This week Matt tackles depth of field – what it is and how it changes an image
Depth of field is generally defined as the area behind and in front of a subject that’s deemed to be acceptably focused and sharp. Images with a shallow depth of field show only a small portion of the scene in focus and the rest blurred, while images with a large depth of field show either the whole scene or the majority of it in focus.
One of the main ways in which we can control depth of field is by adjusting the aperture of the lens. As a general rule, a wide aperture with a smaller f/stop such as f/1.8 or f/2 will produce a shallow depth of field, while a smaller aperture with a higher f/ stop such as f/11 or f/16 will produce images with a more expansive depth of field. Aperture is, however, not the only thing which determines how much depth of field your images are likely to show.
Something else which determines depth-of-field is the distance between yourself and the subject. If you capture an image of a flower or another subject with a macro lens, you’ll find a fairly standard aperture such as f/4 or f/5.6 will give you far shallower depth of field than an image taken further away from the subject with exactly the same aperture. Similarly, if you use a wide aperture and focus far into the distance, you’ll find your depth of field much greater than if you were to focus closer.
It’s often thought that wideangle lenses produce more depth of field than longer lenses, but this isn’t quite the case. The reason for this is that for a fair comparison to be made, the subject would need to occupy the same proportion of the frame. So, you would need to move closer to the subject with a wideangle lens or further away with a telephoto lens. Trying to take the same image like this, using the same, aperture, would reveal depth of field to only vary marginally if at all.
Many photographers avoid using particularly small apertures such as f/16 or f/22 as an effect known as diffraction affects most lenses when used at these settings. This effect blurs the image slightly and reduces the fine details in the scene, so a wider aperture and a change in focus position will often be used instead. Narrow apertures are, however, very useful in product and macro photography where the closer subject distance requires this to keep everything in focus.
Your camera may have a control that lets you preview depth of field before you take an image, and this can be useful if you’re trying to isolate a specific subject from its surroundings or make sure a range of distance are in focus.
It’s important to remember what you consider to be acceptably sharp is also influenced by the size at which you view or print an image, and the distance from which you view it. It’s more difficult to see subjects slightly out of focus when they are viewed at a smaller size, just as its more difficult to appreciate this when you are further away from them.