Originally Posted by neilL
Isn't a Barlow lens pretty much the same as a Premier Reticle booster for Leupolds?
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...
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.
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.