The Truth About Depth of Field and What Affects It

Depth of field is a frequently misunderstood subject, particularly when it comes to what actually affects it and how it changes based on your technique. This awesome video will show you everything you need to know about the physics behind depth of field.

Coming to you from Gerard Undone, this great video will teach you all about depth of field and what actually affects it, while showing you an easy way to think about the physics behind it. There's quite a lot of misinformation out there regarding how depth of field works and what actually affects it, and having a working understanding of it is crucial to operating as a photographer and employing your technical knowledge in a way to helps you achieve the artistic results you envision. In the video, you'll hear him talk about the entrance pupil. This is the image of the aperture as seen from the front of lens (also known as the effective aperture); you can think about it as the aperture after being affected by the magnification of the lens elements in front of it. It's a great video that should really help you better understand a commonly misunderstood subject; check it out! 

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12 Comments

Matthias Kirk's picture

Nice presentation. A shame he got his physics wrong.

Maximilian Sulzer's picture

What did he get wrong?

Matthias Kirk's picture

Honestly, I do not know where to begin, the whole thing is pretty incoherent. The biggest mistake is the statement that there is no direct correlation between DOF and focal length. If looking into a physics book isn't your thing, you can easily check it out with the help of the DOF simulator.

Maximilian Sulzer's picture

I'm genuinely asking about the exact point and the correct physics behind it.

If i check the DOF simulator and i set a fixed field of view and a fix aperture then the DOF is the same at any focal length.
at f/2
DOF@ Focal length 35mm, Subject distance 1,55 m = 14,5 cm
DOF@ Focal length 150mm, Subject distance 6,64 m = 14,5 cm

"...and i set a fixed field of view and a fix aperture then the DOF is the same at any focal length".

Key words: "fixed field of view". This is always the example people try to use when they say focal length doesn't affect depth of field. But whenever you maintain a "fixed field of view", you're adjusting 2 different variables to do this... focal length AND distance to subject. It's just that when you alter both of them at the same time in relation to your subject, they're effectively canceling each other out.

Think about your example:

DOF@ Focal length 35mm, Subject distance 1,55 m = 14,5 cm
DOF@ Focal length 150mm, Subject distance 6,64 m = 14,5 cm

If focal length truly doesn't matter then you should have different results in this scenario, not the same. If you're saying distance to subject is the only thing that matters then 1.5m vs 6.6m would show different results. If the results ARE the same, there must be another variable which is canceling out distance to subject in order to achieve the same picture.

I also think the definition of focal length is incorrect, as it's to do with the distance between where the rays converge in the lens and the sensor (when focused at infinity)... not the distance between where light enters the lens and where the rays converge.

Not saying he's totally wrong about everything he says, as there is some truth mixed in here, but I think this is a pretty common example of where people go wrong when trying to exclude focal length as a variable. "Fixed field of view" is a caveat that's required for this explanation to seem accurate, but I think it's used to draw a false conclusion.

Kirk Darling's picture

"I also think the definition of focal length is incorrect, as it's to do with the distance between where the rays converge in the lens and the sensor (when focused at infinity)... not the distance between where light enters the lens and where the rays converge."

You're right that he's wrong. He would be correct with the very simplest lens formulations (such as from 100 years ago) but not with modern retrofocus wide angle and telephoto (as opposed to simple "long focus") lenses, which may be physically much longer or shorter than the focal length indicates and in which the point of convergence may not even be within the series of lens groups.

Maximilian Sulzer's picture

Okay you're right that there are two variables and each has an effect (3 if you include the sensor in the same way).
That's how he explains it too. He doesn't say it doesn't matter, he says for a given subject (field of view) Sensor size and Focal length are not the deciding factor? That's how i understood it.

The baseline (for me) is, what would be the point in not containing the same composition.
Let's say you're shooting a portrait. Then switching from 35mm to 150mm sure gives you smaller DOF if you don't move. But then it will be a close up of the persons nose.

"That's how he explains it too."

Not exactly. He says that it's a misconception that focal length affects depth of field and then sets about using a very specific example (fixed field of view) that, on the surface, appears to support his statement. The problem is, "focal length does not affect depth of field" is not the same thing as "focal length, when used to maintain a fixed field of view, is offset by distance to subject".

"The baseline (for me) is, what would be the point in not containing the same composition."

The point is that "the point" is irrelevant . A use case doesn't change how something works, it merely describes certain parameters that, when taken together, produce a certain result. That is not the same thing as defining how an individual component works independently of that use case.

Matthias Kirk's picture

That result is indeed correct. But it has nothing to do with the claim that the only factors that directly affect DOF are pupil size and distance. You had to change distance, diameter AND focal length to get to those results.

The problem with his explanation seems to be that he does not understand what DOF actually is and that magnification is an very important part of the definition. His definition of DOF is "how easily rays converge" and that isn't at all the definition of depth of field that is used in optics.

Depth of field is the distance about the plane of focus where objects appear acceptably sharp in an IMAGE. That is not at all what his explanations and oversimplified graphics demonstrate. They show how precisely the REAL WORLD can be resolved at a given distance at a given aperture and that is something completely different. For example if you can resolve 10 cm in 100m distance it will appear sharp in an wide angle shot and blurry with a telephoto lens because of the different magnification.

This false definition leads him to false claims like "...sensor size has no impact on your depth of field...". This is easily disproven by doing the math or by changing the sensor size in the DOF Simulator (you will find that your DOF is divided by half if you change the sensor from FF to MFT leaving everything else as it is).

Another false claim you can check for yourself is that "DOF only depends on pupil size and subject distance".

A good approximation for moderate to large distances is given by the formula

DOF=2*N*c*f^2*s^2 / (f^4 - N*c^2*s^2)

With the f-number N, the focal length f, the circle of confusion c and the subject distance s.

You can substitute the f-number N=f/D to introduce a fixed pupil size D and still get a function that is dependent on s,D AND f (and c but that isn't important for this point to make).
If math isn't your thing plug the numbers into the DOF Simulator:

For example 100mm f4 and 50mm f2 have the same pupil size of 25mm, right? The approximation given by the above formula says that DOF should be roughly reciprocal to the focal length with all other parameters remaining fixed. Let's check for 5 meters in the DOF Simulator, shall we?

5m
50mm f/2 -> DOF=91.7cm
100mm f/4 -> DOF=45.1cm

No surprise there.

I could go on. But I think I made my point here.

Hope this helps.

R. P.'s picture

He is right if one takes the same composition as a starting point. Then and only then the entrance pupil and distance2subject are the two factors that influence dof.

Matthias Kirk's picture

I am not convinced that even that is correct. You have to change focal length and distance to get the same composition and then change the pupil size to get the same DOF. How does it follow from that that the focal length does not affect the DOF? Because the f-number stays the same? By that logic you can just as easily make the claim that pupil size does not affect the DOF.

Kirk Darling's picture

This guy--as most these days--also excludes the final factor that changes depth of field: The magnification of the final image display.

The magnification of the final image display always increases the amount of visually perceived blur (that is, reduces the amount of visually perceived sharpness).

We've all seen the cases where an image looked sharp on the LCD screen, but was actually blurry when enlarged.

That phenomenon is always at work. Sharpness always decreases with enlargement, which means that areas that start out blurred are perceived as even more blurred with enlargement, if the viewing distance remains the same.

DoF tables are always designed with a magnification and viewing distance as part of the algorithm, usually enlargement to 8x10 viewed at reading distance.

DoF actually decreases from what's given in the DoF table if a greater enlargement is intended without a corresponding increase of viewing distance.