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FOV versus Minimum Zoom on an 80+ MM scope (1 Viewer)

cjmassey

Member
Hello:

First time poster on the Scopes side of the forum. I had gotten great advice last year in the forum for my binoculars purchase (Vortex Razor 8x42). I am now in the market for a full size scope. I am a beginning birder. I'm looking for something that punches above its weight for the money. I can afford the big guns, but don't want to pay an extra $1000 for the last few increments of sharpness, etc.

I was focusing in on scopes in the $1200-$1800 range (like Razor 85mm 27-60X and Nikon 82MM ED 20-60X). I talked to a gentleman who has owned an optics store for many years and has led 100s of group birding trips, and he advised me that I should get a scope with a minimum zoom of at least 20X, for the reasons of a) being able to pull back the zoom to find objects at close range ( a bird in a close bush), and b) when other people are using the scope, a lower power zoom will allow better depth of field so refocusing will not be need for each person.

Based on his comments, I removed the Razor 85mm 27-60X from consideration (minimum zoom >20X). I am now rethinking this since I was so pleased with my Razor binos versus the upper level glass when I demoed them, and because I started thinking that isn't FOV at minimum zoom more important than magnification?

For example, the two scopes above have listed FOV at their minimum magnification of 117 FT (Vortex Razor 85 MM @ 27X) and 111 FT (Nikon 82 MM ED @20X), a nominal difference. For his second point, I will usually be using my scope by myself or one other person, so this point is not a big consideration. For this reason I am thinking about adding the Razor back to the list. Am I thinking about this correctly?

Thanks
 

jring

Well-known member
Hi,

regarding minimum magnification for a scope the two arguments for going low are in my opinion dubious.

As you found out yourself, the true field at the low magnification end is the relevant number for finding a bird while scanning with the scope (at any distance). That is mainly defined by the eyepiece construction and zoom range (the classic 3x for 20-60x magnification or 2.x for the more modern wide angle zoom EPs like in the Razor). In most cases, a fixed magnification wide angle EP for 20 or 30x will be best for scanning - if one is available.

As for the higher chance of group members being able to enjoy the view w/o refocusing at low magnification - that is probably true, although it really depends mostly on whether the person aiming the scope and setting the initial focus has close to normal eyesight (whether corrected or not) in the first place. Then others members of the group with close normal eyesight might be able to get a usable image w/o refocusing and those with glasses hopefully are corrected to normal too. In my experience with my zoom rarely leaving the 52x maximum setting (I scan with bins, memorize the position and can aim at the position with the cable tie trick), some users might need a little adjustment...
If you don't plan to use the scope to show rarities on a guided walk, that is not really an issue.

As for your the Nikon 82m ED scope you mentioned - if that is the current MONARCH 82ED-A - they also offer a 30-60x wide angle zoom which offers roughly the same true field at 30x as the standard one at 20x (2.0 deg vs 2.1) and has a higher true field at 60x (1.2 vs 1.0 deg). It shows a bit more lateral CA though... Some people in the Nikon forum are discussing adapting the Baader Mk4 astro zoom to it, which probably is the best solution.
If you were considering the older Nikon Fieldscope 82 ED, there are no wide angle zoom EPs for that and the normal zoom EPs are known to be quite narrow and not ideal (to put it mildly) for use with glasses. The classic Nikon fieldscope series is most often used with their fixed mag wide angle EPs.

Joachim
 

Tringa45

Well-known member
Europe
Field of view is one aspect and depth of field another. At 20x the depth of field is more than double that at 30x, so if you want a better view of songbirds at moderate distances than your binoculars would offer, the increased FoV and DoF at the lower magnification would make it easier to locate the birds.

However, even at 20x the DoF is still very shallow (about 1/6th that of your 8x binoculars), so the likelyhood is high that any two viewers would need different settings for optimum focus.

John
 

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