Depth of focus/field

[fordfanjpn]

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This isn't strictly an M8 question, but since this is the only place I hang out these days, and since you guys seem to have all the answers, I thought I'd give it a shot.

 

Back in the (good) old days, we made a distinction between depth of field and depth of focus. However, it seems that these days the term "depth of field" has fallen by the wayside and "depth of focus" is being used (incorrectly I would have thought) in its place. Did something change over the years and I just didn't see the memo? Is it OK now to say "depth of focus" when what I really mean is "depth of field"? I'm also wondering if this apparent change in the lingo is due to the advent of digital. Can someone bring me up to speed on this? Thanks.

 

Bill

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[DavidStone]

This isn't strictly an M8 question, but since this is the only place I hang out these days, and since you guys seem to have all the answers, I thought I'd give it a shot.

 

Back in the (good) old days, we made a distinction between depth of field and depth of focus. However, it seems that these days the term "depth of field" has fallen by the wayside and "depth of focus" is being used (incorrectly I would have thought) in its place. Did something change over the years and I just didn't see the memo? Is it OK now to say "depth of focus" when what I really mean is "depth of field"? I'm also wondering if this apparent change in the lingo is due to the advent of digital. Can someone bring me up to speed on this? Thanks.

 

Bill

 

The confusion between these two terms is nothing new, it goes back as far as I can remember. But they refer to distinct and separate phenomena and the distinction needs to be retained, as it does with all technical terms. I always made sure that my students knew one end of a lens from the other.

 

David

[Shootist]

I don't think it has anything to do with digital other then there are more people getting involved in higher end photographic equipment and taking tons more photos without having a good background in photography.

You can't open a Sunday newspaper without seeing adds for 5-20 cameras of all shapes and sizes. Wasn't like that when film was the only choice.

[fordfanjpn]

The confusion between these two terms is nothing new, it goes back as far as I can remember. But they refer to distinct and separate phenomena and the distinction needs to be retained, as it does with all technical terms. I always made sure that my students knew one end of a lens from the other.

 

David

 

Thanks David. I was away from photography for many years before digital brought me back into the fold, and I was beginning to think that the terminology had changed in the interim. Glad to see that it hasn't. So I guess I'll continue to use "depth of field" where other people are using "depth of focus" and allow for the colloquial use of those terms.

 

Bill

[lars_bergquist]

I do wish Leica would have the courage to scrap their present d.o.f. scales based on the pitifully inadequate 1/30mm circle of confusion. It was obsolete already in 1939. Now I find it no problem to use 1/60th – just multiply the f-stop figures by 2 – but this must be confusing to novices. Also, it is a matter of honesty.

 

The old man from the Hyperfocal Age (early Mesozoic)

[giordano]

I do wish Leica would have the courage to scrap their present d.o.f. scales based on the pitifully inadequate 1/30mm circle of confusion. It was obsolete already in 1939. Now I find it no problem to use 1/60th – just multiply the f-stop figures by 2 – but this must be confusing to novices. Also, it is a matter of honesty.

 

I'm never quite sure about this. 1/30mm is still valid for the modest enlargements from full frame that were originally assumed and that still account for most of the prints produced in the world. The equivalent for the M8 would be 1/40mm, and for 1.5x crop cameras 1/45mm, wouldn't it? And then there would be the confusion caused to novices by 'new' lenses seemingly having less depth of field than older ones, or Leica lenses having less than Canons. On balance I feel we're better off with a common standard.

 

Lars, you'll no doubt remember that in 1939 Leica DOF scales assumed 1/30mm while Contax ones assumed 1/20mm. I wasn't around then: how much confusion did that cause?

[jaapv]

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I'm with Lars here; those scales and 1/30th were conceived for 6x9 cm prints and the thick film emulsions of the day, and were never changed for fear the public would think lenses got "worse". The main reason for confusion, however is the idea that DOF is an absolute, valid for passport portraits to billboards. The camera makers contribute to this misconception. For instance, Canon defines their AF accuracy as "within 1/2 DOF".

[Pedro]

I do wish Leica would have the courage to scrap their present d.o.f. scales based on the pitifully inadequate 1/30mm circle of confusion. It was obsolete already in 1939. Now I find it no problem to use 1/60th – just multiply the f-stop figures by 2 – but this must be confusing to novices. Also, it is a matter of honesty.

 

The old man from the Hyperfocal Age (early Mesozoic)

 

Could I have a more detailed explanation on this issue? Thanks.

[jaapv]

An old post of mine:

Quote:

DOF: The reality of an illusion.

 

DOF is a subject that causes heated discussion in photographic circles. It is, of course, next to light and shape, one of the main photographic symbols to express ourselves.

There is a simple mathematical approach that is expressed in DOF scales on lenses and DOF tables in manuals, but, as always, that is not the whole story – by a fair margin.

 

DOF as a phenomenon is childishly simple. The human eye is a rather imperfect instrument for judging sharpness, so with a resolution of about 5 lp at 75 cm everything that is higher resolved appears sharp. So now the compications start. It readily confuses contrast with sharpness, the only reason that sharpening algorithms in postprocessing actually work.. So a photograph at noon at the beach will appear to have a deeper DOF than one on a misty morning. Of course, a photograph is, in reality sharp only in one plane, which is theoretically infinitely thin, but at least as thin as the state of correction of the lens and the quality of the receiving medium, be it film or sensor, allows. Lens manufacturers, in their quest for simplification and standardization have decided, in the 1920-ies, that an unsharpness of 0.03 mm on 35 mm film would be judged the measure of DOF. That leads us to the first set of complications.:

1. Without knowing the end enlargement of the photo one takes and without taking the contrast into consideration, judging the amount of DOF is actually rather hit and –mostly- miss.

2. As DOF is solely dependent on field of view, the “enlargement” of the focal length of the lens, which is responsible for the apparent deep DOF of wideangle-lenses and shallow DOF of long lenses gets into play, so the subsequent crop will influence the DOF in as much that if one crops a 28 mm shot down to the FOV of a 90 mm lens, the DOF will be exactly the same as that 90 mm lens would have produced.

3. Film is not without thickness. In reality a COC of 0.03 mm will act like a torch shining into a murky plate of soup. It will produce a cone, diffractions, reflections, if the light strikes the film at an angle it will turn into an oblong, etc., the net result being a larger diffuse spot. This is complicated by the fact that the films we have now are much thinner and higher resolving than we had in the 1920íes.

4. Digital sensors react far more like the ideal thin receiving medium than film, causing the COC’s to be even less diffused.

5. The net result is that the DOF produced now, and especially with modern lenses (of which I will write later) is more pronounced than it is historically. It is safe to assume that it is about 70% of the scale indicated on your lens. Btw. let’s not forget that it is not divided equally in foreground and background. The real division is, for simple mathematical reasons, 1/3-2/3, more or less, depending on subject distance.

All this caused me to call DOF in another context and another forum a RBU <rubber band unit>, which got me heavily flamed.

 

Then we get to the real controversial point, and that is the effect of individual lenses on DOF,

which relates to the elusive “boke”, which aptly translates to "chaos" or "confusion" I'm told, and to the rendering of out of focus picture elements.

In general the lens is corrected optimally for the plane of sharpness only, which means that aberrations like chromatic aberration and astigmatism increase quickly as sharpness decreases. Add this to my plate of soup effect and the magnitude of possibilities gets so large that only using the lens in practice will give any firm grasp of its (lack of) qualities.

The result is that, in extreme cases of not too well corrected lenses, there will be double contours, rings and general unpleasantness in the unsharp areas. That gives bad Boke. More elegantly, but still not optimally corrected lenses, and this applies to a large number of the older lenses used by Leica fans, will produce generally soft and smoothly changing unsharp areas where the forms as such are undistorted. (did I mention geometrical distortion with the aberrations? This is the three-dimensional variant;)) That are lenses with a good boke. Then there are the newest, highly corrected lenses, like the Leica ASPH’s, APO’s etc. Those define the unsharp areas so well that they will break up the contours, giving rise to harsh boke.

Film will behave differently than sensors, as explained above. So a sweet lens on film may be unpredictably disappointing for digital and the other way around.

Could I have a more detailed explanation on this issue? Thanks.

[lars_bergquist]

I'm with Lars here; those scales and 1/30th were conceived for 6x9 cm prints and the thick film emulsions of the day, and were never changed for fear the public would think lenses got "worse". The main reason for confusion, however is the idea that DOF is an absolute, valid for passport portraits to billboards. The camera makers contribute to this misconception. For instance, Canon defines their AF accuracy as "within 1/2 DOF".

 

It all started with the insight that a print, viewed at normal distance (25–30cm or 10–12") looks sharp if its circle of confusion – the smallest representation of a point, if you will – is no larger than 1/10th of a mm. The Leitz people assumed during the late 1920's that the maximum enlargement of the Leica neg would be 3x, print size 108x75mm! So the acceptable c.o.c. in the neg, for a sharp print, would be .1 / 3 = .033 = 1/30th of a mm, and they based their d.o.f. calculation on that. But their customers were making 24x30cm prints already during the 1930's. Still, Leitz wouldn't bite the bullet and change their scales and tables, probably (as suggested above) because they feared that prospective customers would believe that lenses had gone bad.

 

The contemporaneous Zeiss standard was originally 1/1000th of the focal length! They did go over to 1/20th of a mm with the Contax. I have actually used medium format cameras with scales based on a c.o.c. of 1/10th, meaning that the manufacturer expected you to do contact prints alone (and 6x9cm contact prints were in fact deemed acceptable by most Sunday photographers even around 1950). The confusion, circular or otherwise, was tolerable because most users set their targets so low.

 

As you can see from the calculation, a c.o.c. of 1/30th of a mm is NOT adequate even for a 10x15cm (4x6") print. But if you set the aperture to, say, 8, and read d.o.f. at the f:4 markings on the lens, then you are in fact operating at an assumed d.o.f. of 1/60th, which would be sufficient for a 20x15cm print. This is what you can normally take in as a full picture at normal viewing or reading distance. Prints larger than that are usually viewed at larger distance, hanging on a wall e.g., unless you are a perverse fuzz-finder with a magnifier. These people can always find the unsharpness they deplore (but love to find) simply by demanding a larger print. Two by three feet will probably satisfy them if they can keep their magnifiers.

 

The old man from the Age of 120 film

[DavidStone]

Jaap wrote:

 

"2. As DOF is solely dependent on field of view, the “enlargement” of the focal length of the lens, which is responsible for the apparent deep DOF of wideangle-lenses and shallow DOF of long lenses gets into play, so the subsequent crop will influence the DOF in as much that if one crops a 28 mm shot down to the FOV of a 90 mm lens, the DOF will be exactly the same as that 90 mm lens would have produced."

 

This is not the case, neither does it take into account the apertures of the respective lenses. Depth of field is directly proportional to the diameter of the lens aperture, regardless of focal length. Thus the apertures in a 50mm lens at f/2 and a 100mm lens at f/4 will have the same diameter (50mm divided by 2 is 25mm, 100mm divided by 4 is also 25mm). The resulting photographs taken with these lenses at these apertures will have an identical depth of field, although the field of view will obviously vary in the ratio 2:1. When a photograph taken with a wide-angle lens is cropped down to give the same angle of view as a photograph taken with a narrower-angle lens, assuming that the photographs are taken from exactly the same position, what will be identical is the perspective, which is a function of viewpoint.

 

David

[jaapv]

I take the aperure into consideration in another part of the post, David. The final enlargement through the the system is certainly relevant. Your explanation does not cover the deep DOF of small sensor/film cameras in relation to large format. But I have a sneaking suspicion that we are saying the same thing through a different approach. I apologize for the use of the word solely. I was not meant to refer to the aperture, which is of course totally relevat.