P3D Re: 2519 - focus

[Duncan Waldron <J.Waldron@xxxxxxxxx>]

On Fri, 23 Jan 1998, John W Roberts wrote:

> [deleted]
> 
> Apologies. As I'm sure you're aware, it may be a worthy accomplishment to
> gain a piece of knowledge, but sometimes it's even more difficult to convey
> that knowledge to another person. 

You're not kidding!

> I have referred to photography texts in
> the past, and probably will do so again in the near future, but I doubt that
> most of the readership of P3D has immediate plans to look up these texts.

Very few normal, sensible people would...

> Since this issue can have a direct impact on how well certain 3D photographs
> turn out, perhaps it would be worthwhile to try to define these terms a little
> more precisely.
> 
> >Let me try another tack (more words...); you use the lens to focus, 
> >therefore depth of focus is near the lens, ie at the focal plane. 
> 
> So for a typical 35mm camera in typical use, depth of focus would be a
> matter of millimeters? And it would be inside the camera?

Depending upon the aperture and the distance focused on, not even
millimetres. Consider a 50mm lens, focused on infinity; if you shift the
focus and move the lens barrel out, say, 1 mm, the lens does not move
appreciably closer to the subject (which is at infinity), but you have
altered the point of focus to around twelve feet - and this by effectively
moving the film away from the lens only 1 mm. In other words, you have an
enormous focus shift, with a very small movement of the focal plane - ie
shallow depth of focus - and yes, inside the camera. 

> 
> >In my example, the depth of field (not focus) is 8 metres - the distance
> >from 2 metres away to 10 metres away. 
> 
> The example from your previous post (depth of field under the following
> conditions is 2 meters to 10 meters, with focus set at 5 meters) appears
> to be much more useful than the restatement in this post (depth of field
> under these conditions is 8 meters). The (2 meters to 10 meters) gives both
> a near distance and a far distance, while the (8 meters) gives neither, and
> for any situation where the focus is set beyond the hyperfocal distance,
> the single-parameter value would be "infinity", without giving any information
> about where the near distance is.

That's absolutely correct, but I was just answering your specific question
on distances from an earlier post. Of course, to be of any value, an
expression of depth of field must indicate near and far points of
acceptable focus. 

> 
> Both you and John B gave what are undoubtedly correct explanations, but they
> were also highly abstract, with the exception of the partial example you gave.
> I think what may be most useful at this point is a set of concrete examples
> of how exactly depth of field and depth of focus would be expressed, with
> nothing left out:
> 
> "Under [the conditions that matter], and with focus set to 5 meters,
>    the depth of field is [one parameter? two parameters? what units?]."
> 
> "Under [the conditions that matter], and with focus set to 5 meters,
>    the depth of focus is [one parameter? two parameters? what units?]."
>    (And are those distances inside the camera?)
> 
> I'm not too concerned with the exact math at the moment - that can be filled
> in later - just with what *kinds* of things we're talking about.
>

This sounds like you are asking for a set of depth of field/depth of focus
tables...? I don't know about exotic things like Realists, but with
conventional cameras, you have the indication of depth of field on the
lens itself, for several apertures. This is a good guide, but *may* be
optimistic. As with so many things, depth of field can be highly
subjective. It depends upon how sharp you personally wish your photographs
to be, either side of the actual point of focus; you may find that the
indicated depth of field at, say f/8, is only actually achieved by
stopping down to f/11. On the other hand, you may be happy with f/5.6
(unlikely). I tend to focus on the nearest and furthest points that I want
sharp, note the respective distances, then make sure they are comfortably
inside the indicator marks on the lens in use. 

  Examples of depth of field are:

  with a 50mm lens:

  focused on 20 feet, depth of field at f/11 is from 11 feet to nearly
  infinity

  ditto, at f/16 is from 9 feet to infinity

  focused on 7 feet, depth of ield at f/11 is from 6 feet to ten feet

  ditto, at f/16 is from 5 feet to 13 feet

The relevant parameters are: lens focal length, aperture in use, distance
focused on; it is stated as being from a near point to a far point, and is
in whatever units you care to use, either feet or metres.

In reality, depth of focus is not something that you would consider when
taking photographs. If the film is flat in the focal plane, and you have
stopped down sufficiently to allow a good depth of field, then depth of
focus just does not become an issue. Only if you are focusing near
infinity, with a wide aperture, with a suspect focal plane (eg poor
pressure plate or buckled film), or are using a bellows camera in close-up
work and are focusing with the rear of the camera (ie by moving the film
plane), do you start to worry about depth of focus. Otherwise - forget
about it. You only need to know what depth of focus *isn't*, when
discussing such matters with pedants :-) Where it becomes a practical
consideration with me, is when I work around f/2.8, photographing the
night sky. On long exposures, the film can pop forward from the focal
plane; in this situation, with minimal depth of focus, the picture can be
ruined. A direct analogy is in projection: when a non-glass mounted slide
pops in the mount, the image on screen goes out of focus. How far did the
slide move? Less than 1 mm, I would think, but with disastrous effect on
screen.

Hope this helps
Duncan Waldron

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