Hi--

(My computer crashed, so I've been late responding to e-mail...)

>Tom Buckley suggested I contact you for some information.
>
>I will soon be acquiring an 800mm f/5.6 ED-IF Nikkor. This
>lens focuses to 30 feet. In addition to larger beasts, I hope to
>use it on small birds as well but will have to get closer than
>30 ft to fill the frame with them.
>
>I have the current Nikon catalog picturing their PK-11A, PK-12,
>PK-13 and PN-11 auto extension rings but don't have any idea
>as to which would be appropriate for my application. I think
>getting the 800 to focus to, say 15-20 feet would be about right.
>Any idea as to which ring would be the best choice? Is there
>a formula or table about this?
>
>Any information you can supply about this would be much
>appreciated.

I don't know, though I suspect a fairly long tube (at least the
1" PK-13) will be needed. Someone responded with a post with a
formula for figuring it out. I suggest setting the lens up on a
tripod, maybe 18' from a target (focused to minumum or mid
distance), then holding the camera body with a space between
the lens rear and body. See how much space there is between the
body and lens when focus is correct. If about 1", then the PK-13
should work; if nearer 2", then the PN-11... Another solution
may be to try a weak close-up lens in the filter slot, if you
can find one thin enough to fit (an optomitrist should be able
to figure out the power needed, and to fit one in a filter rim,
if you have a spare).