Cameras and Lenses
Field of view
Another beginner's question:
I often read comments regarding the narrow field of view of some binoculars, particularly as a criticism of for example 10x42 v 8/8.5x42.
Why is the difference so appreciable?
A popular 10x42 has the following specs:
FOV@1000m - 112m
FOV real - 6.4*
FOV apparent - 60*
A popular 8.5x42 has the following specs:
FOV@1000m - 133m
FOV real - 7.6*
FOV apparent - 60*
The difference in linear FOV is 21m. Given that a lot of emphasis is also placed on the close focus ability of binoculars, of you scale back the linear FOV to closer distances, the difference is just 2.1m at 100m, 1.05m at 50m, or 0.53m @ 25m.
On paper it would seem that the difference is insignificant (assuming the maths is correct 
). Or, doesn't it work like that and is there another factor at play here?
The numbers are correct.
The difference in field of view is about 20%, which does make a perceptible impact.
That said, people seem to get comfortable with whatever field their glass provides pretty quickly, so it is not a killer issue, at least imho. Certainly the range of fields available is narrow, maybe from 5.5* to 8.5*.
There are no 12* field of view binocs out there now afaik, although if memory serves there were some ultra wide angle Japanese porros sold in the 60s that had a 625ft FoV at 1000 yards, over 13*. Eye relief was under 10mm, unfortunately.
These are some of my thoughts about extra wide AFOV/FOV.
I use optics for three main purposes:
1. Measuring optics, such as range finders, test and surveying instruments, which will not be discussed here, but are considered detail optics.
2. Detail optics, these are for ID’s or study of details, reading.
3. Scanning optics, for sweeping larger areas, in general just locating movements.
For detail optics such as my pocket binoculars, which are just pulled out when I want to have a closer look at something, already located, and just need more detail. This consists of about 90% of my binocular usage.
Scanning optics is what I typically use on weekends and when just searching large areas for movement or signs of things that may be of interest. Searching for random birds, etc.
For detail binoculars I like small to moderate FOV/AFOV for various reasons, mostly image quality. For myself, I am content most of the time with AFOV’s on the order of 50-55° or so, but you have to keep in mind that 95% of my optics use is working with instruments around 30x50 and FOV’s around 1.5 degrees and exit pupils about 1.3 mm, so 6.5 degree FOV’s and 2.5 to 4 mm exit pupils seem very large in comparison. I also tend to center everything on axis since about all my optics use involves a crosshair reticule.
Most of the discussion on Bird Forum seems to be in favor of very large FOV’s and AFOV’s along with greater eye relief. But there is a lot of discontent with the edge performance of the larger AFOV/FOV’s. It has to be kept in mind that a large FOV has to be a compromise of optical performance or a great deal more expensive to make the necessary corrections (Swarovision, for example).
The following considerations need to be accounted for with wide angle FOV’s:
1. As you get farther off axis, natural vignetting increases exponentially as the angle increases (darker edges).
2. Field curvature increases with off axis magnitude.
3. Astigmatism increases with off axis magnitude.
4. Shorter eye relief for the same size eye lens.
5. Increased transverse CA.
In short, smaller AFOV’s equal sharper, brighter images (eyepieces below 55 degrees AFOV are a lot easier to build and quite a bit more economical) and large AFOV’s equal larger scanning area, but with greater image degradation past 45-50 degree AFOV plus a premium cost for the larger FOV if well corrected.
I like a large AFOV as well as the next fellow, but I am well aware of the fact that good edges with large eye relief is going to cost me $$$.