How deep is your field?

Professional photographers often talk about depth of field, but what do we mean and why does it matter?

Simplistically, depth of field (DoF) is distance between the closest and farthest objects from the camera which appear sharply focussed. It's usually said that the things which affect DoF include the focal length of the lens in use, the aperture which is set on the lens and the closeness of the nearest object. You can use it to control the sharpness and thereby the visual impact of objects in different parts of your pictures.

Use a lens with a wide aperture, you get shallow DoF and can throw items in the foreground or background out of focus at will. Use a small aperture and you can maintain sharpness over a huge range of distances - look up 'the f/64 club' sometime for one extreme. On many older lenses, in the days before auto-focus and ultra wide angle zooms, the manufacturer used to helpfully mark a depth of field scale on the lens body. The idea was that you could gauge the depth of field by comparing the marked lines against the aperture marks. You could turn the focus ring until it matched the distance for the closest object you wanted sharp against one line and the furthest against the other, adjusting the aperture to the value indicated as necessary. In theory, that meant you could quite accurately control the way people would view your images.

But that's only half the story...

More fundamentally, DoF depends on the capabilities of your own eyes. Your eyes, like any optical system, have limited resolving power. There's a very simple experiment you can do to test this:

Using a ruler and a wide-nibbed pencil or pen, draw a pair of thick parallel lines on a sheet of paper with a gap between them of exactly 1mm. Pin the paper to a wall or door and walk away a few paces. Check to see if you can still see the gap between the lines. Yes? Then try again from a bit further away. Repeat until you are just too far away to see two lines, instead seeing just one, thick line. Measure the distance you've walked from the paper and you have a fairly accurate guide to your own optical resolving power. You can easily convert this to an angle if you treat the distance between the two lines as the short side of a triangle and the distance from you to the paper as the long sides.

Now, think about a photographic print. It's most likely made up of points rather than line, but the same principle applies - if the angle between two dots on the print is smaller than the angle your eyes can resolve, then you will just see them as one dot, not two (or more).

So, putting the concepts of the DoF of the camera and the resolving power of your eyes together, what you get is an understanding that it's not actually necessary (or possible) for any part of the image to be absolutely sharp. In fact, anything which is rendered sufficiently sharp on the print that your eyes can't see any unsharpness  will look sharp to you anyway. Sounds obvious, really, but that leads to several perhaps surprising conclusions that photographic reference books seldom mention.

- How sharp an image appears to you depends on both it's scale and your proximity to the print. Just like the lines on the paper.
- The DoF marked or suggested on the lens assumes a 'standard' size print viewed at a 'standard' distance. Any other viewing arrangements make it a nonsense.
- In general terms, any picture viewed smaller or from further away will look sharper overall, and the apparent depth of field will be greater as a result.

So, by all means use DoF as a way of controlling the relative sharpness of objects in a composition, but don't be slave to the concept, Depth of field is not absolute.

How can you reduce the amount of adjustment your pictures need?


I was sitting in my studio earlier today, working on some of the boring, paperwork that goes with running a business, when in waked a typical amateur snapper asking for help. It's always a bit irritating, because I have work to do, but we all started as amateurs. And we all have to learn somehow. At least this bloke was asking someone who might actually have sensible answer. Me. Oh well, paperwork can wait, I suppose.

His bits are the bits in quotes. Mine are in brackets...

'I took some photos at my cousins wedding (oh dear, hear we go...) in the West Indies (hmmm, so why wasn't I invited then?) and they all look awful. Red faces. No details in the white dress. (Yes, I know. And shadows so deep you'd break a leg if you fell in) I'm spending ages trying to improve them in Photoshop. What could I do to cut that down?'

Back to me.

Well, there are a whole lot of things to say about that, but just in terms of photography, the main problem is that you're probably letting the camera do the thinking for you. Don't.

Him 'Huh?'

Do things the old fashioned way with the photographer in charge. It's the only way to get things right, especially in difficult conditions.

Let's think about the way a modern, all electronic camera looks at a scene. It doesn't see a picture, just areas of light, shade and colour. It has no idea what they represent. It can only compare them to a standard model and see if they are close enough to what it expects. If there's a lot of dark areas such as big, deep shadows, it tries to make the whole picture lighter. If there are lots of blue/green areas such as the sea, it tries to add the opposite colour, red, to balance it up. It tries to make everything conform to the most boring, ordinary scene it can possibly imagine. And it's imagination isn't very good.

Take control. Put the camera in a manual mode, so you control the exposure. Force it to accept the colour you want by telling it the kind of light you're using. Then use a decent, incident light meter to measure the light actually falling on the scene, so it's not fooled by awkward subjects like a white dress.

The most important thing about photography is light. Light and control. Light, control and empathy. The three most important things about photography are light, control, empathy and composition. Oh bugger...

What are apertures and why are they important?

To a photographer, the aperture is the opening in the lens through which light passes into a camera. It's most important physical property (okay, it's sole physical property of any interest!) is it's size, because that has a number of rather important effects on the pictures you make.

First of all, I'll be specific about what I mean and don't mean by the 'size' of an aperture. I don't mean a simple measurement across the opening. I do mean a measurement of that opening as a proportion of another physical property of the lens, it's focal length.

On most lenses, the maker will mark the focal length on the front. A typical standard lens on a traditional 35mm SLR might have '50mm f/2.8' engraved around the front, for example.

So, what does that all mean? It means the lens has a focal length of 50mm and a maximum aperture of f/2.8, which is the same as saying 50mm/2.8, which is roughly 18mm.

Fascinating, eh? No, not really. Unless you're actually fascinated by numbers.
The point is not the number itself, but what that effect that has on your pictures.

The aperture affects how fast light comes through the hole - a bit like turning on a tap. Open it wide and water (or light) comes streaming through. Keep it small and water is reduced to a trickle. This helps control the amount of exposure on your film or digital sensor. Using the water analogy again, getting the exposure right is like filling a bucket from a tap. You need to fill it right to the brim, not half full, but you also don't want it to overflow. You can do that by opening the tap wide for a short time, or leaving it dripping for a much longer time. To get a full bucket, it doesn't really matter which way you do it. Exposing your film or sensor is the same. You can let in light very fast for a short time, or slower for a longer time. So, a big aperture means we can have fast exposures. A small aperture means we need slower exposures.

The opening also affects 'depth of field'. Big depth of field means you can keep lots of things 'in focus' at different distances at the same time. Small depth of field means you can keep just one thing in focus at one distance and let things at other distances blur. More about that another time...

So, use the aperture to control exposure. That's a technical use. Use the aperture to control depth of field. That's potentially a creative use as it lets you control how people see the things in your pictures.

Last thing for today - why not just give the aperture as a straightforward measurement? Why give it as a proportion of the focal length - what's often referred to as an 'f-number'? The answer is to do with an interesting and very useful property of lenses. Any lens at the same f-number lets light pass at exactly the same rate. That means that it's very easy to measure the exposure as a combination of an 'f-number' and a time, because it doesn't matter which lens is used.