MARCUS MCADAM

PHOTOGRAPHY

Elgol Skye Depth of Field

If there is one element of photography which is misunderstood, then it has to be depth of field. Thinking you understand something, but misunderstanding it, is worse than not understanding it all at, as you end up using settings and techniques which are probably doing the opposite to your intentions.

In this first of my three part tutorial on depth of field we will look at what it actually is and what influences it. Put simply, depth of field is the “area of acceptable sharpness” in an image. The worrying word here is “acceptable” as what is acceptable to one person, may not be acceptable to another. It also fails to take into consideration the degree of enlargement, and the distance the image will be viewed from - both essential when deciding the required depth of field. The greater the enlargement, the more obvious any soft areas will become, and the closer the image is viewed, the more scrutiny is possible of any areas which are even slightly out of focus. Add to this the fact that the calibration of the depth of field system was created in the days of 35mm film, which is hardly appropriate for today’s super high resolution DSLRs.

Ask photographers what depth of field is, and the most popular answer will be that it is “the area of an image which is sharply in focus”. This is actually incorrect, as it is only possible to bring a single plane into sharp focus. On a standard lens (one which cannot be tilted) this plane is always parallel to the sensor in the camera. A specialised tilt lens allows this plane to be tilted, making it possible to sharply focus on a nearby subject at the bottom of the frame and a distant subject at the top of the frame, but there is still only a single plane of sharp focus through the scene. We will look at tilt lenses in part 3 of this series on depth of field. Regardless of lens used, everything on a different plane to the one which has been focused will always be out of focus, but to varying degrees. The further an object is from the plane of focus, the less sharp it will be rendered.

When we adjust the aperture of our lens, we are not changing the area which is sharply focused - we are simply altering how out of focus the out of focus areas appear. It is important to realise that no matter what settings we use, anything not precisely on the plane of focus will never be truly sharp. Everything not on the plane of focus appears as a fuzzy circle rather than a sharp point of light. These fuzzy circles are known as circles of confusion - the more fuzzy they are, the more out of focus they appear. As we change the aperture of a lens, these circles of confusion adjust with the size of the aperture, which is why at smaller apertures, more of the image appears sharper. A small circle of confusion may appear to be sharp when examined on the camera’s screen or at low magnifications, but on closer inspection on a larger screen it will become apparent that such areas are not truly sharp.

COC Large

In the above diagram, we see the lens and the sensor. The converging green lines between the lens and the sensor represent a cone of light from an object at infinity. As you can see this cone is currently being focused to a fine point in front of the sensor. The red lines represent an object in the far foreground, and its cone is currently being focused to a fine point onto the sensor. The blue lines represent an object close to the lens and this cone is currently being focused to a fine point behind the sensor. If we were to look at these cones end on (as the sensor sees them) then as we move away from the fine point of the cone (in either direction) the point of light becomes bigger and softer.

The three coloured circles below the graph show (crudely) how each is perceived by the sensor. The green object is very out of focus, the red object is sharp, and the blue object is reasonably out of focus.

As we focus a lens, it moves closer or further away from the camera’s sensor to bring different distances into sharp focus. If you focus a lens to infinity, it moves closer to the sensor (ie - it would align the point where the green cone comes to a fine point with the sensor). If we then focus onto closer objects the lens moves away from the sensor (ie - it would align the point where the blue cone comes to a fine point with the sensor).

Let’s assume we are focused as the diagram suggests - on the middle distance red subject. As we can see, objects further away (such as infinity) are appearing as large circles of confusion, and objects closer are also appearing as circles of confusion. If we now adjust the aperture to a small value of f/16, this has the affect of making all the cones (and therefore the circles of confusion) smaller.

COC Small

The result is that they look like they are not as out of focus as they were at the larger aperture. When they get small enough, circles of confusion can appear to be fine points of light, but the key thing to understand is that they are still out of focus. It doesn’t matter how small our aperture is, we will never be able to make these circles of confusion become finely focused points of light. So reducing the aperture size gives the impression that more of our scene is being sharply focused, but in fact this is not the case. Enlarge the image enough and you will soon see the out of focus areas are still out of focus.

 
Once we accept that depth of field is subjective, there are three elements which influence it….

1 - Aperture
2 - Focal length
3 - Distance to plane of focus.

There are some simple laws which help us determine which of the above can assist us when trying to control depth of field. Pay attention for this bit as it can be a little daunting...

Firstly we have aperture, which if we double its value (changing it by 2 stops) then we double the depth of field. For example, if we start with f/8 and we double the value to f/16 (an adjustment of 2 stops), we double our depth of field. If however we halve the aperture value (going 2 stops the other way to f/4) then we halve the depth of field. Whatever aperture value you have, if you alter it by 2 stops, the number will either double of halve, depending which way you go. Doing so essentially doubles or halves the circles of confusion and in effect doubles the depth of field - easy to remember.

Secondly we have focal length, which if we double it, will reduce our depth of field to 1/4 of our original depth of field. For example, if when using a 50mm lens at f/8 we have a depth of field of 2 metres, when using a 100mm lens at f/8 (focused at the same distance) we will now have a depth of field of 0.5 metres. Halving the focal length increases depth of field by a factor of 2 (doubling twice).

Lastly we have our distance to subject, which if we double it, increases our depth of field by a factor of 2 (doubling twice). For example, if our distance to subject is 5 metres which gives us a depth of field of 1 metre, and we double our distance to 10 metres (keeping focal length and aperture fixed) then we will now have a depth of field of 4 metres.

The more astute among you will have noticed that the last two influences cancel each other out. For example, if we are using a 50mm lens to take a portrait of someone, and we decide that we want to increase the depth of field by doubling our distance to them, we would now need to use a 100mm lens to bring the subject back to the same size in our frame. So by doubling our distance to the subject we increased our depth of field by a factor of 2, but by then doubling the focal length to kep the subject the right size in the frame, we reduced our depth of field by 1/4 and ended up back where we began - bummer! As these two influences cancel each other out, we tend to use aperture to control depth of field, as it has no opposite element to interfere with it.


So if depth of field is the “area of acceptable sharpness”, who says what's acceptable? Well, someone a very long time ago decided that any circle of confusion smaller than 0.05mm on a 35mm film negative was no longer a confusion, but should be accepted as a fine point of light. However, as previously stated, what was acceptable back then is unlikely to be acceptable now, and also if we enlarged such a small circle of confusion enough, we would see that it is not a finely focused point of light.

So when considering depth of field, you first have to ask yourself how big is the image going to be reproduced, and how far is the likely viewing distance going to be? This is the first question I will ask a commercial client (maybe after “how much am I being paid?"). If they want an out of focus but recognisable background, then I will use completely different settings if they tell me the image is for a website than I would if the image was intended for a bus shelter billboard. A web image is never going to be viewed very large, so out of focus areas will need to be very out of focus to have the desired appearance. In the bus shelter billboard situation, the image will be large and people will be able to view it from close up, so out of focus areas will be more obviously soft.

If you are shooting an image for your Facebook page or Instagram account, then it’s easy to give the impression that everything is sharp as any small circles of confusion will appear to be fine focused points. You probably won’t need to use f/16 to get the right depth of field, and f/8 may do the trick. If however you are taking a photo which you want to print large and frame on your wall, then it will be difficult to render everything sharp, as even small circles of confusion will be obvious.

Glencoe depth of field

I shot the above image for online purposes only, so used f/5.6 as I knew it would only ever be seen at small enlargements. Focus is on the cottage, meaning the foreground boulder and background are both out of focus. Even though this is obvious when viewed on a large screen, you cannot clearly tell here. Had I taken it to be sold in my gallery then I would have used something around f/11 to tighten up all the circles of confusion.

If you have one of those depth of field apps on your phone, or a chart in your bag, you can throw them away (although keep the phone, as it may be useful for other things). They are flawed because they won’t ask you the key questions - what’s the enlargement of the print, and how close is it going to be viewed?

In part 2 we will look at where to focus in a scene, something called the hyperlocal technique, and when you will and will not want to use it.

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