tr
The article shows your formula as the one to be used with positive (Kodak portra) type single element lenses. I used the d = 30mm also from the article. I agree, take it out in the field and check it out.
Harry
P.S. Based on the calcs in the article the minus in (-250mm) is ignored in the formula.

<font size=-1>[ This Message was edited by: worldphoto on 2004-02-15 21:19 ]</font>

<font size=-1>[ This Message was edited by: worldphoto on 2004-02-15 21:21 ]</font>

The formula seems applicable to either positive or negative supplementary lenses; if one works through it with "regard to sign," one gets plausible answers.

I got a set of "Telek" lenses from Craig Camera a couple of years ago. These discussions have persuaded me that, as soon as the temperature rises and the wind dies down, it is time to take them out and see what they can do...

I had a "Telek" -4 when I was in college (you could still get them new, though my dealer had to order one for me). It worked very well with an old 124mm. Kodak "Anastigmat" that I scavenged from a junker 616 folding camera and mounted on a board for my "Century" -- pretty much pushed the bellows to its limits, but produced very usable images.

Thanks for that formula, t.r. Very handy for computing the capabilities of these "add-on" lenses, of which I have complete (I think) sets: Kodak Telek -1 thru -4 and Portra +1,2,3. When will the warm weather return??

T.R. & Harry,

Thanks! I'll have to try out that math (now, where did I put my calculator...)

--Micah in NC