3D high fidelity - or: the curse of flat field
When comparing the best 8x42 binoculars, one of my most impressive moments was when I observed a stork about a 100 meters away with the Swarovision and then the Leica Ultravid.
The Leica had a much deeper, natural rendering of space where the Swarovision compressed the scene almost into one plane, like a cartoon.
And the phenomenon becomes even more impressive focusing on closer objects.
Other quite shocking moments I had recently when comparing my Leica Ultravid HD Plus 8x32 with the Nikon EDG 8x42 and Noctivid 8x42. The Leica just drags you into the image where the Nikon and Noctivid present a cartoonish, strangely and strongly compressed rendering of the scene. As my friend J. put it: With the Ultravid you see every branch where it should be, whereas with the Nikon and Noctivid you keep guessing where the branches are exactly. Even worse was the comparison of the Leica Ultravid HD Plus 7x42, the Zeiss Victory FL 7x42 and the Nikon EDG 7x42. I stopped testing the Nikon because the flatness of space in its image seemed unacceptable to me.
This discussion is not about differences in stereo base and true stereopsis - of course porros with their very wide stereo base (defined by the distance of the optical axes) render much more 3D than AK prism glasses, which render more 3D than SP prism glasses. The porros´ disadvantage is that the brain needs to work harder to merge the two images into a 3D one and especially so for close objects. SP and AK prism glasses are much easier, more comfortable to use.
So let´s mainly look at SP prism bins, which basically have the same stereo base.
The phenomenon of a natural, "rounded", deep rendering of space and objects is long known to photographers. It occurs in 2D images and is somehow caused by the optical properties of a single (non stereoscopic) lens system. The most common term for it is probably the "Zeiss 3D pop". Nikon recently published some interviews with lens designers where they coin the term "3D high fidelity" which seems very appropriate to me (although frankly I was unable to really understand their argument).
I was very pleased to find a demo video of Hollywood cinematographer Shane Hurlbut demonstrating a comparison of the latest flat field Leica lenses (about 20K a piece) vs the classic Cooke S4 lenses (about 10K). His opinion was that the flat Leica glass is only good "for shooting sheet metal" - but not human faces for a movie. At 6:10 you will find footage of an actress filmed with both lenses... Hurlbut suggests that field flattening and reduced distortion is the cause for flat images.He is probably right, although the phenomenon might be more complex.
Anyway it seems that field curvature, especially when the edges of the field curve away from the viewer, is important for 3D high fidelity. A classic lens example is the famous Zeiss Distagon 28/2 with its almost surreal 3D pop. But a trace of astigmatism and other aberrations may also be necessary for good 3D. In this respect, many modern lenses are overcorrected for the sake of MTF ("contrast transfer"), sacrificing 3D pop. Too many modern lenses are designed with a flattened field - to yield good edge performance at open aperture in the MTF performance - which of course is obtained with 2D test charts. Generally we are not interested in images of 2D planes but of 3D scenes and objects, especially human faces. Basically, field flatness is useless for 99.9% of photography (astrophotography being one obvious exception).
Here is an example for 85mm lenses - scroll down to the close portraits of a woman. To my eyes she looks most beautiful photographed with the Nikon, closely followed by the Canon, both classic curved field designs. But pictured with the high end, ultra expensive, technically superb and strongly field flattened Zeiss Otus her face becomes wider and much flatter.
Interestingly, the new Zeiss Milvus 85/1.4 which does not have the strong field flattening of the Zeiss Otus has extreme 3D pop - take almost any picture from the flickr group... for example this one.
Here is an example from a macro lens with great 3D pop, the Fujinon X 80/2.8. I photographed a light pattern and then defocused towards infinity. What you see is that the central part of the image bulges towards the viewer when defocused whereas the edges curve away, giving a strong 3D effect.
The 3D rendering qualities of a lens have become a main criterion for me when choosing lenses for my work as a filmmaker - because to have this 3D pop is paramount for drawing an audience into a 2D scene. 3D high fidelity definitely helps creating an emotional response when watching an image. Some of my lenses with best 3D pop: Anything from Fuji X (16-55, 50-140, 80 macro, 56/1.2), Zeiss Distagon 25/2, Distagon 35/1.4. Counterexamples: A technically great lens which I had to get rid of due to flat imagery is the Canon 16-35/4 L. The Sigma 24/1.4 Art had to go immediately after I compared to the Zeiss 25/2. Also, I tested a 25K Canon Cinezoom vs a similar one from Fujinon and the Canon was awfully flat. Lens designers from Angenieux, Cooke, Fujinon and Zeiss seem to have a particularly strong emphasis on 3D high fidelity. There are some possible drawbacks to this approach, such as weak edge performance and a swirly bokeh. As always, everything is a compromise, but hopefully an intelligent, balanced one.
Back to binoculars. Comparing 7/8x42s the Leica Ultravids, the Zeiss HT and Zeiss Victory FL (7x42) have good 3D pop being classic curved field designs. The Swaro SLC should be in that group, too, but I don´t remember that one well. Comparing the Leica 7x42 to the Zeiss Victory 7x42 is interesting because both have curved fields but the Leica has a much better correction of other aberrations in the outer field which is what I suspect an ideal approach - curved field and good aberration control but not overcorrection of all aberrations. The Zeiss SF is not too bad, as the field flattening is a somewhat irregular, wavy one. The binoculars with strong field flattening - Nikon EDG , Swarovision and Leica Noctivid are pretty bad in 3D high fidelity, with a very flat rendering. This is the curse of flat field.
I for sure take a lot of pleasure from a deep, well rounded 3D image when using my Ultravids. With my Nikon EDG, I just miss that special something that the Ultravids deliver in spades. As always, the choice of a binocular will always be a compromise and the Nikon has a lot of other great qualities to offer, but it does have that Achilles heel of field flattening.