View Full Version : Figuring out coverage?

3-Sep-2011, 14:01

Newbie question: How do I figure out the coverage of a lens? The only measurements I have are a diameter of 5.3cm and length of 5.8cm. Someone's trying to sell it to me and I want to make sure that it will cover 6x7 (or larger).

Also, I'm guessing I wouldn't be able to figure out the max aperture without more info, correct?



3-Sep-2011, 14:18
Maybe post some information like the make/model as I'm sure someone here has either used it or knows the specs?

3-Sep-2011, 14:24
It's a Darlot, but other than that, that's all the info I have. Not much, I know.

3-Sep-2011, 15:56
You need any of the following two and you can calculate the other with trigonometry:
a) Angle of view
b) Image Circle
c) Focal Length

3-Sep-2011, 21:40
with those dimensions the lens is *probably* about 6" focal length and my *guess* is that you'd have black corners on a 4x5 neg focused to infinity but probably just cover focused close for a portrait or still life

if you mean 6x7cm then I'd *guess*you'll have full coverage, but you may miss out on obvious swirl because that's in the periphere of the field

I'd strongly suggest you get more information from the seller because I won't be held responsible if you aren't happy with the result !!!

I'm guessing those results based on the lens being an f4 petzval because that's such a common value for the petzvals of that vintage and what everybody seems so keen on
if the lens is a rapid rectilinear design you're looking at a longer focal length [probably 10 to 11 in] and almost certainly f8
if it's an R.R. you'd have coverage whether it's 6x7 cm or 6x7 inch

do you have an idea which design you're looking at ?

Richard Rankin
3-Sep-2011, 22:34
A Darlot less than 6cm long is not likely a petzval. Darlot did some landscape lenses, but the one I have, at least, is shorter than that. They also did 'single view' lenses and some RR types. My guess would be that it is probably an RR type - Rapide Hemispherique. If you can get the glass diameter, you could check here, starting around page 16, and see what matches up.



3-Sep-2011, 23:36
in fact, I think olwick means the lens currently on ebay that I was planning to bid on
it's short because there is no hood but I think it's a small petzval
given the dimensions I guess it could be a shorter focal length and more like f3 ???
well.... I probably have too many old lenses I never use