Friday, June 12, 2009

Is Image Stabilization Really Necessary with a Faster Shutter?

Unfortunately the question inherently assumes that you can always simply choose a fast enough shutter speed, which may not be a valid assumption. Under normal lighting and shooting conditions it may be difficult to always shoot at 1/200th second when using a 200mm lens. While image stabilization technologies claim that you can shoot 3 or 4 stops slower, 1/25th to 1/12th of a second using a 200mm lens, it doesn't always work.  The assumption is that your hand shakes at the frequencies that particular technology is designed to cancel out, and that there are no other movements, like in your whole arm or body.  Reality isn't quite that tidy; often your image-stabilized shots aren't as sharp as you'd like them to be.

It gets worse form there.  Ever shoot with a polarizer, such as to reduce reflections off of surfaces such as leaves or skin? You'll need two more stops of light.

Then consider if you might ever want to add a teleconverter later. Suddenly you get hit with the double whammy of needing 1/400th second exposures AND your widest aperture drops two stops, such as from f/4 to f/8. You'd need a camera with incredible high ISO performance to gain 3 full stops of light to compensate for these changes! And that's assuming that you were fine in all shooting conditions (which I'm highly skeptical of) before adding the teleconverter.

The question also assumes that your shutter speed for a single frame at your best possible exposure is all that matters, which clearly is not true if you ever may choose to use Exposure Bracketing and later want to use your lightest (slowest shutter) shot, which may be 2 full stops, 4 times longer than your main, center exposure.

Then consider shooting an HDR sequence, not an unreasonable thing to attempt on a camera such as the Canon 40D which shoots 6.5 fps. Using Automatic Exposure Bracketing to capture 3 exposures up to 2 stops apart each, you'll probably shoot in Aperture priority mode, and end up with at least one shot exceeding your ability to maintain a sufficiently fast and shake-less shutter speed.

Note that these points compound each other... available light, teleconverters, exposure bracketing, and HDR... all favoring having the widest possible aperture and having image stabilization if you'll ever shoot without a tripod.

Regarding the correct shutter speed to aim for when shooting without IS, consider that the actual pixel size on the 40D is very close to the pixel size on a full frame model such as the 5D Mark II, so if the general rule of "1 divided by the focal length" works for 35mm and full frame digital cameras, the exact same amount of shake (or not) will be experienced by the 40D. In other words, 1/200th second should work fine and no special conversion is needed to address the 1.6X "crop factor".

Now let's look at real numbers. On a sunny day with a 200mm lens your f/16 1/100th second exposure (at ISO 100) must be 1/200th f/11. Oops, you're already losing options such as depth of field. Want to perform exposure bracketing? Even using an increment of only one stop forces you to change to f/8 1/400th second so your lightest exposure will still be 1/200th or faster. Want to use a polarizer? You'll need to shoot at f/4. At this point, forget teleconverters (unless you have IS). Of course you can bump up your ISO sensitivity to some extent that varies by camera model, but clearly you'll have lighting situations that may be less than full sunny day conditions as well. You can see how quickly you can run out of light when using telephoto lenses, and increasing the shutter speed and bumping up the ISO become the critical options.

Wide telephoto lens apertures and image stabilization are two features that can ward off the point at which your shooting becomes impractical and unproductive.