Re: F number

From: Len Philpot (len_at_philpot.org)
Date: 11/20/04 

Date: Sat, 20 Nov 2004 08:08:53 -0600

In article <%AHnd.4961$1u.525@twister.nyroc.rr.com>,
adfrenchremoveallthis@nycap.rr.com says...
> "eric" <eric.archambeault@videotron.ca> wrote in message
> news:G4znd.54475$Uy.1273097@weber.videotron.net...
> > Hi Eric,
> >
> > Low F is more bright, and Hi F give more contrast. [SNIP]
>
> The image brightness depends on aperture, not f/ratio. F/ratio has no
> relation to contrast, although an obstructed instrument of high or "slow:
> f/ratio is likely to have a smaller obstruction, resulting in a slight
> increase in contrast.

Absolutely. It's all too easy to look at the focal ratio and forget
that it's just that - A /ratio/. For someone new to telescopes (and
particularly familiar with camera lenses), it's easy to get things
backward. A camera lens set at different f stops is varying aperture
with a constant aperture, while comparing two, e.g. 6" telescopes of
different f ratios is a difference of focal length, not aperture.
Varying either against the other is what changes the f ratio.

I suspect some people might try an 8" f8 scope with eyepiece X, then
an 8" f4, also with eyepiece X and come to the conclusing that the f4
is instrinsically brighter. However, it's actually just operating at
half the manification and therefore four times the brightness. Take
an eyepiece of X/4 and try it in the f8 and it will be just a bright. 

-- 
 -- Len Philpot -> len@philpot.org     <--
 ------ ><> -----> http://philpot.org/ <--

Relevant Pages

  • Re: digital camera as exposure meter
    ... Example for a 1:4 key to fill ratio. ... Select the desired exposure setting (i.e., ... Make test exposures and adjust the strobe power to ... the DSLR aperture for whatever ratio of light is ...
    (rec.photo.digital.slr-systems)
  • Re: Get Me Started (again...)
    ... designed for visual use and the higher end models especially tend to come with low profile focusers designed for eyepiece use. ... In order to do imaging with one of these on a platform drive, you will need to push the focal plane further out of the focuser by either shortening the poles on a Truss design, or pushing the mirror forward in a solid tube design. ... For children this is especially difficult as you will need some means to get them up to the eyepiece of larger apertures. ... Give him as much aperture as possible and digital setting circles. ...
    (sci.astro.amateur)
  • Re: TeleVue-60 versus Orion Dobs
    ... Small aperture, short focal length, on altazimuth mount for touring. ... three levels of expense in time, energy and money (blood, sweat, and tears ... Eyepiece requirement*: ... Mix of expensive, highly corrected, wide field designs, and inexpensive, ...
    (sci.astro.amateur)
  • Re: Explain a DOB principle
    ... a 10mm eyepiece will give you 99.5x magnification. ... In other words, the higher the F ratio, the less ... coma you'll have in the scope. ...
    (sci.astro.amateur)
  • Re: FINALLY! The shot Ive been waiting for!
    ... I used ISO 200 @ 1/125 sec but I should have bumped up the ISO ... They were both at infinity with room to spare, so they would both be in focus no matter what lens or aperture. ... The truth is, I would have been better off using the camera adapter since the increase in magnification was not worth the loss of aperture, and the eyepiece introduces a lot more glass to look through. ...
    (rec.photo.digital)