The 50mm's are sure nice when it comes to weight and bulk. Because most 50mm's eyepieces are limited to around 30x, resolution isn't bad at those powers. It's under poorish light they really fall down.
The 60-66mm class practically double the weight and take up a lot more room in the pack. I've yet to see a spotter in this class where resolution doesn't begin to quickly fall off above 45x (although that's the top range for most of them). Including the Swaro and Leica 65mm's.
But they do much better in varying light conditions, and the higher magnification definitely helps.
It's important to note, atmospheric conditions--heat waves, mist/drizzle, failing light, windy conditions that make it difficult to hold the spotter steady, often means you're turning the magnification down anyway.
77-95mm class are ridiculously heavy, big, and....expensive. I've seen too many clients practically have heart attacks at altitude with overloaded daypacks--then again, half of the "daypacks" that have become popular are big enough to camp out for 3-4 days....
Keep in mind, most of the low and mid priced spotters can show a lot of variability in optical quality from one unit to the next.
The (top end) "name" brands like Nikon, Swaro, Leica and Zeiss don't show nearly as much variability in my experience. Plus I have a lot more confidence in their quality of build after seeing the rate at which low and mid priced optics break........
In the 50mm:
Both examples of the Opticron MM3 were the best I looked through--and I did get to compare one to the latest 50mm Nikon ED. I also had the opportunity to compare a different unit of the Nikon to a Razor 50mm, and definitely liked the Nikon better. Vortex has the best warranty, Nikon's is allegedly good, don't know anything about the Opticron.
In my book, the best compromise in weight vs quality of image is the Nikon 60mm Fieldscope ED. Although the "all body" focusing ring makes it more difficult to focus while holding the body still, nor does it stick out like focusing knobs just asking to be bent or plumb broke off (I've seen a Vortex 65mm knob bent when a horse banged its head into a guy's pack).
65mmm:
The Leica is my fav, but their warranty is hit and miss, and any significant repair requires a trip back to the motherland via slow boat. The Swaro and Ziess are a close second,the Swaro has a superb warranty and has a full blown repair facility in the States. The Leica has the dual focusing knobs recessed into the body--the best of both worlds. The Swaro has the focusing ring, Zeiss the knobs.
The Razor HD and the Vanguard Endeavor are, well,...good. The Vanguard can be had for $450 currently. We know the Vortex warranty, haven't heard anything about the Vanguard warranty.
In other words, all it takes is money.........
The problem is, once we get to spend some time looking through those breathtakingly expensive optics, the others aren't as much fun to look through. So if you don't look through anything costing more than $1200, a guy can be a lot happier with the others.....
Casey
