alanr0

alanr0

Lives in United Kingdom Harlow, United Kingdom
Joined on May 13, 2005

alanr0's recent activity

  • Leonard is correct.  Magnesium fluoride is an inorganic material and unlikely to dissolve in acetone.  However acetone  will dissolve certain plastics and paints.  According to wikipedia magnesium ...
  • Replied in Hmmm... Still?
    My reading is that Zemax image space f/# is for infinite object side distance.  I thought this was clear from the wording above, but the the user guide adds. " Note that infinite conjugates are ...
  • Posted in News & Rumours but didn't garner much intelligent comment 2 months ago. It is a way of manufacturing pure silica glass at the relatively low temperature of 1100 C, compared with 2000 C ...
  • Replied in Hmmm... Still?
    Agreed. I don't know where you are coming from here. I made no mention of "relative" or "corrected" focal length. "Working f-number" is defined irrespective of the magnification and image quality.  ...
  • OK, I (finally) see more of where you are coming from. For a well-corrected photographic lens the second (rear) principal surface is a sphere of radius f , centred on the focal point. Whatever the ...
  • No argument about the physics.  I am poking a stick into the definition of f-number, aware that there may be attempts to bite my arm off. I don't believe there any ambiguity if we use working f-number
  • This is getting to the root of the problem. What are the limits on usablility? My concern is that an unqualified "f-number" does not not appear to have a universally accepted definition. If one ...
  • Hi Jack, Agreed. Reading between the lines of both the Focal Encyclopedia and the Wikipedia entries, I would not be surprised to find "standard" definitions of f-number which reflect traditional ...
  • Most folk posting here would accept that 1/(2 NA) is a "sensible" definition of f-number.  I am curious to know when (or if) it became the formal or "official" definition of f-number? The Focal ...
  • The manual I linked is for the Brownie 2 and 2a.  Page 2 of the manual shows the lens (E) on the same face as as the two viewfinders (F), with the shutter release (C) adjacent. The back of the ...
  • It is a window so that the user can see the frame numbers printed on the film's backing paper.  After each exposure the film must be wound on until the next consecutive number is visible in the ...
  • RichRMA wrote: BBC News : "It doesn't look very exciting - a cardboard box about 5in (13cm) tall, covered in leatherette, with a small round opening at the front." The text describes a box. The ...
  • Replied in Binary prefixes
    Binary prefixes : a note for pedants (readers with a life may wish to move on): G (giga) prefix is defined by the International System of Quantities (ISQ) as 10^9: 1000,000,000. Gi (gibi) prefix is ...
  • Agreed - there is no problem for real-world imaging, or even for solar concentrators where the heated element has an opaque surface. In both cases each point on the detector or collector surface ...
  • I had assumed that recent participants in this thread could visualize a sine curve without the aid of a calculator. For those that can't, my second paragraph includes an explicit reference to a ...
  • It occurs to me that one of the reasons for the (NA< refractive index) limit is that most optical sensors are attached to something flat and opaque, so they can't accept rays more than 90 degrees ...
  • For a parabolic reflector, distance from the focus to the mirror vertex is what most folk consider the focal length .  How much the ratio f/D tells you about the optics and image formation is a ...
  • You probably mean 0.001 radian, not degree, unless f=57.3 m. Just out of personal curiosity, are you guys are deliberately winding each other up?  Joe has consistently talked about f/D ratios, and ...
  • Replied in Forum navigation
    Right hand side near the top of the column (left of 'latest reviews'): Click "Flat view" to toggle between flat and threaded views. Many of us prefer to see the thread arranged logically in ...
  • If you have not tried this already, a really useful exercise is to drop a coin or a lens cap onto a sheet of white paper or some other plain surface.  Aim the camera slightly away from the coin, ...
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Last post Sep 29, 2012
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