good bokeh is completely circular, and the greater the number of aperture blades, the more circular the opening. Lens makers may also add a curve to the edges of the blades to get closer to a truly circular opening.
Bokeh is not the same as blur. Bokeh is the quality of the blur.