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Old 9th September 2013, 01:54 PM
peterh peterh is offline
Stoopit forriner.
Join Date: Aug 2013
Location: The Netherlands
Posts: 199

Originally Posted by RobF View Post
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).
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.
The Filthy Pig: Air Arms 100 (1990) + AA Olympic trigger + Welham reg + Custom Shop CS1000 stock. Status: leaking.
The Unknown Quantity: AA EV2 Mk4. + BSA Platinum 10-50x60 + Jon Harris wheel. Status: sorted up to 25 yds.
The Limping Dog: Air Arms Shamal (1989) + unknown origin sporter stock + Optisan Cobra 3-9x42. Status: slowly leaking; needs work.
The Ox: HW97k + Venom Lazaglide. Status: working just fine; needs scope.
with apologies to NJR100

Last edited by peterh; 9th September 2013 at 02:02 PM.
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