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Photography Question 

blurry photos with a zoom lens

I recently purchased a Sigma 70-300 mm lens for my Minolta Maxxum 5 camera. I went out and took a couple of rolls, using a tripod, and 90% of the shots came back blurry! I used 400 and 800 speed film. With my regular 28-80 lens, using the same film and no tripod, I never had blurry shots. What could the problem be?

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7/15/2003 9:36:14 AM

Maynard  McKillen   Dear Heather:
Let's rule out one possible cause. Use a loupe or powerful magnifier to examine your negatives and make sure they are, in fact, blurry. Paper curl, paper movement during exposure, and misaligned, defective or incorrectly chosen optics are problems that occur at film processing labs, and any or all of these can cause blurry photos, even when the negatives are sharp.
Then, too, does this blur seem to result from the subject being out of focus, or from rapid subject movement? And are you willing to confess which tripod you used? Also, were you shooting indoors or out, with flash or with available light?
I take it the image you saw in the viewfinder looked sharp, and the correct focus confirmation light appeared, too.
Did you document the f/stop, shutter speed and focal length you used for each frame? Such info can be useful when diagnosing problems.
One rule of thumb you may know of is to use a shutter speed whose denominator is a number of greater value than the focal length you are shooting at. For example, if you shoot at a focal length of 200mm, use a shutter speed of 1/250 or higher. If you shoot at a focal length of 300mm, use a shutter speed of 1/500 or higher. Being a rule of thumb, it's not foolproof, but it does tilt the odds in your favor. And, if you were shooting with flash, your camera cannot shoot above a certain shutter speed (1/125th of a second for that model), which could result in blurry photos if you were using a focal length like 150mm or even higher.
You are certainly magnifying your subject when you use this lens, especially at the upper end of the focal length range, which means you are also magnifying any movement your camera/lens makes, and any movement yout subject makes.
Imagine fixing your camera to a tripod and using your 28-90, set at 28mm, to photograph a cat walking sideways to the camera, six feet away (No panning allowed.) If he's moving at a foot per second, no tall order for most domestic felines, he'll still be in the viewfinder by the time you press the shutter button, and at a shutter speed of 1/125th of a second (a denominator well above the 28mm focal length), you'll freeze him in mid stride.
Try that with your 70-300 set at 300 and challenges arise. Your angle of view is so much narrower that Rex (assuming he's an orange tabby) may enter and leave your viewfinder before you press the shutter button. His speed may not have changed, but distance he travelled as a percentage of the distance across the viewfinder has changed. Even if you do capture him at 300mm, 1/125th of a second may not freeze him.
Finally, did you focus manually, use single shot mode, or continuous/predictive mode?

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7/16/2003 8:11:08 PM

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