I've written on optics before, though probably not as much as I should have; and Steve H. is currently looking for some scope advice.
There's lots of good feedback over there, so let me summarize the advice he's already received; and then just raise a few points not yet addressed.
First things first, what kind of rifle are we talking about, and what is he going to do with it?
Steve has suggested that he's interested in a Savage FVSS in .223 or .22-250, to do some varminting; from prairie dogs, up into the 'yote/javelina range; out to about 300 yards maximum. He also wants to do basic target shooting at the same ranges.
I think that Savage is an excellent choice here; and happens to be a recommendation I've made to others looking an accurate rifle at a reasonable price. Unsurprisingly, considering I shoot with a number of them, most of the folks in Steves comments tend to agree.
There is a caveat to that recommendation though, and I'll get to it in a minute, because it has specifically to do with the scopes Steve is interested in.
After doing some basic research, Steve is thinking about several scopes from Nikon. The general consensus in his comments is that Nikon and Burris offer good optics, at good prices; that other lower end manufacturers are a mixed bag (you may get a good scope, you may not); and that unless you are looking to get into serious competition, or high value hunting, going much higher end is probably a waste of money.
Personally, I'd agree with the consensus there entirely. I think Nikon offers a great price for what you're getting (I actually just purchased a Nikon ProStaff 3-9x40 for my 10/22 in fact). I also second the recommendations for Burris. I believe that those two brands offer the best value proposition in sport optics right now; especially in the Monarch, and Fullfield II product lines.
Now, the scopes he is currently considering, are the Nikon Monarch 4-16x50 and the 6-24x50; and this is where I want to step in with a few points.
A lot of people don't understand that your choice of stock, is very important to your choice of scope (and vice verse). A stock should be suited to the shooting you are going to do with it, and a scope should be suited to both the shooting type, and the stock type.
Why is the stock specifically important to the scope?
In a word: Consistency
You need to have proper optical alignment and a good cheek weld, to ensure consistency in your shooting.
A 50mm or larger objective lens will generally require extra high rings (on most guns); and therefore a high comb, Monte Carlo, or target type stock to get a proper cheek weld.
That's perfectly OK if that's the type of stock you want, but the Savage rifle in question doesn't have one; it has a low drop straight comb (the American standard basically) sporting stock.
Of course, if you want to re-stock, you've opened up a lot of options; and actually for the purpose intended, that may not be a bad idea.
The factory Savage stock isn't too bad; it's a stable composite, free floated and pillar bedded, so it's about as good as you can get without going to a hand fitted custom stock; but it IS basically a slightly modified standard sporter stock.
So why is that a bad thing?
Well, it isn't if your out hunting in the field; but for varminting and long range target shooting, you'll generally be either seated or prone.
If you're going to be doing any varminting, or any long range shooting at small targets (or small circles on paper); a sporter stock isn't the best choice, because it is optimized for fast mounting and fast followup shots, preferably offhand (standing up). When you are holding a sporter stock in shooting position, you are putting a fair bit of opposing tension on the stock to hold it steady, by design.
These are all good things for a rifle out in the field; but on the bench, they induce a slight wobble, and are fatiguing over time.
A proper target or varmint stock is designed to minimize errors induced by the shooter, and reduce shooter fatigue during long shooting sessions. They are designed to be shot from a stable rest, and to keep the shooters hand, forearm, shoulder, neck, and head, all in as neutral a position as possible; with just enough body contact to ensure consistent sight picture and trigger pull.
Importantly to this specific situation, high comb targer stocks allow for better cheek weld and optical alignment, with large objective lens scopes. Larger objective lenses need higher rings, and higher rings need higher combs to get that good cheek weld.
If we presume that you don't want to change stocks; you're probably going to want to go with an objective lens smaller than 50mm. A smaller objective means lower rings. The lower the rings, the lower the cheekweld, and therefore the more drop (or lower cheekpiece) the stock can have and still maintain proper optical alignment.
A 46mm scope can generally be mounted on high rings (rather than extra high as required by 50mm and larger). A 42mm scope can generally be mounted on medium rings, but may require high. Some rifles could take a 40mm scope on low rings, but most would require medium rings.
For the purposes of this discussion, I don't think we want to talk about scopes with smaller than 40mm objective lenses; since they would generally be of low magnification.
These are very general rules of thumb of course, and will vary between scope designs, from gun to gun; and with the degree of magnification, and eye relief involved.
Generally speaking, more magnification means longer eye relief (though this is not always the case). longer eye relief means mounting further forward, which generally allows for lower rings, because the bell of the scope will clear the smaller barrel diameter further along a tapered barrel (as most rifles have). Shorter eye relief means mounting further back, which generally requires higher rings for the bell to clear the larger diameter of the barrel, closer to the receiver.
Actually, some rifle designs are notorious for needing very high rings, relative to the size of the scope. Also, in general, semi-auto rifles need one size higher ring for a given scope size than bolt action rifles do (because they tend to have longer, and larger, receivers).
Which blends nicely into, the second big topic I want to address; that of magnification (and I've hit this subject several times before in other postings by the way).
There's a little basic optics rule of thumb you should know. Take the objective diameter, and divide it by the maximum magnification. That will, in most optics, be the diameter of the exit pupil.
On someone with healthy eyes in near darkness, the eyes pupil will dilate to a maximum diameter of between 6 and 7.5mm; so any objective/magnification combination that results in an exit pupil of more than 6.5-7mm is probably wasted (at least in rifle scope terms. For telescopes it's a different story).
Again, on someone with healthy eyes, in bright sunlight, the pupil will contract to somewhere from 2.5mm to under 2mm.
The ideal situation is that the exit pupil size, is just slightly smaller than the dilation of your eyes pupil. This gives you the full image, and the maximum amount of light available through the scope.
Generally speaking, when a scope has an exit pupil smaller than 2.5mm at a given magnification; that scope will be quite dim, and not very sharp (at that magnification), except in very bright light.
Of course, in bright sunlight, almost any optics that keep zero will work OK; because you're still getting plenty of light, even with a small exit pupil. It's in marginal light (the 3/4 of the day NOT surrounding high noon) where you want bigger, and better glass.
Generally speaking, the better glass you buy, the less diffusion and diffraction you have(brighter and sharper images). To an extent, you can compensate for a smaller exit pupil by having better glass, but it will cost you serious money to do so.
Taking all that into account, as a rule I like to keep my minimum exit pupil to between 2.5mm and 4mm; and I prefer to keep it to 6mm if I can.
I personally think the ideal short to medium range scope is a fixed power 6x42.
Generally speaking (basically, unless you're doing some very long range shooting), for variable power scopes I'd recommend either a 3-9x or 4-12x in scopes with a 3x zoom, or a 2.5-10 or 3-12 in scopes with a 4x zoom (different models and manufacturers have different zoom multipliers). Then, you chose your objective size based on what the lighting conditions you expect to use the thing will be.
Getting back to Steves rifle, the one big point I'd like to raise about what he's thinking about; is that he's looking at too much magnification for how and what he plans to shoot.
As I said before, Steve is considering a 4-16x50 or a 6-24x50.
I have a couple points where I'd object to those scopes.
First, the only way I'd ever get a 20x or higher scope, is with a 50mm objective (and 30mm tube); and even then only in the very best brands (Nightforce, Swarovski, Zeiss, Schmidt and Bender etc...) would I consider the brightness and sharpness to be acceptable.
More fundamentally though, you just don't need that much magnification for 300 yards; even for shooting a prairie dog.
Beginners tend to assume that higher magnification scopes will help them to shoot better; however beyond a basic level of magnification to see the target clearly, this is generally not true.
Greater magnification of the target, also means greater magnification of the errors you induce while breathing, moving.. even your heartbeat (and in an 18x or greater scope, you will definitely see your heartbeat).
What you want, is to have just enough magnification that you can clearly see the spot on the target you want to hit, at the range you want to shoot at. That spot, should be half the size of a circle matching the accuracy of your rifle at the range you want to shoot at.
So, if you are going to shoot at 300 yards, and you have a gun capable of shooting 1 moa; you want a scope that will allow you to clearly aim at a spot 1.5" in diameter (aim small miss small).
For most people, that's about a 6x scope by the way; but if you're a little more nearsighted, you can compensate with 8x or even 10x.
The one thing that higher magnifications are good for at low to medium ranges, is calling your shots without a spotting scope. You may only need 6x to hit your spot, but a .224" hole is pretty damn small at 300 yards. Cranking up to 16x magnification should allow you to read your point of impact; presuming your scope is of sufficient quality (to be more geeky, presuming your scope has sufficient resolving power) and your eyes and the light are good enough.
I don't mean any of this to suggest that people shouldn't buy high magnification scopes at all; just that you shouldn't buy more magnification than you really need, because you pay for it in brightness, in sharpness, in induced errors, and in hard earned cash.
For almost everybody, a 12x is going to do everything they'll ever need... maybe at most a 16x.
In fact for most peoples budgets, a 16x or higher will actually be a worse choice for short and medium range shooting; because you won't be able to buy a big enough, bright enough, high enough quality scope to make use of that extra magnification.
Get out past 600 yards and we're talking an entirely different shooting regime; where high magnification scopes make sense. They also make sense for precision benchrest shooting. But for most people, who will never shoot beyond 300 yards; they are probably doing more harm than good.
If you need that extra magnification you already know it; and you also know that you're going to pay for it.