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09-16-2019, 04:47 AM - 2 Likes   #1
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Vintage Zoom Lenses and Input Focal Length

Most will be aware that you need to tell the camera the focal length of the lens you are using, if the camera hasn't actually been told the value by the lens itself (i.e. so those vintage manual focus lenses). And that this due to need for the SR mechanism to correctly counter blur caused by camera shake.

Obviously a problem for zoom lenses, unless you want to be constantly changing the value to match the focal length you choose for taking a shot.

Some may remember I came up with a formula a few years ago, for the choice of best "compromise" value - as follows:

FL = 2*min*max/(min+max) (where min and max are the extremes of the lens in question).

This was to equalise the amount of blur reduction you get at the extremes of the zoom range.

HOWEVER, since you need more blur reduction at the long end, it might be better to equalise the actual amount of blur you get for a given amount of camera shake.

So, a new formula that does this - surprisingly it comes out as (min+max)/2, i.e. the average of the focal length extremes. This works well for lenses with zoom ratios up to 3x. Above that, you would actually get blur magnification when zoomed-out. In fact, for a 3x zoom fully zoomed-out, you would end up with the SR mechanism applying twice the amount of blur, but in the opposite direction (so a net blur equal to that you get without SR).

As an example, for a 28-70 zoom (a zoom ratio of 2.5x), the first formula yields a figure of 40mm, the new one a figure of 49mm.

As always, it's as well to remind that an FL entered which is more than 2x the actual shooting FL, will result in more blur than you would get with SR turned off.

09-16-2019, 06:54 AM   #2
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Just a comparison of the two methods:

For a lens with a zoom ratio of 2x, method 1 yields 3x blur reduction at each end, method 2 yields 2x blur reduction at the short end and 4x reduction at the long end.

For a lens with a zoom ratio of 3x, method 1 yields 2x blur reduction at each end, method 2 yields zero blur reduction at the short end and 3x reduction at the long end.

Somewhere between the two methods may be best compromise - though obviously it's best if you can match FL for every shot.
09-16-2019, 04:50 PM   #3
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huh, I always went with the shortest shooting focal length as then it won't ever be working against me (I.e., if I put in 75mm for a 50-100 zoom, at 50mm it's over-correcting and actually causing motion blur in the opposite direction). What has your experience been on the various methods?
09-16-2019, 10:29 PM   #4
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QuoteOriginally posted by fehknt Quote
huh, I always went with the shortest shooting focal length as then it won't ever be working against me (I.e., if I put in 75mm for a 50-100 zoom, at 50mm it's over-correcting and actually causing motion blur in the opposite direction). What has your experience been on the various methods?
Yep, it's over-correcting by 50%, so you'd still be getting 2x blur reduction. And, when you zoom in at 100mm, it under-corrects, but you still get get 4x reduction. Whereas, if you had input the shortest FL, at 100mm, you would get only 2x reduction.

You only really need to worry if it over-corrects by more than 100% (in other words, more than 2x the input focal length), in which case you end up with more blur than you would have had with SR switched off.

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