Photo Basics – Aperture & Depth of Field

This is going to be the first in a series of articles on the basics of photography. As I speak with newer photographers and check out various internet photo discussion boards, an understanding of the basics of photography (e.g., depth of field, exposure) seems to be missing.

The digital era of photography has ushered in a huge new generation of people interested in photography. And that’s terrific! Many have jumped in without any background in film photography; or perhaps more relevant manual photography, and seem frustrated and disheartened by the results they’re getting. So perhaps this new series of articles will be helpful to some.

Why start with depth of field? Why not start with exposure? I could have started with exposure. The aperture is a key part of DoF and the aperture is a key part of exposure.  Understanding what the aperture does before moving to exposure is, I think, important.  DoF and the concepts surrounding it are applicable to all cameras/lenses.  In order to really take creative advantage of DoF; however, you need a camera that allows you to set the aperture manually.  Fully automated P&S cameras don’t give you that level of control.

I’m going to try to keep the technical side of the discussion to a minimum and focus on practical concerns but there will need to be some technical commentary.

First let’s define DoF.  Depth of field is the appearance of sharpness in an image from front to back.  If we look at a photo we’ll often see parts of it that are sharp and parts that are a bit blurred or softer.  The image below is an example.

Example of Depth of Field (click for larger version)

We see that the chocolate on the left and parts of both slices of orange peel near the chocolate are in focus but the chocolate on the right at the front and the orange peel at the left back are soft and out of focus.  That’s depth of field.

When people talk about depth of field they often talk about it in terms of the image on the film or digital sensor (i.e., at the image plane).  Unfortunately this isn’t the proper way to look at DoF.  Warning – technical aspect coming up. The ‘standard’ for measuring DoF is an 8″x10″ print.  More on that later, but for now just know that there is a difference between DoF at the image plane and in that ‘standard’ 8″x10″ print.

How is DoF created?  Largely by the aperture setting of the lens.  What’s the aperture?  All lenses have an iris inside that opens and closes when the shutter opens and closes.  This iris opening is the lens aperture.  Think of it like the pupil of your eye.  If you look in the mirror and move your eye so more or less light hits they eye your pupil gets smaller or larger in response to the amount of light.  This is similar to the aperture in a lens.  The difference is your pupil works in real time whereas the lens aperture is fixed while the photo is being taken.  So how is DoF created?  Warning – technical issue coming up. DoF is a product of 2 things – magnification and aperture.  Magnification is a product of 2 things – focal length of the lens and distance to subject.  More on this below.

Apertures are measured in f-stops.  These f-stops are denoted by numbers.  These numbers work in an arithmetic progression. Warning – technical issue coming up. This arithmetic progression is based on the square root of 2 (or 1.4 – it’s actually 1.414213…. but for simplicity we use 1.4). Why the square root of 2?  It has to do with the properties of light and the intensity of light as we allow more or less light to shine on our subject or allow more or less light into the lens.  It also has to do with something called the Inverse Square Law.  For purposes of this article, simply know that each 1 full stop change in the aperture equates to double or half the amount of light getting through the lens to the film or sensor.  The progression of f-stops is as follows:  1; 1.4; 2; 2.8; 4; 5.6; 8; 11; 16; 22; 32… You can keep going but for modern DSLR or 35mm film lenses f32 is generally the limit.  The smaller the number, the larger the aperture opening.  So f1.4 is a larger opening than f8 which is a larger opening than f16.  This is important.  The smaller the opening (i.e., larger the number) the greater DoF will be.  Got that?  Small number = larger aperture opening.  Smaller aperture opening = more DoF.  You can see it visually in the diagram on this site.

That’s the aperture part of DoF.  Now let’s look at magnification.

Every focal length magnifies parts of your scene to a different extent.  Shorter focal lengths have less magnification.  Longer focal lengths have higher magnification.  We’re not going to discuss specialised lenses like macro lenses in this article.  We also said that distance to subject affects magnification.  A shorter distance to the subject means higher magnification.  A greater distance to your subject means less magnification.  Can we offset a longer distance (less magnification) with a longer focal length lens (more magnification) and have the same DoF as with a shorter focal length lens at a closer distance?  Yes we can.

We can see then, that the aperture and magnification (focal length & distance to subject) work in combination to give us DoF.  We can see also, that we can adjust one or both factors to increase or decrease DoF to suit our needs.

How does it work in practice?  Let’s take a look.  The series of photos below were shot with the same focal length and the same distance to subject (same magnification) but the aperture was changed to adjust the DoF.

The first image was shot at f.28.

F2.8 (click for larger version)

We plainly see that DoF is very limited.  Only a small part of the stamen of the flower is in focus and everything else is soft.

Moving to f5.6.

F5.6 (click for larger version)

More of the staem of the flower is now in focus and we begin to see a bit more definition in the pink centre part of the flower as well as in the petals.

At f8 this trend continues.

F8 (click for larger version)

Now nearly all of the stamen is in focus and even the branch behind the flower is beginning to show some increased definition.

Moving on to F16

F16 (click for larger version)

At this point much of the flower is now in focus as is the branch behind the flower.  There’s still a little detail left to reveal back there but not a lot.

We get to that detail by f32.

F32 (click for larger version)

The details and veining of the petals on the flower are now clearly visible as is the branch behind the flower.  This has taken us actually too far because looking closely, we can see there’s a bit of noise visible and there’s a bit of distortion visible due to a phenomenon called diffraction.

Clearly we see that smaller aperture openings (larger numbers) lead to increased DoF.  In this case magnifcation was held constant.  You should do your own testing to see how changing aperture and/or magnification changes DoF.  Only by doing this kind of testing will you get comforatble with the concept and be able to use it creatively in your photography.

I said we’d get back to the ‘standard’ for measuring DoF.  One of the biggest myths in photography is the mistaken idea that smaller sensors produce more DoF.  That’s absolutely false.

DSLR cameras come in basically two types.  So-called ‘full frame’ cameras have sensors that are the same size as a frame of 35mm film.  APS-C cameras have a sensor that’s roughly 2/3 the size of a frame of 35mm film.  This smaller sensor in the APS-C cameras means that any lens will have a narrower field of view. Field of view is the amount of a scene that the sensor takes in.  This diagram shows both the relative sensor sizes and the field of view comparison.  This narrower field of view means less of the scene is captured with the same focal length standing in the same place on a camera with a smaller sensor.  The analogy holds for compact cameras as well.  It also holds when comparing film formats of different sizes.

In order to get the same scene in view we have to reduce magnification.  We can do that by using a shorter focal length, by moving further away from our subject or some combination of the two.  What happens when we reduce magnification?  We increase DoF.  The important thing to not here though is that we’re increasing DoF at the image plane. Remember that’s not the ‘standard’ for measuring DoF though.  The ‘standard’ is viewing an 8″x10″ print at a normal viewing distance (about 1 foot).  To take it further, the standard is something called ‘circles of confusion’ of 1/100″ or smaller in that standard print.  What’s a circle of confusion?  The aperture opening in the lens is a circle.  As a result, if you were to look at a photo at high magnification you’d see that the image is made of of millions of dots.  These dots are smallest at the exact point or plane of focus.  As we move away from the point of focus, these dots get larger.  Any part of an image is considered to be in focus or not depending on the size of those dots or circles.   The smaller the aperture opening, the smaller the circles on the film or sensor which is why smaller apertures give greater depth of field.  The consensus is that a dot of 1/100″ or smaller in the standard print from normal viewing distance for someone with normal vision is considered in focus.  How’s that all follow through?  If we make that standard print of the full frame image and the APS-C image, the APS-C image has to be ‘enlarged’ more to get to the same print size.  The APS-C image has to be enlarged more because it’s smaller to start with.  That extra enlargement of the APS-C image will offset the increased DoF at the image plane somewhat but not entirely and in the two ‘standard’ prints the smaller sensored image will show greater depth of field.  But this is solely because of the difference in magnification.  The reduction in the size of the circles of confusion aren’t proportional to the reduction in magnification.  The difference in depth of field is the ratio of the size of the sensors.  So an APS-C camera will have about 1.5 or 1.6 times the depth of field of a full frame camera.  A 4/3 sensor will have about double the depth of field of a full frame camera.

So, in closing, what is all this palaver about depth of field good for?  Well depth of field is a terrific creative tool.  It allows you to isolate a subject or part of a scene from the background to make it stand out.  It also allows you to ensure everything in your image appears sharp.  Isolating a subject can be very useful in closer up photography, which includes macro photography.  On the other hand, in a large landscape vista you’ll generally want most things to appear sharp.  So your choice of focal length and subject distance (for magnification) and aperture setting will help you achieve either of these results.  If we go back to the first image in this article, I purposely used a shallow depth of field to isolate the chocloate on the left and have the rest of the shot more blurred.  I did this by using a longer focal length from a close distance and a large aperture.  In the image below, on the other hand, I wanted good detail throughout so used a smaller aperture, a shorter focal length and from a longer distance to achieve it.

St. Michael's Cathedral Ceiling Detail, Toronto (click for larger version)

So now, go out and practice with your own camera and see how depth of field can work for your creative vision.  As always, feedback is most appreciated.  If you like this article or any others on the site, use the buttons below to share it on any of several social networks or via email.

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