Depth of Field - Useful Rules of Thumb - Tom Axford 20230507

The way that most photographers calculate the depth of field for a particular situation is to use a depth of field calculator such as dofmaster.com. You feed in the type of camera, the focal length, the aperture (f-number) and the subject distance and it gives you the depth of field and the hyperfocal distance. However, it becomes very tedious if you have to recalculate the depth of field every time you change the aperture or zoom the lens or move the camera a bit closer or a bit further away. That is where these simple rules of thumb can be very helpful.

When taking a portrait, suppose you decide you want to use a longer focal length to get a different perspective. You move back to keep your subject the same size in the frame. In these circumstances Rule 1 tells you that the depth of field remains approximately the same.

Rule 1
If the subject remains the same size in the frame, then the depth of field remains approximately the same as you change the focal length and subject distance, but keep the f-number unchanged.

Rule 2
If you keep the focal length and the subject distance fixed and only change the aperture (f-number), then the depth of field is approximately proportional to the f-number.

For example, in going from f/4 to f/8, the depth of field approximately doubles.

Rule 3
If you keep the focal length and the aperture (f-number) fixed and only change the subject distance, then the depth of field changes approximately in proportion to the square of the distance.

For example, for a subject 2 metres from the camera there will be approximately 4 times as much depth of field as for a subject 1 metre from the camera.

Limitations
These rules work for any type of camera, including phone cameras. In other words, they work for any size of sensor, but you must keep to one sensor size. The rules do not work when you switch from one sensor size to another.

In most circumstances these rules work very well. However, they break down if the the subject distance gets close to the hyperfocal distance or greater (when the depth of field is infinite). In practice, the rules work best for subject distances that are less than one third of the hyperfocal distance.

They may also break down if the subject is very close to the camera (for macro photography).

These rules are far too complicated for me to apply in the field, but I'm glad they work for you! :)

David

Hi,

I use the DoF scales on my lenses most of the time. I focus on what I want to be in focus on both the near side and the far side. Then set the aperture so that I get that range on the DoF marks.

I do notice that most auto focus lenses do not have proper DoF scales though. So now one needs to do something different. For the most part, my lenses both manual and auto focus do have DoF scales. It's one of my required items for a lens I buy to have. For the fee which do not have DoF scales, I have enough experience with the ones which do to make a decent aperture choice.

Stan

Too complicated for me. Nothing beats practice and learning through experience what apertures to use for your kit and shooting scenarios. You eventually end up with an unofficial DOF table in your head that’s good enough.

For example, if I’m doing a group portrait with my 35mm prime I know what the largest aperture I can get away with is to keep them all in focus and depending how I’ve framed them. If in doubt I will shoot at my guesstimated aperture then stop down an extra stop and rattle off a few extra frames as well.

Those markings are usually computed for a 30 um circle of confusion or more. Do you derate them?

Agreed. Use the DOF table in your head during shooting. Inform that table with the charts & graphs, as well as experience. Well put BurnImage.

Is anyone using the object-field technique?

Yes, I always think in terms of the object field myself.

However, the rules of thumb that I gave are intended to be helpful as a supplement to using common DoF calculators.

Hi,

Nope. I just use them as I always have from the beginning.

Such as this tractor shot for the featured machine of the year for an antique farm equipment club. A Pre AI Nikkor 85/1.8 on a Nikon Df. I focus on the very front and very back and noted the distance and set the aperture to what would fit between the lines. Kept it simple.

I was happy with it. They were ecstatic.

Stan

Just as important as the aperture etc. while shooting, which you can put into a DoF calculator, is the size of the print and the viewing distance or, combined, the angular size of the print as seen by the viewer.

As an example, take this image which was IIRC part of the launch campaign of the Canon 1Ds Mark III. All three examples are the same image at different sizes of enlargement and if you stay behind your computer at the same distance the only variable will be the 'print size'.

This first image is the entire image, both the ear and the eye seem sharp:

The second is enlarged a bit further, you can see the ear becoming slightly unsharp already:

The third is at 100% (one pixel of the image is displayed by one pixel of the screen) and now you can see that the ear is extremely blurry.

If you have the room, now stand up and move backward while looking at the third image and see the DoF getting deeper while the image gets smaller for you as the viewer.

If you're serious about DoF you'll have to know in advance how the image will be viewed and even what the eyesight of your viewers will be.

AFAIK the markings on the lens was devised in the 1930s for small-sized prints. I guess they work for printing on T-shirts as well :).

You mean something like:
DOF ~= (size of subject)^2 * (f/ number) / 10
?
[for FF, in SI units]

Yep, I chant that to myself every morning. Followed by 3 Hail Marys and Self Flagellation. I follow that up with a hearty breakfast. Then I go out and shoot at f2.8 or f5.6 and nail it. :-)

Object-field technique:
Set your aperture equal to the size of the smallest near objects you’d like resolved, then focus on infinity.
(aperture size: focal length divided by the aperture f-number).

That's one way of doing it. Throws away a fair amount of sharpness. Doesn't consider print size or viewing distance. Assumes detail at infinity is important.

I guess focus bracketing is the only other simple alternative :).

For my personal use I keep a small crib sheet in my wallet that tells me that the DoF for a 80x60cm frame on my MFT cameras is 20cm at f/4 (using a CoC ratio of 2000 instead of the more common 1500).

I use another rule of thumb (Rule 4, if you like) that tells me that the DoF is proportional to the square of the frame size. Then I don't need to resort to DoF calculators at all.

I prefer to use frame size rather than subject distance.