The Ultimate “Real World” guide to depth of field.

Anyone with anything more than a passing interest in photography knows that the aperture controls depth of field in photography. But that is anything but the full story. Did you know that there are three other measurable things that affect depth of field? – Focal Length, Focussing Distance & Sensor Size – and one unmeasurable one (how far away the background is). Are these incontestable facts? No, not really. There are several ways of looking at this subject, and according to an extremely technical viewpoint, focal length doesn’t in fact affect depth of field. I’m going to disprove that here, just for the hell of it, but I’m normally not interested in jostling with the pixel peepers. I’m more interested in the real world, with a camera in my hand, and there, focal length certainly does affect depth of field. Confused? Intrigued? Read on. In this post, I’m taking the real-world approach, and will explain my reasoning throughout.

Was I taught this stuff when I completed my Diploma of Photography 10 years ago? No! Well, not much of it. Is that a scandal? Most definitely, as regardless of how technically minded you are, the rules of depth of field haven’t changed at all in the last few decades, and understanding and controlling depth of field is one of the primary creative options in photography.

Photographers NEED to understand this. And not just on a superficial level. Especially if you make a living from photography, or it’s something you want to get good at! Grab a cup of tea and settle in. This might take a while!

Depth of Field

OK, so what is depth of field? It’s most commonly defined as ‘the area of acceptable focus in an image. I suppose this begs the question, ‘when is it unacceptable? Why, when it’s blurry of course! The main thing to understand here is that focus doesn’t just start and stop on the head of a pin – it gradually goes from being sharp and in focus to being blurry and out of focus. Hence the definition of acceptable focus.

Now, if you want to get technical about it, this area of acceptable focus is defined by the Circle of Confusion (I know, right, who makes this stuff up?) The best definition I have seen of the Circle of Confusion is: “A group of photographers sitting around trying to understand Depth of Field” Just kidding…

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Seriously though, a definition of the circle of confusion reads something like this on Wikipedia –

circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. It is also known as disk of confusioncircle of indistinctnessblur circle, or blur spot.

In photography, the circle of confusion (CoC) is used to determine the depth of field, the part of an image that is acceptably sharp. A standard value of CoC is often associated with each image format, but the most appropriate value depends on visual acuity, viewing conditions, and the amount of enlargement.  

So, the circle of confusion (pictured above) defines what acceptable focus is, but only pixel peepers really need worry about it. In the real world, with a camera in your hand, it’s just not that relevant..

Now I know some of you are probably thinking: “Hang on, didn’t he say this was the Ultimate Guide to Depth of Field?”

No, I said it was “The Ultimate Real World Guide to Depth of Field”, and there’s an absolutely huge difference. Pixel peepers might be interested in the scientific minutia behind every law of physics that controls photography, but most photographers aren’t. And neither do they need to be. Some are very interested, of course, but in my experience, these are usually middle aged men with $30,000 Medium Format Phase One Digital Backs who have way too much money and not much of a clue about how to take a good photograph. On the other hand, they can talk about the circle of confusion all afternoon, as well as the various merits of different sensors and so on and so forth. Let’s just leave them on the tour bus of their highly expensive weekend workshop, and get back to business…

Then again, if on the off-chance you do want a good technical explanation of the circle of confusion, I’d recommend Cambridge in Colour’s excellent tutorial. http://www.cambridgeincolour.com/tutorials/depth-of-field.htm or this http://paulorenato.com/index.php/151 or for the really technically insane amongst us, try this one http://www.normankoren.com/Tutorials/MTF6.html

Later, we will talk about the quality of the blurry part of the photo but depth of field is technically about the bits in focus, so let’s leave that discussion for later.

Aperture

OK, a logical place to start a discussion on depth of field is the aperture. And here it is very simple. The larger the actual aperture (the smaller the number) the less depth of field you get.

So, on an f/2.8 lens, choosing an aperture of f/2.8 will give you the least amount of depth of field for that lens, while choosing f/22 (or whatever the smallest aperture is on the lens) would give you the largest amount of depth of field.

If only controlling depth of field were as easy as controlling aperture… But depth of field is a convoluted son of a diaphragm, and we need to look at the other factors affecting depth of field to see their influence overall.

But this is the simplest starting point. The largest aperture gives us minimum depth of field and the smallest gives us maximum depth of field, but what field of view the lens has, and how close it’s focusing also have a huge impact on the final resultant depth of field (not to forget the senor size). With a 100mm lens focused at 1 meter, the depth of field would be very narrow, but with a 16mm lens at 1 meter, the depth of field would be apparently much greater (I should say ‘apparently’ here or the aperture police will come and say I’m incorrect and that the effect of focal length on depth of field is apparently (excuse the pun) an illusion caused by magnification, and that if you crop in to the same size, depth of field at various focal lengths are almost the same.) Be that as it may be, (or not be as is actually the case), in the real world, focal length has an immediate and visible effect on depth of field, so I’m siding with the non-technical crowd on this one.

OK, clear as mud so far? Let’s relax a little with a shot showing the influence of JUST aperture on depth of field. Focussing distance and focal length (and of course sensor size) haven’t changed. Only aperture. Simple right? Yes, it is. OK, ready to continue?

Focussing Distance

Focussing distance is pretty straight forward as well, and is not under question. The closer you focus, the less depth of field you get at any given aperture. It is measurable and consistent. Like every other factor in depth of field, its effect is not linear. Its effect is most obvious at very close focussing distances. If you take two photos, one focussed at 15 meters and one focussed at 10 meters, you won’t see much of a difference at all… But when you get very close (like with macro lenses) the differences are considerable, with only a small change in distance. A related aspect of focussing distance is the concept of hyperfocal distance, but that has its own section a little later.

This photo was taken with a 100mm lens on a full frame camera at a very close focussing distance (app 20cm) See how narrow the depth of field is. It’s only a few mms. On a full frame dslr, it would be practically impossible to get less depth of field than this.

Focal Length

This is the elephant in the room when it comes to depth of field. Technically, they say it is not a major influence on depth of field (yes, I know that’s probably a bit of a shock to some people, no more so than some of my students I am sure.) But I have been teaching the simple version for years, and this ‘truth’ is only for the boffins. In the daily world of photography, focal length IS a major and measurable factor. And so, I continue to treat it as such. But technically, and here we go back to the pixel peepers, if the subject occupies the same fraction of the image, the total depth of field is virtually constant with focal length! In other words, focal length appears to influence depth of field, but it is really just an expression of magnification.

OK, technical arguments aside, how does focal length affect depth of field in the real world? Simply put, the longer the focal length, the less depth of field you will get at any given aperture. A shot taken at f5.6 with an 18mm lens will appear to be pretty much all in focus, but if you zoom in to 200mm, the photo will have a blurred background (if you are focussing on something relatively close to the camera). How blurred it is will depend on the sensor you have, the aperture you have chosen and both the focusing distance to the subject and the distance to the background, but just the one simple change of focal length will change the apparent depth of field all by itself, and quite dramatically at that.

This photo shows the difference in depth of field with JUST changing the focal length. Aperture and focussing distance remain the same. The shot on the left was shot at 70mm, while the one on the right was shot at 200mm. There is an obvious reduction in the depth of field in the shot, right? Well, the pixel peepers, as I have said before, try to say that it’s almost completely just an effect of magnification, and that if you crop in to the same size, the depth of field will be almost the same. So, let’s do that and see if they are right.

So here on the left, I have cropped in on the 70mm shot, to the same field of view as the 200mm shot. On the right is the uncropped 200mm shot. Your honour, I rest my case.

Sensor Size

The larger the sensor, the less depth of field. And so a full frame DSLR camera with the same settings will give an image with less depth of field than a camera with an APS-C sensor. It’s not really common knowledge, but there are at least 18 different sensor sizes in common use, ranging from 4.54mm in width (Apple iPhone 5) all the way through to 70.41mm (Phase One P 65+),

Obviously, you can’t change your sensor from shot to shot, so this is not a variable in depth of field that you can adjust. But it might influence your next camera purchase.

Click on the thumbnail to see a larger version of the comparative sizes of sensors.

Distance to background

I like this one. It can’t really be measured (as depth of field is really concerned with the in focus part, rather than how out of focus the background is). But it’s very logical, and so makes perfect sense. Getting everything in focus in a shot isn’t half as difficult as  getting very blurry backgrounds. When using a crop sensor and kit lenses that are not so fast; that only open to f/5.6 for example, then one would need to have the background a lot further away to even start to get it to begin to blur. So, for that minimum depth of field look, consider how far your background is from the subject. The further the better is the easy approach. With a full frame sensor and a fast lens, it need only be a few meters, but again, the further the background is away from the subject, the blurrier that background will become.

If you’re looking to get a blurry background, you need to remember to allow some distance to the background. Even if you shoot wide open on an f/2.8 lens like these shots, if your subject is too close to the background, the background doens’t have much chance to blur out. The first shot, the model is aproximately 1mt. from the wall.In the second shot, he is closer to 3mts away.

Quality of blur (Bokeh)

The quality of blur in photography and in particular, the quality of blurred specular reflections and light sources, is known as ‘bokeh’, which comes from the Japanese word boke, which means blur or haze. Lenses have different quality bokeh, depending on their internal aperture irises, and the number of blades in the iris. A circular bokeh is considered most desirable, yet most cheaper lenses exhibit a more polygonal shape with 5-8 sides. This is obviously more important on lenses designed for shallow depth off field, like macro and telephoto lenses. Some lens manufacturers even make lenses designed with specific controls to change the rendering of the bokeh.

If you are a landscape photographer who constantly shoots with wide angle lenses at f/11, then rest easy. You’re not likely to have ever seen bokeh, and you are not likely to. But for a portrait photographer, shooting with medium focal lengths and wide apertures, it’s a very important factor. Click on the images below to see them larger.