Re: [APML] Need Help Focusing Telephoto Lens

[Anthony Ayiomamitis <anthony@perseus.gr>]

Kevin,

       The knife-edge focuser works on the principle that you have two of the
same camera bodies. One has the KE focuser which you use to focus, you then very
carefyl remove the camera body and install the one with the film. Of course, all
of this applies if you build one yourself. If purchasing one from a third party,
you will specify the type and model of camera you propose to use so that they
will provide you with a KE which matches the distance to your camera's film
plane precisely.

        I think I have read somewhere that some cameras (medium format?) have
removable backs which means one camera body but two backs.

Anthony.

Kevin Wigell wrote:

> Hello, APMLer's, I need some help. I'm fairly new at AP and have been mostly
> lurking here for several months. My problem is getting consistent focus when
> I'm piggybacking my 35mm camera with a 300mm telephoto lens. Every once in a
> great while I'll get some in-focus shots, but most of the time I end up with
> little donuts instead of stars. I think I've finally mastered tracking, as
> the donuts are nice and round.
>
> Ironically, I usually have no problem with focus at prime focus of my 10"
> SCT. I use a Hartmann mask and that seems to work. With my telephoto lens,
> lately I have been trying focusing on a very distant terrestrial object
> during the day, then firmly taping the lens in place, then checking again
> that the distant object is still in focus. The lens in question is stopped
> down during exposure by one stop to f/5.6.
>
> So, can someone please tell me how I can achieve consistent focus on stars
> with this lens?
>
> As an aside, could someone also please explain a little bit about how
> knife-edge focusing works? I understand the principle perfectly, I think
> (when at perfect focus, a sharp edge should extinguish a star image at the
> film plane instantaneously), but how does this work in practice? Does one
> have to do the KE focusing before loading the film? If not, then how the
> heck do you do it with film loaded? And if so, can you do KE focusing a
> second time without removing the film? And how do you know when the
> knife-edge is right at the film plane?
>
> I am *so* confused.
>
> Thanks in advance for any help,
> Kevin Wigell
>
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