DOF – Depth of Field Explained.

Depth of field (DOF) can be a confusing concept when you first start out in photography but it is also one of the most important things to understand and can dramatically change the look of your images.

DOF in simple terms.

The most basic (and accurate) way to think about ‘depth of field’ is how much is in focus. The title image of this post demonstrates this well.

All three images are taken from the same distance, with the same focus point, the only difference if the aperture.

Focus.

We need to go back a little fist so that this all makes sense.

Any optical device has to focus. If you’ve ever used a telescope, binoculars, a camera or even magnifying glass you know there is a point of focus.

Using binoculars you will roll the little wheel in the middle to get whatever you are looking at in focus. If you then look at something closer or further away you will need to adjust that wheel again to select a different focus point.

This is even true with our eyes. We don’t notice it so much because our eyes can change focus so quickly.

All good so far?

Let’s move on to Depth of Field.

Ok, now you have that clear in your mind. The focal point it the distance that is at the optimum focus………

Depth of field describes how far in front of that point and how far behind that point is still in ‘acceptable’ focus. As you look at objects closer or further away from that point (or plane) they gradually become less focused.

In some cases, it happens quite quickly and others it is a very gradual fall off. Photographers use this effect to bring attention to a specific part of an image by making only a small part in focus or to tell a bigger story by making everything in focus.

The difference between portrait photography and landscape photography is a really good example of this.

With a portrait you might want to have just your subject in focus and remove any background distractions, this makes it clear to the person looking at the image exactly what the photographer wants you to see.

In landscape photography, you might want the viewer to see every last detail in the shot and tell the complete story of the entire scene. Like everything in photography, this is often done completely opposite for creative reasons but that’s an article for another day.

How to effect Depth of Field.

There are a few ways to affect the depth of field but the two main ways a photographer will do this is lens aperture and relative distance.

Aperture.

Aperture or f stop, This is part of your exposure triangle (see this article). Your lens has a few specifications one is the maximum aperture, and just to confuse matters the smaller the number the larger the aperture.

This is because the aperture is a calculation between the focal length and the diameter of the glass, that’s for another discussion. For now, think about it this way. The lens opening is controlled by blades (see diagram below). The widest aperture has the biggest opening. as the blades close the opening becomes smaller and the aperture/f stop number gets bigger.

In this example, we’ll look at one of my favourite lenses the 50mm f1.4.

The verticle blue line is the focus, the red horizontal lines shows how far in front or behind will be in focus.

Depth of Field Explained

This is nothing like to scale but you start to get the idea. The wider the aperture (lower f-number) the less is in focus in front and behind the subject. A narrower aperture (higher f-number) will have more in focus in front and behind your subject.

If you take a picture of somebody’s face with something like the 50mm f1.4 and you fill the frame with their head, you will be about 50cm away from them on a full-frame camera. That would give you a depth of field of less than a centimetre. So if you focus on the eye, you would see the nose going out of focus pretty quickly and if they are not straight on you will certainly have the other eye out of focus.

At f8 you should have the face all in focus but the focus will be getting quite soft by the time you get to the ears. Remember this example is at about half a meter, that changes as your distance from the subject changes.

Depth of Field Explained

This is a good simple way to look at depth of field in terms of aperture. The first image (left) is taken at f1.4. The second (middle) image is shot at f5.6. The last (right) image was taken at f22.

Personally I like the middle photograph, The DOF is enough that you can make out the person behind but it is still clear that the focus is on the coin.

The first is a little too abstract for me and the last a little too clear, but that is the beauty of photography, we all like different things. I would probably like it better at f4 but I wanted to demonstrate even (ish) transformations.

Relative Distance.

So the closer you are to your subject, the shallower the depth of field will be. So, of course, the further away you are the larger it will be. The same 50mm lens at f1.4 but at 5m from your subject will give you a depth of field of around 85cm.

As we said before at 0.5m away that depth of field is more like 1cm and at 10m your DOF is around 2.5m. I’ll cover that more in a DOF table below.

This is where the relative distance starts to become very relevant. There are 3 key elements in the photograph in this scenario. The camera, the subject and the background.

For this example, we’re not going to do as Arthur Fellig said, “f8 and be there”. Let’s try f5.6 because that’s where a many kit lens will be. The camera stays in the same position, let’s say 10m from our background, a colourful graffitied wall. Let’s make the model do all the work for this portrait.

First – We start with our model leaning on the wall, at 10m it’s a good full-body portrait, we have everything in good sharp focus. The model is nice and clear, the graffiti wall is perfectly crisp and clean.

In fact, anything a little past halfway is probably in fairly good focus.

f5.6 @ 10m you have around 24m DOF (4m in front & 20m Behind).

Second – You bring them halfway to the 5m mark, surprisingly here the background is still almost in focus, it’s just starting to soften up, that’ll make more sense when we look at the shape of DOF later.

So bring them another metre closer. at 4m from you and 6m from the background, you are starting to get some separation from the background as the graffiti starts to blur, you can still tell what is behind but it’s subtle enough to draw your focus onto your model. You now have a nice torso shot.

f5.6 @ 4m you have around 2.3m DOF (0.85m in front & 1.45m Behind).

Third – Bring them to about a metre from the camera. Here you are getting a little more than a headshot but importantly your depth of field is down to about 20cm. This means that the wall is now becoming a blur of colour and your picture is clearly focused on the model.

f5.6 @ 1m you have around 13cm DOF (6cm in front & 7cm Behind).

Depth of field Relative Distance

The depth of field in the 10m example extends about 20m behind the subject but rather than having a picture that extends off the screen I have cut it off just behind the tree.

You can see by the red lines how the depth of field changes with relevant distance and how you can use this to include or exclude the background.

How this helps the average jo.

We all like beautiful effect that “pro’s” get from their expensive lenses but if you use relative distance you can get something similar with any kit lens, it just takes some planning and a little more work.

The standard canon kit lens is an 18-55 f3.5-5.6. A headshot taken from 2m at 55m f5.6 with a background 15-20m away will give a really soft bokeh effect behind the subject. If we can do it at f8 and 10m, f5.6 at 20m will be completely out of focus.

That said I have always and probably will continue to recommend the 50mm f1.8 to anybody who wants to get better images without breaking the bank. Find it at Amazon

If you want something a little nicer check out the f1.4 here

DOF - Depth of Field Explained.

The shape of Depth of Field.

Not everything is made even and that includes your DOF. There are a few things that alter this but as a general rule of thumb whatever your depth of field is that will be split one-third in front of and two thirds behind the focal point. The closer you are to the subject the closer to 50/50 that gets. Once you get out around ‘hyperfocal point’ the rules change a little again and in theory, you achieve infinity focal behind the subject.

As a rough and ready guide – Really close up think 50/50 and out at a little more think in 1/3rds.

Depth of Field Table.

A handy guide based on a 50mm lens on a full-frame camera. The two figures are for the distance in front and the distance behind that will be in focus.

For example f5.6 @ 3m = 50/70cm or 50cm in front and 75cm behind.

1m2m3m5m10m20m
f1.42/2cm6/7cm14/16cm39/46cm1.4/2m5/15m
f2.83/3cm12/14cm27/33cm0.7/1m2.5/5m8/42m
f5.66/7cm23/31cm50/75cm1.25/2.5m4/20m11m/∞
f1111/15cm40/70cm0.9/2m2/10m5.5m/∞14.5m/∞
f2220/35cm70cm/2.2m1.3/12m2.9m/∞7.3m/∞17m/∞

Whatever smartphone ecosystem you have there will be a depth of field calculator app. With that, you will be able to calculate every variable for just about every camera and lens combination out there.

Macro Lenses.

This is a bit of an additional. Once you start getting into macro lenses where you can focus very close to the subject the depth of field gets ridiculously narrow. Even at f5.6, you can be at just a few millimetres of focus. This can give you some super wild effects and brings a new bunch of challenges to the table but we’ll cover that another time.


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