Digital Camera Dilema - Depth of field

[Chuck Vaughn]

Very rarely does a subject come up on this list about which I know
something. This one I do, so here goes.

Depth of Field and Depth of Focus, although both f/ ratio dependent,
are not the same thing. Depth of focus is the range of focus over
which the image will have its maximum sharpness. At f/2 that is about
+/- .001" ranging up to +/- .005" at f/10. The smaller the depth of
focus the flatter the film has to held which is a problem with
increasingly large formats.

Depth of field is the distance over which the subject is in focus.

Depth of field does not change with focus. It either will be equally
good or equally bad over the same distance.

The paragraph below confusing. I ran this by a professional photographer
and fellow astrophotographer to get his take on it. Even though they
say "Depth of Focus", they really mean Depth of Field, which is the
modern terminology. Despite the comment about "equally sharp definition",
the 2" lens that produces an image 1/2 the size of the 4" lens cannot
possibly be as sharp when enlarged to match the size of the latter.
In addition, since this was written in 1939, they are talking about
photographic prints and the greater enlargement will be grainier than
the lesser enlargement. What is true is that the depth of field of
the 2" f/2 lens will be the same when the image is enlarged to match
the 4" f/4 image.

Personally, I view maintaing the depth of field as not much of an
advantage when compared to loss of resolution to the longer focal length
lens.

There's something else everyone should know about digital cameras. Most
of them use a single shot RGB CCD which means that there is not red, green
and blue data for each pixel. Instead they use a Bayer pattern of red, green
and blue filters over the pixels and interpolate the missing colors at
each pixel location. This means 2/3 of the data is interpolated. Although
the image looks good, there is some loss of resolution with this method.
Very high end digital cameras use a prism to split the light in 3 directions
and onto 3 three CCDs so there is true RGB data for the entire image.

Chuck Vaughn <aa6g at aa6g.org>

-----------------------------------

> 
> Here is the situation with regard to depth of field, quoted from
> the _Handbook of Photography_ (Henney & Dudley, 1939):
> 
> ----------------------------------------------------------------------------
> Depth of Focus, Including Enlargement of the Print.
> 
> It can easily be shwon that if we photograph the same object with two lenses
> of different focal lengths and diamters, if we subsequently enlarge the
> smaller picture to make it the same size as the larger picture, and if we
> insist on equally sharp definition in the two final equal-sized pictures,
> then the depths of focus of the two cameras will be proportional solely to
> the _diameters_ of the two lenses. Thus an f/2 lens of 2-in. focus and an
> f/4 lens of 4-in. focus both have a diameter of 1 in. The 2-in.lens forms
> a picture half as large as the 4-in. lens, but after enlargement to make
> them equal in size, the depth of focus of each will turn out to be the same.
> This property constitutes the real advantage of the miniature camera, in
> that it permits the use of a fast lens without the loss of depth of focus.
> ----------------------------------------------------------------------------
> 
> It is thus true that digital cameras where the CCD is smaller than a 35mm
> film do indeed have a (theoretical) advantage in depth of field--provided
> that the resolution of the CCD is high enough to make use of the advan-
> tage.
> 
> Ken Philip
> fnkwp at uaf.edu


 
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