ssv1761982 said:
What is really the gain on tow comparable scopes with different tube sizes?
I have 6.5 x 20 older Leupolds. There is a 6.5 x 20 30mm with the side focus on sale locally. I like it but is it worth the $$? What would I really gain?
Given the above, all the same optics and stuff, the only thing you will gain from a 30mm tube over the 1 inch (25.4mm) tube is more adjustment range, and perhaps a small amount of greater strength in the tube. If you plan on using the tube to pound in nails, that last bit may be of some use, at least for a little while.
The amount of light entering the tube is a direct function of the size of the objective lens. The bigger the objective lens the more light that comes in. From that point on, it's all about how the light is managed in the scope in how much light will get to the eyepiece and how it will be presented to the eye.
Let's compare a 30mm scope to a 1-inch scope with the same objective lens size and the same magnification range. If the 30mm scope has a greater range of adjustment, it simply means that internally the lenses are the same size as the 1 inch scope. It will not perform any better than the latter scope, all else being equal.
You see, everything in optics is governed by well understood rules of physics and a riflescope is the expression of the manufacturer's decisions on the various trade-offs it made when putting it together. You get to decide which parameter you can change to get the performance you desire.
For example, I already stated the larger the objective, the more light that comes into the system. So, you want the biggest objective you can get but the trade-off is a bigger, bulkier scope. Not sure I would want to go hunting deer with a 100mm lens, that makes for a BIG, heavy scope. Think spotting scope size here. Also, your eye is a marvel of trade-offs and limitations and one of them is the size of the pupil. At some point your eye can't handle the amount of incoming light so there is no need to always have a massive objective lens, especially when you figure out that the size of the exit pupil, what is presented to your eye, is a direct result of the objective size divided by the magnification.
A young person's eye can use an exit pupil size of about 7mm, anything more than that is "wasted." As you age, that size diminishes by a millimeter or more over time. So taking our putative 100mm objective and a magnification of 3X, you can see that 33.3 mm exit pupil is overkill. On the other hand, at 25X on a nice day, you will have a really bright picture.
So one of the trade-offs is the size of the objective Vs the magnification. In daylight, on a sunny day, your pupil will constrict and maybe 1 or 2 mm is more than enough for a nice bright picture so you 100mm scope with a 50X magnification is performing, well, brilliantly. But at night, it sucks.
Another optical rule is that the larger the lens the more light that goes through it. We know that already from the objective lens discussion, but this also applied to the internal lenses. What? You thought the only important lens was the objective and that from that point on light was like water through a pipe? A riflescope is very different from a regular telescope such as a spotting scope, because you have a reticle that can be adjusted internally. In the olden days the crosshairs were moved to the desired point of aim inside the scope; they were not centered in the image presented to your eye Another method was external adjustment of the scope until the always centered crosshairs were finally on the desired point of aim.
Nowadays, most riflescopes have an internal tube, with a reticle centered at the front (FFP) or at the rear, behind the zoom lenses (SFP) and this tube is moved around or "adjusted" using the elevation and windage knobs. The internal lenses have to be of a size that will fit in the internal tube and still allow for adjustment room. If the lenses are the same size in a 1-inch tube and in a 30mm tube, the latter will have more adjustment rooms, WITH THE SAME SIZE internal lenses and internal tube and the 1-inch scope.
Now, when you compare a 1-inch to a 30mm scope, if they have the same adjustment range, then you can presume that the 30mm scope has larger internal lenses, especially if it's from a reputable vendor. For example, my NF NXS has the same (low) adjustment range (45MOA total) as most 1-inch scope. To me that means bigger internal lenses, better optical performance, especially at higher magnifications.
My March-X scope has a 34mm tube, but virtually the same adjustment range as the NXS; once again, bigger internal lenses and better optical performance, and I see the difference in the early mornings and heavily overcast competition days.
So the rule here is bigger lenses (in and out) provide better optical performance, but there are costs ($ and otherwise) associated with that. You can see an example of that with the sports photographers' camera lenses. Have you seen, held and priced these lenses? If you think a March-X is expensive at $3500, you will fall down when you see the prices of those lenses.
Please note that I tried to use simpler concepts in the above post and I didn't cover all aspects of a rifle scope and optics. I just tried to address the misconceptions presented earlier.
Caveat emptor is a great rule to live by.
ETC: I had mistyped 25.2mm, when it's in fact 25.4. I am mortified by that typo.
