Ok, I have been looking at the exact same binoculars – Zeiss Victory FL 10x56 and Swarovski EL 10x50. I have compared them side-by-side, and they are both extremely good binoculars. I don’t think that you would be unhappy with either, but there are differences between the two. Forgive me if this is a bit long winded, but if I am going to spend £1500-1800 on anything, you can bet I will agonise and over-analyse the thing!
Firstly, 8x or 10x? You get a wider field of view with the FL 8x42, FL 8x56 and EL 8.5x42, compared to any of the 10x binoculars from the same makers. Yes, some people find that they can see more detail with 8/8.5x because there is less hand-held image shake. But, when you get into low light, assuming a decent aperture of 4mm+ you will resolve more detail in low light with 10x than you will with 8x because you get a bigger image. The visual acuity of the eye is so poor in low light that magnification trumps aperture (assuming that the aperture is big enough to let sufficient light through for you to see. Yes, you might get a brighter image with 8x because of a larger aperture letting more light in, but believe me, looking at a deer in the almost dark, most of the brightness will be from the background and the deer will appear black. With 10x, you are more likely to see more detail. That is the basis of the twighlight factor specifications for the binocular. It is not a perfect mathematical formula, but comparing the binoculars that people actually buy, it is a useful one.
I have 10x42 EL at the moment and I think that you are correct to be looking at 10x50 or 10x56. Why? Because they offer a larger aperture and consequently offer an easier view. Part of the reason that 8x42 and 8.5x42 offer such a pleasing view is due to the 5.25/5mm aperture. I would not personally buy another 10x42 for dawn/dusk viewing, because 10x50/56 offers a much more relaxed view and lets so much more light through.
So what is the real difference between the Swarovski EL and the Zeiss FL? They are both excellent in their own way, but the design engineers made a series of choices which reflect the different design philosophies. The Zeiss uses Abbe-Konig prisms in the x42 and x56 models, while Swarovski uses Schmidt Pecan prisms (which Zeiss also uses in the x32 models). While there may be arguments over the numbers, apparently Abbe-Konig prisms are more efficient, offering around 2% better light transmission (I read that Zeiss reckons its x32 models have 2% lower light transmission compared to the x42 models simply because of the different prisms). The different prisms also dictate the shape of the binocular barrels, with the Swarovski being narrower than the Zeiss FL.
Compared to the Zeiss Victory FL, the latest Swarovision EL also includes extra eyepiece elements to compensate for distortion at the edge of the field of view in order to produce its signature ‘flat field of view’ with ‘sharp’ edges. This provides stunning static images, no question. However, there are two trade-offs in producing this flat field image. Firstly, and possibly most importantly, the compound distortion used to achieve the sharp edges means that when panning, you can set up a globe effect (also known as rolling ball) where you can see the image bending/rotating at the edge of the field of view (in most binoculars, this is countered by varying amounts of pincushion distortion). Some people find this distracting, even nauseating, while some don’t notice it. Using the EL 10x50, it jumped out at me when panning along the edge of woodland and bramble covered banks, but I did not get to use it for long enough to know whether I would get used to it or not. It is distinctly different to the pre-swarovision EL, Zeiss FL and Leica Ultravid HD. Secondly, the more eyepiece elements you have, the more glass-to-air surfaces you have and the more light transmission is reduced. I have not seen measurements for the EL50, but I think that it is commonly accepted that Swarovski designed the EL for maximising colour rendition and contrast, which it does very well, rather than transmission (the opposite was true for the SLC). Whether you can see the 2-3% or so difference in transmission is a moot point.
The Zeiss Victory FL does not have these additional flat field eyepiece elements, but (as for all binoculars, including the EL) uses pincushion distortion to correct for the rolling globe/rolling ball effect produced when panning. The binocular was designed to produce maximum light transmission and a superlative images across the centre of the field of view. This is achieves very well, with great colour definition and high contrast. However, in contrast to the Swarovski, the edges of the field of view are not clear. I could see a ring around the edge of the field of view which was not in focus (astigmatism). Just as some people find the Swarovision sharp edges (rolling ball) distracting, some people find the relatively soft edges of the Zeiss FL distracting. But, no-one objects to the superlative centre-field image in the Zeiss FL.
I haven’t seen light transmission curve graphs for the FL 10x56 or EL 10x50, but these graphs show the difference between the 8.5x42 EL Swarovision and the 8x56 FL. For info, the visible part of the spectrum is between 380 and 750 nm.
