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Now with Stabilization
11 April 2008
Sometimes, the inevitability of a working life gets in the way. Unfortunately, this week is the final week of the dreaded “tax season” here in the good old U-S-of-A, and the outlook of a worthwhile Friday post looked “hazy” according to my magic 8-ball, and finally passed to “Get real.”
However, I do bring you hope. I have finally (thanks to the glory of credit card rewards) purchased both the Tamron 17-50mm f/2.8 and the Canon 55-250mm f/4-5.6 IS lenses that I talked about in my previous post.
My initial thoughts on both are fairly positive, though oddly moreso with the Tamron. I understand that the Canon 55-250mm is, for all intents and purposes, a budget lens. For that budget, there is a terrific optical quality - but the autofocus speed leaves much to be desired. Shots lagged for more than a second while the AF hunted for its target in lower light. This isn’t really the issue I make of it for most people, as you wouldn’t be shooting a very long lens whose maximum aperture is 4.0 in low-light situations often. Its outdoor daytime speed is much snappier - I just felt it should be mentioned exactly where the “budget” starts to show.
The Tamron 17-50mm, on the other hand, I could not be happier with. The lens used to be plagued with AF low-light problems, but at least on my copy it acts responsive and finds targets quickly even at wide apertures and low light. In a side benefit, I find the viewfinder to be much brighter with this lens than with the old kit lens - a side benefit of the wide max aperture. The Canon 350D-400D series is plagued with a notably dim viewfinder, so this makes a world of difference.
I hope to get out on Sunday and take some good shots with each lens for illustration, as well as to test Canon’s remarkably bold statement of four whole stops of gain from the IS.
In fact, let’s go into a little bit of what the real gain is for Image Stabilization (IS on Canon lenses, VR on Nikon). Many people seem to expect it to “fix” the blur in a bad image, and in a way that’s true, but that blur can only be from the camera end - not the target.
Normally, you must use a shutter speed of about 1/focal length to prevent ‘hand shake’, or the minute movements of your hand from shaking the camera during exposure. So, if you’re shooting at 55mm on your lens, you need about 1/55 (1/60) shutter speed to assuredly prevent blur. The longer the lens, the faster that shutter has to be.
But when you get out to 200-250mm as many telephoto lenses do, that means you need a shutter speed of 1/250th just to eliminate the shake - which is a pretty fast picture. Coupled with the fact that telephoto lenses naturally have a more narrow aperture (most are around f/5.6 at that range), you are already not letting in a lot of light. Since your shutter speed must be 250 or faster, you only have two options - crank the ISO up to 800-1600 (maybe higher), or miss the shot.
See, shutter speed and ISO, unbeknown to many amateurs, are both also measured in “stops.” This helps make the relationship and the give and take of the big three in photography - ISO, Shutter and Aperture. If you shrink your aperture by one full “stop” (f/4 to f/5.6, for example), you are subtracting one stop of light (roughly illustrated by the light meter as -1 if your image was properly exposed). To bring it back up to 0 (proper), you can either increase your ISO by one stop (go from 400 to 800), or increase your shutter speed by a stop (250 to 125). Conveniently, though aperture uses weird decimals for its stop values, shutter and ISO are just halved or doubled roughly.
So, when you go out to that 200mm and your aperture shrinks to f/5.6, you may find yourself suddenly needing to go to a shutter speed of 1/125th - the address of Hand Shake City.
IS/VR allows you to avoid this - at a small caveat. Canon’s boast of four full stops on the lens means that I can take a photo with a shutter speed of 1/30th at the full 250mm and not blur. The lens does this by voodoo magic or by decoupling the focusing element from parts of the lens construction…I’m not sure which, but my bet is on voodoo.
The trick is, it only works for shake at the camera end. This means tiny tripod bumps, hand shake, and mirror vibrations. It will not fix an image where the subject is moving.
Let’s say that together: IS/VR will not prevent blur of a moving subject.
Good. With that, I’m off to do some taxes and hopefully take some pictures this sunday. Tuesday’s post may be a bit delayed, but rest assured it will appear - even if it has to be Wednesday morning.
April 18th, 2008 at 9:53 am
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