Both side-focus and an adjustable objective essentially correct the focus of the reticle, but the side-focus does it inside the scope, rather than by moving the objective lens. Generally an adjustable objective is considered more precise and perhaps tougher, but side-focus is certainly handier--at least for right-handed shooters!

I don't know why a side-focus scope would be dimmer than an adjustable-objective scope, and haven't noticed it in using a number of side-focus scopes. Is the reticle in your older scope a plain plex-type, and the reticle in the new scope a "ballistic" type? Usually plain plexes are wire, while ballistic reticles are etched. The glass that reticles are etched on is usually uncoated, and can slightly dim the image, though the difference normally isn't vast.

The reticle in the older scope is a Nikoplex, as is the reticle in the newer scope. This scope was unbelievably dim, as in the Bushnell Trophy 3-9x40 that we mounted on his .22 LR and sighted in at the same session was much, much brighter, as was the Nikon Prostaff spotting scope we were using. I didn't say anything because I didn't want to offend the BIL, but I certainly made note of it. I'll check it out again while we're hunting together next weekend to see if it was as bad as I'm certain it was. I'm a big fan of Nikon optics and was disappointed to see how dim that scope was!

That's very interesting. I wonder if something's wrong with it.

I am wondering about this too. I am looking for a scope that has very good low light performance and it seems to me that things appear "farther" in poor light. The natural response is to want to increase magnification, but then you must also increase the objective lens diameter.

Not necessarily, but sometimes. It depends on the magnification and the person's eyes.

The twilight factor formula is an attempt to resolve these factors.

MD,

Do you have the Twilight Factor Formula handy?

TF is the square root of magnification x objective lens diameter (in millimeters).

A small adjustment:

TF is the square root of (magnification x objective lens diameter)

Thanks!

so should I buy a Conquest 3x9x50 or a 3.5 x10x50 for my next 223 caliber deer hunting rifle? No shots are ever over 150 yards but I do like the big image in the 50mm scopes. The 3.5 x 10 is an inch longer and 3 ounces heavier but my eyes are getting weaker.