[Laser] Re: CONCLUSION TO >>> focal length question

[Tim Toast]

It sounds like you have a good idea of what's going on
with all that, George.
I only meant 'fast' in the photographic optics sense,
and not in how it relates to the speed of the various
detectors. I guess we are all using 'fast' optics
generally. Since the signal to noise ratio is so
important, you reach a point of diminishing returns
when going to faster and faster lenses. And eventually
you'll lose the advantage of 'fastness' when the SNR
(and field of view) gets too big.

In photography they talk about a lens being fast if
the focal length is less than twice the diameter of
the lens (< F 1:2) But it could also be used when
comparing two lenses, say an F4.5 and a F5.5 lens for
example. The F4.5 lens would be considered the faster
of the two. Now, how that applies to lenses with
detectors other than photographic film, i'm not really
sure. With film, the light can build up over time
(integrate) and produce an image, and it happens
quicker with a 'fast' lens. But with a typical
photodiode detector, the way we normally use them, the
light does not integrate - as far as i know. Long
integration times tend to destroy modulation anyway,
so it's not something we normally think about with our
modulated light beams.


George Vastianos wrote:
COMMENTS: TIM ABOUT THE FASTER.... I BELIEVE THAT
DEPENDS FROM
WHAT KIND OF SENSOR YOU ARE USING...ACCORDING TO SPEED
ORDER
(FROM FASTER TO SLOWER) WE HAVE: PHOTOTRANSISTORS,
PHOTODIODES
AND PHOTORESISTORS



Tim Toast
http://www.aladal.net/toast/exp.html

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