Brighter cameras - higher shutter speeds? (1 Viewer)

Hi!

I understand high shutter speed is a good thing when digiscoping, and a 77-85 mm scope collects more light than a smaller one. But how is it with the light collecting capacities on the camera objective? I'm just an amateur on fotography (so excuse me if I'm totally wrong here), but isn't it so that a bright camera with a low f-number (big aperture) result in higher shutter speeds in a given situation than a camera with a high f-number?

Maby then there is digital cameras who is better in this aspect than for example coolpix 4500 (biggest aperture 5,1 at 4x zoom)? The Olympus C-4000 zoom has a biggest aperture 2,8 as another example. I have no idea if this camera is good for digiscoping though! (Has anyone tried it?)

You're right about the F2.8 lens being faster (brighter) than a F5.1 lens on its own but I'm not sure whether this applies equally when digiscoping.

I'm also just an amateur who may be completely wrong but I believe that when you hold a camera against the 'scope you are, in effect, creating a 'new' lens which would have to have the iris positioned in a new part of the 'lens' in order to function correctly.

As it is I think it just stops light rays around the edge of the image (when stopped-down) without actually reducing the intensity of light reaching the camera's sensor (or film).

Therefore, I would say that, providing there is no image cut-off, the maximum aperture of the camera lens will make no difference to the amount of light passing through as it is just, in effect, a few more elements in the lens-train. (There will be some light-loss through the normal process of going through the camera lens elements but I think it will be the same for any lens speed).

I'll sit back, now, and let the experts shoot my theory down in flames!

Adey

Falco,
In conventional camera body + lens setups the image is designed to focus onto the film plane within the camera body. The real snag with standard digiscoping techniques is the restricted diameter of the telescope eyepiece objective lens giving the vignetting effect you will have read in threads on this forum.
The consequence is that to reduce the incidence of this vignetting we all end up using a camera with a lens small enough to be a near match for most telescope eyepieces. The unfortunate side effect (as you state) is a small aperture (f no) with the additional restrictions this imposes.
I have tried other cameras (digital and real ones) with proper f1.4 lens for digiscoping but it just made me cry. The larger lens were of no asset at all.
The only serious but inconvenient answer is for true marriage of camera body and lens to entirely replace the viewing eyepiece on your Swarovski/Leica/Zeiss (delete as appropriate) scope.
Some genius may eventually design a solution but until then we will struggle on with our Coolpix 4500 or similar.

Thank you very much for your answers!

ralphj said:

Falco,
In conventional camera body + lens setups the image is designed to focus onto the film plane within the camera body. The real snag with standard digiscoping techniques is the restricted diameter of the telescope eyepiece objective lens giving the vignetting effect you will have read in threads on this forum.

Click to expand...

This is said frequently, but everything I've seen and tested shows otherwise. I've found that vignetting can be predicted pretty well by considering four factors. I put this page together some time ago, and so far the four factors discussed are very good predictors of vignetting.

http://www.jayandwanda.com/digiscope/vignette/vignetting.html

There is a loose correlation between lens diameter and vignetting which is probably what led people the conclusion that lens diameter is the main issue. But that loose correlation isn't the real issue and doesn't make that great of a predictor of vignetting.

ralphj said:

The consequence is that to reduce the incidence of this vignetting we all end up using a camera with a lens small enough to be a near match for most telescope eyepieces. The unfortunate side effect (as you state) is a small aperture (f no) with the additional restrictions this imposes.
I have tried other cameras (digital and real ones) with proper f1.4 lens for digiscoping but it just made me cry. The larger lens were of no asset at all.
The only serious but inconvenient answer is for true marriage of camera body and lens to entirely replace the viewing eyepiece on your Swarovski/Leica/Zeiss (delete as appropriate) scope.
Some genius may eventually design a solution but until then we will struggle on with our Coolpix 4500 or similar.

Click to expand...

Larger f-number lenses often need more "eye relief" from the eyepiece because their "entrance pupil" is more deeply placed. Where a camera lens' entrance pupil is located and how it might actually change location while a camera lens is zoomed is one of those things that can usually only be answered through direct experimenting.

Note that the new CP5400 is a poor digiscoping camera but its lens is of a similar diameter to that of the CP5000 which is a good digiscoping camera.

The best estimate we can easily make on light throughput is to calculate the exit pupil diameter of your scope and eyepiece combination (ie. 80mm @ 20X = 4mm) and see how it compares to your cameras aperture diameter (ie. 16mm @ f4 = 4mm). In this case, the fastest usable f-number on the camera is f4. An f2 lens would not result in a faster shutter speed. But an F8 lens would surely reduce the shutter speed. Likewise, if the 80mm had a 15x eyepiece (5.333mm exit pupil diameter) then a 16mm f3 camera lens woulld be able to make use of the extra light.

So the idea is to have a large enough camera iris diameter to fully use the light from the scope - which is represented by the scope/eyepiece's exit pupil. Either the scope/eyepiece or the camera lens iris can be the limiting factor in how much light is passed on to the CCD.

You can calculate a lot of these parameters using my online digiscoping calculator.

http://www.jayandwanda.com/digiscope/digiscope_calc.html