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ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

Zeiss uses AK prisms - so why not an open bridge design? (1 Viewer)

S

spitfiretriple

Well-known member
Zeiss has used, and continues to use, Abbe-Koenig prisms, while almost all of the rest of the roof world uses Schmidt-Pechan. AK prisms are lighter, and give a brighter image, but are longer, making for a longer bin. The longer tubes however, would be perfectly suited to an open bridge design. Why doesn't Zeiss follow Swarovski and move to open bridge? Or have I effectively just answered my own question?
 
The AKs are not quite straight-through, so the objective end of the barrels are farther apart than if SPs were used. This results in a unique ergonomic feature. There's lots of thumb room under the FL, though you can't see this in the usual view from above.

It annoys me that advertisements almost never show the underneath view, so you don't know what you're in for regarding space between barrels, thumb grooves, thumb support ridges that stick up, and such.
Ron
 
Ron,

Now the penny's dropped! I was always surprised by the amount of thumb room between the comparatively fat barrels of my 10x42 FLs.
Dependent upon the IPD setting, the objective spacing is 5-9mm greater than the IPD.
On my 7x42 Swarovski SLCs it is 2-3mm greater and on 7x42 Meopta B1s it is 5-6mm.

John
 
And that's why Zeiss AK bins have a slight "3D effect" somewhere between a porro and a SP roof prism.

Interesting that some other SP roofs also have some widening of the objectives.
 
I still think SP's are exactly straight through. I just measured my Leica, and I get that the eyepiece separation is precisely the same as the objective separation, measuring to about 1/32" accuracy.
John, could you recheck?
Ron
 
I think It's possible to configure SP's with a little offset. i haven't measured it but I've noticed that the Nikon 8x32 LXL appears to be offset so that the objectives are slightly closer together than the eyepieces.
 
I measured my Nikon 8x32LX and with IPD set the whole way open I measured about 17mm difference and 12mm at closest setting with the eyepieces being wider. Just approximate of course.
Regards,Steve
 
henry link said:
I think It's possible to configure SP's with a little offset. i haven't measured it but I've noticed that the Nikon 8x32 LXL appears to be offset so that the objectives are slightly closer together than the eyepieces.
Click to expand...

Henry,

I also found this to be true with the full sized LXs, even more so. Not sure how they do it since they use SP prisms.

Other roofs I've tried look like two straight tubes attached together, and if you look down the barrels from the EPs, the center of the objectives seem to roughly line up with the center of EPs, but not with the LX.

Not sure how much this contributed to the good 3-D effect, but the 8x42 LX had the best 3-D effect I've seen through a roof prism bin. The ZR 7x36 ED2 is also good in this regard.

Despite the offset on the 8x32 LX, the 3-D effect is minimal. The image looks very compressed, making it more difficult to judge distances between objects than the 8x42 model.
 
brocknroller said:
Despite the offset on the 8x32 LX, the 3-D effect is minimal. The image looks very compressed, making it more difficult to judge distances between objects than the 8x42 model.
Click to expand...

Hello Brock,

The way Steve described it , "I measured about 17mm difference and 12mm at closest setting with the eyepieces being wider," there would be no stereo effect just the opposite. That would explain the compression, just as it is in reverse Porro binoculars.

Happy bird watching,
Arthur
 
Yup, for my 8x32 LX L when the EPD is set to 65 mm the objectives are actually 61 mm apart (i.e., -6% center to center). So there would be a reverse 3-D effect making the image appear larger with less apparent depth. For comparison:

8x32 LX L > 65/61
8x30 SLC > 65/65
7x42 BGATP > 65/73
10x42 SLC > 65/68

The 7x42 BGATP, and to a lesser extent the 10x42 SLC, have enhanced stereo effect, which I think is often confused with them having greater "depth of field" then magnification would allow for.

Ed
 
For my 8x56 FL: eyepieces 65mm, objectives 79mm. Bigger prisms, more offset.

In the diagrams below you can see how the larger prisms of the 8x56 Victory (not FL) increase the offset compared to the 8x56 Classic.
 

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I daresay the exact dimensions of the prisms (SP or AK) are not carved in stone (or glass). Subject to maintaining a workable light path (bearing in mind internal reflection angles etc), I would argue there is a bit of leeway for engineers to play with the prism geometry. That could be enough to account for slightly different geometries in different bins.
 
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henry link said:
In the diagrams below you can see how the larger prisms of the 8x56 Victory (not FL) increase the offset compared to the 8x56 Classic.
Click to expand...

Thanks for that, Henry.
Interesting to see the the simple construction of the 8x56 Classic, which no doubt contributes to its excellent transmission values, and its narrow FOV.
Can you hazard a guess at its focal ratio?

John
 
John,

Yes, it has the fewest elements a binocular can possibly have. The only way to squeeze out a little better light transmission would be to cement the prism cluster. To measure the focal length just follow the line drawn through the optics from around the middle of the objective doublet glass to the "X" at the eyepiece fieldstop. Looks like the focal length is about 200mm, so around f/3.6.

Henry

Edit: Ron Watson was kind enough to send me an email pointing out that I had failed to account for the refractive index of the prism glass in my measurement. That increases the focal length to 238mm, f/4.25. I hope Ron will post his excellent image of his measurements of the light path through the 8x56 Classic.
 
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Henry,

That's faster than I imagined but I was probably fooled by the overall length.
Perhaps the shorter Victory with its telephoto or Petzval constuction is not significantly faster.

John
 
The long appearance is deceiving isn't it? The AK prism shortens the overall length by only about 20mm.

I agree that because of the complex objective design the effective focal length of the Victory might not be much shorter, if any at all. A simple measurement of the line indicates about f/3, which seems too fast, but a focal ratio somewhere below f/3.5 might explain some of the complaints about the optics of the original Victories.

Henry
 
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Pinewood said:
Hello Brock,

The way Steve described it , "I measured about 17mm difference and 12mm at closest setting with the eyepieces being wider," there would be no stereo effect just the opposite. That would explain the compression, just as it is in reverse Porro binoculars.

Happy bird watching,
Arthur
Click to expand...

...and also the larger apparent image. The image scale in the 8x32 LX looked larger than in the 8x42 model. The barrels ("stubs") of the 8x32 are very close together, in fact, the 8x32 LX fit between the barrels of the 8x50 Octarem! I wish I had taken a "Mutt and Jeff" photo of the two of them together.

The Octarem's wide separation btwn the barrels and large offset btwn the EPs and the objectives made for very impressive 3-D views.

This is my chief beef with roofs, particularly with the compacts and midsized formats - you lose the wonderful 3-D effect you get with even cheap porros, which makes the landscape look more natural and "real" looking.

Besides the aesthetics, I also find it easier to spot "camouflaged" birds such as mourning doves, which blend in easily with the gray tree bark, because the better depth perception in porros makes the birds stand out from the background.
 
Brock,

Just to demonstrate that there's never universal agreement I find that I don't like the wide spacing of Porro objectives. The 3-D effect doesn't charm me enough to make up for the poorer right and left field overlap at close distances. Perhaps that's because much of my birding is in close woodland areas.

Henry
 
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