The two most important concepts regarding birding lenses are the focal length and the aperture.  You can think of the focal length as being a measure of the amount of magnification that the lens provides (i.e., to what degree it makes a small bird appear larger), whereas the aperture influences (among other things) how bright the resulting image will be.  (We’ll be refining and augmenting these basic definitions as we progress through this chapter, but for now these will serve as a good starting point for our discussion of these most fundamental concepts about telephoto lenses).
    Unlike with binoculars and spotting scopes, where the magnification is specified explicitly as, e.g., 8× or 10× (in the case of binoculars) or perhaps 30× to 60× (in the case of spotting scopes), in photography the issue of magnification is confounded by several factors, including the focal length of the lens, the degree of cropping performed either by the camera’s imaging sensor or by software in postprocess, and any enlargement done by the photo lab when making prints.  For this reason, magnification ratings for lenses are rarely given (and are typically wrong when they are given—so beware!). 
    One thing you can firmly rely on is that longer focal lengths enlarge the bird more than smaller focal lengths.  Thus, for birds that are either small, or distant, or both, you’ll want to use a lens with a longer focal length (i.e., a “longer lens”) than one better suited for large birds close-up.  For warblers and other tiny birds in the wild, a good focal length in practice is around 800mm (give or take a hundred mm), whereas for herons and egrets at a distance of perhaps 10 or 15 feet, a much smaller focal length in the 50-200mm range may be more useful.



    As suggested by part A of Figure 3.1.1 (above), the overall length of a telephoto lens is typically indicative of the relative focal length of the lens: longer lenses most often have longer focal lengths and therefore higher magnification factors than shorter lenses.  This is only a very general rule of thumb, however, since there are a number of special optical designs that can result in large focal lengths in a relatively small package (such as catadioptrics or diffractive optics), and also because the issue of zoom lenses complicates the issue as well.  We’ll address these latter exceptions in due course.
    The important point to be conveyed by part A of the above figure is that it is both the length and the width of the lens that (very broadly speaking) largely determine the brightness of the image.  A long lens that is very narrow will thus tend to produce high magnification but dark images.  Conversely, a short lens with a very wide objective diameter (i.e., the wide end of the lens) will tend to produce only modest magnification, but should be very bright.  Later we’ll consider a number of caveats to this general rule.
    First, let’s dispense with a common (though rather mundane) source of confusion regarding terminology.  To an optician or physicist, a “lens” is a single piece of thin glass with (typically) two convex surfaces.  To a photographer, a “lens” is a complete package consisting of a plastic or metal outer casing enclosing perhaps 10 or 20 glass elements (each of which a physicist would call a “lens”).  Unfortunately, we’ll have to use both of these definitions of “lens” at various points in this discussion, though we’ll try to use the term “lens” only for the complete package, with the individual (physicist’s) “lenses” inside that package being referred to as “glass elements” or “optical elements”. 
    One exception to this rule is the term “objective lens” or “objective lens element”, which refers to the big piece of glass at the widest end of the lens package.  It’s the diameter of the objective lens element that largely determines the light-gathering capacity of a camera lens.  However, this effect (of large-diameter objective lenses to let in more light) is highly dependent on the focal length of the complete lens package.  In fact, it’s the ratio of the focal length to the objective lens diameter which effectively dictates the overall brightness of the image that is projected onto the camera’s imaging sensor.  It’s for this reason that photographers define a special term for this ratio—a term which you’ll see again and again throughout this book: the aperture.  As we’ll soon see, this term is an unfortunate carryover of ancient photographic tradition which has resulted in no small amount of confusion by newcomers.  Hopefully, we can mitigate the amount of confusion with a simple mnemonic, which we’ll come to very shortly.
    But first, just to keep things concrete, let’s consider a quick example.  One of the most popular lenses among professional bird photographers is the 600mm f/4 lens.  This lens has a focal length of 600mm and an objective lens diameter of about 6 inches.  Since one inch is equal to approximately 25 millimeters, the diameter is roughly 150mm.  Applying the formula from part B of the figure above, we get:


Thus, for a lens with a 600mm focal length and a 150mm objective lens diamter, we say that the aperture of this lens is f/4.  One way to think of this f-notation is to think of the “f” in f/4 as standing for “focal length”.  In that case, f/4 would mean “focal length divided by 4”, which in the example above would give us 600/4 = 150, or the objective lens diameter.  Thus, the “aperture” is what we would intuitively expect it to be: the size of the opening through which light is collected.
    Notationally, the aperture of the lens in the preceding example is emphatically not 4—rather, it’s f/4.  That is, the f-number is “4” but the aperture is “f/4”.  That’s important, because while you’d expect a larger “aperture” (hole that lets in light) to result in brighter images, larger f-numbers (such as 8 instead of 4) actually let in less light than smaller f-numbers.  That’s because the f-number occurs in the denominator of the f-notation.  Given two apertures, f/a and f/b, if a>b, then f/a will be smaller than f/b, since a and b are in the denominator, and thus f/a will be a darker aperture than f/b, since it denotes a smaller objective lens diameter, which would let in less light. 
    Let’s consider a few more concrete examples.  In Figure 3.1.2, below, we show four super-telephoto lenses: two 400mm lenses (f/4 and f/5.6), a 600mm lens (f/4) and an 800mm lens (f/5.6).  These are all common birding lenses.  Considering first the two 400mm lenses (both at left), we can see from the figure that the f/4 lens is wider than the f/5.6 lens.  The f/4 lens will thus let in more light, allowing faster shutter speeds that will be better able to freeze the action of the bird and thereby produce sharper images of non-static subjects.  The f/4 lens will also be more desirable in early morning or late evening, or in cloudy conditions, when the faster shutter speeds allowed by f/4 will help to overcome any hand-shake that you experience when taking the shot (if you’re not using a tripod).  Of course, the wider lens will also be heavier and may therefore cause more hand-shake, but as we’ll discuss in section 3.5, image stabilization can to some degree compensate for these issues.




Compared to the 400mm lenses depicted in the figure, the 600mm and 800mm lenses are obviously longer.  To compare widths, let’s first consider lenses of the same aperture (f-number).  Comparing the 400mm f/4 lens to the 600mm f/4 lens, we can see that the 600mm lens is definitely wider than the 400mm lens.  But because 600mm is longer than 400mm, the increased width of the 600mm lens still only results in an aperture of f/4, so that both lenses will produce roughly the same brightness in the resulting images, despite the 600’s wider opening.  Similarly, while the 800mm lens shown above is much wider than the 400mm f/5.6 lens, the fact that they both result in a length/width ratio of 5.6 means that they have the same f-number, and should therefore produce images of roughly the same brightness (all other things being equal). 
    Keep in mind that the issue of brightness typically translates to shutter speed or other camera settings.  In the figure above, you can see that the 800mm and 600mm lenses have the same objective lens diameter (6 inches), but since they have different focal lengths, the apertures end up being different (f/4 versus f/5.6), so the brightness will be different: the 800mm lens will require slower shutter speeds or higher ISO settings than the 600mm, because it lets in less light per unit time than the 600mm, despite having the same objective lens diameter.  In other words, your photos will be darker when taken through the f/5.6 lens than when taken through the f/4 lens, unless you compensate via camera settings (such as shutter speed or ISO).  But the act of compensating via camera settings can sometimes have unfortunate consequences: for poorly lit scenes, you may be forced to use a shutter speed that is so slow that the bird looks indistinct, due to motion blur (motion blur is discussed in Chapter 6), or if you instead just increase the ISO you may end up with an image that is very noisy.  In this way, the aperture of a lens can seriously affect the quality of your images, and this is why professionals generally prefer lenses with the largest apertures.
    Up to this point, we’ve been talking about the aperture of a lens—as if lenses have only a single, fixed f-number.  In truth, what we’ve really been talking about is the maximum aperture of a lens.  Most camera lenses have a built-in iris, or diaphram, which can open or close to various sizes to restrict the amount of light entering the lens, as illustrated in Figure 3.1.3 below.  In this way, the aperture of a lens can be reduced, when needed.



In the figure above, we’re looking through an f/2.8 lens with the iris set to different settings.  In the leftmost image, the iris is wide open, giving us an effect aperture (f/2.8) that is the same as the maximum aperture of the lens.  In the middle image, we’ve closed the iris (or stopped down) quite a bit, resulting in an effective aperture of f/11.  In this middle image you can see that the iris is not perfectly circular: in this case it’s a heptagon (i.e., like a hexagon or octagon, but with 7 sides); we’ll comment on this later, in section 3.7.  In the rightmost image, we’ve stopped down even further, to f/32, and now you can see that the opening is very small indeed.  For practical bird photography, apertures smaller than f/11 are rarely used.
    In addition to letting in more light, a lens with a large maximum aperture (i.e., a smaller f-number) also permits the use of shallower depth-of-field (DOF), which can be useful in isolating your subject from the background and/or foreground.  This can make a big difference in the aesthetics of your photos, since the bird will stand out more, and the background will be less likely to distract the viewer from the beauty of the bird.  The photo below shows five female Boat-tailed Grackles (Quiscalus major) perched on a wooden railing at a park; because I was using an f/4 lens and was shooting wide open (i.e., at maximum aperture), I was able to isolate one bird from the group, despite the other birds being mere inches away from the subject in focus.




In practice, effects like that shown above can be achieved in software if you don’t have a wide-aperture lens.  If I had instead shot this image at f/11, the other birds would probably have been at least partially in focus, distracting the viewer’s attention away from my intended subject (the single bird I was focusing on).  To get the effect illustrated above, but at f/11, I could have opened the image in Photoshop, painstakingly traced the outline of the second bird from the right, and then applied a lens-blur filter to the rest of the image.  This can take a lot of time, though, so it’s much easier to achieve this kind of effect with a wide-aperture lens (if you have one).
    To briefly summarize, the maximum aperture of a lens can affect the depth of field of your images, as well as the maximum brightness, which in turn affects the shutter speeds and/or ISO settings you’ll end up using with that lens, and these latter settings can affect the image quality due to motion blur or pixel noise.  For these reasons, lenses with wider maximum apertures (smaller f-numbers) such as f/4 or f/5.6 are preferable over those with smaller maximum apertures.
    For real-life birding situations, however, the focal length typically trumps the maximum aperture of a lens in overall importance, simply because focal length translates into magnification.  In the majority of birding situations you’ll find that you need as much magnification as possible, in order to capture enough detail on the bird to make a pleasing image.  There are of course exceptions (such as large birds at close range—e.g., ostriches at a zoo), but for capturing warblers and other small birds in the wild, larger focal lengths are almost always preferred.  As the figure below suggests, the most popular focal length for pro bird photographers is 800mm, with the useful range being 400mm to 800mm.




The figure above is a histogram generated by tabulating the focal lengths used by a particular pro bird photographer (who shall remain unnamed, but is very famous and highly successful).  The height of each red bar is proportional to the number of photos that were taken at that focal length.  In this case, we can see that 800mm was used more often than 400mm or 600mm combined.  Other focal lengths outside of this range were rarely used, implying that lenses smaller than 400mm are not very useful for general-purpose birding (though they may certainly be useful in special situations).
    Figure 3.1.6, below, should give you some intuition for how focal lengths translate to magnification and “reach”.  On the left you can see a tripod-mounted camera (a Sigma 800mm f/5.6 lens with a 1.4× teleconverter attached) and a bird (an osprey) perched high above in a tree.  The image on the right was taken after the bird had moved to a more open branch at about the same distance from the camera.  This image was taken at an effective focal length of 1120mm. 




Compare this with the image shown above in Figure 3.1.4 of the grackle shot with a 400mm lens at close range (~12 feet).  In general, I find that my 400mm lens is most useful for birds a very close range, such as that grackle at 12 feet, or for large birds in flight (herons and storks, etc.), while for birds at larger distances I prefer something in the 800mm to 1200mm range (such as my 600mm f/4 lens with a 1.4× or 2.0× teleconverter).  Although I've used focal lengths as small as 70mm for photographing wild (or semi-wild) birds, these situations certainly don't span the range of possible birding scenarios.
    Unfortunately, large-focal-length lenses tend to be very, very expensive.  Canon’s 500mm and 600mm lenses run about US $5000 to $8000 new, and Nikon’s long lenses tend to be even pricier than that.  Even Sigma’s “third party” 800mm f/5.6 lens runs about $6000 or $7000 (new).  There are, however, several options available to the budget-minded birder.  First, there’s the possibility of using a 400mm f/5.6 lens with a teleconverter to achieve 560mm (with 1.4× TC) or 800mm (with 2.0× TC) fairly cheaply: a new 400mm f/5.6 lens runs around $1000 or $1500 for brand name, or less for third-party, and teleconverters range from $100 to $400.  The quality of such a setup will, of course, be no match for a $5000+ name-brand lens, especially when using a 2× TC.  Furthermore, this setup will require you to focus manually, which can be very difficult if you have less-than-perfect eyesight, or if you’re trying to capture birds in flight.  (Note that with a pro body you’d be able to use autofocus with an f/5.6 lens attached to a 1.4× TC, but not with an f/5.6 lens attached to a 2× TC; also, pro bodies run $4000+ new).
    Two other options, both of which require manual focusing, make use of cheap optics, but can sometimes produce fairly good images with some effort.  The first we’ve already discussed: attaching your camera to an astronomical telescope such as the 1800mm focal-length Maksutov telescope shown in section 2.2.  Large-focal-length mirror telescopes such as Maksutovs and Schmidt-Cassegrains can be had, new, for US $600 or so, and can be extremely sharp.  Unfortunately, they require manual focusing, provide no image stabilization (which becomes more essential at the high focal lengths these lenses provide), and typically have a maximum aperture of around f/12.  Smaller versions of these mirror telescopes have been developed and marketed specifically as camera lenses, though these have a reputation for producing low-quality images.  Such a mirror lens is shown below; these lenses often cost as little as $150 and provide 500mm of focal length.  Just keep in mind that with photographic equipment you generally get what you pay for.



One final alternative for budget-constrained birders requiring large focal lengths is the use of lenses from such manufacturers as Opteka, who offer long lenses with very small apertures at very low prices.  One such model is a zoom lens that can go from 650mm to 1300mm, with apertures also zooming from f/8 down to f/16.  This model is available for under $300. 
    If you’re tempted to follow one of these cheaper routes, just be sure to find out about any and all shortcomings of the lens/telescope before buying (or keep your receipt so you can return it for a refund).  In addition to the smaller apertures, many of these lenses have fairly large minimum focus distances (or MFD—see section 3.13.6); which means that if the bird is closer than this distance you won’t be able to focus on it (even manually).  Reasonable MFD’s for bird photography are in the range of 10 to 20 feet.