Problems with - Stack Mode>Mean - in CS4

<p>Having searched this forum I've been unable to come up with an answer to this problem.<br>

I want to use Photoshop CS4 to do some mean values of images to remove people from some images and to work on some long exposures. However, each time I come to the processing the Stack Mode option is greyed out and I can't go further - see the attached screen shot.<br>

I've followed a couple of online tutorials and the process seems to be straightforward and CS4 should do it, I add my images to layers, convert into a smart oobject and then I should be able to go to stack mode to take the mean mean, but that's where it all falls over.<br>

How can I get round the problem? Am I making some schoolboy error?<br>

Thanks for any advice.</p><div></div>

<p>Hi Peter<br>

This link may help: https://forums.adobe.com/thread/1701845?start=0&tstart=0</p>

Thanks John, I think you've nailed it

<p>I've now been able to access a computer with CS6 on it and have done my stack process. - thanks to all who have looked at this thread.</p><div></div>

<p>You're welcome Peter.</p>