Re: Real reticle elevation
The value that determines how far the erector has to move inside the scope to create a certain movement of the reticle on target is the focal length of the objective system of the scope.
Magnification of a scope is determined by objective focal length, (possibly variable) magnification of the erector system and ocular focal length. What matters to the user is the magnification of the scope as a whole, so objective focal length is a number that isn't stated in the spec sheet of a scope, but it is a very important number for calculations necesary to determine the thread pitch of the elevation and windage adjustment (as you rightly speculated) and the dimensions of the reticle to properly subtend pretetermined angular values.
The logic behind this is actually pretty easy to grasp. As an example let's assume that a scope has an objective focal length of 100mm (which is a realistic value for a low- to mid-powered scope), and we want to determine how much we have to move the erector/reticle internally to achieve an adjustment of 1/10 mrad.
The proportion between the movement on target and the internal movement is the same as the proportion of the target distance and the focal length of the objective system. Using 100m as a reference distance, we get 100mm/100.000mm=0.01. 0.1mrad at 100m equals 10mm, so internal movement would have to be 0.01mm or 1/100 of a mm for 0.1mrad of adjustment (that's about 0.0004" for the imperial guys out there).
There's some interesting things to learn from that: Firstly, those mechanical parts inside a scope have to be made with f@*#&ing tight tolerances to guarantee correct subtension of click values and tracking across the adjustment range. Remember that a deviation of the mechanics inside the scope of 0.01mm will cause a shift in POI of 0.1mrad on target.
Furthermore, the higher the objective focal length (which tends to be higher in higher powered scopes), the more room for elevation is necessary inside the tube. That's why scopes with higher magnification from the same series tend to have less elevation. If you create a 2-10x and a 4-20x by using a different objective system, the latter will either have half the amount of elevation or require double the space for movement inside the tube, all other things being equal.
As I said, the same formula used for the reticle adjustment figures applies to the actual dimensions of the structures on the reticle. Assuming a first focal plane reticle, crosshairs subtending 0.05mrad on target will actually measure 0.005mm (that's about 1/10 of a cunt hair in imperial units) on the glass reticle, still assuming 100mm objective focal length. These are dimensions that are at the edge of what is possible to manufacture economically, and that's why in some cases you simply cannot make a reticle as fine as some would like it.
I mentioned earlier that objective focal length is just one part of the actual magnification of a scope, and one could think that you could just use a lower objective focal length the achieve lots of elevation within the space restrictions inside the tube and adjust the other values accordingly to keep high magnification. That is possible to a certain degree (and there are designs out there with surprisingly large elevation range in a small tube), but you will run into problems with optical quality, because taking a small first focal plane image created by a short focal length objective and magnifying it afterwards is kind of like trying to make a big print from a small low-resolution photograph, the result is a big image but image quality will suffer, and, on the mechanical side, reticle structures need to be finer and there is less room for mechanical tolerances in the adjustment.
This is just a side note to show how many considerations and tradeoffs are involved in balancing the different performance criteria like loverall length, tube size and weight, elevation travel and optical quality to create a complete package. Everything comes at a price, so be sure that every time someone on the internet demands a short, lightweigt scope with hig magnification and a large zoom ratio, lots of adjustment, superior image quality and a thin FFP reticle, he'll cause some engineer somewhere to pull out some more of his hair.
Regards,
David
