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Depth of field Determinants

A pink sunray (everlasting) - with shallow depth of field - King's Park, Perth, Western Australia

What determines depth of field (DOF)?

How can depth of field (DOF), be controlled while shooting in the field or studio?

Though there are many resources on the web listing and explaining the factors that control DOF, most people struggle to control depth of field the moment they run out of “Aperture options,”.

What I’m trying to do here in this article is to give the reader a few practical tips as to how the various factors that determine the depth of field could be controlled in the field to achieve the desired DOF ‘look’.

During the next few paragraphs, I will try to explain some of the concepts behind the depth of field. If you are not the nerdy type, or not bothered, please feel free to jump straight to the practical tips.

Let’s start by reminding ourselves something that is very important; a fact: A photograph is made for humans, and whenever we talk about the visual impact of a photograph, we talk about how an image is assessed by the human eye – not by machines or mathematical formulae that forget the biology and the physics of human vision.

Everything related to depth of field starts with the resolution power of the human eye!

Resolution power can be defined as the minimum distance between two points that allows the eye (or an optical instrument) to clearly identify those two points as two distinctly separate points.
While this minimal distance is quite important, what really happens is two light rays coming (or get reflected) from those two points get identified as two light rays because they form the minimal required angle for the normal human eye to identify them as two different rays. This angle, which is about one arc minute (one sixtieth of a degree), is defined as the angular resolution of the human eye.

Two points kept 0.1mm (one tenth of a millimetre) apart at 25 cm away from a normal eye can form this critical angle of 1 arc second and hence can appear as two distinct points.
If the person looking at the two points of our example and then takes his or her head back just a few centimetres the angle between the rays that pass through the pupil of the eye drops below the critical angle and the two points start to appear as a single point.
In other words 0.1mm or smaller circle at a 25 cm distance appears to us as a dot while anything above 0.1mm at a 25cm distance will appear as a small disc/circle.
This is the reason why we see stars that are thousands of kilometres wide as mere dots in the sky.

What has this got to do with depth of field?

Well, everything!…here’s the reason;
When you focus on your subject and take a picture, everything on either side of the plane of focus gets slightly out of focus and appear blurry; but if two rays coming from a slightly out of focus point on the final print (or the monitor) can create an angle of one arc minute or less, on the eye, that point would still appear as a point even though it is not on the plane of focus.

Therefore, achieving acceptable depth of field is all about making sure that, a point placed away from the plane of focus still appears as a point on the final print, (or on the monitor) at the intended viewing distance.

In other words, this simply means, acceptable depth is all about how much an out of focus point object is magnified on the final presented image, and whether we still identify it as a point or something bigger than a point, i.e., a little disk, instead of a point.

Here is one important point that seems to have confused many people who have written articles about depth of field on the web: Depth of field is always dependent on the size of the final image (i.e., magnification)

Say you set your camera lens and the distance to have a particular depth of field on an 8”x10” print.
What does this mean?
A point object that was not on the plane of focus would fall within the critical angle of one arc minute or less when the final 8”x10” print is viewed at the intended distance. Or the point object would be less than 0.1mm, if the intended viewing distance was 25cm.
If someone were to enlarge this image to 16”x 20” our “point” would become twice as big, and would not appear as a point if viewed at the previous distance. For the larger image to have the same depth, the viewer must move back to fulfil the critical angle of one arc minute, or we would have to produce a different image of the same subject with a different camera, lens and distance settings to make our point appear as a point on the larger image.

(A great place to start for anyone who wants to go a bit deep would be Introduction to Optics by Frank A. Jenkins and Harvey E. White)

Milkmaids - Gidgegannup, Western Australia
Milkmaids – Gidgegannup, Western Australia

So what determines depth of field?

 

1. Aperture

Everyone knows how to control this and therefore I’m not going to discuss this any further.

 

2. Subject Distance

This is probably the most underused depth of field determinant and therefore I will discuss how to use the subject to image plane distance to get the look and the feel one is after.

If the aperture, focal length, and the camera (sensor size) remain the same the effective depth of field increases with distance. So, if you want to more depth of field you have to move back from your subject with your camera and lens or move in, to have a narrower DOF!
Easy, isn’t it?

Now, what if you want to place someone or something against a background that you want to make blurry while keeping the subject sharp?
Get close to your subject as much as possible and separate the background from the subject as much as you can.
In other words, keep a lot of distance between the background and the subject and only a little distance between the subject and the camera!

Most beginners place their subjects very close to backgrounds even when they try to create blurry backgrounds.
Placing the background close to the subject maximises the chances of that background staying within the acceptable depth of field, which is the thing to do to create a sharp background.
To make it blurry, background must be placed away from the subject while you move close to the subject to minimise the DOF.

 

3. Focal Length

This could be a can of worms!
Does Focal length determine the depth of field?
Yes, absolutely – it is simple physics; focal length changes the magnification and therefore the DOF.

A hypothesis that states the focal length does not change depth of field is getting popularised on the web at the moment of writing this article. A couple of highly reputable websites I personally visit have tried to prove this point by capturing images with different focal lengths and tied to prove the assumption “if the image size remains the same, then at any given aperture all lenses will give the same depth of field”.

While I really respect those authors, they have made basic errors by changing the subject to camera distance in order to keep the subject size constant. They have conveniently disregarded the all the basic formulae in geometric optics and the fact that focal length does change the magnification.

Lenses with longer focal lengths magnify more than the shorter focal lengths. Therefore, they also magnify, “out of focus points” away from the plane of focus more than shorter focal lengths, making them much larger on the final image.

Here are few numbers for sceptics, using a very accurate DOF calculator. Feel free to do it yourself.

Ex 1:
Camera/Senor: 36mm x 24mm
Assumed viewing distance 25cm
Focal Length 50mm
Aperture: f/8
Distance to the subject: 10m
DOF : 24.13m – Everything from 6m to 30.12m is going to be in acceptable focus

Ex 2:
Camera/Senor: 36mm x 24mm
Assumed viewing distance 25cm
Focal Length 100mm
Aperture: f/8
Distance to the subject: 10m
DOF : 3.42m – Everything from 8.58m to 11.99m is going to be in acceptable focus

Note only the Focal length was changed in the example 2.

So if you want a narrow or less depth of field, increase the focal length, meaning use a longer lens; and if you want more DOF use a wider lens.

Here’s an image shot at 200mm

200mm-DOF-Visible-Range

Same scene at 70mm, only the zoom ring was rotated while keeping the aperture, camera to subject distance and the camera unchanged. Cropped to give it the similar dimensions as the above image. (Compare the sharpness of trees and the grass between the bench and the trees).

 

70mm-DOF-Visible-Range

This is bit of cheating – Enlargement of the second image contributes to the apparent reduction of DOF.

 

Here are the same two images without cropping.

70mm – no cropping

 

200mm – no cropping

 

4. Sensor size

Larger sensors provide shorter or less DOF and smaller sensors provide more DOF for a given aperture, subject distance, and focal length, simply because smaller sensors create a smaller blur circle, or a Circle of Confusion (CoC), provided both final images would be enlarged to the exact same size.

So, if you want to create a creamy background (beautiful bokeh) for your subject;
shoot wide open, get close to the subject to keep it sharp and increase the distance between the subject and the background, use a longer focal length and a larger sensor body.
Do the exact opposite if you want front to back sharpness; (and to take it a step further, learn how to set your lens to Hyperfocal Distance).

 

Please remember that you have four independently controllable, (provided you have two cameras with different sensor sizes and more than a single prime lens) variables to achieve the DOF look you are after.

 

  1. Aperture
  2. Your Feet, to move forward or backwards to change the camera to subject distance
  3. Different Focal Lengths
  4. Sensor Size

 

Learn to use all of them without depending only on aperture to control the depth of field.

 

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10 Responses

  1. One question. You quote a website of saying “If the image size remains the same, then at any given aperture all lenses will give the same depth of field”.

    In your example you use two different focal lengths and crop the 70mm photo to be the same field of view as the 200mm. Yes, the DOF will be a different.

    What if you shoot at 200mm and then change the zoom to 70mm and move closer to your subject to get the same field of view, without cropping? Would the DOF be equal?

    1. What if you shoot at 200mm and then change the zoom to 70mm and move closer to your subject to get the same field of view, without cropping? Would the DOF be equal?

      No it wouldn’t be, that was the incorrect assumption made by the very reputable website.

      Changing the focal length changes the magnification, and changing the distance changes the magnification again. Assuming that the change in magnification due to focal length change was going to get nullified by the magnification change due to change in distance, simply because field of view was kept the “same” was the basic error made in the assumption I have quoted.
      Try it yourself and see, or use any of the decent depth of field calculators available online to find out the answer easy way. Let me know if you really want to get in to the mathematics of it, I will send the titles of few books I really like.

  2. Thank you sir, this one i read earlier. I think year or so ago. just now i read it again and it solved another problem. mobile Phone macro vs DSLR Macro considering DOF.
    I was thinking too hard to solve it and now this says the point i have missed. Thank you Again

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