For eyepiece projection with our Morpheus and Hyperion (zoom) eyepieces, we recommend a distance between eyepiece and T-ring of 40 mm on full-frame cameras, in addition to the 55 mm that were the flange focal distance of all "SLR" and DSLR camera bodies for decades when a T-ring was placed in front. Only with a T-ring that maintains this distance can old lenses with a T-thread deliver sharp images. Therefore, it has been tried to take this distance into account in all designs, because this is the only way to ensure that the sharpness performance is optimal. Of course, all this is only valid to a limited extent today - because the DSLR standard is losing it importance and the term „T-ring“ is all too often reduced to the mechanical thread and not the correct distances. In our brochure on digiscopy, we have therefore stated the back focal distance of 55mm. With a DSLR (full frame or APS-C), this distance is also maintained in good approximation with modern, narrow T-rings, which are intended for use on telescopes, where every millimetre often has to be haggled. For Micro Fourthirds cameras, for example, we have a special adapter that allows both an adaptation with the 55mm standard support dimension and a much shorter adaptation.
Mirrorless system cameras have a much shorter back focal distance than comparatively large SLR cameras. The available T-adapters are usually for use with telescopes with a moving focuser. Here the shorter back focal distance must be compensated by additional extension sleeves, if a certain distance has to be kept.
The example adaptations in our digiscoping brochure thus apply to all T-adapters that follow the T2 standard and deliver a back focal distance of 55mm.
The indicated distances of
- min. 40mm spacer sleeve plus 55mm T-2 flange focal distance for full format
- min. 30mm spacer sleeve plus 55mm T-2 flange focal distance for APS-C
- min. 15mm spacer sleeve plus 55mm T-2 flange focal distance for MFT
were empirically determined (Canon full format, Nikon APS-C (slightly larger sensor than Canon APS-C), Panasonic MFT) and are the values at which the image looked good, i.e. no (disturbing) distortion could be detected in nature photography. However, only the Hyperion Zoom without Barlow lens was used. For decades, we had in principle recommended the 40mm extension; with the smaller sensors, shorter distances (and less magnification) are also possible. The overall length of an adapter ring from the M43 eyepiece thread to T-2 was neglected, or it was regarded as part of the eyepiece.
With shorter overall distances than indicated above, the sharpness in the edges is catastrophic. Only with these distances the image is enlarged in such a way that the entire sensor can be used.
When working with an additional Barlow lens for even more magnification, the resulting effective focal length increases extremely and the system becomes more difficult to control. The thick stack of lenses does not make the image any better, and the telescope opening must provide sufficient resolution, also in relation to the pixel size of the camera. For highest magnifications we therefore recommend the FFC instead of the eyepiece projection in combination with a barlow lens – more on this below.
What can easily be overlooked is that, depending on the camera type and the telescope aperture ratio, a more or less pronounced vignetting is unavoidable.
We regret that we can’t give a binding (or universal) recommendation for an eyepiece-Barlow-lens-combination - for no single eyepiece on the market - where vignetting does not occur. Eyepiece projection is basically a compromise and has been "invented" for planetary photography since the beginning of all photography in order to be able to image a small object in the center of the image with excellent sharpness and very long focal length. The further development of the eyepiece also made it possible to enlarge the usable image area - however, this method is still a compromise. We therefore do not claim that you can achieve perfect sharpness over the entire image field with an eyepiece and a Barlow lens. We also do not claim that you can achieve a completely unvignetted image field!
For exactly this reason - because there has been no solution from anyone in the world (and still is not) - we asked Carl Zeiss-Jena in 1990 to calculate for us a plano barlow lens of any complexity for medium format cameras. Ultimately, this was only possible with two additional crystal substrates made of genuine calcium fluorite (not comparable to simple ED glass). For the Baader Fluorite Flatfield Converter (FFC) Barlow - which can produce an image circle of 90 mm from 4x to 8x - an aperture ratio of 1:10 - as is the case with many refractors - was the basis for calculation at that time. The 90 mm image circle was due to the fact that the famous "PENTAX 6x7" medium format camera was available at the time - with a 90 mm image diagonal.
To this day, this FFC converter lens system is "the sharpest of all Barlow lenses" in the world. And since the full 42mm format uses less than half of the FFC’s 90mm image circle, the FFC works so well even at an aperture ratio of f/5 that it delivers diffraction-limited sharpness even with CMOS chips with ~ 4my pixel size (if the telescope optics and seeing are good enough). However, this only works so well due to the large image distance. If you are looking at the "Baader FFC-Barlow", you will see that the image distance has to be varied between 80 and 150 mm (or even more). So you pay for the excellent optical performance with a very long mechanical construction. Without an excellently stable focuser, this can only cause frustration. Anyhow - You will surely find enough judgments which testify that there is no other projection lens system with sharper - and not vignetting - imaging.
So if photography with very long focal lengths is your main focus, then you would have to replace the combination of the eyepiece with a 2.25x Barlow for a FFC - in addition to the armada of extension rings, which are necessary if you want to use this projection method without mechanical distortion.