We all know the low light exposure conundrum. The more you open up the aperture, in any lens, but most especially in long telephoto lenses, the more shallow (to the point of being razor thin) your depth of field becomes. With bird photography, that means (depending on the pose) that much of the bird’s little body is simply NOT in focus.
When I’m out in the State Game Area in the early morning, there’s usually lots of glorious sunshine allowing me to tighten up the aperture to 8.0 or smaller. Here at home, light filters down on a sunny day to accommodate a comfortable aperture of f/5 (with low shutter speed –assuming there’s no wind– and relatively high ISOs). But when it’s cloudy and rainy, the aperture on my 500 mm f/4L II IS lens is mostly wide open. I could swap out lenses and use my 300 mm lens (2.8 L wide open), but then I sacrifice amplification. I’d much rather have high noise in an image that does not need cropping/zooming in post processing.
What’s a “Fast” Lens?
Lenses on which the aperture opens wide and thus allows more light on the focal plane are referred to as “fast” lenses. The more that aperture can open to let in more light, the “faster” the lens. The more light transmitting through the lens, the more flexibility with exposure parameters the photographer has.
NOTE: In aperture speak, when you “stop down”, you are letting in less light…. making the aperture smaller (by moving to a larger number)– and consequently, getting a deeper depth of field. Remember: The f/ represents the size of the hole allowing light into the lens. The smaller the f number, the larger the aperture.
Shallow Depth of Field
When I was photographing this Blue Headed Vireo, I had the 500mm lens wide open to f/4. See photo above. The head and eyes of the vireo are sharp because those bird parts were within the in-focus DOF arena. Those bird parts that were too close or too far away from the sensor are blurred because at f/4 there are only a few millimeters on which to lock focus.
To get an idea of how thin your depth of field can be at wide open apertures, see this DOF calculator. And, of course, if you want this information readily available in the field, there are DOF calculator apps available for iPhones and Droids.
Entire Subject Parallel to the Sensor
In the photo below, shot at f/5, most of the bird’s body is parallel to the sensor, and consequently, overall sharpness is good.
A Good Telephoto Lens
High end prime (fixed) lenses are usually “faster” than zoom lenses. Faster lenses are usually larger, heavier (I usually prefer a tripod with fast 300mm+ lenses) and much more expensive because of the sophisticated optics and highly responsive autofocus and metering technology incorporated into these lenses. A good telephoto lens will always be distinguished by its largest aperture setting. On a cloudy day, that means a lot.
To read more about image sharpness, please press this link.