[schj]

I have gotten into the habit of turning the camera off during lens changes, which supposedly is a good idea from the perspective of dust, but it also means I get prompted to enter focal length when I turn the camera on. So I rarely forget any more.

If you really think it through, you will realize that entering to *low* a value might hamper the effectiveness of SR, but it won't ever be worse than turning it off. Entering too *high* a value could definitely cause the SR to become counterproductive, but only if entering a value *much* larger than the actual focal length. I'm guessing entering 2X the actual focal length would basically nullify SR by moving the sensor twice as far as necessary, which would just reverse the direction of the shake. Anything more than 2X and you'd be getting more blur than with SR turned off. But still, you're not going to hurt anything in the camera; just the IQ of any images taken with the much-too-high setting.

I have experienced that going from my 500mm mirror lens to a M28/2.8 and leaving the focal length to 500. All pictures where blurry until I switched off/on and entered the correct length.

[m42man]

It is not quite as simple as that. If you read the patents you'll realise how much cleverness goes into moving the sensor plate at the right velocity and direction during the exposure

Yes, I'm sure there's an awful lot of cleverness - the SR algorithms have to take rotational acceleration values from the shake-sensing accelerometers, and then, as you're saying, figure out directions and speeds. (That no doubt accounts for the delay in the green SR indication in the viewfinder.)

Assuming the camera's made the right deductions concerning rotational velocity, then the only other factor affecting how fast it moves the sensor is focal length. The amount of blur will be (1 - {value you input}/{actual FL})*{blur with SR off}. (A negative value just means the blur is in the opposite direction to the original blur.) So, obviously, you can end up with more blur than with SR off when the value you input is higher than the actual value. Conversely, for values lower than the actual FL, the blur you end can't ever be greater than the original blur.

If there is inaccuracy in the actual focal length of the lens, this will add to any error in the camera's deductions, and that's why it's always best to err on the low side when setting your FL value.

[m42man]

As a simplified formula for one setting, you might just add 1/3 to the shortest focal length of your zoom - it won't be too far out...

Actually, it's even easier to remember a setting of 40mm for a 28-70, 28-80 type zoom, and 100mm for a 70-210, 80-200, 75-150 type of zoom. In each case you'll either be bang on, or slightly conservative - and those focal lengths must surely account for the vast majority of vintage zooms.

[m42man]

Finally, a word of caution: we're used to getting full SR when we're shooting; although the SR mechanism isn't perfect, we can easily end up with anything up to about 4 stops-worth of shake reduction, which is 16x. Using a vintage zoom lens with the prescribed compromise FL setting, we may get only 2x shake reduction when operating at extremes of the zoom range. Now, 2x is better than none at all, but it's nowhere near as good as 16x. In fact, lacking any means of comparison, it may even seem that the resulting images haven't had any shake reduction - or worse, that the shake had been increased. The only way of evaluating SR effectiveness is to take loads of shots of the same subject, with and without SR switched on. You have been warned!

[toidpentax]

Sorry to resurrect this thread but my question applies... Should we be entering the equivalent focal length or the actual focal length of the lens? i.e. 50mm film lens or the APS-C equivalent.


Thanks

[m42man]

Sorry to resurrect this thread but my question applies... Should we be entering the equivalent focal length or the actual focal length of the lens? i.e. 50mm film lens or the APS-C equivalent.


Thanks

It's the actual focal length. A 50mm lens is a 50mm lens whatever camera you mount it on.

[Kerrowdown]

Yep, I'm led to believe the focal length of the lens does effect how much SR works for that lens and camera combination.

So I've always put in the correct lens length when I change over, as my A series lenses don't do it for me automatically.