"To calculate effective coverage as related to 35mm lenses;
fl divided by diagonal of format, multiplied by 50"
43 gives a better (closer to reality) than 50
quite right Dan
Originally Posted by Dan Fromm
by using Pythagoras the diagonal of a 24x36mm rectangle is 43.266615
had you heard of this calculation before Dan? i kind of made it up, i thought
I've recently been looking at a copy Handbook of Photography by Henney and Dudley, copyright 1939. It has illustrations of all the clasic lens designs and a number of formulas. Plus a lot of discussion of how the elements in these designs function.
You may be able to find a cheap copy at a used book shop, or perhaps a library sale.
Since you’ve made several lenses from the Greene bible, you are already successfully launched on a career of trial and error. Your hunt for “sources for designs of the lenses” is a pointless exercise.
For example, you can look at http://dioptrique.info/ for all the details on lens’ composition, BUT you’ll never find the elements on the shelves of Anchor Optics or Surplus Shed or any other place that I know of. They just won’t have the correct combination of focal length, surface curvature, glass composition, etc for you to reconstruct the classic 19th century lenses, let alone anything since. Better for you to build on your experience, already knowing that a duplet is better than a single element, and see what you can build from available pieces. Try a triplet, and see what placing a negative element between two positive yields. All trial and error, but you are already a lucky guy.
FYI the John Evans book is available on Surplus Shed under books … bend those rays … J
look at the on line catalog or ask to be sent the hard copy...
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Pythagoras' theorem is very old, quite well-known.
Originally Posted by Ray Heath
Your shortcut for comparing diagonal angles of view isn't, AFAIK, widely known but I've been using it for years. I mean, it is obvious.
Ray: thanks for the info!
most of my home-mades i'll be mounting in front of a packard shutter, so i'll be able to use slightly faster shutter speeds.
how do you mount the actual lens in the tube? since they're all different sizes. i've been using foamcore as suggested by the Primitive Photography book, although i'm not convinced it's the best way.
Walter: that sounds a lot like the 'portrait lens' i just built. asymmetrical duplet. the larger element goes in front, and the focal length is going to be less than either of the two elements by themselves. there's a formula somewhere for that, but i don't have it handy.
Originally Posted by walter23
Thank you for that link!
Nikon must be using a different convention for their eyepiece diopters, because when I apply that formula I get a very different focal length from the one I measure. Eg., my +2 diopter measures as a 320mm lens, but 1/2 = 0.5 meters or 500mm. My +3 is not 333mm but rather 200mm or so.
Originally Posted by Dan Fromm
As an aside; this is really weird; I'm being hit by a huge dose of deja vu. I wonder if I had some conversation about diopters 3 or 4 years ago that I've forgotten all about. I have images of street photography with a diopter in my head. Maybe zombies or something too. Go figure. Must be a neurological misfire.
Last edited by walter23; 11-11-2007 at 04:14 PM. Click to view previous post history.
The universe is a haunted house. -Coil