How to estimate DOF without focal length or distance

How to estimate DOF without needing to know the focal length or camera to subject distance

The subject of focus seems to crop up quite a bit. People ask "why isn't my picture sharper?". There are many reasons why a picture might not be as sharp as it could be (camera shake, focusing at the wrong distance, DOF, poor lens, the subject near the edge etc), and the factor that needs to be known is DOF.

Many replies ask "how did you focus" and "what was the DOF?".

Estimating DOF is useful for for two reasons:
1. For assessing someone else's pictures.
2. For deciding what aperture to use when taking a picture.

So far I have only seen a few people mention that DOF is NOT changed by changing the focal length of the lens. Yes, really. I know some people already know that, and won't be surprised. Yes, the range of distances that will be considered "in focus" does not change with focal length. A longer focal length decreases the Angle of View (AOV), and also "enlarges" the background, making the out of focus regions look more out of focus.

So to get a certain image, one will choose a focal length for AOV and the background.
And, given a certain framing of the subject, one will pick the aperture for the DOF of the subject wanted.

Most people get directed towards the well-known online DOF tables. Enter focal length and distance from camera to subject, CoC, and get the DOF out.
But why is focal length requested when it doesn't affect the DOF? Because the tables combine focal length and distance. All they really need is the framing size. The size of the subject that will fill the frame.

I wanted a quick reference table for DOF, so used the online DOF table, and created one for a 200mm lens. Then I created another one for a 70mm lens.
And then went duhhhhh, look they are the same, why didn't I remember that focal length doesn't change the DOF?!
I wouldn't have noticed they were the same is I hadn't used distances from camera to subject calculated for the same framing of people. But I did, so I did

And that is why I created this table:

I have never once (in well over 40 years as a photographer) used a depth of field calculator or a table such as the one above to figure out what I wanted to do about depth of field control in my images.

While it's true that my old Nikon lenses had depth of field markings on them, I seldom even resorted to using them. Mostly what I would do if depth of field was at all critical for an image is use the depth of field preview button to get an idea of what the image would look like. Otherwise, I just use experience and gut feeling to decide what aperture setting to use.

The bottom line is that, after a little experimentation, a photographer who is paying attention to his/her work can easily get a good feel for what works and what does not.

Here are some background articles:

Do Wide Angle Lenses Really Have Greater Depth of Field Than Telephotos?
http://www.luminous-landscape.com/tutorial​s/dof2.shtml

http://www.cambridgein​colour.com/tutorials/d​epth-of-field.htm

This is very good. Bernie talks about DOF being related to subject size:
http://berniesumption.​com …depth-of-field-for-geeks/

This is a more complete table than mine, but I think mine is more useful day-to-day for portrait photographers:
http://berniesumption.​com …ter-depth-of-field-table/

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Other threads where this has come up:

DOF question for you pros
https://photography-on-the.net …p?p=12524665&po​stcount=12

Lens, subject framing and DOF
https://photography-on-the.net …showthread.php?​p=12701136

OT: remembering or estimating Depth Of Field on the hoof
http://www.flickr.com …discuss/7215762​7276165884

SkipD wrote in post #12269241
I have never once (in well over 40 years as a photographer) used a depth of field calculator or a table such as the one above to figure out what I wanted to do about depth of field control in my images.

While it's true that my old Nikon lenses had depth of field markings on them, I seldom even resorted to using them. Mostly what I would do if depth of field was at all critical for an image is use the depth of field preview button to get an idea of what the image would look like. Otherwise, I just use experience and gut feeling to decide what aperture setting to use.

The bottom line is that, after a little experimentation, a photographer who is paying attention to his/her work can easily get a good feel for what works and what does not.

Skip,
I agree, I don't look at my table much now! But I think for people new to photography with narrow DOF (not point and shoots), hopefully, my table will make the learning experience easier and quicker.
And when asked a question, I can point to this article rather than having to write the explanation out again

CaptivatedByBeauty wrote in post #12269302
Skip,
I agree, I don't look at my table much now! But I think for people new to photography with narrow DOF (not point and shoots), hopefully, my table will make the learning experience easier and quicker.
And when asked a question, I can point to this article rather than having to write the explanation out again

That does make sense of course. That's why a group of us wrote the tutorial on perspective. It's saved us a lot of typing.

I would say that instead of stating "focal length doesn't matter" when discussing DOF, wouldn't it be more accurate to phrase it "changing your focal length and changing your distance to the subject will have the same affect on your depth of field"? Yeah, you can talk about the framing, but you change the framing by either changing your focal length or your distance to the subject. I don't want to bicker about this, it just seems like you can point out both factors. Like you said, your approach may work well for portraits, but one thing that is important, and that is that your choice between changing your focal length or your distance can make a big difference in the perspective, which you want to consider, and still retain your DOF considerations. Just my thoughts.

I'm just curious why you would need this when you could just press the DOF/aperture test button on the camera.

I have to admit to making oversimplified statements about 'DOF does not change when the subject size stays the same in the frame, as FL changes." I just used the Cambridge Color flexible DOF calculator to calculate DOF at a series of FL (15mm, 30mm, 60mm, 120mm), and at different subject distances (calculated where subject distance: FL = n, where n= 50<300). Here is a plot