Focal Length/Aperture Questions

Being new to digital photography using a telescope as a telephoto lens I hope someone can enlighten me...when a digicam is used for afocal imaging through a telescope, what is the combined effect on "real" aperture considering the camera aperture or F-stop and the telescope's focal length and/or aperture. I know that photographic "f-stop" and telescope "f-number" are exactly not the same, but what happens when I shoot my CP4500 at any given aperture through my 80mm/f6 telescope (using a Scopetronix adaptive EP). The camera allows selection of aperture priority, shutter priority, full auto, or full manual. It would seem that using aperture priority would be best for control of depth of field, but how can one determine the effective camera f-stop in this case?

Then of course there's the issue of the camera's zoom setting combined with the scope's focal length. I assume you add the two to get the resultant focal length...is that correct?

Thanks,
Jim

Regarding these questions and others, consider joining the focused board, which is "digiscopingbirds@yahoogroups.com," with regular input by people who do this all the time, including those who originated the technique.

Jim,

While I haven't actually done much digiscoping yet, I will take a stab at this one. I do know a fair bit about afocal photography. Determining the effective focal length of the scope/ eyepiece/ camera combination is the easy part. You've got a 480mm f.l. scope w/ an 18mm eyepiece which yields 26.66x you then multiply that by the focal length of the camera (I'm going to use the 35mm equivalents), 38 - 155mm. So you get the equivent of a 1013mm to 4133mm lens in 35mm format. Now you can simply divide that by the aperture of the telescope (80mm) to get the effective aperture. In this case we get about an 1013mm F 12.7 to 4133 F 51.7 These should be approximately the right values excluding light loss from lens elements in the light path which should be minimal. Here is where things get kind of muddy the scope/ eyepiece you have yields a 3mm exit pupil which happens to be almost exactly the same as the true aperture of the camera's lens when it is set to wide angle so, in theory it really is operating at f 12.7 so if you close down one f -stop, you will be at f 18, etc. At the full zoom setting though, the true aperture in the camera's lens is about six millimeters so it is effectively not using 3/4 of the light from the scope, reducing the maximum effective aperture of the system from f 51.7 to f 103.4 I have noticed that, with the coolpix cameras (I used a friend's 4500 for some afocal planet shots) the high zoom settings result in a really, really dim image and this would seem to agree with the results above. The easiest way to check this would be to aim the camera/ scope setup (set to f2.6 , aperture priority) at a gray card on a sunny day, normal exposure at ISO 100 setting would be F16 so you should get a reading of about 1/150 sec at the f 12.7 effective aperture. The reading at maxium aperture zoomed all the way out should be about 1/4 sec.

I would note that, at the extreme focal lengths we are talking about, no amount of reducing aperture is going to give you much depth of field, so you might as well use wide aperture to keep the motion bluring of the subject and camera to a minimum.

Joplin