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The accuracy of lens size

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#1 DarkDisplay

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Posted 06 March 2013 - 08:28 PM

I can't understand why the supposed lens size on many binoculars is not accurate. A 10x50, for example, may actually be less than that. It might be made very well. Yet, the stated lens size is misleading. The observer would not really be getting a 10x50 view. This matters.

Two questions for those more informed than I am:

1. Why is this so common and is there any way it could be prevented?

2. Are there companies that are accurate (as is possible) when presenting the lens size for a binocular?

Much thanks for your comments.

Best wishes,
Frank

#2 EdZ

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Posted 06 March 2013 - 08:33 PM

all the objective lens sizes are accurately made. That's not where the problem lies. It's most commonly in the prisms, which are often too small to pass the entire light cone from the objective lens. Bigger prisms raises a whole bunch of additional design issues, so it's not easily solved.

edz

#3 hallelujah

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Posted 06 March 2013 - 08:49 PM

A 10x50, for example, may actually be less than that.

Two questions for those more informed than I am:

2. Are there companies that are accurate (as is possible) when presenting the lens size for a binocular?

Much thanks for your comments.
Frank


Take a look at these under the aperture heading:

http://www.cloudynig...hp?item_id=1770

Stan

#4 GlennLeDrew

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Posted 06 March 2013 - 10:23 PM

The physical clear aperture of most binoculars are as stated, with a few being perhaps 1mm smaller due to the construction of the lens cell.

The far greater problem, ad Ed pointed out, is effective aperture restriction. While we commonly think this results from too-small front prism openings, it's really because of the manufacturer's insistence (spurred by customer demand for compactness) on using objectives of excessively short focal length. If instead of the f/3.75-4 lenses so commonly used, f/4.5 or so would redress this unnecessary affliction.

#5 Jon Isaacs

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Posted 07 March 2013 - 06:25 AM

The physical clear aperture of most binoculars are as stated, with a few being perhaps 1mm smaller due to the construction of the lens cell.

The far greater problem, ad Ed pointed out, is effective aperture restriction. While we commonly think this results from too-small front prism openings, it's really because of the manufacturer's insistence (spurred by customer demand for compactness) on using objectives of excessively short focal length. If instead of the f/3.75-4 lenses so commonly used, f/4.5 or so would redress this unnecessary affliction.


The barrels themselves can be the restriction, there is not enough clear aperture for that F/3.75 focal ratio. This is probably a result of designing around the undersized prisms.

Jon

#6 DarkDisplay

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Posted 07 March 2013 - 07:32 AM

"...so it's not easily solved."

I suppose companies could put 10x50(more or less), 15x70(almost), etc. on their binos. A little sarcasm, but it's true. Some companies probably do their best. Others may not really care about accuracy. I prefer a binocular that is what it's supposed to be.

Best wishes,
Frank

#7 Tony Flanders

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Posted 07 March 2013 - 07:40 AM

I suppose companies could put 10x50(more or less), 15x70(almost), etc. on their binos. A little sarcasm, but it's true. Some companies probably do their best. Others may not really care about accuracy. I prefer a binocular that is what it's supposed to be.


I don't think this is any accident. Premium-priced binoculars today run at or very near their advertised aperture. And almost all binoculars from the 1970s or earlier ran at or near their advertised aperture.

#8 DarkDisplay

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Posted 07 March 2013 - 08:32 AM

Yes, there was a time when people took pride in the quality of the product they made. It seems that many companies today aren't like that.

Best wishes,
Frank

#9 Jon Isaacs

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Posted 07 March 2013 - 09:04 AM

Yes, there was a time when people took pride in the quality of the product they made. It seems that many companies today aren't like that.

Best wishes,
Frank


I think it more about getting what you pay for.

There have always been poorly made, under performing, "junk" binoculars. We tend to remember to the "typical" binoculars from earlier eras as the ones that are still around today, those are the ones good enough to last 40 or 50 years, the ones that were good enough for someone to hang onto for 40 or 50 years. When I travel the garage sale route, I seem pretty poor quality plastic binoculars and stamped metal binoculars from years gone by.

It's more about what the buyer is willing to accept. There have been numerous posts here about the Celestron 15x70s, Celestron is making something people, knowledgeable people, are willing to buy. The reality is, a 15x63 for $75 is still a good value and offers reasonable performance (if collimated).

If you want to purchase binoculars made with pride, there are many, many out there to choose from. But they cost more than $75.

Jon

#10 Tony Flanders

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Posted 07 March 2013 - 10:09 AM

There have always been poorly made, under performing, "junk" binoculars.


You can say that again! I still have the pair of binoculars my parents gave me when I was a young child, and they're just awful. However, they were surely marketed as kid's binoculars.

On the other hand, I have a pair of 10x50 Celestron Ultimas from the late 90s that are really quite good, both mechanically and optically. But their effective aperture is actually 46 mm. Not terrible, but enough to make a difference.

#11 GlennLeDrew

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Posted 07 March 2013 - 11:47 AM

Here's something I noted which often differentiates older from newer Porro binos.

The older ones usually employed 25mm prisms, which moreover had notable angled slices cut off on one or both sides of the apex. This allowed the field stop to be placed closer to the rear prism aperture, which in turn allows the objective to be located commensurately farther forward of the front prism aperture. A more forwardly positioned objective means its light cone is a bit narrower where it meets the front prism aperture, this lessening (or obviating) aperture reduction.

Modern binos of lower cost mostly seem to use 22mm wide prisms, which sometimes do not have the larger angled slice off the apex. The latter feature notwithstanding, the narrower prism often does not allow the field stop to be placed nearer to the rear prism aperture, thus forcing a somewhat closer positioning of the objective to the front prism aperture. And this tends to result in aperture reduction.

It's not so much about the quality as it is about dimensioning of components. If a smaller prism is used, which forces the eyepiece's field stop farther back, a slightly longer focal length objective ensures full aperture performance.

Of course, when quality is bad, a potential affliction is turned edges on lenses and prisms, due presumably to too-vigorous polishing. (I've seen pretty egregious examples of this defect.) If there is turned edge on the objectives, a reduced aperture is a benefit, for it masks the image-damaging periphery.

#12 Mark9473

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Posted 07 March 2013 - 02:15 PM

That made me smile, Glenn, "Porto" binoculars. A bit out of fashion, but still, ... ;)

#13 KennyJ

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Posted 07 March 2013 - 02:25 PM

Aren't these new "Porto" binoculars the ones made in Portugal? :-)

Kenny

#14 ronharper

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Posted 07 March 2013 - 03:20 PM

And aged 30 years in oak casks?

#15 GlennLeDrew

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Posted 07 March 2013 - 06:08 PM

Ha ha, guys. I've gotta watch what I type like a hawk, because of the auto-correction in my iPhone. But I dare not disable it 'cause otherwise I'd really be doing a lot more correcting myself. A blessing and a curse, but I more of the former if I'm honest. :grin:






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