The most basic description of ‘depth of field’ is the portion of a scene which appears in sharp focus.
Now, if you will excuse the potential for plaigiarism, I would like to quote the Wikipedia, as the following provides an accurate and succinct description, all be it with a little science and jargon thrown in, which I will later demystify for you all:
From here on in we shall be reffering to depth of field as DOF. Basically, I am a lazy typist.
“Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.
For some images, such as landscapes, a large DOF may be appropriate, while for others, such as portraits, a small DOF may be more effective.
The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number (relative aperture). Except at close-up distances, DOF is approximately determined by the subject magnification and the lens f-number. For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF.”
OK, so that last paragraph has some techno-babble in it; granted. But the first two paragraphs are pretty straightforward to understand, I hope. As for the jargon and geek-speak in the last paragraph, let me explain:
“The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number (relative aperture).” You are focussed on an object 5m away. This is the subject distance. The ‘plane that is perfectly in focus’ is an invisible line extending up and down at 90-degrees to the object you are focussing on, otherwise known as ‘the plane of focus’. If you can imagine that invisible line extending up and down from, say, the flower you are focussing on 5metres away…. just hold that thought, because I am about to expand on it farther down this page.
“For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF.” - Have you ever focussed on a person who is close to you and who has a deep background behind them? Have you noticed that it takes a very small aperture [large number, f/11 for instance] to get both the person and background in focus? This is you increasing the magnification by moving closer to a subject, and it results in less DOF. Similarly putting a longer lens on the camera or zooming in to, say, 200mm, with your zoom lens, results in less DOF. Instead of you moving closer, you have moved optically closer with the use of the longer focal-length lens. Again, less DOF is the result. A wide-angle lens [e.g. 28mm or wider] makes the world look farther away than it does to your eye. This is a decrease in magnification and explains why almost everything appears sharp through wide-angle lenses, even though you may have only selected a wide aperture [e.g. f/2.8]. See the image below for an illustration of how moving relative to objects can increase/decreas depth of field.
The right-hand image was shot 4metres from the bollards, focussed on the nearest bollard at an aperture of f/2 and shows the rear of the two out of focus. For the left-hand shot, I stepped back to 10metres and now the rear of the two bollards looks much more in focus. Both images were shot at the same, wide-open aperture of f/2 on an 85mm prime lens.
The bit about ‘close-up distances’ refers to extreme close-up, or macro photography. Different rules apply here. I will be writing a piece all about Macro Photography soon, in which all will be explained.
Clear so far? The pictures in the gallery at the foot of the page will hopefully make sure all of this has gone in.
It is about now that you should get your head around a core concept of DOF, called ‘hyperfocal distance’. For some of you I hope this will prove to be a revelation in how you approach certain kinds of photography, namely street and landscape photography, but frankly it has benefits well beyond just these two genres.
When focus is set to the hyperfocal distance, the DOF extends from half the hyperfocal distance to infinity, and is the largest DOF possible for a given f-number.
What, then, is hyperfocal distance? I could talk about this for several pages but that would bore the shit out of you. However, I will do that at some point in a separate piece. For now, if you want the full-blown explanation there is no better website to visit than DOFMaster.com. They also have some great tools for those of you who don’t have lenses with a DOF/hyperfocal scale on them [see below]
Simply put, there is a point with any lens – at a given aperture – after which everything appears to be in focus. This is the hyperfocal distance. Come back halfway from that point and you have DOF all the way to infinity. Not all lenses have DOF scales on them. A lot of modern lenses for digital SLRs dont, in fact. Some just have the markings for the two smallest apertures. See the links section at the bottom of the page for some great aids to figuring all this stuff out if your lens doesn’t have the scale on it. Including a great app for the iPhone, which has just come out.
The picture shows a lens set to f/22 and with the infinity mark placed level with the left-hand marker for f/22. The hyperfocal distance is shown above the middle white marker. The distance shown above the right-hand marker for f/22 is the near-focus point.
The lens above, then, is being used at f/22 and as you can see from the scale, by setting the lens by using the DOF scale we now have DOF from 0.7m [70cm] all the way to infinity. If we were shooting a landscape with this lens set like this we might usually focus the lens on infinity [the infinity mark over the white, middle point of the focus barrel]. That would be fine and you would get a sharp landscape in the distance and also a little way back from the far distance. But, focussing the lens in the way we see above means that you now have a landscape that is in sharp focus all the way from 70cm in front of you to infinity. If you were shooting street photography and didn’t fancy holding the camera up to your face all the time, alerting people to the fact you were taking their picture, you could pre-focus the lens as we see above and shoot from the hip. No need to touch the lens to focus just as long as you keep the bits of the world you want to be sharp more than 70cm away from your camera.
This technique is called pre-focussing or Zone Focussing. I love it and I shoot a lot of my street photography like this. It means I don’t have to constantly have the camera up to my face. However, zone focussing is only really effective at apertures smaller than f/8, depending on the lens. So it may not suit every situation. But it is most certainly the way to squeeze the maximum DOF from your lenses.
OK, so what does all this techno-bollocks mean in practical situations?
The photos in the gallery below will hopefully put some visual shape on all of this verbal. Apologies for the prolonged explanation but photography, if you want to learn it properly, has a fair amount of science involved. I’m sparing you most of that but some verbal explanation is required.
So, here are some situations and solutions:
You want to shoot a landscape and have as much of the scene in focus as possible. Nothing much is moving so shutter-speed is not too much if an issue but you do not have a tripod, so the speed still needs to be above, say, 1/60th sec to hold without camera shake. You want a good quality image to print so the ISO needs to be kept low; 200ISO for instance.
Meter the light [we shall go into exposure in another article but let's say the perfect exposure comes out at 1/250th sec at f/8]. Select a small aperture, small enough to give you a shutter-speed of 1/60th sec. In this case it will be f/16 @ 1/60th sec. If you have a DOF scale on the lens, set infinity to the far distance marker for f/16. You should be in manual-focus mode to do this. Or, with the camera on auto-focus, focus one-third of the way into the scene. We shall explain this later, in another article; it’s called ‘focussing by thirds’. These two methods will give you a sharp picture with focus all the way from one third of the way into your landscape and all the way back to the horizon.
You are shooting pictures of flowers in a crowded flower-bed. The background behind the flower you are really interested in is cluttered and busy. You only want the one flower to be the main subject of the photo.
Ideally you would shoot this with a macro lens, 105mm for instance, or at least with a zoom that allowed to you to maintain some distance from the subject and crop out as much of the background as possible. You would also, again ideally, want a lens with a large maximum aperture. However, the standard kit-lenses that come with most DSLRs are not like this, having variable apertures. So, let’s assume you are using one of those; a 17-85mm lens with a variable aperture of f/3.5 – f/5.6. The great difficulty with these lenses is that we want, with this subject, to get as close as possible. That would mean selecting 85mm on the zoom. But, at 85mm you are getting the smaller end of the variable aperture; f/5.6. Isn’t life a bitch? However, the closer you physically get to the subject, the less DOF you are going to get with the object relative to its background. So, zoom into 85mm and select the largest aperture available, in this case f/5.6. Your flower will stand out from the background. Alternatively, go out and buy a constant-aperture zoom with a decent sized maximum aperture [f/2.8] or get a macro lens.
You are shooting a portrait. You are in good daylight. You want to print out the photo to A4 at least. You want a good, natural perspective for the person’s face; they are very self-conscious of the shape of their features.
Wide-angle lenses [16-28mm] are very bad for portraits. Not only do you need to up your subject’s nose to get close enough to get their face to fill the frame but they also distort as you get closer. Not good for our self-conscious model. The ideal focal length for this on 35mm film cameras and full-frame DSLRs would be either 85mm or 135mm. These render faces with a natural perspective and allow you some distance from the subject. On a DX or cropped sensor [all but the most expensive DSLRs] 85mm equates to 130mm; perfect. If you have your kit lens on, get as far back as the zoom allows you so you can fill the frame with your subject’s face. Select the largest aperture the lens has. Bingo! Natural face, background defocussed. Remember though that if you are using a lens with an aperture that opens up as far as f/1.4 for instance, that you may not get sharpness across the whole of your subject’s face. At apertures of that size, DOF is very limited, or ‘shallow’. So, you may want to select f/1.8, f/2 or f/2.8 to get the best focus on the whole face but still keep the background out of focus.
Right, that’s enough talking…. here are some pictures to illustrate the whole lot:.
- The Wikipedia pages on Depth of Field
- The Wikipedia entries on Shallow Focus
- The Wikipedia entries on Deep Focus
- The Wikipedia entries for Hyperfocal Distance
- Alfie’s set of Flickr pictures shot with the wide-aperture Nikon 85mm f/1.4 lens
- Alfie’s set of Flickr pictures shot with the wide-aperture Nikon 50mm f/1.2 lens
- Make your own DOF calculator at DOFMaster.com
- Online DOF calculator at DOFMaster.com
- The PhotoCalc application for the iPhone: loads of useful tools including a hyperfocal distance calculator, sunset times for every city etc etc