How much of FOV should be in focus?

[EricH]

Hi! I just bought Nikon Monarch 8x36 binocs and am trying to decide whether to keep them or try some others. I noticed that the inner and outer parts of the FOV don't come into focus together.

If I focus on something in the center of the FOV, then move the binocs so that the object is halfway between the center and the edge, the image becomes slightly out of focus. This becomes progressively worse as I move the object further from the center. I can bring the object back into focus by adjusting the focus knob, though of course its dimmer near the edge.

Am I just experiencing a limited sweet spot? Is this typical for binocs in this price range? Its no better in this regard than my $70 compacts - I was expecting better.

Thanks for any comments!

[ND2000]

EricH -

Yes you are just experiencing a limited sweet spot and yes it's typical for roof prism binoculars in that price range. My experience is you need to spend about $700+ to get a really large sweet spot. Only if you spend $1,000+ can you get real edge-to-edge sharpness, and even then the outer ~10% is not razor sharp.

If your budget keeps you under $500, I would strongly urge you to consider a pair of porro prism binoculars. Most roofs are no good at that price.

ND2000

[ThoLa]

Hi! I just bought Nikon Monarch 8x36 binocs and am trying to decide whether to keep them or try some others. I noticed that the inner and outer parts of the FOV don't come into focus together.

If I focus on something in the center of the FOV, then move the binocs so that the object is halfway between the center and the edge, the image becomes slightly out of focus. This becomes progressively worse as I move the object further from the center. I can bring the object back into focus by adjusting the focus knob, though of course its dimmer near the edge.

Am I just experiencing a limited sweet spot? Is this typical for binocs in this price range? Its no better in this regard than my $70 compacts - I was expecting better.

Thanks for any comments!

It's perfectly normal.
It is called field curvature.
The human eye has it as well.
The human eye can focus reasonably well (at the same time) over an area of about 2 degrees.
having a "sweet spot" much larger than this does not make much sense, as it is much easier to keep a pair of binoculars steady and turn the head than to correct this defect by engineering.
All spherical lenses suffer from field curvature.
Even the most expensive binos have it.
The wider the fov becomes, the more pronounced it will be.
Tom

[EricH]

Thanks for the replies. I was wanted to make sure this wasn't abnormal.

[Tero]

I would test the 8x36 and 8x42 side by side, if you can. Despite the smaller fov, the 8x42 may be more pleasing.