In Chapter 2 we reviewed the basic technology underlying today’s DSLR’s.  As we now consider the effective operation of the camera in the field, we’ll rely on a number of concepts introduced in that earlier chapter.  Though we’ll briefly review essential concepts where necessary, it may be useful to skim through Chapter 2 at various points in order to most effectively digest the material presented here.

6.1 F-stops, Shutter Speeds, and ISO

The very first task in learning to use your DSLR is to master the skills involved in obtaining a good exposure—that is, in getting a photo in which the bird is lit well enough that you can discern sufficient detail in the bird’s plumage and other features.  Of course, the “correct” exposure of a bird depends to a very large degree on what you’re trying to achieve, aesthetically, with the image that you’re crafting: you may want the scene to have a dreary, subdued mood, or you might be trying to achieve a “high key” effect in which the photo is excessively bright, for artistic reasons.  For now, however, we’ll concentrate on the more basic task of maximizing the amount of detail visible in the bird, and leave the more artistic considerations for later.
    As explained in Chapter 2, there are three settings that you can manipulate in the camera in order to affect the exposure (brightness) of an image: the aperture, the shutter speed, and the ISO setting.  The aperture is the easiest to understand: a wider hole, or aperture, will let in more light per unit time, resulting in brighter images.  The aperture that we’re talking about here is the diaphram (or iris) located within the lens.  This diaphram can be opened or closed to varying sizes, resulting in changes to the amount of light that passes through the lens.  Recall from section 3.1 that a larger aperture (wider hole) corresponds to a smaller f-number; thus, f/8 lets in more light than f/11.



    Light passing through the lens (and diaphram) only reaches the imaging sensor when the shutter is open, and the amount of time that the shutter is open is dependent on the shutter speed.  Obviously, if the shutter opens and closes very quickly, less light will reach the sensor than if the shutter opens and closes slowly.  Thus, a faster shutter speed results in a darker exposure.



    Finally, we have the ISO setting.  Recall from section 2.5 that the ISO setting controls the amount by which the analog output of the sensor (the signal, as well as any attendant noise) is amplified before being converted to digital pixel encodings.  Because higher signal values result in brighter pixels in the resulting image, turning up the ISO (i.e., increasing the amount of amplification) will cause the resulting image to look brighter (though possibly also noisier).
    We thus have the following simple rules:

If you want a brighter image...	If you want a darker image...
decrease the shutter speed	increase the shutter speed
enlarge the aperture (decrease the f-number)	constrict the aperture (increase f-number)
increase the ISO	decrease the ISO

For example, if you take a photo (in manual exposure mode—section 6.3) and then, upon reviewing the image on the camera’s LCD, decide that it’s too dark, you can make sure the next photo (of the same scene) turns out brighter than this one by either increasing the ISO, enlarging the aperture (i.e., using a smaller f-number), or decreasing the shutter speed (or some equivalent combination).  If after making the adjustment(s) to these settings, you find that the next photo is still too dark, then you can simply adjust one or more of the settings further and try again, and repeat this process until you get an image that has just the right amount of exposure.  This trial-and-error process may seem slow and wasteful, but in time (with experience in the field) you’ll get better at adjusting these three settings rapidly and in the proper amounts, so as to hone in on the right values with fewer trials.



    Now we need to consider some of the complications involved in adjusting these three parameters (aperture, shutter speed, ISO).  The good news is that there isn’t a single, unique combination of the three parameters that results in the “correct” exposure.  Once you’ve found a setting that gives you an exposure that you like, it’s possible to change two or more of the parameters (in the proper way) and still get the same overall level of brightness.  For example, if you were to turn the f-number down one “click” on your camera’s aperture dial (say, from f/8 to f/7.1), and then turn the shutter speed up one click on your camera’s shutter-speed dial (say, from 1/320 to 1/400), you should find that the resulting exposure hasn’t changed.  That’s because the larger aperture (smaller f-number) has been counterbalanced by the increase in shutter speed.  This is called reciprocity.  Reciprocity works by trading off “stops” of light between parameters.  A stop of light corresponds to a doubling/halving of the shutter speed (or ISO), or multiplying/dividing the f-number by 1.4 (the square root of 2).  In practice, you can usually forget about the exact definitions (multiples of 2 versus 1.4) and just think in terms of clicks on the parameter dials.
    So the good news is that you’re not looking for a needle in a haystack: a number of different combinations of ISO / aperture / shutter speed will typically give you a good exposure, and you just need to find one of those combinations.  The bad news is that there are secondary effects of adjusting these parameters—that is, other effects besides just changing the exposure.  As we explained in section 3.7, for example, enlarging the aperture not only increases the amount of light collected, it also results in a shallower depth of field (DOF).  Sometimes you’ll want to do both (increase brightness and decrease DOF), and in those cases you can achieve that by adjusting the aperture.  If, however, you don’t want to change the DOF, then you’ll have to resort to changing one of the other two parameters (ISO or shutter speed) in order to adjust the exposure.



    Similarly, changing the ISO or shutter speed can have other effects on the resulting image.  Increasing the ISO can increase the noise level (as explained in section 2.5), while decreasing the shutter speed can decrease the sharpness of the image, due to motion blur.  Thus, the challenge is in finding the combination of these three parameters which gives you an acceptable exposure, without producing too much noise (via high ISO) or motion blur (via low shutter speeds); in addition, if you’re trying to achieve a specific effect via DOF (such as isolating the bird via a shallow DOF), then you’ll want to find a combination of parameters that doesn’t violate your desired DOF.
    To make matters worse, there are limits on how high or low you can set each of these parameters, and it’s not uncommon to run into some of those limits in the field.  Native ISO values are typically limited to a range of 100 to 1600; “expanded” ISO ranges typically are only “simulated” and should generally be avoided if possible.  Apertures are likewise limited at both the high and low ends of the range, though it’s the upper limit that is most often encountered in the field, since the objective lens diameter limits how wide the aperture can be made.  Though shutter speeds are also limited at the high end (typically 1/8000 sec for pro bodies) as well as at the low end (typically 30 seconds), in practice you’ll want to avoid going below 1/125 or 1/160 sec (plus or minus, depending on whether you’re using a tripod and/or image stabilization) in order to avoid image blur (or 1/640 to 1/1600 for birds in flight, or even just small birds frantically foraging).



    Fortunately, as we’ll see in section 6.3, there are several exposure modes that you can use to make it easier to manage the three parameters affecting exposure.  What you’ll typically find in the field, however, is that you only need to search for your optimal parameters at the beginning of the day’s session, and then to gradually adjust individual parameters a little bit throughout the session, as the lighting changes (i.e., as cloud cover comes and goes).  Furthermore, if you do most of your birding at the same place several days in a row, you’ll often find that establishing your initial settings at the start of each session is easier, since they’ll likely be the same settings you used at the start of yesterday’s session, and may be just a few “clicks” (on the parameter dials on your camera) away from whatever setting the camera was left in when you turned it off at the end of the last session.



    One trick that can help you to become faster at adjusting exposure parameters is to reassign the camera’s dials and buttons (if your camera allows you to do that) to particular exposure parameters, depending on your shooting style and exposure mode.  For example, if you find that in a particular birding situation you tend to adjust the ISO and aperture more often than you adjust the shutter speed, then you could assign those two parameters (ISO and aperture) to the two dials or joysticks on your camera that are most easily within your reach.  My current camera has two dials, and I typically assign shutter speed and aperture to these; adjusting the ISO can then be done by first pressing a button and then turning the dial that normally changes the shutter speed.  However, if I’m birding in a dim environment, such that I’m limited primarily by my shutter speed, then I may set the shutter speed to its lowest practical value (say, 1/160 sec) and then assign the two dials to ISO and aperture.  Unfortunately, not all cameras allow you to reassign their controls in this way.
    Note that when trying to freeze birds, a faster shutter speed isn’t always the ideal solution.  As previously mentioned, birds in flight often need a shutter speed of 1/640 to 1/800 sec for slow-flapping birds or 1/1600 to 1/2000 sec or faster for fast-flapping birds.  Even the fast head movements of foraging birds (especially warblers and small shorebirds) can require speeds of 1/1600 or more to freeze.  When these speeds aren’t feasible, due to exposure constraints, flash can sometimes be used to freeze the bird by keeping ambient light low and using a short flash duration (see section 7.7).  In the case of foraging birds (i.e., those not in flight), if you’re already using flash for fill light then it’s often preferable to keep your shutter speed below the sync speed (to maximize effective flash output and minimize recycle time), and just try to get the bird while it’s not moving.  I’ve found that for shooting frantically feeding warblers, 1/1600 sec sometimes isn’t fast enough to eliminate motion blur, so I instead shoot at 1/300 sec (the sync speed of my camera) and take lots and lots of photos in hopes of getting the occasional lucky shot when the bird is perfectly still.  This is sometimes the only way to fully dispel the shadows in the scene.  Flash is dicussed in greater detail in Chapter 7.