Spotting Scope Lens Size Comparison

Discussion in 'Shooting Gear and Storage' started by D.B. Cooper, Feb 25, 2017.

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  1. D.B. Cooper

    D.B. Cooper Member

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    Regarding spotting scope objective lens diameter, is there much practical difference in image quality (at dawn/dusk) between a 60 and 80mm lens? At 60x, the exit pupil will only be 0.3mm larger on the 80. (1mm larger at 20x). I've only used scopes on ranges, so I'm looking for feedback from the group. I'm comparing the Vortex Diamondback in the two lens sizes. The 60mm is half the weight, and $100 less; is the performance difference worth all that? 3rd runner up is the Redfield Rampage. (I have a Redfield scope and binos already and generally happy with them, but the spotter is so cheap I have to wonder why.)
     
  2. rabid wombat

    rabid wombat Member

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    Think area, rather than diameter. Remember, 3.1417 x diam^2. Consider ~ 20,000 mm^2 vs ~11,000 mm^2. You have a lot more light coming through the larger glass.

    Seperately, quality matters as much (if not more than) as size. An 80 mm crappy glass will do a lot worse than a much smaller good glass.

    I am blessed, as my wife is a pro photographer. This allows me to have good glass, too. I bought a 80mm Swarovski spotting scope through Cabela's black hole. Getting it home, I started looking at birds on the wire behind the house. The bird was OK, but I read the embossing on the wire. WOW - I was reading the black on black molded embossing of wire.

    If possible, test both side by side. Ideally in the light you expect to use it. You are much better buying quality, rather than size. Buy once, cry once.
     
  3. rabid wombat

    rabid wombat Member

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  4. D.B. Cooper

    D.B. Cooper Member

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    Thanks for the web link; I never even heard of that before.

    Also, thanks for making math relevant. (I hate math, and I hate it when math is relevant, BUT...it definitely puts it into perspective.)

    60mm is 445.265 mm sq. 80 mm is 791.582 mm sq., or 56.25% larger in area.

    I was hung up on the 0.3mm difference in exit pupil size, thinking it was relatively insignificant, but even that amounts to a 59% increase in area.

    That probably settles it.
     
  5. rabid wombat

    rabid wombat Member

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    Yep, pi x*r^2. I screwed that up, but you figured that out - sorry. Please remember glass quality. Nothing better than what you see - test together if you can!
     
  6. rabid wombat

    rabid wombat Member

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    By the way DB, you should be able to afford what you want. If not, the book rights should be top shelf :)
     
  7. Bart B.

    Bart B. Member

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    That link in post #3 is for photographic lens images on sensors, not for telescopes and the human eye.

    The Dawe's limit is for telescopes: https://en.m.wikipedia.org/wiki/Dawes'_limit

    R = 116/D

    R in arcseconds (60 per MOA) where D is the diameter of the clear objective lens in millimeters.

    A 50mm lens highest resolving is about 1/26th MOA.

    Lens numbers, glass quality and coatings reduce the resolution a little.
     
    Last edited: Feb 25, 2017
  8. D.B. Cooper

    D.B. Cooper Member

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    Actually, I didn't. I calculated using diameter, so I should go back and redo it.
     
  9. Newtosavage

    Newtosavage Member

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    Lens size is only part of the equation. Coatings and quality of the glass are the other. Give me a great lens and great coatings over a larger lens any day.
     
  10. Walkalong

    Walkalong Moderator Staff Member

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    Yes, it is hard to compare between brands because of coating differences and lens quality.

    That said, between the same model, and between comparable brands, 80 vs 60 is a big difference in my opinion. Especially if you will be using it in low light. In bright light you can get away with many things you cannot in low light.
     
  11. WhiteFoot

    WhiteFoot Member

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    I think that there is a big difference as well. I would go with the larger one. Unless you're packing in miles and miles to hunt, where every ounce and inch counts, you won't regret it. I love my 80mm and I backpack hunt with it. Worth it IMO.
     
  12. Howard Roark

    Howard Roark Member

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    There's a formula for comparing light gathering power between two scopes based on objective diameter.

    LGP= (D1/D2)^

    (80/60)^ = 1.78

    The 80mm gathers 1.78 times as much light as the 60mm. How much is lost through the lenses is another story as pointed out above.
     
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  13. D.B. Cooper

    D.B. Cooper Member

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    Guess I was away for a bit. Busy week at work. Anyway, given the mathematical comparisons, and that, for the most part, this is a range tool, with about 10 days afield each year...and most of that on a 4 wheeler or in a boat...I went with an 80mm Vortex Diamondback. Bottom of their line, for sure, but better than all of the "budget" optics out there.
     
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