Cameras and Lenses
A question about full field sharpness?
Hi all!
The bins that force me to ask this question are Nikon Hgl 8x20. I'd like to know if this is normal: I focus on a book cover 5-6 meters away focusing on the text in the center of the field of view, then I move the bins and/or eyes to see focused text in outer part of FOV, near the edge(without changing the focus) and it isn't sharp anymore.
Is it normal? (Objects observed near the edge of FOV are sharp when I focus specifically on them.) About 50-60% from center sharpness remains the same but in outer 40% it is sharp on closer objects. It is less visible on far objects(Proportions change to 70/80 to 30/20). This is probably a lame question,but I'd like to get some clarification about this. And I hope I made myself clear
You are quite observant. This common aberration is called field curvature. Binoculars differ in the amount of it, and in how it is mixed with the other edge-blurring aberrations, coma and astigmatism, which cannot be "focused away". Every binocular I have seen has more or less of what you describe, and in that same direction.
I have often used a 3-pound 7x50 that was difficult to focus because the eyepieces focused individually. I made good use of that binocular's field curvature, which happened to be rather strong, but was not complicated with the other aberrations. By freely placing the object somewhere between the center of the field and the edge, I could get a useful range of focus. The penalty I paid was increased lateral color, the closer to the edge I got.
Ron
FWIW, your HGLs show less field curvature and edge distortion than most. It's one of their strong points.