scampo said:
The 6900 is almost the same as the S602, if i remember, so digiscoping will prove a disappointment as the front lens element is too large and moves when the camera is switched on.
It may very well be true that the S602 is disappointment for digiscoping, but it is not because it has a large diameter lens that moves.
The main reason that some cameras are poor digiscoping performers is that their entrance pupils are set back further and or move back further when the lens is zoomed. Look into the front of the S602 (or any camera) and locate the iris/shutter mechanism. Now zoom the camera. If the iris/shutter seem to get sucked way back, the camera will almost surely have problems digiscoping since the entrance pupil is usually located near the iris/shutter mechanism. If possible, do the same with a CP99x/4500 so you have some sense of reference. You will note that on the CP99x/4500, the iris moves back and forth. Its movement is fairly limited and it stays more forward as compared to many other cameras.
If you look down the front of the CP99x/4500, you may also observe that the camera's front lens element also moves. You can get the same effective result with many other external lens cameras by using a simple adapter tube. The tube effectively makes the lens "internal". The real issue isn't internal or external, but is whether the camera has or can be equipped with a robust filter mount that the camera lens can freely move behind. My externally zooming CP5000 works pretty well for digiscoping.
The size of the front element is also not the issue. People are finding it quite possible to digiscope with DSLRs, and they have very large front elements. I've demonstrated this myself with a regular SLR and its 50mm lens which is very large.
The lens size and external movement are probably wrongly attributed as the problems for a couple reasons. The Coolpix 99x/4500 series are the most successful digiscoping cameras and they have a small and internally moving lens. Many others that do not work have do, in fact, have large diameter lenses that move exernally. But what is missed is that the large diameter lenses are usually the result of faster lenses or zooms with greater range. The zooms with greater range are usually external zooms and they usually have elements that "telescope" out quite fare. These designs are more likely to require that the lens entrance pupil moves through a greater distance or that the entrance pupil be more deeply placed.
So compact lenses of modest zoom ratio (3-4x) are more likely to have digiscoping friendly designs because its is more likely the entrance pupils on such lenses will not have to move as far or be as deeply located.
A good case that shows how lens size and configuration is an unreliable predictor is the differences between the CP5000 and the CP5400. The CP5400 came with a completely new 4x (vs. 3x on the CP5000) lens design. Both cameras have similar sized front elements and both zoom externally. But the CP5000 is a good digiscoping camera (hampered a bit by its widish zoom) while the CP5400 (which has a more digiscoping friendly zoom range) does not work well with typical birding eyepieces. A glance down the lens while zooming shows that the iris/shutter just keeps falling deep into the camera as the lens is zoomed.
We usually refer to a "good" camera as one that works with typical eyepieces that have around 20mm of eye relief. If the scope can be equipped with an eyepiece having enough eye relief to compensate for the camera's deep entrance pupil, "bad" digiscoping cameras can perform similarly to "good" ones.
Kowa has recently released a number of eyepieces that have extra long eye reliefs including one that has 57mm of eye relief designed specifically for digiscoping and/or videoscoping. There is also the quite popular Scopetronix Maxview 40 that gives around 32mm of eye relief. Depending on which scope you have, there is an outside chance that your S602 can work well if a special eyepiece with vey long eye relief is used.
