Originally posted by UncleVanya It might not matter if the body can read the focal length as much as you might think. Use the tool linked in the link mentioned in this thread to set an optimal single setting for manual zooms. There are multiple optimization choices that influence the decision, and the tool output gives you the info so you can decide what focal length setting is optimized for your use.
Setting IBIS on a manual zoom with SCIENCE! - PentaxForums.com
In the marketplace recently there were listings for a Sigma 120-400, a 50-500, as well as a 100-300 all of which offer the automation to inform the body of the focal length in use.
Hmm, the linked post states, from the article it's referring to: "IBIS for a 28-200 (per this paper) should be set to 75mm for optimal effect".
This is bunkum, and should be ignored.
If you input a value which is greater than twice the actual focal length, the camera will move the sensor such that you end up with more blur than with SR turned off.
There are two formulae you
can use though:
1. This equalises the blur reduction at each end of the zoom range, and is intrinsically safe (i.e. will never magnify blur):
2*min*max/(min+max)
2. This equalises the amount of blur at each end of the zoom range, so is better at the long end - but is only safe for zoom ratios up to 3:1 - just take the average, i.e.
(min+max)/2
There's another, more sophisticated formula which optimises for random zoom settings, and this can be found over at dpr. You end up with a value somewhere between the two above, but be careful, as some (large) zoom ratios will end up with an unsafe value.