Do 8x binoculars always need more lenses than 7x?

[Swissboy]

At least twice in different threads, the following topic has come up: It was stated that 7x binoculars are particularly bright because they have one lens element less than 8x models. I'd like to know why this is possible and whether this holds for all brands and objective diameters.
As a second question then, I wonder whether higher magnifications like 10x or 12x need even more lenses than 8x models. And if not, why is the 7x an exception (if it really is one)?

[AlanFrench]

At least twice in different threads, the following topic has come up: It was stated that 7x binoculars are particularly bright because they have one lens element less than 8x models. I'd like to know why this is possible and whether this holds for all brands and objective diameters.
As a second question then, I wonder whether higher magnifications like 10x or 12x need even more lenses than 8x models. And if not, why is the 7x an exception (if it really is one)?

There is no reason the eyepiece of an 8x (or a 10x) binocular needs more elements than a 7x pair. There may, however, be a correlation between apparent field and how many elements are in the eyepiece. In the world of astronomy you pay a premium for eyepieces with large apparent fields for a simple reason - you need to do more work to get good images at the edge of a wider field, and this means a more complex design with more elements.

With modern coatings, which are very effective, and glasses, which are very transparent across the spectrum, one or two elements more or less is unlikely to make enough of a difference to be perceived.

Clear skies, Alan

[Pileatus]

There is no reason the eyepiece of an 8x (or a 10x) binocular needs more elements than a 7x pair. There may, however, be a correlation between apparent field and how many elements are in the eyepiece. In the world of astronomy you pay a premium for eyepieces with large apparent fields for a simple reason - you need to do more work to get good images at the edge of a wider field, and this means a more complex design with more elements.

With modern coatings, which are very effective, and glasses, which are very transparent across the spectrum, one or two elements more or less is unlikely to make enough of a difference to be perceived.

Clear skies, Alan
Alan,

Ultravid models:

8 lens elements
7X42, 8X50

9 lens elements
10X50, 10x42, 8x42

11 lens elements
12X50

Anyone know why?


John

[AlanFrench]

Alan,

Ultravid models:

8 lens elements
7X42, 8X50

9 lens elements
10X50, 10x42, 8x42

11 lens elements
12X50

Anyone know why?


John

John,

The number of elements increases with increasing apparent field.

8x50, 8 elements, apparent field 53 degrees
7x42, 8 elements, apparent field 56 degrees
8x42, 9 elements, apparent field 59 degrees
8x32, 9 elements, apparent field 62 degrees
10x42, 9 elements, apparent field 63 degrees
10x50, 9 elements, apparent field 66 degrees
10x32 and 12x50, 11 elements, apparent field 68 degrees

The apparent field is based on Leica's published true fields (apparent field equals true field times magnification). This relationship is an approximation, but this should not change the general trend.

It takes a bit more to get good correction at the edge of a 68 degree apparent field than at the edge of a 53 degree apparent field, which tends to mean adding more elements.

There is some correlation with power since increasing magnifications reduce the true field, and if you use an eyepiece with a wider apparent field you can "buck the trend" a bit.

Clear skies, Alan

[ceasar]

I think the reason 7X bins appear to give a brighter view is because of the exit pupils they generally have. You rarely see a 7X bin with an exit pupil less than 5mm and 6mm and 7mm exit pupils are also common in 7 power bins. 8X's usually stop at 5mm or slightly highter and 10X's above 4.2mm aren't commonly used by birders.

The number of elements an eyepiece has, and the design of the eyepiece, usually determines it's FOV.

According to a formula I found in the televue website (www.televue.com. Click on "eyepieces.) the "true field of view" can be calculated by dividing the diameter of the field stop of the eyepiece by the focal length of the binocular and then multiplying by 57.3. This is an interesting site and well worth visiting although it is an astronomy site. As Alan says wide field eyepieces have more elements and are harder to correct than narrow ones and this will show up in the price. One interesting piece of info I came up with is that eyepieces do not magnify linearly across the field and a factor involving geometric pincushion distortion is applied to the lens to correct this using a formula known only to the manufacturer. Admittedly, this is a bit hairy for me. Maybe Henry or Iaparoli could explain it further.

Yours in confusion,
Bob

[henry link]

I agree with Alan's analysis. I'll stick my neck out and say that the eyepieces probably consist of a 4 element in 3 groups, a 5 element in 3 groups and a 6 element in 3 or 4 groups. The difference in light transmission between the 4 element (7x42) and 5 element (8x42) should be less than 1%. I know I can't see a difference that small. About 3% is my threshold to see any difference at all in a controlled situation, switching back and forth quickly between two binoculars. In any case the sample variation of the coatings transmission of different specimens of the same binocular is on the order of 2 or 3%, so that is a more likely explanation for any visible difference between two Ultravids in light bright enough so that exit pupil is not a factor.

Bob,

It's true that magnification varies over the field if pincushion or barrel distortion is present. In fact increased magnification toward the edge of the field is what causes pincushion and decreased magnification toward the edge causes barrel. The Televue Panoptic and Nagler eyepieces employ huge amounts of pincushion to keep stars sharp at the edge, but the result is a real field about 10-15% smaller (compared to the apparent field) than it would be with no distortion.

[AlanFrench]

I think the reason 7X bins appear to give a brighter view is because of the exit pupils they generally have. You rarely see a 7X bin with an exit pupil less than 5mm and 6mm and 7mm exit pupils are also common in 7 power bins. 8X's usually stop at 5mm or slightly highter and 10X's above 4.2mm aren't commonly used by birders.
[SNIP]
Bob

Bob,

Keep in mind that during the day your eye is the geat equalizer. In the light of day, your eye is generally only open to 2 or 3 millimeters. If the binocular has an exit pupil larger than your eye's pupil, then some of the binocular's light it blocked from entering your eye and never makes it to the retina.

Clear skies, Alan

[bradshrike]

Swissboy, I was the one that has brought this up several times on several threads... I never meant to imply it was the only reason a 7x is brighter, but it contributes to it... obviously the larger exit pupil of the 7x contributes even more. It goes without saying, everytime light has to pass through an extra glass element light transmission is diminished slightly. Obviously, at the top end of binos (I see the top as the Ultravid) the difference is tiny. However, the 7x42 Ultravid is mind blowing. My next favorite is the 8x42... the brightness of the 7x42 over the 8x42 is minor but noticeable.

[ceasar]

Bob,

Keep in mind that during the day your eye is the geat equalizer. In the light of day, your eye is generally only open to 2 or 3 millimeters. If the binocular has an exit pupil larger than your eye's pupil, then some of the binocular's light it blocked from entering your eye and never makes it to the retina.

Clear skies, Alan

Yeah Alan,
I'd have to agree with you on that.

Now, I might be mistaken on this, but I think I picked up something in one of your prior posts that indicated you are an amateur astronomer. I don't know how far you go back in it, but I can remember, in the days before super coatings and computers, that experienced observers recommended using eyepieces constructed of no more than 3 elements in order to minimize light loss. When Plossl eyepieces first came out they were first received with skepticism. As you would know, they have 4 elements in 2 groups.

Your analysis along with Henry's seems to indicate that, despite the vast improvement in coatings, all things being equal, an eyepiece with fewer elements will give a brighter view than one with more elements.

Now, how do we explain the fact that Bins, both porro's and roofs, so often have very wide fields and long eye relief in a 7 x 42 format? I've always wondered about that.

Any thoughts on that? Anybody.

Happy Thanksgiving,
Bob

[Curtis Croulet]

experienced observers recommended using eyepieces constructed of no more than 3 elements in order to minimize light loss. When Plossl eyepieces first came out they were first received with skepticism. As you would know, they have 4 elements in 2 groups.

Hmm. I don't ever remember a recommendation for 3 elements. Orthos have 4, and they were once widely considered to be the "best" type for general observing.

[henry link]

Bob,

The long eye relief in most 7x42's is a natural byproduct of the eyepieces that are used. Their eyepieces have long focal lengths and the apparent fields are seldom above 60 degrees, usually less. Given the same design a 24mm eyepiece with a 56 degree field that might be used on a 7x42 will have longer eye relief than a 16.8mm eyepiece with a 64 degree field that might be used on the same binocular body to make a 10x42. The real field of a 7x42 may be wide because of low magnification, but the apparent field will be limited by the size of the prisms and how large a fieldstop can be accomodated by the eyepiece housing.

I agree that simple eyepieces can be a bit brighter than complex ones, although I don't think anyone could see the difference between a 3 element Kellner and a 4 element Plossl with the same coatings. After all they both have 4 glass to air surfaces. The difference in transmission between the two would be much less than 1%. On the other hand it's not that difficult, for instance, to see the difference between a T* coated Zeiss 3 element aspheric eyepiece with 4 surfaces and almost 99% transmission compared to a T* coated Zeiss Diascope zoom eyepiece with 10 elements, 10 surfaces and 93-94% transmission.

[ceasar]

Hmm. I don't ever remember a recommendation for 3 elements. Orthos have 4, and they were once widely considered to be the "best" type for general observing.

Hi Curtis,
You probably don't "go back" as far as me, geezerly speaking. I was remembering back 30 or more years ago when long focal length refractors andNewtonian/Cassegrain designed reflectors were popular for use on the moon and planets and splitting doubles stars was a big deal and no one heard of DSO's and Wide Field views. The 3 element Kellner was the eyepiece of choice then. I recall comments to that effect from astronomy books of that period, or earlier. Ortho's and Plossl's came around a bit later. Even today the Edmund RKE (a reversed Kellner design) is one of the best eyepieces for this type of astronomy. Sadly, I see that Edmund has discontinued them. Their 26mm was a dandy!

Cordially,
Bob

And Henry, thanks to you also for your comments on the 7x42's FOV and eyerelief.

[Curtis Croulet]

Orthos have been around since about 1890. The oldest issue of Sky & Telescope in my possession, Jan 1959, has ads for orthoscopic eyepieces. Plössls came into amateurs' consciousness during the 1960s in the form of eyepieces from Clavé. Plössls remained expensive rarities, at least in the US, until Tele Vue introduced theirs around 1980. When I bought my Cave 8-inch reflector in 1973 (previously I'd used a scope made by my father), I specified and got orthos rather than the Kellners that ordinarily came with the scope ("lightweight deluxe"). As to geezerhood, I "discovered" astronomy in late 1954 just after turning 10. I know people were using Kellners during the 60s and 70s, but it was because they couldn't afford anything better. Doubtless some justified their use by claiming they were "better" because of having fewer elements, but economy was the main reason for choosing them.

[ceasar]

I'm a bit older than you and discovered astronomy when I was 9 or 10 also. Ortho's have been around for a long time as you stated. I believe that's where the argument originated between 3 and 4 element eyepieces. Cost was always a factor I'm sure. As Henry Link stated above, with coatings, the difference is now minimal. I don't know when coating eyepieces became common and affordable, but previous to coatings light loss through reflections was probably more noticable. Anyway, this is a bird site. I originally brought it up as a possibility for why 7x bins were brighter than 8x's. Henry and Alan cleared that up.
Bob

[Curtis Croulet]

To be honest, until you brought it up, I never noticed that 7x, 8x etc. binoculars from the same product line used eyepieces with different numbers of elements. I just assumed they adjusted the curves of the eyepiece lenses.

[Swissboy]

Thank you all for this most illuminating thread. It is clear that my original question was not referring to exit pupil size as a reason for the differences. I see mostly this was understood as I had intended it to be.

[etc]

Most interesting thread.