Rifle Scopes Why, technically, does optical clarity breakdown with Magnification?

[demolitionman]

Why, technically, does optical clarity breakdown with Magnification? For example; figure I would want to buy a fixed power scope in 21 power versus 12 power. Field of View differences aside, I understand that it is typically reccomended one not run an extreme high power fixed scope because the clarity suffers and often time vignetting of the image edges becomes an issue as well. My friend and I are both working on scope upgrades and I tried explaining this to him and he thinks I'm silly. Specifically I'm referencing the SWFA fixed power scopes and the Bushnell Elite fixed power scopes. For lower end/mid priced glass I'm sure the negative attributes at higher power magnification are much more pronounced. For myself and for my shoot'n partner, why would you talk a 1000yd steel target (12"x12") gun into a 12 or 14 power over a 16x or 21x?

From my own experience I find 14x to be more than sufficient to 1000yds. I'm certain a 12 power like an SWFA would be plenty capable too. How can I explain this to my friend? Really I'm wanting to learn and I'm wanting him to go through this thread as well so he realizes a shooter doesn't have to have a 20+power scope to bang steel.

 

[billyburl2]

If two scopes have the same prescription glass, I.E. the SS fixed powers, and the only difference is the amount of magnification, the first thing to be magnified is the imperfections in the glass. On top of this, the exit pupil, is the diameter of the objective divided by the number of X's. So a 12x42 has a 3.5mm exit, but a 20x42 only has a 2.2mm.

 

[CanPopper]

There are several reasons.

1. When light passes through glass, it breaks up into many different colors. Scopes compensate for this by using either more glass at the objective (doublet or triplet) or using high index glass (think HD glass or Flourite) to bring the colors back together. These solutions aren't perfect so at higher magnification, you are more likely to see the separation of colors, epecially at the interface between bright and dark backgrounds.
2. Variable magnification scopes have more lenses than fixed power. So when light passes from one lens to the next, some of it will not make it through and some of it will separate. A fixed power with fewer lenses will inherently have less light loss and light separation (when all else is equal).
3. At higher magnification, the exit pupil becomes smaller and smaller. The human eye could have about 6mm of pupil opening in darkness and 2mm in bright lighter. At very high magnification, the exit pupil of the scope could be smaller than the pupil opening of the eye resulting in mismatched impedance (eye's ability to see light isn't fully utilized). You figure this number out by taking objective size divided by magnification. So for a 2mm pupil and a 50mm objective, you can go to 25x before you start "wasting" the eye's ability to bring in light.

A good argument for fixed powered scopes is mechanically, they are simpler so they are more durable by default. Simply fewer lenses and moving parts to break. They make good bugout scopes.