How to avoid soft image at longest focal length with telephoto zoom lens?

Question

I have a Tamron 70 - 300mm lens. I have noticed that the best focus I can achieve at the 300mm setting is not as sharp as the focus I usually get at a lower setting. Is this a common problem? Is there any way to work around this? The camera I use is a Canon 1000D.

Answer

Every lens has a different sharpness. Some zooms are better at the beginning of the range, some in the middle, some at the end. Maybe yours isn't at its best on the end. Check online reviews, there sure be someone who noted that.

If not, some other things can be checked:

• sharpness change depending on the aperture size. How is the sharpness when closing aperture two or 3 stops down? For example, going to f8 or f11 if your lens is max f4.
• at 300mm and more, any move can produce a slight defocus. What speed are you shooting at? A basic rule is to use 1/zoom seconds. In your case, use a speed greater than 1/300 of a second.
• back or front focus problem: if the back/front focus is not correct, you will lose focus while changing the zoom factor. If you don't know what I'm talking about, check with a pro. Most of the time you can't do anything about it on your own.

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