Originally posted by newarts If you know about what focal length you'll actually use, enter it. Otherwise enter the average focal length for the range you might use; it can make a big difference in image quality.

Focal.length.SR.optimal = (max+min)/2

It is the theoretically optimal focal length for minimizing mean squared shake error for random shots between max and min.

Think about what happens to other formulas in the case of very large or very small focal lengths; using a formula often offered, 2MaxMin/(max+min), for large max results in 2min - which makes no sense at all, conversely, if min=0, 2MaxMin/(max+min) = 0 which also makes no sense.

**Using average focal length for SR is better than not using SR for hand-held photos.**

I'm afraid you're wrong!

If max is (very) "large", then the value

*approaches* 2*min, and the net effect of the SR accordingly approaches zero (which is OK). If the "lens" is a flat plate of glass, then the FL will be infinite, and the result would indeed be 2*min, so you'd get zero net SR. But a flat plate of glass isn't actually a lens.

Likewise, a "lens" with a FL of zero isn't actually a lens, so

*that* assertion is also meaningless.

However, for a real lens (i.e. with non-zero and non-infinite FL), the formula holds true.

As for your (max+min)/2 formula, you can easily see that a large value of max will result in a value approaching max/2, so for a zoom ratio of greater than 2x, you'd be setting a value more than 2*min, which means that the camera's SR would be

*magnifying* any camera shake. You can demonstrate this crudely by mounting, say, a manual 50mm lens and setting a value of 200mm. Now take a few shots at, say, 1/60s and compare the results with shots taken with SR switched off.

Having said that, a great many vintage zooms have a zoom ratio of 3x or less, so using the formula (max+min)/2 wouldn't be the end of the world - either you end up with a value that's close to the "optimum" value anyway, or you just get zero net SR when you set the zoom to minimum FL (if the zoom ratio is exactly 3x). But this formula does fail for zoom ratios greater than 3x.

The golden rule to remember is: you get shake magnification if you input a focal length value greater than 2x the actual focal length that you're going to be shooting!