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The viewfinder is usually not 100% accurate in representing the final image you will get and I've lost a few shots while using a too narrow DOF. I would imagine that cameras, knowing the focal length, aperture and distance to the subject (via focus mechanism?) could compute the depth of field, which would help photographer get better results.

Or as many modern lenses lack hyperfocal distance charts, which are useful for shooting landscapes, it would be useful to have it in camera too, especially since you don't need to know the distance to subject and calculation is much simpler in that case.

Why don't any cameras do this then?

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Some do actually. You are right that most do not though. The main problem is that there is no such thing as an exact depth-of-field. DOF actually depends on the viewing size, actually the angle extent of the image as being seen. Most DOF tables assume a fixed 8x10" print seen as from 12" away for someone with 20/20 vision. Today software allow you to enter these values but you would have to do that for your camera too.

When you switch to Manual Focus mode on a Fuji X100S or one of their mirrorless cameras you see a focus-distance scale at the bottom of the EVF. The thin line is the focus distance and the blue bar shows you the depth-of-field for an unspecified print-size and viewing distance. Notice how when in Aperture priority or Manual exposure mode, the blue bar changes size.

Plenty of DSLRs - above entry-level models - have a DOF-preview button which stops down to show you the image at the selected aperture. This of course darkens the image. Most modern ones can also stop down the aperture in Live-View which make it easier to judge DOF.

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First, a word about what depth-of-field is and is not:

In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eys, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image.

Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the focus plane.

Since depth-of-field is dependent upon viewing size and distance as well as the visual acuity of the viewer it is hard for a camera to indicate depth-of-field if it doesn't know what the display size of the photo will be. Assuming the standard 8x10 viewed at 10 inches by a person with 20/20 vision is probably a little too broad. Because of this it has never been a priority to the users and designers who advise major manufacturers on what features are desired in upcoming models. Most of the photographers in those groups are advanced enough to have a feel for the distances/focal length/apertures they use the most and probably don't see a need for it. They also understand how to use distance scales on lenses that are marked well enough to be usable. Unfortunately, lenses with usable distance and DoF markings seem to be getting more and more uncommon. This seems to indicate the market in general doesn't demand such a feature.

Although it is not in-camera, these tools from DepthOfField master.com are simple, easy to use, and the price is right (free).

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At the point where you are actually setting up the shot, it is easier to just push the Depth of Field preview button which stops down the lens for you so you can see the depth of field.

On cameras that don't have a depth of field preview button, they aren't expecting photographers to be caring about that level of detail I'd guess.

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+1 for teaching me what this button was used for :o) –  LeFauve Feb 5 at 1:22
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As far as I am aware, they don't bother with this, because most lenses already do have DOF markings, for example, the Nikkor 1.4 AF-S:

enter image description here

Note the 16 11 | 11 16 scale, which is there to give an idea of the DOF at f11 and f16, across the scale.

Basically, if its already there, why try to re-invent it in software?

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In my experience more newer lenses than not do not have usable aperture indexing on the microscopic distance scales you see on most AF lenses, if the lens even has a distance scale. Your example is extremely limited in usability compared to a typical 50mm f/1.4 lens from 30-40 years ago that would have had a much larger distance scale and more precise aperture markings that probably started somewhere around f/5.6. –  Michael Clark Feb 5 at 9:52
    
Isn't it hyperfocal distance markings? They're hardly useful implemented like that, but my point was also about about DOF at subject distance, which is impossible to implement mechanically. –  Andrew Feb 5 at 10:00
    
no, they are even referred to in the user manual (for that lens) as "Depth of Field Markings" - Not sure what you mean, Subject distance is marked on the distance scale.. –  Darkcat Studios Feb 5 at 10:03
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@MichaelClark - please note in this case I did specify "to give an IDEA of the DOF " - they really are "dumbing down" lenses aren't they! –  Darkcat Studios Feb 5 at 10:05
    
Andrew, it is not impossible to implement mechanically. As long as the distance scale is marked logarithmically (just as the focusing action works) there are many wonderful old lenses that did this quite well. google.com/… –  Michael Clark Feb 5 at 10:06
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distance to subject (via focus mechanism?) Not necessarily with contrast detect systems.

Secondly, Phase Detection is passive. It's only active when the shutter button is half pressed. So, it'll only tell you the reading at the moment of focusing. You could use Continuous AF but could slow the process down.

Personally, I think it's not needed and maybe a whole lot of other people do to. Who knows, it could be a niche feature for future DSLR's but I can't see it taking off.

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Is this an answer to a different question?? –  Darkcat Studios Feb 5 at 9:23
    
Totally. Because Andrew asked about the camera measuring the subject distance, which phase detection does. But contrast detection does not. I got the impression he was asking about actual calculations. If phase detection was to do this, it would slow the AF process down. –  BBking Feb 6 at 2:33
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I have a Contax RX which has no auto-focus but provide focus assistance and DOF information (and a DOF preview button).

As it date back to 1994 the technology should be widely spread today... as it is not the case one can deduce that there is not enough people asking for it.

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