[LargeFormat] A really big lens

[Michael Briggs]

On 10-Oct-2004 Jim Hemenway wrote:
> If only I had a 16x20 field camera for this!
> 
> http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3842947706&category=30076
> 
> Well... that and the required $9,750


If you had a 16x20 camera and this lens, I think you would get only a small
spot in the center of the film illuminated.

From the auction description: "De Oude Delft (The Old Delft),   LARGE LENS G
11377 E150/0.75 RAYXAR".   "RAYXAR" suggests a lens related to X-rays, probably
imaging a phosphor screen.  150/0.75 suggests 150 mm f0.75.  This would fit
with imaging x-ray screens because, at least in medicine, one wants to reduce
the x-ray exposure as much as possible.

Searching the web, I find pages on similar lenses:

http://home.intekom.com/franlet/astronomyPAGE69.htm
http://home.intekom.com/franlet/astronomyPAGE68.htm
http://www.macrolenses.de/ml_detail.php?ObjektiveNr=158

The Dutch name of the lens is because they were used on old Philips x-ray
machines.

From the optical diagram for a 65 mm lens on the first link, the film is placed
only 0.8 mm behind the rear element.  The auction photos show the rear cell to
have a much smaller diameter than the front cell, perhaps 3 or 4 inches
diameter.   So the image can't be larger than about 4 inches in diameter.

This is not a large format lens -- it is a super-fast lens.  f0.75 is close to
the theoretical limit for a lens in air: f0.5.   This is why the front element
is so huge.

Auctions purporting to sell satellite optics should be considered sceptically. 
Would a space agency sell a spare lens?  More likely they would keep it for
future use or for display in a museum.  It more likely that the seller is
misinformed or a fraud.

--Michael