The Canon EOS and Photography Forums

This is a question for Bob.  I know that you have a 101mm Televue apo.  I have been experimenting using mine as a 504mm lens, using live view and focussing manually using a laptop and it seems fine.  I also have a a 2" 4x powermate and I wondered if you had any experience of such a setup.  I realise that it will be too slow for autofocus.  My Gibralter tripod will provide a nice steady support and the thought of an effective focal length of 3000mm is tempting.

The subject will be birds relying on the birds settling on the spot on whcih the telescope is trained and focussed.

Thanks

Ken

I've never tried it but I suspect that it won't be an easy combination to use! I know the 4x powermate is designed for visual use, so I'm not sure how well it will do photographically, but I guess if you have one it's easy enough to try.

I've used stacked teleconverters on a Genesis to give a 2.8x multiplier and the results weren't great, even though the TCs should be optimized for photo use. Focusing gets tricky and you're starting to push the diffraction limit. With a crop sensor DSLR you start to see significant diffraction softening by f16 and a 4x on the Genesis puts it at f20.

Also, if you're using the extra focal length so you can be further from your subjects, then shooting through extra air (at ground level) will blur the image due to turbulance, especially since exposure will tend to get long at f20.

So I'd say try it since you have the gear. You might get lucky and there have certainly been some usable shots taken using lower quality digiscoping techniques with spotting scopes and digicams shooting through the eyepiece. They must be operating at very long effective focal lengths and very slow effective aperture.

Thanks Bob, as you say nothing lost in giving it a shot.  I had not appreciated the difraction issue.  Certainly looking at the Televue website, a straight Genesis can give some great bird pictures.

Ken