Depth of field 2/3 behind and 1/3 in front???

[Abitconfused]

I have read that for any point of focus (excluding infinity I presume) a depth of field exists with 2/3 of its range located behind the POF and 1/3 of its range in front of the POF. If this is true, is the sharpest point within this range at the POF or at the center of the DOF?

[dan marchant]

There is a point at which focus is achieved (the plane of focus). Everything in front or behind that is increasingly out of focus to some degree. The answer to your question is that the position of the plane of focus alters within the DoF depending on the settings used. The one thirds/two thirds is just a general rule of thumb for estimating. If you use a Depth of field calculator you will see that the ratios actually change quite dramatically.

Depth of field is the area in front and behind the plane of focus where, although objects are not "in focus" they is considered to be "acceptably in focus" - as in not too out of focus. Obviously your measure of what is or isn't acceptably in focus will be different from someone else's.

[Brev00]

The point of focus is the sharpest point--but as a plane, not just a single point, as dan mentioned. In other words, the depth of field is not usually a circular orbit around this pof. Different depths of field will have different shapes with the 2/3, 1/3 thing being a generalization and one that can be used to good effect--since we are not talking about exact distances here. The one area that is routinely sited as diverging from this generalization is macro where the dof is more in the 1/2, 1/2 neighborhood (so hard to get much sharp that close!). Which is why, when the dof is too tight for my desires, I will try to get my subject as flat as possible to my camera (parallel to the sensor) so I don't have to worry about that vague depth of field.

[Manfred M]

The 2/3 : 1/3 is true for wide angle lenses, but gets down to around 50:50 for longer ones.

https://www.cambridgeincolour.com/tu...h-of-field.htm

[William W]

I have read that for any point of focus (excluding infinity I presume) a depth of field exists with 2/3 of its range located behind the POF and 1/3 of its range in front of the POF. If this is true

No.
This is a generalization or a Rule of Thumb.
A better generalization or Rule of Thumb would be to say: For MOST NORMAL to WIDE FRAMING of The Shot, the DoF extends 2/3 behind and 1/3 in front of the Plane of Sharp Focus.
*
As already mentioned it is the PLANE of Sharp Focus (not the 'point' of sharp focus).
The Plane of Sharp Focus may be curved.
*
Arguably the best Rule of Thumb to use is that which is based on the Axiom of DoF:
1. DoF is related to the FRAMING of the SHOT.
2. The TIGHTER the FRAMING of THE SHOT, then the closer the DOF becomes to being ½ behind and ½ in front of the Plan of Sharp Focus.
*
Apropos what FRAMING of “The Shot” means. It is how THE SHOT appears in the VIEWFINDER (FRAMING it is NOT how tight the shot is CROPPED in POST PRODUCTION:
An example of a TIGHT Shot is: 'An Head Shot' and here the DoF is spread about ½ in Front and ½ Behind:

*
An example of a WIDE Shot (or sometimes termed a 'LONG Shot' or a BROAD Shot) is a group of people sitting in rows and here the DoF is spread about ⅓ in Front and ⅔ Behind:

*
And an example of a MID Shot (or sometimes a 'Normal' Shot) is a Full Length person standing, with a bit of air above and below:

It is at about this MID SHOT, that the DoF becomes about ⅓ in Front and ⅔ Behind the Plane of Sharp Focus.
*
Note that FRAMING is NOT about the FL of the LENS. One can have a TIGHT SHOT with a Wide Angle Lens and a BROAD SHOT with a Telephoto Lens, etc.
*
As already mentioned, for MACRO PHOTOGRAPHY (which is could be thought of as ‘Extremely Tight FRAMING’) the DoF is for all practical purposes is: ½ in Front and ½ Behind the Plane of Sharp Focus:


WW

[chauncey]

It would be remiss of me not to throw a monkey wrench into the equation...
all of the above responses are correct, providing your Auto Focus is spot on target
and does not exhibit back/front focusing characteristics.

[ajohnw]

Another monkey wrench is that these approximations are based on formulae that are also approximations and also assumptions about a certain size of the final image. The other problem of course is that these are fractions of some distance influenced by what f ratio the lens is actually set at.

There is another approach, not well known, that throws it all on it's head and works on the basis of what detail needs to be resolved in a scene. It's far more usable without carrying a phone application to perform the calculations.

http://www.trenholm.org/hmmerk/DOFR.html

There is more information and examples of the results about on the site. I did this to summarise it a bit more clearly.



I found it interesting as long ago shooting film me and a number of others always focused on the horizon or maybe a bit short and chose an aperture that we knew would work. This method refines this in as much it gives the size of things in the foreground that will be resolved, Pixel counts and final print size don't enter into it other than that the camera may not be able to record the detail. It's just another use of the approximate formulae used for calculating hyperfocal and other dof related calculations.

John
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[William W]

The 2/3 : 1/3 is true for wide angle lenses, but gets down to around 50:50 for longer ones.

No. As I understand the meaning of that statement, it is not correct.
It is not really about the Focal Length of the Lens.

What the Focal Length of the lens does affect, is that it moves back the point in the FRAMING of the shot where the ratio begins to vary: the Aperture has an effect similar to this, also.

For example using a telephoto lens for the shot of the Girl playing hockey as opposed to using a normal lens for the SAME FRAMING, there will be a difference in the DoF Ratios.

But even, so at around that FRAMING or a bit wider framing, if one using a telephoto lens, there will still begin to be the movement towards two thirds / one third ratio and away from the half/ half ratio.

Similarly the same relationship occurs if the APERURE of the lens is relatively LARGE. But again this is only a slight difference in the general rule of where the ratio of the DoF spread begins to move as I described above and which is based upon the axiom of DoF.

***

The axiom of DoF in simple terms is:

For most common shots the DoF will be CONSTANT for any same camera FORMAT for any same APERURE used provided the FRAMING of ‘The Shot’ is the same.

This Axiom is very important as it can release the Photographer from DoF tables and allow one to learn a set of common DoF by rote. For example learn three apertures for three different but typical FRAMINGS that one often uses.

***

Manfred, please see explanation I wrote above. I don't believe the CiC link supports your statement.

For example a 200mm lens on a 5D shooting a scene where the lens is focussed at 600ft will be about 1/3 :: 2/3, that is because that is a WIDE SHOT, even though a telephoto lens is being used.

It is a commonly stated that DoF is dependent upon the Focal Length of the Lens and also it is a common comment that the ultimate ratio of the spread of the DoF is also dependent upon the FL of the lens (the latter is what I understood your statement to mean).

I guess these misbeliefs are because not many Photographers use telephoto lenses for a Wide Shot.

If I have misinterpreted the meaning of your statement, please advise.

WW

[William W]

. . . these approximations are based on formulae that are also approximations and also assumptions about a certain size of the final image. . .

Of course approximations - but importantly often knowing the ‘approximation’ is very useful for real world shooting.

***

There is another approach, not well known, that throws it all on it's head and works on the basis of what detail needs to be resolved in a scene. It's far more usable without carrying a phone application to perform the calculations.

I tend to shoot a lot of people photos.

I have found learning the DoF for three apertures, combined with knowing at what point in the FRAMING of the shot that the DoF ratios change, (as I have described above) the most very simplest method to learn and to apply relevant DoF computations to my practice of The Craft, especially to ensure that the shot is ‘safe’ when shooting on the hop or under the pressure of time.

Here is an example of one of my Cheat Sheets for my 135 Format Cameras


Those who actually check the figures can work out the CoC used and hopefully will also note that I have rounded the DoF figures to be "safe" - AND - also rounded the DoF figures so that they can be easy to remember by rote, as one can see the numbers form an easily remembered pattern.

WW

[Manfred M]

Bill - taking the snippet out of the CiC article, I believe it is saying what I am; (given a whole gamut of assumptions, including focal length and f-stop).

The question isn't so much as to the absolute DoF (and I agree with you about the framing being the driver of the DoF, rather than the focal length), which indeed remains constant. The question deals with the distribution of the DoF in front of and behind the focal plane. I understand that this is not constant, but depends on the focal length being used, as per the part of the CiC article shown below.

"Note how there is indeed a subtle change for the smallest focal lengths. This is a real effect, but is negligible compared to both aperture and focusing distance. Even though the total depth of field is virtually constant, the fraction of the depth of field which is in front of and behind the focus distance does change with focal length, as demonstrated below:

Distribution of the Depth of Field

Focal Length (mm) Rear Front
10 70.2 % 29.8 %
20 60.1 % 39.9 %
50 54.0 % 46.0 %
100 52.0 % 48.0 %
200 51.0 % 49.0 %
400 50.5 % 49.5 %"

[MrB]

Play around with the Depth of Field Calculator for yourself here on CiC:

https://www.cambridgeincolour.com/tu...calculator.htm

and you will see that the 1/3:2/3 "rule" is a rule that, as a rule, isn't a rule.

Cheers.
Philip

[William W]

Bill - taking the snippet out of the CiC article, I believe it is saying what I am; (given a whole gamut of assumptions, including focal length and f-stop).

OK.

I have carefully re-read the CiC Tutorial.
I now better understand your reference.

EXPLANATION:

The CiC tutorial is taking a STATIC FRAMING and then is comparing different Focal Length Lenses at that ONE STATIC FRAMING.

This can be easily seen by the table – the FL doubles and the Focus Distance doubles – thus the FRAMING is the same for all those shots in the table.

HOWEVER what I wrote here is correct: “What the Focal Length of the lens does affect, is that it moves back the point in the FRAMING of the shot, where the ratio begins to vary.”

This fact can be evidenced by the example which I gave, using a 200mm lens and focussing at 600ft – the DoF is NOT spread at ½ :: ½, but the spread of the DoF is closer to 1/3 :: 2/3

***

So in summary and using practical examples (taking the CiC table as a base):

We use a 50mm lens at 2.5 meters on a 5D series camera and that will give us about an HALF SHOT – the spread of DoF will be 54 :: 46 as per the CiC tutorial.

And (as per what I wrote) that would become close to 66 :: 33 when the FRAMING of the shot gets to about a Full Length Shot using a 50mm lens - actually it is tad broader than a Full Length shot, but certainly when the FRAMING is as wide as the group of kids in the swimming pool example – the DoF spread is certainly about 67 :: 33.

BUT on the other hand, as per the CiC Tutorial, I do not doubt that when using a 200mm lens for that HALF SHOT the DoF distribution is 51 :: 49 . . . and when using a 200mm lens for a FULL LENGTH SHOT (like the Girl playing Hockey) the distribution will be (a guess) 55 :: 45.

BUT what I am getting at is:
 firstly the 200mm lens will get to about 67 :: 33 DoF distribution if FRAMING of THE SHOT is BROAD enough.
 Secondly the effect that a telephoto lens has compared to a normal lens, is that a telephoto lens requires the FRAMING to be BROADER, before the change in the spread of the DoF is noticed.

Conversely it is the opposite for a Wide Angle Lens.

Does that make sense to you?

WW

[dje]

In addition to all of the good discussion above, could I just add a comment on one common situation - a wide landscape shot (small aperture) where DOF is required to extend between a few metres and infinity. In this situation, the 1/3, 2/3 rule of thumb is of little use and pretty much meaningless. The focus distance really doesn't have much effect on the inner range of DOF provided it (the focus distance) is say 10m away or more. And as long as it's further away than the hyperfocal distance, DOF to infinity will be achieved. In my opinion, in this situation, you are best to focus on something in the image you want to be as sharp as possible.

Dave

[Manfred M]

Does that make sense to you?

Yes it does Bill.

The problem with trying to explain something like DoF really means identifying the assumptions that are made. I find that the best solution is to get to know your gear so well that these discussions become irrelevent because you know how to line up your shot to get the result that you want.

I found that when I was shooting film, I would plan my shots carefully and focus using the hyperfocal distance to bring more of the foreground into focus and blur the background. The long rotation (330° or so) on the focusing ring and DoF markings, this was a relatively painless and practical way to play with DoF.

With a autofocus and modern zoom lenses, this is no longer a practical route. The focus rotation on many modern lenses is in the 120° range and DoF markings on the lens have disappeared (although this was often the case on zooms). It's much more complicated go through these convolutions on many modern cameras; so I take the "test shot" approach (obviously magnifying the results on the camera screen; can't tell otherwise). So much simpler...

[ajohnw]

Of course approximations - but importantly often knowing the ‘approximation’ is very useful for real world shooting.

***



I tend to shoot a lot of people photos.

I have found learning the DoF for three apertures, combined with knowing at what point in the FRAMING of the shot that the DoF ratios change, (as I have described above) the most very simplest method to learn and to apply relevant DoF computations to my practice of The Craft, especially to ensure that the shot is ‘safe’ when shooting on the hop or under the pressure of time.

Here is an example of one of my Cheat Sheets for my 135 Format Cameras


Those who actually check the figures can work out the CoC used and hopefully will also note that I have rounded the DoF figures to be "safe" - AND - also rounded the DoF figures so that they can be easy to remember by rote, as one can see the numbers form an easily remembered pattern.

WW

That is excellent. Bill. I have had several attempts to try and get new people away from worrying about hyperfocal distances etc. Even the CinC tutorial points that out (hope I remember correctly).

The interesting thing about the chart I posted is that it doesn't depend on distance only fractions of some distance which is much easier to judge, There is also no getting round what the near resolution will be either. The only question is how well that needs to be resolved. He gives example on the site, his books can be freely downloaded now, but essentially say the aperture is 2mm 2mm items will show a "controlled" amount of blur, recognisable but blurred. 10mm items will be sharp. For landscapes he advocates focusing to infinity and using the same calculation. Everything from camera to infinity will then be resolved down to the same limit. Might pay to remember that he was a view camera man so had more pixels than we can even dream about. He does point out that there is no way a camera can resolve say 2mm at infinity but also mentions what use of the hyperfocal distance will do to shots. On the other hand say the camera is focused 1/3 or the way into the shot. The 2/3 beyond the focus point will double the resolution figure, a 1/4 way in treble it. 1/2 way in match it. Say there was a lampost 2 miles away. Can't measure it but it might be safe to assume it was say 180mm dia and focus to some fraction of the distance on purpose to account for that. A silly example as the aperture has been set to resolve the foreground so may as well focus on the far distance as the resolution of the foreground will be the same where ever the camera is focused. It's not a silly example in situations that are closer and distances can be roughly estimated. One example he gives is photographing an animal. He can tell that the tail will be shown but that the hair on it will be sketchy. The method is also of use for deliberately blurring backgrounds or quickly determining if this is possible.

The wide angle shot with close foreground, often pebbles etc always gets mentioned in relationship to hyperfocal distance. Well if the aperture is too big they wont be resolved clearly full stop if the method is correct. They wont be resolved if the camera sensor can't either. That's one of the odd facts about the method. It takes no notice of print size and circles of confusion only if it's physically possible to resolve things and after a fashion how wekk. Say the pebbles became 1/2mm grains of sand. Not much point in trying to resolve these with a wide angle lens only to be sure of capturing undulations. Maybe there is a log. near. If bought into sharp focus then the effect on near and far can be estimated or say the log has detail down to 6mm an aperture can be set to capture it focusing where ever some one fancies but infinity is always going to be an easier sum to do in the head.

I said if correct as I haven't tried it yet but when I try ultra wide angle close to the ground I most certainly will. The only thing that can be said is that the maths behind it seem to be as sound as the other method with far less complications when it's used in practice. In many cases the maths is simple 8mm lens 1mm resolution F8. The other way would be to say well I can't quite work it out exactly in my head but I now that Focal Length / F ratio is finer than I need. I can still do fractions in my head - I think.

John
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[William W]

. . . The problem with trying to explain something like DoF really means identifying the assumptions that are made. I find that the best solution is to get to know your gear so well that these discussions become irrelevent because you know how to line up your shot to get the result that you want. . .

I think that there is another matter too: there are limitations of communication by the written word.

And these limitation are especially noticeable on forums, where, sometimes the only way to flesh out an understanding on issues is to continue the conversation in a totally un-emotional manner and especially without reverting to personal attacks.

Thank you (and others too) for this conversation. Hopefully we’ve left something that is useful to others.

Bill

[Glenn NK]

I have read that for any point of focus (excluding infinity I presume) a depth of field exists with 2/3 of its range located behind the POF and 1/3 of its range in front of the POF. If this is true, is the sharpest point within this range at the POF or at the center of the DOF?

As has been amply demonstrated, this bit of folklore might not be of much use.

The "sharpest point within this range" is where one focuses. But beware - some lenses have a flat plane of focus and for the odd lens, it is a curved surface and not planar. When the camera's axis is perpendicular to a brick wall, all the bricks should be in focus, but some lenses this does not hold. Perhaps one of the best examples is the Canon 24-70 f/2.8 (older version). This referred to as Field Curvature.

One of the better landscape photographers doing commercial work is Tim Fitzharris (I have his book on landscape photography). He states, "The image will be most arresting if it displays sharply from front to back. This can be accomplished by shooting at the smallest aperture to maximize depth of field, and by focusing about one third of the way into the picture space to center the in-focus zone over the framed area. Use your cameras' depth-of-field preview feature to check results in the viewfinder".

However keep in mind that he is using wide angle lenses for landscape photography (see Manfred's chart), so the 1/3 advice must be tempered with knowledge of one's lenses (again, read Manfred's comments about knowing one's gear).

Perhaps his best advice Fitzharris offers is, "Use the depth of field preview to view the scene at shooting aperture. Make sure the viewfinder is well hooded, give your eyes a few moments to adjust to the dimness and examine the scene carefully. Start with focus at infinity and back off until the most distant features begin to lose sharpness. Reverse focus a smidge and then examine the foremost picture elements for adequate detail. If they are not sharp, adjust to a smaller aperture, or move the tripod back from the foreground features and repeat the procedure".

Glenn

[ajohnw]

I think that there is another matter too: there are limitations of communication by the written word.

And these limitation are especially noticeable on forums, where, sometimes the only way to flesh out an understanding on issues is to continue the conversation in a totally un-emotional manner and especially without reverting to personal attacks.

Thank you (and others too) for this conversation. Hopefully we’ve left something that is useful to others.

Bill

There is a big problem with written word in forums especially and that is just how much to type out. I'm glad that I could provide a link to something that provides far more detail along with examples. The only remaining question really is how many "resolutions" in it's terms are needed to say make a blade of grass in the foreground sharp. Final image size may have some impact on that. Debayering and sharpening will have an effect as well. Might turn out that 3 resolutions are fine for grass in web shots or even less..

Bill's approach could be extended. Another cheat sheet for say squirrels and another for elephants. That would give some one an idea how things vary. Too much trouble but a professional can often make these judgements without even really thinking about it - repetition. Probably plenty of mistakes learning to do that as well.

Personally I feel books in areas such as photography don't help much either. They are not really written for the good of mankind or to pass on genuine knowledge. They are written to make money. If some one comes up with a new angle on something they may make even more money. Not that there really are any new things to say about some areas of photography especially when it comes to actually taking the shot.

Things are generally simplified as well. DOF and hyperfocal distance are a good example of that. They all at a basic level assume that the final image size will be 10x8in but of late that has turned to 12x8in viewed from a certain distance. Make the final image bigger and the available DOF reduces, make it smaller and it increases. Worse still this assumes that the image is a print which has higher resolution than a PC monitor but we don't usually view those from 250mm nor say 24x16in prints. This is why decent dof calculators not only have settings for specific camera format sizes but also circles of confusion sizes. This can be rather important if some one wants to crop say a bird out of a much larger image. Being fair though the dof for a 10x8in print does still hold for a 20x16 providing it's viewed from more than 500mm rather the 250mm and is the full camera frame. Personally I don't think anyone has managed to figure out what all of this means when images are debayered, PP's and displayed on a PC screen. It's even possible to loose detail that the camera captured.

If people want to try scanning a scene or worse still focusing at F16 using DOF preview I suspect they will find that the magnification available in dslr's just isn't high enough to be sure. Focusing means working either way and guessing the middle. It is a technique that was used on view cameras (plate cameras) along with a hood and a special purpose magnifier. My view on this goes back to film days. I might use DOF preview if shooting a relatively shallow DOF, never on from here to eternity DOF. Others may find it works out I haven't.. I also found that when it comes to focusing on "infinity" lens markings are useless. What ever the far distance is has to do.

It is possible to get a good idea what has actually been captured after the shot has been taken and the probable largest final image size. Taking my E-M5 as an example the rear screen is 2in tall and the max preview magnification is 14x so I can judge final image sizes up to 28in tall. This is why you will see some pro's fiddling with the preview at times when they are taking shots. They will be judging other things too but at least this way however they came to some setting or the other they can directly see what they have done. It's too late when they get back to the PC. Some will still do this even though they are capable of looking at something, setting the camera and taking a shot as it's the only really safe thing to do. Film was much tougher in this respect.

A magnified live view could also be used to check focus before the shot is taken even just to check that the settings that have been derived some how do have the expected results. Personally I feel it would be better to check after taking the shot. That approach can be used to check any aspect of the settings. Taking Bills example. Say they are elephants at various distances. There may be a need to take and preview several shots at different distances to establish useful DOF apertures. If you do this often enough you will remember what apertures to use. On the other had you could carry a laser range finder about and a dof calculator or just guess. Knowing that elephants are going to be shot and likely ranges crib sheets could be pre prepared as well. That one reason I like the idea. Say I forked out 600 quid a ticket for a formulae one meeting and took my camera. I would most definitely want some idea of the camera settings I would need before I went. Trouble is I would want 2 or 3 tickets and might get wet at that price. I hate watching football (unusual in the UK) but again if I wanted to go to a match I would want some idea of the settings I would need - Bill's just done that. This sort of approach makes far more sense than assuming if you read about DOF that you will suddenly start getting miraculous results. That is highly unlikely.

John
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[William W]

. . . Bill's approach could be extended. Another cheat sheet for say squirrels and another for elephants. . . Say they are elephants at various distances. There may be a need to take and preview several shots at different distances to establish useful DOF apertures. If you do this often enough you will remember what apertures to use.

(Hypothetical)
When I have to take a portrait of a group of Elephants, or squirrels - I would frame up a person at the same Shooting Distance. I know the DoF for people shots.

(Not Hypothetical)
One of my photographic pastimes is collecting Church Interiors using Available Light and One Frame only (i.e. NOT using HDRI, though I usually make an exposure bracket).

To calculate the DoF that I have for any particular shot, I can reference the height of person, even if there are no people in the shot, even if there are no people in the Church.

For example the Altar sits at chest height, Pew is at waist height, etc. So, for example if I am taking a tight shot inside the Sanctuary, I can still adequately reckon the DoF for the shot using my memorized cheat sheets based on people’s dimensions.

The same applies to other building interior and exteriors that I fancy. etc. The archway of a normal door is a bit bigger than a person; the bed is a bit longer than a person.

Also, if the shooting scenario allows – and for most shots of any subject within 20~50ft it usually easily does – I can move and place MYSELF against the Subject (Object) being photographed to reckon its dimensions relative to ME and then I can compute DoF using my method above.

WW

[jcuknz]

Play around with the Depth of Field Calculator for yourself here on CiC:

https://www.cambridgeincolour.com/tu...calculator.htm

and you will see that the 1/3:2/3 "rule" is a rule that, as a rule, isn't a rule.Cheers.
Philip

Before there were wonderful DOF calculators I used a Hyperfocal distance table and using a 50mm lens, becuase it was the one I had on my SLR, I discovered that the 33/66% 'rule applies out in the field but from about 25 feet this gradually changes to 50/50 by the time you get close to around 7feet.
Apparently this changes depending on the lens being used [ focal length ] but I never bothered to repeat the exercise with other lens that I got with digital. It doesn't really impact on my photography so is of little interest to me