--- Don Weiss <[EMAIL PROTECTED]> wrote:
> >From: Bob Meyer <[EMAIL PROTECTED]>
>
> >f-number has absolutely nothing to do with beams of
> >light, axial or otherwise. Nor does it have
> anything
> >to do with light transmission, vignetting or
> filters.
>
> The included angle of the cone of rays converging at
> a
> point on the focal plane determines the depth of
> field. This in turn is determined by the numerical
> aperture (as well as the focal length and subject
> distance).
You have the relationship backwards, though. f-stop
afffects DOF, not the other way around. And DOF has
nothing to do with light transmission, which is what
we were discussing.
>
> >An f-stop is simply the ratio between the focal
> length
> >of the lens and the diameter of the aperture.
> Nothing
> >more, nothing less.
>
>
> I have always assumed that the most important
> aspect of lens aperture was the following
> desideratum:
>
> "When a lens is set to a given aperture or reports
> that aperture to the camera, it shall transmit
> the same amount of light as a _theoretical lossless_
> lens at that given aperture."
Well, you can assume that, but it's not true. What
you're asking for is called a T-stop, and is used
extensively in the movie industry, but never caught on
in the still photo market (although a few vendors did
try).
>
> An honest lens designer/marketer would design
> his lens to behave this way. It implies that he
> would
> design his (real-world) lens with enough _extra
> numerical aperture_ to compensate for its
> internal transmission losses. Thus, if he designed
> a lens whose maximum aperture would be advertised
> (both to the consumer and to the camera exposure
> electronics) as an f/2.8, the geometry of the lens
> might have to be f/2.4 to _transmit as much light_
> as a theoretical f/2.8.
This has nothing to do with honesty, but with the
definition of f-stop. Truth be told, with internal
metering this is a non-issue. It is an issue with
hand held meters, but less of an issue than it used to
be (because modern coatings allow much higher
tranmission levels. In the days before coated optics,
actual transmission could vary by more than a full
stop from the physical, marked f-stop).
>
> Sure. This means that the lens actually exhibits
> less
> depth of field than the theoretical lossless f/2.8
> lens,
> but it does something that is more important: _it
> transmits as much light_. An unavoidable tradeoff,
> and that's the direction I would want it in.
Actually, for the reasons I stated above, I think your
suggestion is worse. Internal metering automatically
accounts for variations in light transmission. But
having DOF vary from lens to lens (at the same focal
length and aperture) would drive me nuts.
Of course, internally focusing zoom lenses vary focal
length as distance changes, so calculating DOF becomes
near impossible anyway.
>
> If lens designers went around calling their lenses
> by the
> geometrically defined aperture, and those lenses
> reported the same to their camera bodies, how
> could the designers of the exposure electronics know
> how much transmission loss to factor in?
> The information on a given lens would be
> unavailable;
THEY DON'T HAVE TO CALCULATE IT!!! The internal meter
simplly measures the amount of light coming in through
the lens. When you put a filter over your lens, do
you have to tell the meter the filter factor? Of
course not. The meter just adjusts based on the fact
that less light comes through. It doesn't matter to
the meter whether the light is lower because its night
time, because you put on a filter, or because the lens
transmits less light.
> it would have to be assumed as some kind of brand
> or industry average. Is that what they really do?
No, see above.
>
> The only way out, if they really do all trade on
> geometrical aperture, would be if the transmission
> loss were negligible; I believe, as you do at least
> for zoom lenses, that it is not.
Actually, with modern coatings, light loss is fairly
negligible, but it doesn't matter anyway (for the
reasons I explained above.
=====
Bob Meyer
I wish I knew what I know now, when I was younger...
http://www.meyerweb.net/epson
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