[UncleVanya]

The Aggregate: Selecting IBIS Zoom Focal Length

I saw this on DPReview in a Sony forum. The person who posted the topic wrote this paper.

Apparently there’s a calculator to help determine an optimal value for a zoom that doesn’t communicate focal length to the body. As an example, IBIS for a 28-200 (per this paper) should be set to 75mm for optimal effect.

[AstroDave]

As an example, IBIS for a 28-200 (per this paper) should be set to 75mm for optimal effect.

At least in that case, the optimum appears to be the "geometric mean" (= square root (product of max and min zoom values) ) of the maximum and minimum focal lengths.

Makes sense - as a multiplicative factor, that setting is the same "distance" from the extreme end settings of the lens.

And, of course, if you tend to use your zoom mostly around maximum or minimum focal lengths, you should set the focal length for something in that vicinity.

---------- Post added 06-04-22 at 06:47 PM ----------

As an example, IBIS for a 28-200 (per this paper) should be set to 75mm for optimal effect.

At least in that case, the optimum appears to be the "geometric mean" (= square root (product of max and min zoom values) ) of the maximum and minimum focal lengths.

Makes sense - as a multiplicative factor, that setting is the same "distance" from the extreme end settings of the lens.

And, of course, if you tend to use your zoom mostly around maximum or minimum focal lengths, you should set the focal length for something in that vicinity.

[UncleVanya]

At least in that case, the optimum appears to be the "geometric mean" (= square root (product of max and min zoom values) ) of the maximum and minimum focal lengths.

Makes sense - as a multiplicative factor, that setting is the same "distance" from the extreme end settings of the lens.

And, of course, if you tend to use your zoom mostly around maximum or minimum focal lengths, you should set the focal length for something in that vicinity.

While it does "make sense" I have never seen anyone suggest that approach before on these forums - the question comes up often and the answers are quite varied.

[ZombieArmy]

My answer is to turn it off. If I'm shooting a manual zoom it's probably in bright daylight.

[m42man]

I've not read the article, but if 75 is its recommended value for a 28-200mm zoom, then the tool is clearly wrong.

Values greater than twice the actual focal length will result in blur magnification - clearly undesirable.

Use instead 2*min*max/(min+max), which is safe for all zoom ratios, or (min+max)/2 (but only for zoom ratios up to 3:1).

[reh321]

I've not read the article, but if 75 is its recommended value for a 28-200mm zoom, then the tool is clearly wrong.

Values greater than twice the actual focal length will result in blur magnification - clearly undesirable.

Use instead 2*min*max/(min+max), which is safe for all zoom ratios, or (min+max)/2 (but only for zoom ratios up to 3:1).

I don’t believe any value is “wrong”.

I accidentally set my “Q” to a value greater than the actual lens focal length - and the photo was fine.
The best action is to hold the system absolutely still without using a tripod. and not depend on the “SR” system.
The SR system should be just a backup in case you do accidentally move it.
I tend to set it to the smallest focal length, because I’m sure that it will “do no harm”.

[m42man]

I don’t believe any value is “wrong”.

I accidentally set my “Q” to a value greater than the actual lens focal length - and the photo was fine.
The best action is to hold the system absolutely still without using a tripod. and not depend on the “SR” system.
The SR system should be just a backup in case you do accidentally move it.
I tend to set it to the smallest focal length, because I’m sure that it will “do no harm”.

Greater than the actual FL is indeed fine, as long as it's less than by a factor of 2. I definitely agree about trying to minimise camera shake, but you do lose a bit of reduction if you set a zoom lens to minimum FL - the "safe" method above is worthwhile - unless you want to shoot at minimum FL a lot of the time.

[mlag]

On a manual 28-200 I would switch my camera on/off quickly and change the focal length to match the next expected scene like wide 30 medium 70 or tele 200... Iwould not be more accurate than these 3 values.
My feeling is that accuracy matters more on the wide end, than on the longer end... hence the 30 70 200

On my pentax 400-600 mm, I set it either on 450 or 600mm, to stay away from values from my other manual tele’s ( a 400, a 500 and 550 (actually a 1,7x300mm)), to keep track of lenses in exif data. As the camera uses steps of 50..., it’s about as accurate as possible to keep track of lens in exif....