Every so often, I find myself having to explain Depth of Field to my other half. She is trying to understand the relationship between shutter speed, aperture DoF.

This is how I explain it, and this is also my own internal mental picture. I think in allegories and metaphors at times.

Depth of field is how much of an image is in focus in front of and behind of the focal point of the lens. The amount of clarity is governed by the speed at which light “pours” onto the film (or sensor). More time = bigger depth of field, so more things are in focus. [cue blank look]

Imagine light is a spoon of honey (or treacle - doesn’t matter). You have a spoon of honey amount that will make the picture work.

You are going to imagine pouring the honey onto some paper to make an image. When all the honey has left the spoon and hit the paper, your picture has been taken. We are going to pour the honey through a board with a hole in it. This hole is the aperture for the camera.

If the hole is big, the honey will go through quickly and will make a splat. Not all the paper has been covered because all the honey hit it in one go. So you get a small depth of field.

If the hole is small, the honey will take a long time to pour through it, and as a result will spread out more and cover more of the paper. The result - more of the image will be in focus because the light has taken.

In effect, pouring honey through a hole seems to be an image that works more than the actual science behind what is happening. I am trying to imagine a way of illustrating the relationship between time, light and the image.

I wasn't aware that time had anything to do with depth of field.

Focal length and aperture affect depth of field- wide angle lenses and small apertures will give a greater depth of field, whereas telephoto lenses and wide apertures give far less depth of field.

I think that I am also right in saying that how near or far away the subject is has an effect because the depth of field is something like from 1/3 the distance from the subject toward the camera to 2/3 the distance beyond the subject. It's been a while since I read up on depth of field, but I don't ever recall time being a factor.

Think about the cone of light formed between the aperture and the sensor. The narrower the aperture, the thinner the cone. An object that's out of focus will have the apex of the cone just in front or just behind the sensor, leaving a blurred circle on the sensor instead. The diameter of this circle determines how out-of-focus the object is. With a narrow aperture, you'd have to go further away from the focal plane to increase the diameter of this circle a significant amount than you would with a wider aperture.

Say, for example's sake, that an object only appears out of focus if the circle is more than 0.1mm wide on the sensor. If we focus on an object 10m away at f/22, you might have to defocus to 11m before the focal point widens to a circle of this size. If however you're on f/11, you may only have to defocus to 10.5m, reducing the depth of field.

Does that make sense?

All correct, tk!

*takes a bow*

Actually, after referring to my photography book, I realised that I had one thing slightly wrong, although it may have appeared correct to the reader.

If the depth of field was 1m and if the object at point B below was the point of focus, an object at point A, 33cm closer to the camera would be in focus, and an object at point C 66cm further away would be in focus. The depth of field projects 2/3 beyond the subject and only 1/3 in front regardless of distance from the lens, which is where I was incorrect.

A.................B..................................C
----------------------------1m------------------------

What I was suggesting in my original post was actually based on a different grasp of the subject, because I hadn't read up on it for a while. What I was suggesting was that this depth of field business was based on the distance of the subject from the lens, which is, as far as I can see, not correct.

Not directly, but time is one of the variables. You need a certain amount of light to expose an image, and how that light hits the sensor or film is controlled by the hole it passes through and the time it is allowed to do that.

What I am trying to do is explain how it works in a way that someone who doesn’t follow this fully can understand. If I get technical, I lose the audience.

Mind you, this is someone who understands the control dial on the washing machine.