PhotographWorks

Q: How do you get a shallow depth of field?

Use a lens with a very wide aperture. And remember that a bigger sensor format will achieve shallower depth of field, whereas the same settings in a smaller format will not have as shallow depth of field.

Q: What is the magic and why would I want shallow depth of field?

Shallow depth of field makes the foreground subject stand out from the background. it works best when the foreground is further from rather than near to the background. And it works best when the subject is near to the camera. If the subject is far from the background but both the subject and the background are far away from the camera then they will merge together and the foreground will not stand out from the background. So shallow depth of field works best when the subject is near the camera.

Q: Help me choose?

Well, f1.8 is considered to be a standard wide aperture. Anything wider than that is where the magic starts. So f1.4 is gong to give good separation between the foreground and the background in an image. A wide aperture means the hole in the lens is big compared to other f stops where the hole is smaller. And the bigger the hole the more shallow the depth of field possible. That is why bigger formats such as full frame or medium format give more shallow depth of field than smaller formats such as APS-C or Micro Four Thirds.

Q: Any downsides?

Yes, you might miss focus completely when the depth of field is very shallow. It is easy to find you focused on a nose or a cheek and not on the eye, for example. Also, wider aperture lenses are more expensive and heavier. They are more expensive to make because the lens element at the front of the lens has to be big enough to open up wide.

And glass is heavy, so wide lenses weigh more.

It’s not necessary to chase super wide apertures on long focal length lenses because they are going to be used at greater distances and longer focal lengths compress the distance between foreground and background. So there is no advantage is chasing something that isn’t going to show.

A long lens with a widest aperture of f4 is as good as anyone needs. Actually, if you are shooting in poor light – maybe wildlife in the early morning or in a wood – then a bigger aperture of say f2.8 is better. But that is for light gathering rather than for separating foreground from background.

Q: Some numbers?

OK. Let’s clear one thing up and get it out of the way. A 50mm lens on a full frame camera and a 35mm lens on APS-C will have the same field of view. So to compare like with like means comparing these two focal lengths.

A camera with an APS-C sensor and a 35mm f1.4 lens and a subject 1.5m away has a depth of field of 10 cm. A full frame camera with a 50mm f1.4 lens and a subject 1.5m away has a depth of field of 7 cm.

A camera with an APS-C sensor with a 35mm f1.4 lens and a subject 2.5m away has a depth of field of 29 cm. A full frame camera with a 50mm f1.4 lens and a subject 2.5m away has a depth of field of 21 cm.

A camera with an APS-C sensor with a 35mm f1.4 lens and a subject 3.5m away has a depth of field of 56 cm. A full frame camera with a 50mm f1.4 lens and a subject 3.5m away has a depth of field of 41 cm.

From these numbers we see that depth of field increases the further away the subject is from the camera and we also see that the difference between full frame and APS-C narrows the further way the subject is.

The street musicians were at a Winter Fair in Cambridge. Two of the men are holding Dino Baffetti melodions or squeezeboxes.

The two versions of the photograph illustrate how post processing can affect depth of field,

Depth of field is the distance between the nearest and the farthest objects that are in focus in a photograph.

And depth of field depends on the circle of confusion, which is a way of describing what the eye can and cannot see.

An image might not be perfectly sharp when viewed close up with a magnifying glass, but at a normal viewing distance the eye can only make out blur when that blur is big enough to be apparent at that normal viewing distance.

One thing that affects how sharp something looks is how much experience a person has at looking at photographs. Once the eye becomes more practised, small difference in sharpness become more obvious.

That said, if one man has perfect vision while another is older, with cataracts forming and wears glasses, then what is sharp to one will be less sharp to the other.

In other words, the circle of confusion is not an exact science, and one man’s blur will be another man’s ‘sharp enough’.

Whatever the circle of confusion is agreed at, it defines the limits of the depth of field.

And depth of field for a given focal length and subject-to-camera distance varies with the camera format.

There are. many formats and I am going to just look at two – the Nikon full frame (24x36mm), and APS-C (23,5×15.6mm) sensors. Each linear dimension of the APS-C is two thirds the length of the full frame. And it is linear dimension, and not the overall area that determines what seems sharp.

When comparing full-frame and APS-C sensors, for a given focal length and distance from camera to subject, the smaller sensor captures a narrower field of view, and that affects depth of field.

For example, a 50mm lens on an APS-C sensor at a given aperture gives a similar field of view as a 75mm lens on a full-frame camera. But the image from the APS-C sensor will have a greater depth of field because the smaller sensor increases the apparent sharpness because the circle of confusion will be smaller.

Which is best depends on what effect you want. If you are shooting portraits then a larger sensor will enable a shallower depth of field, and for landscapes a smaller sensor will give greater depth of field..

So if you want the foreground to ‘pop’ as it is called, to appear separated from the background, then use the largest aperture and the biggest sensor.

And that is where post processing in Lightroom comes in because it can do that after the photo is taken. So a careful use of background blur preset in Lightroom can give APS-C sensors the best of both worlds.