Manually focused lens gives blurred images and then sharp images - without being touched

I have been taking photos at a dance show rehearsals using two camera – sat at one of them framing and shooting, whilst the other camera was on a tripod fired by a hand-held wireless control.

The remote one was a 350D with 18-55STM lens – the lens set to manual focus and test shots were perfect. But when I fired the remote, about 1 in 4 shots is blurred.

It seems like the lens adjusts focus temporarily whilst the shot is taken. A subsequent shot a coulple of seconds later it perfectly sharp.

I wondered if the camera had gone into sleep mode and then when I fired the remote, it woke up. During the wake up, the electronic focusing was still adjusting – despite being switched to manual.

Throughout the few hours shooting I did not adjust the lens at all - so after a blurred shot, the lens returned to the correct focus setting.

GreenPig wrote:

I have been taking photos at a dance show rehearsals using two camera – sat at one of them framing and shooting, whilst the other camera was on a tripod fired by a hand-held wireless control.

The remote one was a 350D with 18-55STM lens – the lens set to manual focus and test shots were perfect. But when I fired the remote, about 1 in 4 shots is blurred.

It seems like the lens adjusts focus temporarily whilst the shot is taken. A subsequent shot a coulple of seconds later it perfectly sharp.

I wondered if the camera had gone into sleep mode and then when I fired the remote, it woke up. During the wake up, the electronic focusing was still adjusting – despite being switched to manual.

Throughout the few hours shooting I did not adjust the lens at all - so after a blurred shot, the lens returned to the correct focus setting.

see p. 4 of the instruction manual for the EF-S 18-55 f/3.5-5.6 IS STM.  The STM lens has it's own sleep mode, and does a focus re-set when it wakes up.  Need to wait a second or so after waking the lens for it to return to the set focus.

http://gdlp01.c-wss.com/gds/8/0300011908/02/efs18-55f35-56isstm-im2-eng.pdf

Seems counter-intuitive that power on/off would have any affect if the lens is set to manual focus, but remember that with STM even manual focus is powered and not a direct mechanical connection to the focus ring.

Thanks Lemming51 - that certainly explains it.
I have now found other stories of the STM sleep mode and all is now clear (or blurred)

If I use the same set-up again then I'll need to remember to wake the lens up before taking a shot - trouble is, I cant find any documentation on how long the sleep time is.