"whisky-dave" wrote in message
...
On Thursday, 1 October 2015 11:11:29 UTC+1, NY wrote:
"whisky-dave" wrote in message
...
Ah! I didn't appreciate that your use of "DoF" wasn't referring to
depth
of
field,

thre are two DoFs in PHOTOGRAPHY. Nowerdays most people that do
photography know of depth of field, when I stared in the 6th form I
looked
DoF up in a photography book.

I'd never heard of depth of focus but a quick google has educated me on
that. Strictly speaking the "London Underground" sign marks the position
of
the film, rather than the range of positions of the film.
I think early cameras such as plate it was importan to make sure the plate
was in teh right place eraly cameras weren't acurratly made.


where the lens
would produce an acceptably sharp image, which I'm sure isn't a constant
and
varies according to focal length of lens.

I don;t think it does. if yuo have any camera with interchangable lens all
of tehm have to be in focus at the same point and that is where the film
or sensor is. In the old days yuo could take the film pack out and replace
it.
If teh fiml/sensor is in teh wrong place the picture will be out of focus.
Those with difital camera don;t consioder this and probley have never even
throught about it.

I doubt whether anyone with a camera with a fixed and defined focal plane
(eg the polished plate across which the film runs, with the 36x24 mm or
120-sized aperture in it) will have thought much about it. After all, the
position of the focal plane is no more adjustable for most film cameras than
for a digital camera.

Where the focal plane mark comes into its own is if you are setting focus of
your lens by tape measure rather than by adjusting the focus ring until the
correct part of the image is in focus on the focussing screen (or letting
the auto-focus do its job).

How certain are you that "depth of focus mark" is the correct term for the
"underground symbol" mark? In camera manuals it's described as "focal
plane". Depth of focus (like depth of field) refers to a *range* of
distances - either side of the focal plane (in the case of depth of focus)
or either side of the subject (in the case of depth of field):
https://encrypted-tbn2.gstatic.com/i...My4vO0bTwyLldN

It is probably technically incorrect to refer to the "underground mark" as
"the depth of focus mark" (because this is a range rather than an absolute
distance) but nevertheless it may be that it is common parlance. If so, fair
enough, although all the references I've seen to the mark on cameras have
called it the focal plane mark.


Since the rigidity of the camera body keeps the lens perpendicular to the
sensor/film and the correct distance away, depth of focus isn't really an
issue for cameras unless you use bellows between lens and film. Position of
focal plane is another matter and you'd use it for specialised focussing as
I described earlier.


I doubt whether all lenses focus IR the same distance closer/further than
visible light. My gut feeling is that it may vary depending on the quality
of the lens (as well, almost certainly, on the focal length). Here's why. A
simple lens made of a single piece of glass focuses different colours of
light at different distances, which results in chromatic aberration. To
counteract this, photographic and telescope lenses have elements made of a
combination of two pieces of glass of different refractive index to minimise
the difference. In general, the more you pay for a lens, the less chromatic
aberration you'll get - ie the smaller will be the depth of focus between
the red and violet ends of the spectrum. Extending this further, a good lens
will correct over a wider range, extending to some part of the IR spectrum.
It may not do it perfectly, but the degree to which it does governs how far
apart in depth the lens will focus visible and IR. For this reason, and the
fact that as you say a longer lens probably has a greater offset between IR
and visible focal planes, I'd expect it to make sense only to mark the two
focus points on the lens focussing scale and not on the camera.

Effectively the underground mark is saying "this is where the sensor/film
is". A lens's focussing scale is marked such that a visible light image will
be focussed at the plane, by virtue of the camera having a fixed and
precisely controlled spacing between mount and film. For IR, a different
lens-dependent offset is needed to counteract the fact that when the lens is
correctly focussed for visible light, IR will be focussed at a different
plane *whose position depends on the lens* both in terms of focal length and
degree of chromatic aberration correction.



How standard is it for the tripod mounting thread to be aligned with the
focal plane.

I'd say never but I've never seen one.

Can;t see the3 point of doing that the tripod mount should be so the
camera balancies better on the tripod and not stressing anyhting.
It's why you have tripod mounts on telephoto lenses and not normally on WA
ones.

The only time when the precise position of the tripod mount is critical (as
far as I am aware) is when taking multiple overlapping photos eg for a
panorama. I have seen special brackets with knurled knobs to move the
rotation point accurately to the position of the sensor if the camera's own
tripod bush isn't in the right place, though I'm not sure how you calibrate
it. It is probably important for movie cameras where the geometry has to
remain correct when panning during filming so a subject moving on a circular
path centred on the rotation point will remain in focus.

I agree that apart from this case, it makes sense to put the tripod point on
the lens (if it's a heavy lens) for better balance.