After reading this I feel there is somethings I need to clarify.
since the 2.5-10 is not first focal plane, the reticle will not be magnified as you twist the wheel. The picture is the only one that I could find for the 2.5-10.
He was curious if the reticle would be hard to see, I don't think that it would be, but you wouldn't be able to subtend it correctly like you would if it was FFP. So the mesurements are going to be different 2.5x, 3.3X, 5x and 10x.
The measurements are only accurate on 10x.
My thoughts on being able to subtend it, would be divided by the correct mag.
On 5x you would have to multiply it by the correct mag. 5 goes into 10 two times. So when measure it, if it's accurate on 10x (10x = .10 milrad{.36" @ 100 yards}) You would have to double the measurement for 5x magnification because it is half. And likewise for 2.5x, you would have to triple that measurement in order to match the measurements of the reticle with the turrets.
But my math is is wrong.
Q: I have an 18 inch KNOWN target that subtends 1mil on my scope which is a second FP set on 3x. How far away, based upon your information, is my target?
Intially I was (over) thinking:
On 3X you are covering 1 mil.
1 mil on 3x would be 3.33 on 10X.
On 10x your target would appear to be 60”
3.33
Equals 500.50 yards.
The amount of math on SFP is unnecessary when you could use FFP in order to subtend the reticle for ranging. Since it would be used for a hunting scope, you could use SFP, but if you wanted to use the reticle to range effectively, you have to do alot of math and conversions.
I was informed that my formula was incorrect.
The target is 18" tall, it doesn't matter if you look at the target on 1X or on 100X, the target is the same size, it never changes.
I was calculating how large the hash mark or mil-dot would be on the target not the actual target size.
The correct formula should be;
18 x 27.778
---------------
3.33
Should be 150.15 yards
1mil at 3x is the same as 3.333333333333335 (approximtely)mils at 10x therefore your distance would be 150yds. Same scope at 5x, 1mil would represent 2mils in actuality, at 20x would represent .5mil. Apparent size of target is meaningless. Target is still 18 inches only the mil representation has changed.