What's causing this?

Yes, exactly my findings. That's why I'm always interested in any claims to the contrary.

'IS' you say? Which brand did you go to in the end? Isn't IS what Canon call their SR? Presumably that's lens-based IS rather than sensor-based (not that it probably makes much difference)?

This is all very interesting and if we ever get to the bottom of it I'll start a thread on the other urban myth, flash trigger voltages.

I'll tell you what ain't an urban myth...

You can get a bloody painful electric shock off a (edit: old) flashgun if the meter probes slip when you're trying to measure the voltage across the contacts.

It was an old Cobra hammerhead of some description. My arm ached for quite a while after

My two £0.02p worth -

I reckon it is down to resonant frequencies. The resonant of a camera when it's hand held is going to be different (probably much lower) when hand held compared to when mounting on a fairly rigid tripod. The SR probably cannot cope with the latter.

Coming back to the topic, the reason for the CA is that the shot is taken against the sky.

G

Real men don't use meters to test flash voltage. They just lick their fingers, and touch the contacts.

Can be fun to watch.

G

And not all real men are equal. The real, real men do it with fingers from both hands, not one hand.

No wussy short, round the hand, circuit for us. No, it's got to be right across the chest for maximum enjoyment.

The SR in the pentax is an oscillating type and from my understanding it creates inertia by oscillating the sensor.
Since the SR actually cause movement it's better you turn it off if you don't have need for it.

I'm not sure about this though so it might be better to check but it's a posiable explanation at least.

http://www.pentaximaging.com/pdf/All_Bodies_Shake_Reduction_012208.pdf

lol ask what you thought was a small question and 3 pages latter. Thanks.

I have never turned the feature off and have never noticed any shake that had not been from me knocking something even with marco's. I will try turning it off and see if it makes any difference now I'm using the K5.

Is this something they ask you to turn off because the first systems where not as good as the newer Pentax systems.

No. Ever since SR was introduced with the K10D, they've told you to turn it off, but never said why.

Consequently there's been any amount of anecdotal and speculative muttering about why it might be, and never a shred of actual evidence that might give some actual information.

Just leave it on. You'll never see any difference, and you'll have the additional benefit that there'll be plenty of people out there that will believe you when you claim it's why your pictures aren't sharp

I also leave it on but if you work with longer focal lengths things get so sensitive.

Would be interesting to see some samples with 300mm with 1.7 TC or with a 500 or 600 mm.

Yes, it would - I only tried up to 100mm when I looked at it all in detail.

I get better results with the Sigma 150-500 and DA*300 with the SR on.

As to the real men thing - I have a box on my bench. It puts out 300Vdc at up to about 100A. Anyone is welcome to demonstrate the real man thing.

Wuss,

Worthing.

Not from Pentax but from Canon.
The system is slightly different but the mechanics behind it are the same it seems.
Instead of lens group use sensor

“The IS mechanism operates by correcting shake. When there is no shake, or when the level of shake is below the threshold of the system’s detection capability, use of the IS feature may actually *add* unwanted blur to the photograph, therefore you should shut it off in this situation. Remember that the IS lens group is normally locked into place. When the IS function is active, the IS lens group is unlocked so it can be moved by the electromagnetic coil surrounding the elements. When there’s not enough motion for the IS system to detect, the result can sometimes be a sort of electronic ‘feedback loop,’ somewhat analogous to the ringing noise of an audio feedback loop we’re all familiar with. As a result, the IS lens group might move while the lens is on a tripod, unless the IS function is switched off and the IS lens group is locked into place.”


So it's maybe a feedback loop, sounds plausible.

Sounds plausible to me.

G