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Juggling with Magnifications (1 Viewer)

Tringa45

Well-known member
Europe
Puzzlingly large differences in weight and overall length of two related 42 mm binoculars provoked this thread, but more on that later.

Magnification is the ratio of objective focal length to eyepiece focal length or alternatively, objective aperture to exit pupil diameter.
If a manufacturer provides choices of different magnifications for a specific design it would make economic sense to achieve as much commonality of components as possible.
If, for instance, a higher magnification were desired this could be attained by either an increase in objective focal length or a decrease in eyepiece focal length.
An example of the former is the Nikon SE, which used 32 mm, 42 mm and 50 mm objectives with corresponding increases in focal length to produce 8x32, 10x42 and 12x50s, all using the same prisms and eyepieces.
Zeiss went both routes with the Conquests of the 2000s. The 8x30 and 10x30 had different eyepieces, but using 45 mm objectives with 1,5x the focal length enabled them to offer a 12x45 and a 15x45.
Another example where Zeiss used both strategies is the 42 mm FL. The 8x42 and 10x42 used different eyepieces in the same body, but the 7x42 was 9 mm shorter, so it has been generally assumed that it used shorter focal length objectives with the eyepieces from the 8x.

Today most variants within a binocular family use the same housing, but just swapping eyepieces is not that simple.
The normally sighted user will try to bring the image plane of the focussed object to near coincidence with the focal plane of the eyepiece so that the image of the object is at or near infinity.
If however one uses an eyepiece with a shorter focal length, its focal plane will be farther distant from the objective because of the fixed housing length.
Most Porro binoculars focus by racking out the eyepieces, but this is not an option on most modern roof prism binoculars with a fixed-length waterproof housing.
A near object would place the image plane further from the objective so close focus is obtained by shortening the focal length of the objective with an internal focussing lens. This can be accomplished by shifting a converging (+ve) lens towards the other objective elements as on the Zeiss SF or a diverging (-ve) lens away from them, as on most other modern roof prism bins.
Whether the travel of focussing lenses is sufficient to compensate for different focal length eyepieces we don't know. For a shorter focal length eyepiece (higher mag.) one could conceivably use a weaker +ve focussing lens or a stronger -ve one and this would not involve great expense.
Many medium-tier binoculars such as the 42 mm Meopta Meostars and the 56 mm Swarovski SLCs would appear to use scaled versions of the same eyepiece design for different magnifications, so with increasing magnification the eye relef and diameter of the eye lens diminishes. The eye relief on the 8x, 10x and 15x56 SLCs is 23 mm , 19,5 mm and 16 mm respectively.
The eyepieces of the Swarovski NL, however, would require completely different designs to maintain 18 mm eye relief and flat fields for 8x, 10x and 12x magnifications.

The two binoculars which puzzled (not only) me were the Swarovski SLCs of the 1990s and 2000s, the 7x42 B and 10x42 WB. The 7x42 is a full 17 mm longer and 80 g heavier than the 10x42. The objective design is unusual with a fixed weak +ve element and a movable achromatic doublet (see cross-section here Differences between 2003/2008 SLC binoculars?) but a scaled down eyepiece for the 10x42 could hardly account for the weight and length differences.
My 7x42 SLC of 2003 vintage, btw, was my first "good" binocular and is still much appreciated despite a few minor shortcomings. It is probably the last binocular I would part with.

John
 
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tenex

reality-based
It's an interesting question how much a focusing lens has to compensate for different eyepiece/objective pairings. (I don't think it depends on exactly where viewers want the image plane, which has been a matter of some dispute?)

What struck me about SLC 7x42 was its use of fewer and thicker elements than usual. One could wonder what inspired such a design.

This classic reassortment of eyepieces and objectives seems to be less common now, as uniform lines of binoculars are replaced by assortments of different models clustered around favorite formats. Others must always have been in much less demand, but may come to be missed if discontinued entirely.
 

Tringa45

Well-known member
Europe
It's an interesting question how much a focusing lens has to compensate for different eyepiece/objective pairings. (I don't think it depends on exactly where viewers want the image plane, which has been a matter of some dispute?)

What struck me about SLC 7x42 was its use of fewer and thicker elements than usual. One could wonder what inspired such a design.

This classic reassortment of eyepieces and objectives seems to be less common now, as uniform lines of binoculars are replaced by assortments of different models clustered around favorite formats. Others must always have been in much less demand, but may come to be missed if discontinued entirely.
If we were to assume an f/4 focal ratio for the objectives of the 7x42 SLC, then they would have focal lengths of 168 mm and the eyepieces focal lengths of 24 mm. With the same objectives the 10x42 would require 16,8 mm eyepieces and their focal planes would have to be moved about 7 mm towards the objectives, otherwise infinity focus could hardly be achieved.

Because the "focussing lens" of the 7x42 is efectively the objective and is comparatively powerful (short focal length) it has a very short travel, which I would estimate at 8 mm. This encompasses a range of 4 m close focus to 7 dioptres past infinity.

The eye relief of the 7x42 and 10x42 is 19 mm and 14 mm respectively, which would support the idea of scaled eyepieces, but would they save another 10 mm length and 80 g?

John
 

William Lewis

Wishing birdwatching paid the bills.
United Kingdom
It gets more puzzling, the 8x56 slc is 1225g the 10x is 1195g but the 15x is 1200g. I'm assuming all the weights in the glass - the 8x has the widest ocular lens but the 15x has the smallest and is still a little heavier than the 10x?
 

Tringa45

Well-known member
Europe
It gets more puzzling, the 8x56 slc is 1225g the 10x is 1195g but the 15x is 1200g. I'm assuming all the weights in the glass - the 8x has the widest ocular lens but the 15x has the smallest and is still a little heavier than the 10x?
Just a theory, but the current generation of SLCs have negative focussing lenses, so they might have to be stronger (thicker at the edges) to reach out to the more distant focal plane of the short focal length 15x eyepieces.

John
 

kimmik

Well-known member
United Kingdom
It gets more puzzling, the 8x56 slc is 1225g the 10x is 1195g but the 15x is 1200g. I'm assuming all the weights in the glass - the 8x has the widest ocular lens but the 15x has the smallest and is still a little heavier than the 10x?
The 15x has 26 elements, while the other two have 24 i believe. The extra one is thought to be a second fluorite necessary for the high magnification.
 

John A Roberts

Well-known member
Australia
SLC Optics

The all new SLC x42 models were introduced in 2010 (with revisions to the covering, focuser and internal objective construction in 2013),
and the all new x56 models followed in 2013.


In terms of the optical construction, what we do know:

The number of lenses per side (see the specification sheet):
• 9 for 8x42 and 10x42;
• 10 for 8x56 and 10x56, and;
• 11 for 15x56.

All have HD glass.
On the x42 models it’s the third lens in the objective (see Clay Taylor of SONA at: Differences between Swarovski SLC and EL Binocular )
So the x56’s also have (at least) one HD lens.

All three x56’s have Abbe-Koenig prisms verses the Schmidt-Pechan prisms used in other Swarovski roof prism binoculars.

And there’s two images showing the optical construction of the x42’s. But none of the x56’s.

- - - -
Possibly/ Likely (?):
• the x56’s all have an extra lens in the objective - perhaps a second HD one (and if only one has an extra HD lens it would be the 15x56), and;
• the 15x56 has it’s additional lens in the eyepiece.
See Roger’s reviews of the 10x56 and 15x56 models, especially regarding CA control and field flatness at: Binocular Reviews

. . . and that’s about it. We just don't know as much about the x56's as we'd like to.


John


p.s. Also see about the x56 eye lenses in post #10 below

New SLC x42.jpg
 

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John A Roberts

Well-known member
Australia
Yes, I’d forgotten about the differing diameters of the eyepiece glass!

See an image from Erik Bakker comparing the three SLC x56 eyepieces, from post #24 at:
Arrival of the new Swarovski SLC 56 family - Binoculars - Cloudy Nights

And two of an 8x56 from a 2021 eBay listing by ruacrous-0.

In his 15x56 review, Roger (see post #7) indicates that its eye lens is about 1 mm less in diameter than that of the 10x56.


John

SLC x56.jpg

SLC 8x56.jpg
 
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William Lewis

Wishing birdwatching paid the bills.
United Kingdom
Either way they are epic. I'm slimming down the optics at the moment and considering what to keep, the 8x56 slc's have the most value in them but if you've ever looked through a pair you'll know why they're last on the for sale list.
 
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