digiscoping image quality: CP7900+ED82 vs CP4500+LeicaAPO77 (1 Viewer)

After having digiscoped during ~1 year with several scopes (Leica 77, Swaro ATS and Nikon ED 82) and many cameras, I believe that now I have gained enough experience to be familiar with (at least) the basics of digiscoping. However, there is something that I still do not understand with this technique and I need your help.

Many of the best digiscoped images that I have seen so far have been shot with a CP4500 on the 20-60 zoom of the Leica APO 77. I personaly owns a Nikon ED 82 and recently acquired the new 30x ocular that has been specifically developed for digiscoping use. When I put my eyes on the new 30x+Nikon ED 82, the image is really excellent, better IMHO than the image obtained with the 20-60 Leica zoom at 20x. I make this comparison, because when it comes to digiscoping, this is exactly the opposite: the image obtained with the CP4500 on the Leica is better IMO than an image obtained with a CP7900 on the Nikon.

here are some examples that I shot recently with a friend. The images obtained with the Leica:
http://www.oiseaux.ch/birdline/imgObs/18436.jpg
http://www.oiseaux.ch/birdline/imgObs/18436_s1.jpg
http://www.oiseaux.ch/birdline/imgObs/18436_s2.jpg
http://www.oiseaux.ch/birdline/imgObs/18437_s1.jpg
http://www.oiseaux.ch/birdline/imgObs/18437.jpg
and some of my images shot with the Nikon from the same location at the same moment:
http://www.oiseaux.ch/birdline/imgObs/18560_s2.jpg
http://www.oiseaux.ch/birdline/imgObs/18560_s1.jpg
http://www.oiseaux.ch/birdline/imgObs/18560.jpg
http://www.oiseaux.ch/birdline/imgObs/18556.jpg

The images obtained with the Leica are IMO better than those obtained with the Nikon. I do not understand why this is the case whereas this is exactly the opposite when I put my eyes on both eyepieces. Note also that
the specifications of the Nikon DS eyepiece 30x are really good on the paper (excellent FOV, better for instance than the FOV of both the Leica wide-angled 32x and zoom 20-60x at 20x, and also excellent eye-relief with 19mm).

What am I missing in the digiscoping scenario ??

thanks in advance for any help,
Pierre

PS: if someone is interested in a full resolution picture obtained with the CP7900 on the 30x DS eyepiece, here is a link:
http://www.pbase.com/padam/image/43690244
Note that the image is untouched with low in-camera sharpening, the image respond well to unsharp masking on PS.

interesting, could be many factors, one is that the leica scope is a TRUE APO construction, the Nikon only has ED-glass, chromatic aberration is still there in the Nikon even though for the eye it is OK. Another factor is that the leica-zoom probably has a more flat field more suitable for digiscoping, a wide-angle eye piece like the nikon 30xW may not be optimal, shake is also a factor, 30x is a bit to much for digiscoping,

gorank said:

interesting, could be many factors, one is that the leica scope is a TRUE APO construction, the Nikon only has ED-glass, chromatic aberration is still there in the Nikon even though for the eye it is OK. Another factor is that the leica-zoom probably has a more flat field more suitable for digiscoping, a wide-angle eye piece like the nikon 30xW may not be optimal, shake is also a factor, 30x is a bit to much for digiscoping,

Click to expand...

thanks for the reply. I doubt CA is a problem since I found the Nikon at least as good in this respect. If the camera can reproduce only a part of the quality I can see with my eyes, I will be happy. Shake was also better controlled on the Nikon with a sturdier tripod and we both used a remote.

Among the different factors you cited, I think the most probable is the flat field (or the lack of with the Nikon). This is a very good point you raised here. The "flatness" of the field can be a very important parameter and can explain a lot of things.

A pity Nikon does not propose a 20x instead of a 60x (who dares will buy a 60x eyepiece for digiscoping ?? I doubt they will sell only one of these....).

----
added afterwards

another argument against a CA problem: I have obtained several images that show some purple fringing with the Leica (not really disturbing but this is not the point), and close to none with the Nikon.

I see a bit of CA in all the nikon example pictures you have posted, it seems that the eye is more forgiving when looking into the scope than the camera, i have noticed the same with my fieldscope ED III 60mm. The leica example looks a little bit better in this aspect. (And a true APO construction should be better). CA also can be produced both in the camera an in the scope and sample variation could be a factor. Especially as an ordinary digital camera lens is not a precision optics instrument.
Another important factor I thought about is exit pupil, the nikon has 82/30 and the Leica 77/20, it means that the area of the exit pupil is about double the size for the leica = about one f-stop difference, could be an advantage for the leica. But it depends also on how much you have to zoom in to avoid vignetting. The CP4500 also has just a bit faster lens than the 7900.

here are the specs, the first number for the Nikon, the second for the Leica zoom at its wide settings:
magnification: 30x, 20x
angle of view: 2.4 degrees, 1.9 degrees
field of view at 1000m: 42m, 34m
exist pupil diameter: 2.7mm, 3.8mm
eye relief: 19mm, 18mm
relative brightness: 7.5, 14.8 (indeed a difference close to one stop)
twilight factor: 49.6, 39.2

but more than the numbers, optically my eyes see a better image in the Nikon that the camera cannot reproduce.
Regarding CA, here are 100% crops of two images digiscoped with the same camera (a Pana FZ3, not the panacee for digiscoping but this is not our point here), the first with the Leica, the second with the Nikon, both with a bird in strong light:
http://www.pbase.com/padam/image/43764392
http://www.pbase.com/padam/image/43764393
Ok, they have not been shot in the same conditions, but from the images I have, I do not believe CA is a problem, both scopes do pretty well. I really have the impression that the image obtained by a camera on the Leica is close to what my eyes can see, whereas the same camera cannot reproduce the image quality given by the Nikon. Really a puzzling problem...

It seems like the wide angle eye piece is optimal for viewing but not for digiscoping, the larger apparent field of view may cause problems for the camera lens resulting in CA and also fall-off in sharpness towards the edges, ie the same as lack of a flat field.

Maybe it's depend on the camera. If you can find a way to connect the CP 4500 to the nikon, and try this combo. I use this combo 30DS/82ED/CP4500 with very good result.
Alain