Seems I remember reading that ER is extended at the expense of FOV, a function of optical engineering.
I recall Bushnell coming out with their 6500 1.25-8x32, a 30mm, 15.8 oz DG type scope. As I recall, Dom (writing frog) was very critical of this scope. Bushnell specs said 6" ER, some write ups used a 5" ER value. At 1.25X, the FOV is 60'. A VX-6 1-6 has a low power FOV of 116', even the VX-6 2-12 has a low power FOV of 57'.
1.25X isn't that far from 1X, but the comparitive FOV is almost half the VX-6. So, for the extra 1" (or 2") ER, they gave up a bunch of FOV, which is an important consideration in a DG type scope.
Apparently the 6500 1.25-8x32 was not that well received.
DF
In the Gun Writers section, John Barsness just did an article that very topic that's referred to as the optical triangle. Basically what it means is there is a balance between ER, FOV and magnification. Some would think that the size of a scopes objective lens would have a bearing on FOV, but I don't believe it does as you can check the specs on Leupold's website and compare two similar scopes with one having a 40mm obj and the other a 50mm obj. Generally, as ER increases, FOV decreases and naturally as magnification increases, FOV shrinks as well.
The good news is that now scope makers are finding if they monkey around with the ocular assembly (size mostly) they can get around the standards once set by this optical triangle. You'll see many new scope are coming out with much larger ocular assemblies, which allows them to use larger lenses with different grind angles to get more ER while still getting good FOV figures. Unfortunately, these larger ocular assemblies create a new set of problems, the most of which is bolt lift clearance. I originally had a 30mm set of low rings for my Rem 700 300 WM, but the bolt clearance was to close to work properly when wearing hunting gloves. I went to Leupold's QR med height rings and now it is perfect, while still sitting low enough to get a great sight picture when you throw up the rifle to your shoulder.
