Can someone explain the DOF rule of 1/3 front, 2/3 back to me?

OfMikeandMen · Apr 17, 2013

I've heard this rule cited so many times, but it makes no sense to me. The rule states that 1/3 in front of your focus will be sharp, and the back 2/3 of your focus will be sharp. But, isn't this dependent on your distance, focal length, and aperture? Wouldn't this amount vary if I used f/1.2 compared to f/22 (with the same focal length and camera-to-subject distance) even if I'm focused at the same point? I'm really confused about this for some reason.

480sparky · Apr 17, 2013

The 1/3 : 2/3 isn't an absolute rule.. it's a general guide. And it's not very accurate, either.

My DOF calculator, when given a FF sensor, 50mm lens set at f/8 and focused at 4 feet, has the near limit at 3.66', and the far limit at 4.41'. That works out to a total of 0.75' DOF, .34 in front and .41 behind the point of focus. Not exactly 1/3 : 2/3.

Ysarex · Apr 17, 2013

The 1/3 front 2/3 back DOF rule is utter nonsense. I don't know where it came from but I suspect it came from someone who didn't understand DOF trying to explain it. Unable to provide an accurate description of what factors influence the DOF distribution they just blurted out, "think 1 third 2 third and you're good to go." Since then ignorance has propagated more of the same.

DOF distributes unevenly with less DOF before the focus plane and more DOF behind the focus plane. The ratio of that uneven distribution ranges from 49.99999999% front/50.00000001% back to 0.00000001% front/99.9999999% back depending on multiple factors. For example; if DOF is any amount before the focus plane but reaches infinity behind the focus plane what's that ratio? What's a fractional percent of infinity?

Somewhere along the continuum 1 third/2 third happens, but let's say you're a landscape photographer who always shoots for max DOF often reaching infinity. 1 third/2 third just doesn't happen there. Let's say you're a macro photographer. 1 third/2 third doesn't happen there either. The best you can do is argue that the rule kind of fits "general" photography kind of "generally." As Sparky said, it's general and not accurate -- yep, I'll second that. It's nonsense.

Joe

bratkinson · Apr 17, 2013

To me, depth of field is more like "how thin do I want the 'wall of focus' (as I think of it) to be?". Trying to figure out in my head 'this f-stop at this distance, etc', is more than I want to think about when shooting.

Obviously, when doing landscapes, cityscapes, and trains (my other big hobby), it's usually daylight and therefore I'm shooting at smaller f-stops (higher f numbers) to keep ISO and shutter speed in the 'reasaonable' range. Practically everything is in focus as a result.

Indoors, where I do most of my shooting these days, I'm thinking "do I want more DOF or less DOF?". Knowing that my 135mm lens, wide open at f2, with a subject about 20' away will be a very, very thin DOF (and produce either stunning results or missed-focus trash), and probably the thinnest I can produce with the lenses in my bag, simply stopping down will produce more DOF. So I'm thinking 'do I want everybody/everything in focus or not?'. It's a subjective call, and I sometimes take multiple shots at various aperture/shutter combinations, just to make sure I get what I want in focus. I don't want to have to stop, do some mental gymnastics (or on a cellphone app), then set up and take the shot. That's a sure fire formula to miss 'the shot'. I recently had occassion to take what I consider the best shot of my 50+ years behind a lens. It was very low-light, spur-of-the moment, no-setup, no-pre-thought, simply aim and click with whatever settings I had in the camera. A second later, 'the shot' was gone forever. 7-8 feet away, 135mm at f2.2, ISO 5000, no flash. DOF is roughly 1-2 inches.

Indoors, flash/no flash photography is a continual challenge with the exposure triangle. My biggest concerns are 'fast enough to stop mine and subject movement' and proper exposure...or close enough exposure that I can fix it in Lightroom. Sometimes I'll intentionally shoot with a slower shutter to get a subjects' arm motions, etc. Sometimes I have no other choice but to shoot slow due to lens aperture and/or ISO & noise considerations. What tradeoffs am I willing to make to get the settings I want, to get the shot as I want it?

Bottom line, unless my primary goal in a series of shots is to have a thin DOF to get subject separation and great bokeh, my shooting is more aimed at getting the desired exposure with DOF wide enough to capture the entire 'setting', or group of people, all in focus. Simply by keeping my aperture somewhere in the middle of the range for whatever lens I have mounted and being about 30 feet away accomplishes this. Note, too, that most lenses have their sharpest focus somewhere in the midrange of their aperture limits.

KmH · Apr 17, 2013

With DoF the point of focus is the sharpest.

Focus sharpness starts dropping off in front of and behind the point of focus. The total DoF distance defines how far in front of and behind the point of focus stays acceptable sharp.
How fast that drop off occurs is what determines how deep the DoF is and the in front of and behind the point of focus distribution.

Use an online DoF calculator and plug in some real world numbers you use - Online Depth of Field Calculator

A 10 mm lems on a 1.5x crop camera set to f/4 and a focus point distance of 2 feet will deliver DoF that is distributed 27%/73%
Change just the lens aperture to f/8 and the DoF distribution changes dramatically to 4%/96%.

A 200 mm lens on a 1.5x crop camera set to f/4 and a focus point distance of 800 feet will also have a DoF distribution that is 27%/73%
The 200 mm lens set to f/8 delivers 9%/91% DoF distribution when focused at 700 feet.

This tutorial might also be helpful - Understanding Depth of Field in Photography

vintagesnaps · Apr 17, 2013

I agree, it doesn't make any sense. There seem to be plenty of so-called rules being perpetuated online that aren't exactly accurate (and that some of us never learned in the first place and I wonder if beginners now will need to unlearn!).

You're right it depends on the lens, the aperture, etc. I wouldn't be concerned about it too much unless you're doing portraits or taking a photo that for some specific reason you want a lot or very little depth of field. If you want to keep your background more out of focus to bring the viewer's attention to your subject then you'd need to adjust settings to use a larger aperture; or you might want a smaller aperture if you want more of the area in front of or behind your subject in focus (or if the entire scene itself is what you want in focus) then adjust settings to be able to get a smaller aperture.

Depth of field seems to get a lot more discussion online that it merits for general use I think. I'd rather get a sharply focused well framed photo and keep my shutter speed fast enough for hand held use, so the depth of field isn't particularly a priority for me.

OfMikeandMen · Apr 17, 2013

So, in other words, if I set my focus point and subject directly in the middle of the frame, then 2/3 of the frame behind them will be focused, and 1/3 of the space in front of them will be in focus? Thanks everyone for your input. I am not necessarily trying to employ this technique, just understand it. Thanks.

480sparky · Apr 17, 2013

OfMikeandMen said:
So, in other words, if I set my focus point and subject directly in the middle of the frame, then 2/3 of the frame behind them will be focused, and 1/3 of the space in front of them will be in focus? Thanks everyone for your input. I am not necessarily trying to employ this technique, just understand it. Thanks.

No. It has nothing to do with 1/3 of the space between you and the subject. The (mythical) 1/3 is 1/3 of the total DOF, as it extends from in front of the subject to behind it. However, the 1/3 : 2/3 'rule' is total bunk.

For instance: I put a 28mm on my camera, set it to f/16 and focus on a subject 5 feet away. My total DOF 18.31 feet.... 2.16' of which extends from the subject towards me, and 16.15' on the other side. Hardly 1/3 : 2/3.

ronlane · Apr 17, 2013

480sparky,

If I understand what you are saying, then when I pose a group of people with the 28mm at f/16, to get the best focus of the group faces, I need to focus on the persons face that is closest to me?

timor · Apr 17, 2013

OfMikeandMen said:
I've heard this rule cited so many times, but it makes no sense to me. The rule states that 1/3 in front of your focus will be sharp, and the back 2/3 of your focus will be sharp. But, isn't this dependent on your distance, focal length, and aperture? Wouldn't this amount vary if I used f/1.2 compared to f/22 (with the same focal length and camera-to-subject distance) even if I'm focused at the same point? I'm really confused about this for some reason.

For each lens it might be different ratio. Actually as different as 3% and 97%.Why don't you check with :
Online Depth of Field Calculator
for your lenses and just remember for most common distances you will be using it.
Or, :sexywink:

, shoot with Pentax SF1n, it is doing it automatically .
Or just download an app for your phone.

480sparky · Apr 17, 2013

ronlane said:
480sparky,

If I understand what you are saying, then when I pose a group of people with the 28mm at f/16, to get the best focus of the group faces, I need to focus on the persons face that is closest to me?

Not necessarily. By focusing on the closest person, you may run the risk of the back row being out of focus. If you have people closer to 2.84' away (gawd knows why you would with a 28mm, but hey, this is all theoretical anyway) or more than 21.15', then your DOF won't get everyone in focus.

amolitor · Apr 17, 2013

You should use the DOF marking on your lens to determine where to place the focus!

Interesting note: The DOF markings on a lens are symmetrical around the distance mark. The distance markings, however, and getting closer together in one direction and farther apart in the other. This is an expression of some interesting optical fact, but I can't quite put my finger on it right now!