Exit Pupil Size and Relative Brightness (1 Viewer)

I have been looking at lenses for my Nikon ED50 other than the 13-40 zoom which I presently use for digiscoping.

According to Nikon's statistics there are considerable differences in Exit Pupil size and Relative Brightness between the various lenses and wondered what effect, if any this would have on their performance for digiscoping.

13/40 zoom: Exit pupil 3.8mm. Relative Brightness 14.4 (both at widest angle)
16x fixed: Exit pupil 3.1mm. Relative Brightness 9.6
20x fixed: Exit pupil 2.5mm. Relative Brightness 6.3
27x fixed: Exit pupil 1.9mm. Relative Brightness 3.6

The differences between the numbers look big but perhaps in the real world they have little effect. Anyone have any thoughts?

Ron

i started off with a 30XDS wide and have since gone over to a 50XDS wide for the majority of my shooting these days.
i find it gives me that little bit extra that was lacking with the 30X without resorting to optical zoom to compensate.
it's a bit darker at magnification but nowhere as bad as some have suggested using anything over 30X for digiscoping can be.

i started off with a 30XDS wide and have since gone over to a 50XDS wide for the majority of my shooting these days.

Click to expand...

These give magnifications of 20x and 27x respectively,on the ED50.

As a very rough guess these might make the difference between 1/100 and 2/100 second, or ISO 400 and 800, perhaps more? I imagine there would be an even bigger difference between the 16x and 27x eyepieces.

To gain a better idea, why not attach your camera to your scope, first at around 16x, then zoom up to around 27x(estimate), take a photo at each setting and note the difference in shutter speeds without altering anything else. The relative brightness between fixed lenses and the zoom should be very similar, if not identical, at the equivalent magnification.

Ron, as you use the Fuji F30 (or is it the F31?), you already have the advantage over most cameras in low light, though there are always situations when we could do with more light to allow faster speeds or lower ISO's (less noisy).

On second thoughts, after looking at those figures for the zoom Ron, it would appear that oddly enough, the zoom has even better relative brightness than the fixed eyepieces at roughly equal mags.

13x on zoom: Relative brightness = 14.4
16x fixed: Relative brightness = 9.6

There isn't that much difference in power between 13x and 16x, yet there is a disproportionately high difference in brightness between the two eyepieces. Like I said, try the camera on the scope at different powers and see how it affects shutter speeds as you zoom up. This should give a rough idea of how higher powers affect your camera settings.

Ron,
We are lucky that as we are using a tripod with electronic shutters this is not such a big issue as it might seem.
Swarov 30x 45x 20x
Exitpupil 2.7 1.8 4.0
relbright 7.1 3.2 16.0
(on an 80 mm scope )
I don't have the 20x eyepiece due to shorter ER which would make it difficult to use on my preferred camera , the Nikon 8400 . It would work well on the F31fd though.
I haven't noticed much operational difference when using the 45x v 30x light-wise though I do like to be at 1/250th second or more if possible so sometimes I have to bump up the iso a bit higher than I would like.
The F31fd gives you lots of iso to play with as well as the full range of camera zoom so I would go for the most magnification I could get. That's what digiscoping is all about after all. Neil.