rangefinding & magnification

Discussion in 'General Airgun Chat' started by poison dwarf, Sep 9, 2013.

  1. poison dwarf

    poison dwarf BANNED

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    What is the generally accepted minimum mag that can be used effectively for range finding?
     
  2. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    Most scopes will rangefind pretty accurately out to 45 yards - even when the mag is as low as say 16x.

    If you're talking about the 45-55 range, then a rough finger in the air guide would be about 32x, but choosing a scope for FT is more than just looking at numbers in a scope spec.
     
  3. RobF

    RobF Administrator Staff Member

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    Really depends on the scope, as it can be the combination of a number of things like the objective size. The trend is the bigger the both, the better.

    Here's the science bit...

    Magnification when quoted, as far as I am led to believe, and not so far contradicted, is a number derived from the focal length of the objective lens cluster divided by the ocular (eye piece). The higher the focal length, the narrower the field of view (the width of what you can see left, right, up and down) and the shallower the depth of field (the amount of stuff you can see behind and in front of the target, in focus... a deep depth of field allows you to say see targets say 10yds either side in focus as well, a shallow one perhaps 2-5 yds).
    So it's entirely possible to have a 'high' mag scope which doesn't differentiate between targets behind or in front as well as another high mag scope which does... all with the same 'mag'.
    Focal lengths of lens clusters are rarely ever published, so it's difficult to know. But certainly comparing a few 50x scopes there's a different field of view between them and a different feeling between them.

    Had an interesting chat with the Khales technical expert in Germany, which confirmed quite a few theories, however I didn't get any actual specific facts to relate or compare to. Once the basic behaviour of the lens focusing is established there are other factors as to how it performs with a wheel, as the actual focusing mechanism can be changed to make it slower, faster, wider gaps etc.

    So I don't think there's an actual number per se to cling to, but going bigger is the general trend on objective and zoom, but scopes with small specifications may pip others for other reasons.

    Try before you buy... but i'd say 32x seems to be the lowest that is used, but many are using 40 or 50 in the mainstream.

    Interesting conversations with Calps & Tool in Germany about how they rangefind, suggesting good reasons why some like some scopes and some like others, due different ways of using them. The trick is to find one that matches how you like to work, being objective in making sure that's the key reason for using that scope and not something else.
     
  4. peterh

    peterh Stoopit forriner.

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    OK, so this would be a first: Rob is contradicted. But this is about a subject that I actually *do* know about, so no biggie. As bad as I shoot, I am a schooled photographer, and had to learn many things about optics that would bore the living daylights out of any sane person.

    A shallow depth of field (which is what you want for rangefinding) is determined by the focal length (the 50 in 10-50x) and the size of the lens opening (the 60 in 10-50x60). Which has nothing whatsoever to do with the eyepiece, and more with that huge bulk of glass at the front of the scope.

    This is not entirely true... the scientific explanation has baffled many, including some "professional" photographers. If you disagree, let's take it to PM and I will explain WHY this is formally not true, and I'll give you a headache in the process.
    But for all practical purposes, to the eye, the larger the magnification, the smaller the depth of focus.

    The effect on the depth of focus of the lens opening is hard, and scientifically correct. The wider your lens opens, the shallower the depth of field.

    This means that, to us FT shooters, a big scope (60mm objective) is not only useful to get more light in, but also to rangefind easier - the shallower the depth of field is, the easier it is to focus accurately.

    It would follow that an 8-32x56 would therefore rangefind better than an 8-32x44. Given that the other optical parameters of the scope would be equal, that would be true.

    "Hmm, a 'would be'. I smell a rat'.
    Yes, you're right. There are more parameters which are not expressed in numbers, and that is where RobF is essentially right. Good news is I can explain it without drilling down into the science of it... I'd have a sore throat and you'd have either a headache or a TL;DR. Probably the latter. So, here goes.
    If we'd have two scopes with equal specs, but one would have better optics (say, a Nikko 10-50x60 Nighteater, and a Nikko Diamond Mk1 10-50x60), you would see that with the latter would be possible to get a tack-sharp image, whereas with the Nighteater you'd never get that last bit of ultimate sharpness. This makes focusing a lot harder than you would think. Focusing is quite tiring for the eye, so the longer it last, the more imprecise it gets. The best scopes "click into focus" just like it sounds - you focus, you focus, and all of a sudden, the image is tack-sharp and you can stop peering down that tube.

    Plus, on top of that, with the first scope the image wouldn't be as bright, and at some point the image would become what they call "milky" (as if you're looking through a white veil), loosing all contrast. Since sharpness is perceived by the virtue of image contrast, a low-contrast view makes focusing ruddy hard.

    And then, JUST when I thought we understood it, I go ahead and complicate matters further.
    There are different ways to render an UNsharp image. Some are considered ugly by photographers, but are actually helping the FT shooter (and the astronomer peering through his Meade, while I'm at it). If you want to know more about this, google on the word "bokeh", and you will be exposed to a realm of silliness on optics. Bokeh (I prefer to call it 'out of focus-rendition, because it is less mystical) is why people like Leitz optics, and old Minolta optics.
    I think that what they would call bad bokeh is actually good for FT shooters - it might help a scope to 'snap into focus'.

    I have seen an 8-32x56 scope that would actually rangefind better at 24x mag - at 32x mag, the image quality degraded to the point where focusing would be prohibitively hard except in an open field. I've also seen a 6-24x50 scope (I recently sold that one) that, just looking at the numbers, would've been not as good for rangefinding, but its optical quality was so much better that, over 40 metres, it actually was more accurate in rangefinding than the 8-32x56 scope. In fact, my Big BSA MK1 rangefinds better than the aforementioned scope at 15x mag.

    So no, answering the numbers doesn't answer the question "which scope should I get". Only one way of knowing: try it out.
     
    Last edited: Sep 9, 2013
  5. RobF

    RobF Administrator Staff Member

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    True, but what I am saying is the actual mag number, the 50x is a figure derived from the focal length of the objective divided by the ocular. So if scope A has a focal length of the objective of a 1000mm and the ocular of 20mm it's a 50x, but that can be the same mag as scope B which has an objective FL of 2000mm and and ocular of 40mm... they perform differently despite having both quoted mags of 50x..., the latter scope B having shallower depth of field at the same mag, even though the objective sizes are the same, because the focal lengths of the lens clusters are different.
     
  6. peterh

    peterh Stoopit forriner.

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    Also true. But the EFFECTIVe focal length (determined by the actual mag number) is only one parameter. The opening (the huge, or not-so-huge, chunks of glass at the front) is another, and it is equally important, so it makes no sense to leave that out.

    As I said, the combined effects of the parameters are enough to make hardened criminals break into tears. There is so much crud involved, it's not funny. Take two identical scopes, forget to coat a few lens surfaces in one, or economise on the coating of all of them, and the resulting differenceis are nothing short of dramatic.

    \Anyway, we all agree on one point: go and have a look at the actual scope rather than at the numbers.
     
    Last edited: Sep 9, 2013
  7. 5teve L

    5teve L STOP ! HAMMER TIME !

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  8. peterh

    peterh Stoopit forriner.

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    That's pretty much okay then. I've had people tell me they felt like this when I tried to explain depth of focus...
     
  9. RobF

    RobF Administrator Staff Member

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    Peter, the focal length isn't effective or derived or determined by a mag number. It's a physical property of a lens due to it's design. It's the distance the lens focuses at when presented with light with parallel rays. It's thus that which thus determines the mag, not the other way around. And the mag is changed by the ocular's lens in proportion to that.

    Here's Celestron's page... ignore the bit about the barlow as that's an additional lens like the lens doublers you can get for rifle scopes. What's important is that you can see the mag is the derived number, not the focal length.

    http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1737

    We change ocular lenses in stargazing so we can 'zoom' into narrow field objects such as galaxies or stars, or far planets, or go a lot wider for things like the moon or sun. From memory my scope has a 24mm ocular or a 12mm ocular, but the focal length of the scope's objective is 2000mm... so it's like a 100x or 200x scope depending on what ocular I use. The mag doesn't drive the focal length, the focal length drives the mag.
     
  10. neilL

    neilL New Member

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    Isn't a Barlow lens pretty much the same as a Premier Reticle booster for Leupolds?

    A higher quality lens would normally have a low Cs (spherical aberration coefficient). Just having a large objective lens doesn't mean it will give a wider field of view or better quality if the manufacturer puts in an aperture to use a narrower angular acceptance.

    A better lens, for me, simply means far less eye strain over a long event. A cheaper lens leads to more errors.

    However, my background in in electron microscopy and where high resolution was measured in angstroms (0.1 nanometres) and the image "focus" is one of many as you play with recombination of the reciprocal lattice :)

    Neil
     
  11. RobF

    RobF Administrator Staff Member

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    I think it is... effectively it doesn't matter which way you skin the cat... boost after or before the ocular. It just means either the mag or the focal length of the ocular changes. Barlows and boosters change the ocular cluster's focal length by decreasing it's overal focal length, which boosts the mag.

    The important thing is that the focal lengths are the prime factor, and mag is derived from that. As much as i'm loathed to stump it up, Wiki has it defined here. http://en.wikipedia.org/wiki/Telescopic_sight

    If i use a my 2000mm telescope with a 10mm lens, it's a 200x mag scope. If I pick up a 1000mm telescope with a 10mm lens it's still a 200x mag scope. But the focal length is what determines the behaviour of the image you see.

    It means that despite a 10-50x56 scope being quoted as the same as another 10-50x56 they may not look the same in terms of field of view and depth of focus as each other.

    Here's some light reading...

    http://www.chuckhawks.com/telescope_focal_length.htm

    In essence though you really (imo) want long focal length... this means the light travelling through the glass gets bent less, giving it an easier time, as bending light brings in aberrations like the colour fringing you see on dark objects against bright backgrounds. Also a longer focal length will have a shallower depth of field. (which is what we want) The downside is that it will have a narrower field view, which manufacturers think we, and also some shooters, like. However I think that's a bit of a red herring as natural point of aim is what gets you on target, and the fovea means your eye is effectively blind to changes outside of a narrow field of view.... in short, the eye and brain don't pick up even really obvious changes outside of an area of about two thumbs width at arms length. You move your eye back and forth, and the brain shuts down so you don't see a blurry mess. It's clever like that.

    For me the critical prime design element of an FT scope has to orientate itself around the focal length of the objective. If the exit pupil is deemed to small once combined with the ocular, then make it bigger. 80mm would be nice. But this yields problems with glass quality. My 300 f2.8 doesn't even use glass for it's front lens element, it uses flourite for it's clarity. Ziess use these on their big Davari range (72mm objective). The other lens tech is ED glass, which is found in high end scopes like March etc... come down in price and you come down in quality. Under good conditions, not so much of a problem. Get it under stress and the differences show. Which is probably why I've never had to down mag from 60x on the March to pick out dark target detail. I'd be lying if i said there haven't been targets i've struggled with, but it's always stayed on 60x to range and I reckon that's way better than my other older scopes.

    Intersting articles here on the effects of focal length on parallax.

    http://www.snipercountry.com/Articles/Parallax.asp#Terminology

    http://www.rimfirebenchrest.com/articles/parallax.html

    But as I said before, the numbers on a box are only a guide because they are derived by the ocular, which is why i've got several scopes which behave differently at 40x, and the HFT boys know certain scopes have different PA error despite being the same mag and objective. But scope manufacturers rarely publish this focal length figue, and i think i've only seen it once. A longer focal length will give a narrower field of view and should rangefind better, all other things being equal. But fiddling with the ocular specs to change eye relief and mag etc means it's a crazy mixed up world.
     
  12. Adam

    Adam Active Member

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    Are you glad you asked? :D :eek:
     
  13. Conor

    Conor Never been banned from sales Staff Member

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    My head hurts real bad!
     
  14. berty177

    berty177 Judge Diva

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    Oh dear

    Too much sun on that fair skin complexion of yours eh fella...?


    Berty
     
  15. Lavant_Lad

    Lavant_Lad Old Git.

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    Ask Rob the time and he will tell you how to make a watch. :D:D:eek:o:eek:o
     
  16. peterh

    peterh Stoopit forriner.

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    True, again.

    But here comes the headache.

    This is all irrelevant, because (as I remember mentioning earlier) the true depth of focus of a lens system... IS NOT DETERMINED (directly) BY THE TRUE FOCAL LENGTH! The real depth of focus is only determined by the lens opening, relative to the focal length.
    And yes, I know that a 200mm tele at 2.8 gives a much shallower PERCEIVED depth of focus than a 24mm at f2.8, but that is the PERCEIVED depth of focus.

    I will see if I can find something that describes this elsewhere, because I really think that this would go way beyond the scope (hmm... pun not intended) of this forum. For us, it is really enough to understand that, the higher the magnification, the shallower the perceived depth of focus, and the easier the rangefinding.
     
  17. Sharpe

    Sharpe Semper fidelis

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  18. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    I believe you'll find the answer is 42 :)
    (But 32 would probably do)
     
  19. RobF

    RobF Administrator Staff Member

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    You can't change the lens opening on a rifle scope. A 56mm lens will always be a 56mm lens the same as another 56mm lens. But a 56mm lens with a focal length of 1000mm will have a different depth of field to that of a 56mm lens with a 500mm. It will have the same exit pupil if you use a 50mm ocular as the other would with a 25mm, and the same derived mag, but the 1000mm will have a shallower depth of field as you see it.

    Size of lens is not an indication of focal length.

    On a camera you have an aperture which opens and closes, and by the act of making it smaller, steeply angled light rays are cut out, only letting those more parallel though. This means you see less blur. But because less light comes through you see less light. You don't have this on a scope. The aperture is fixed. So with 2 lenses of the same diameter but different lengths with the same mag as another with only the focal length changing you see different effects.

    On a camera the depth of field on a 300m focal length lens is the same as 100mm lens with the same f stop/aperture only if you enlarge the 100mm shot to the same size as the 300mm. But on a scope the mag is derived from the focal lengths. So if you zoom 10x on a 1000mm scope it's the same as zooming 10x on a 500mm scope, they both zoom 10x but the 500mm scope wi present a wider field of view to the eye, and thus the depth of field in relation will appear less because the image will be the same size as presented to the eye.

    Look here. http://www.luminous-landscape.com/tutorials/dof2.shtml

    The 400mm lens has the same blur amount as the lower focal lengths as the author demonstrates because the aperture remains the same. But as the focal length lowers the image gets smaller. And because it gets smaller the ability for you to pick out differences in focus between the gremlin and the tower becomes more difficult. This is because the field of view changes with focal length and you are packing in a wider view for the same image size.
    With scopes the image size is fixed by the mag and which fixes the exit pupil/image size.

    So for two scopes, both with 56mm lenses, both with 50x mag, we will see different images with different field of view, with different presentation of the same depth of field, if those scopes use lens clusters with different focal lengths.

    Because we don't know focal lengths of lenses, it's therefore difficult to say how a scope presents its image and thus depth of field. Thus scopes which may have apparent smaller objectives, with the same mag, may work better and why other scopes with less mag may work just as well or worse.

    Generally the trend is larger objective larger mag is better. But there are scopes that buck that and I have 2 right here from the same manufacturer but different releases that do that. Same mag, same lens size, different field of view. Different ability to determine depth of field.

    I also hold that a march set on 50x will have an apparent easier time in showing differences in focus over the same ranges as a s&b 50x ft, assuming I'm correct in my memory that a s&b has a wider field of view than the march at 50x
     
  20. Conor

    Conor Never been banned from sales Staff Member

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    correct Rob,
    S&B had a much wider firld of viewthan the March that some mistakenly imagine and determine that the S&B has a brighter image than the March.

    Anyway I have a x6 lazer rangefinder here thats pretty accurate ;) only a 20mm objective too
     

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