Table. 3.13.1: Angle of view for some common
birding focal lengths. Angles are measured
diagonally across the imaging frame.
As far as choosing a lens, FOV and AOV are essentially useless when comparing units of the same focal length. In the field, the above information can be useful, however, in that it tells you that in order to double your field of view you need to back up to twice your current distance to the bird (or vice-versa).
It’s an unfortunate fact of the economics of lens manufacturing that most manufacturers don’t put much effort into testing the units coming off the assembly line for defects before they’re packaged and shipped off to retailers. Camera and lens companies have discovered that it’s cheaper to just ship all products directly from the assembly line without any testing, and to allow consumers to do the testing for them. Companies rely on the fact that many consumers aren’t discerning enough to notice defects anyway, so by effectively paying for QA (quality assurance) activities only for those units returned by the few discerning consumers in the population, they can save many thousands of dollars per annum. In other words, they're hoping you're too stupid to notice any defects in their product.
Though I’ve purchased a relatively small number of cameras and lenses during my very brief photography career, I’ve encountered a disproportionate number of brand-new items that turned out to be defective—including items that the manufacturer agreed were defective after I sent them in for repairs. These include both name-brand and third-party items. As just a few examples, I bought two pro-sumer Canon cameras that front/back-focused, a Canon teleconverter that back-focused, and a Sigma 800mm lens that fell apart and had to be held together with rubber bands during the peak of the spring birding season. In all cases, the manufacturers honored their warranties by servicing the units free-of-charge.
The moral of this story is that any brand-new lens or camera, no matter who manufactured it, can turn out to be defective. Unfortunately, defects in optical equipment can be very subtle, so that you may not know for sure whether the issues in image quality that you’re noticing are due to manufacturing defects or to your technique. Issues affecting sharpness are especially difficult to diagnose, since there’s typically no standard to compare against: you may think the images produced by the lens lack sharpness, but whether that’s due to poor lens design (in which case all units of that model will perform poorly) or due to individual unit variation (i.e., random manufacturing defects) is generally hard to determine without significant effort.
The issue is confounded by the fact that what may appear to be a lens issue may actually turn out to be an issue with the camera that the lens is attached to. This is especially true of sharpness and focusing isues. Sharpness, or resolution, can be limited by the pixel size of the camera’s sensor as well as by the precision of the lens’ manufacture. Focusing problems can likewise arise from either component, with the assignment of blame being even more difficult when the camera and lens were manufactured by two different companies (i.e., when using a third-party lens with a name-brand camera). In the latter case, the lens manufacturer will have had to reverse-engineer the AF communication protocol used by the camera, so as to render their lens compatible with the name-brand camera. Sometimes this reverse-engineering process works as planned, and sometimes it doesn’t. When it doesn’t work entirely as planned, the name-brand camera coupled with the third-party lens may occasionally behave erratically, producing out-of-focus images or even refusing to focus at all in particular circumstances. In these cases it may be very difficult to decide whether to send the camera or the lens in for servicing, when a defect is suspected.
Manufacturers of lenses sometimes like to publish graphs called MTF charts to try to convince consumers that their lenses are sharper (or “contrastier”) than those of competitors. MTF is another one of those acronyms for which, fortunately, you really don’t need to know what it stands for. The short version of the MTF story is that (1) it’s a good idea, in theory, to compare lenses based on their MTF charts, but (2) in practice the MTF charts published by different manufacturers aren’t comparable because either (a) they’re scaled differently, or (b) one or both of the manufacturers have fudged their MTF charts.
Canon is one of the culprits of MTF fudging. As the fine print on their web site will tell you, their MTF charts are typically theoretical MTF charts, meaning that a published chart doesn’t show you the actual measured optical performance of a given lens, or even an averaging across a number of empirically tested lenses, but rather is a prediction based on details of the lens design. In essence, their MTF charts can be ignored. And because different manufacturers follow different protocols for constructing their published MTF charts, the other companies’ MTF charts can be ignored as well. In summary, then, I don’t recommend even looking at the MTF charts when choosing a birding lens. What’s more relevant is whether you can find a number of example images taken through a given lens showing you that the lens can deliver the image quality that you want.
In the field, lenses can receive quite a beating. When on long hikes, it’s very hard to ensure that your lens doesn’t bump into a few tree branches here and there. Even through normal, responsible use of a lens, some parts will receive a fair amount of stress over time, such as the lens mount. For hand-held lenses, the weight of the lens itself can exert a lot of torque on the lens mount, particularly if you hold the camera horizontally without fully supporting the weight of the lens. For tripod-mounted lenses, the lens mount can still receive a lot of stress during operation of the camera, for example when you rotate the camera to level the image. Over time, these forces can cause parts to become loose, especially in less solidly-constructed lenses.
During my first season birding with the Sigma 800mm f/5.6, the lens mount became loose, allowing the camera (and therefore the sensor plane) to wobble around quite a bit. Even a small amount of wobbling of the sensor plane could result in (partially or completely) out-of-focus images. Because it was the peak of the warbler migration, I was loathe to send in the lens for service, so I temporarily fixed the problem with a system of hefty rubber bands, which worked fine. After the migration was over, I sent in the lens to Sigma and they fixed it at no cost since it was under warranty. It’s worth noting that I’ve never had a problem like that with a name-brand lens—only with third-party units.
As a further testament to the build quality of name-brand lenses, I once dropped my Canon 400mm f/5.6 prime lens on a boulder from a height of 3 feet. The lens body—not the lens hood—soundly struck the boulder, but the lens continued to work fine after that, producing images fully as sharp as before.
It’s worth noting here that mirror lenses typically can’t take the same kind of abuse that “real” camera lenses (i.e., refractors) can take. Mirror lenses using a Schmidt-Cassegrain design in particular tend to be very delicate, and may require re-collimation from time-to-time to maintain sharpness, since the angle of the main mirror can shift over time, simply through normal use.