There are a few other useful features that some lenses offer, which you may want to take into consideration when shopping for a birding lens.  Just keep in mind that these are far less important than the basic lens parameters that we’ve already discussed: focal length, maximum aperture, and overall image quality.

3.8.1 Full-time Manual Override

Most lenses have a switch that turns autofocus on and off.  If you find that autofocus doesn’t work well for the shot you’re trying to take (for example, if there’s a stick in front of part of the bird), then you have to flip that switch on the lens before manually focusing.  Some newer lenses feature a full-time manual override mechanism, which means that you can adjust the focus manually any time you want, without first flipping the manual/AF switch on the lens.  This is only useful in one-shot mode, after the camera has finished autofocusing, and while you still have your finger on the shutter-release button.  At this point you can manually adjust the focus and then press the shutter-release button the rest of the way to take the photo.  Note that if you manually adjust the focus before pressing the shutter-release button halfway, then the autofocus will still kick in (if you haven’t disabled it via the switch on the lens) when you press the shutter-release button, which usually isn’t what you want.  The product literature for lenses having full-time manual override make it sound like an indispensible feature, but in truth I rarely use it. 

3.8.2 Range Limiter Switches

A more useful feature is the range limiter switch, which many of the cheaper lenses don’t have, and even on the more expensive lenses it’s often not implemented as well as it could be.  This is a switch that allows you to limit the lens’ focus capability to a range of distances.  If you’re at a birding location in which all the bird are far away, you may be able to improve the autofocus acquisition speed by limiting the lens so as to focus only subjects at a distance.  Conversely, if all the birds are very close, you may improve AF speed by limiting the lens to only focus on nearby subjects.  The limiter switch typically has a threshold distance, N, which might be 20 or 30 feet or so, and may allow you to limit the lens to focus only at distances greater than N feet, or less than N feet, or to allow focusing at any distance (i.e., limiting disabled). 
    In the figure below is the control panel for a Canon 600mm lens.  The top switch is the focus limiter switch.  The leftmost setting disables the limiter, so that the lens can focus at any distance.  This is the setting that I use 99.9% of the time.  The middle setting allows you to focus only on subjects between 5.5 and 16.2 meters away from the lens, and the rightmost setting permits focusing only on subjects further than 16 meters.



    The way the limiter improves AF speed is to keep the lens from from racking in and out over its entire focus range when the camera has lost focus.  Recall from section 2.6 that SLR cameras use a trick to quickly determine exactly how much the lens’ focusing element has to be adjusted, and in which direction, in order to bring a subject immediately into focus.  For subjects with poor contrast, however, this system fails.  It can also fail if the current setting of the lens’ focusing element is far from where it needs to be in order for the subject to be in focus.  In either case, the camera will typically rack the lens’ focusing element in and out over its full range, searching for a high-contrast subject to focus on.  For some lenses this can be a very slow process.  By limiting the lens’ focusing range, you can speed up this process, since the camera has a reduced range to search through in order to find the subject.  If you’ve already got the bird almost in focus, however, the camera should be able to adjust the focusing element without racking (though somethings it does anyway, especially in low light, and especially for cameras will poor AF), and then the limiter switch will have no effect on AF speed.
    In practice, I rarely use this feature, because I always forget to turn limiting off after I move on to the next bird.  If the next bird is outside the focus range I’ve set on the limiter switch, then the lens won’t be able to focus on that next bird.  It may take a moment then to realize why the camera isn’t focusing on the bird, and by the time you realize the problem and change the setting on the limiter switch, the bird may be gone.  For that reason, I only use the limiter switch in very challenging lighting conditions, where the AF system is having trouble finding the bird and hence doing a lot of racking.

3.8.3 Lens Hoods

A very simple, but very useful, lens feature is the lens hood.  Most super-telephoto lenses (i.e., those with focal length of 300mm or more) have them.  The hood is the part of the lens beyond the last glass element (the objective lens element).  It shields the lens from the direct rays of the sunlight that can cause glare and reduce image contrast.




In the figure above is shown a 400mm f/4 lens with and without the lens hood attached.  As you can see, the lens looks much less impressive without the hood, and indeed most people don’t realize that about a third of this lens (with the hood attached) is empty.  A nice, long hood like this is very effective at reducing glare, and also helps to protect the lens itself.  With the hood in place, the objective lens element is far less likely to come into contact with tree branches or other objects that may scratch the lens.  Also, if you drop the lens, there’s a possibility that it will land on the lens hood, which may prevent damage to the lens itself.
    Not all lens hoods are removable like the one shown above.  Removable hoods are good for cleaning, since it’s easier to get at the glass to clean it with the hood removed.  However, I never leave the hood off in the field; only during cleaning of the lens.  Note also that lens hoods for expensive lenses are themselves typically expensive, so it’s a good idea to take care of them.