So could you tell me if my sweetspot is 325mm will it be better to crop and get a sharper picture or shoot at400 with no crop?
If you want to utilise the sweetspot of your lens, whether it be in terms of focal length or aperture, for the maximum quality that this lens is capable of then go for it. Anything else is a compromise - but such is life. The 50D gives you 15.1MP to play with so it's up to you to decide how many you want to throw away by cropping and this will, or should, be determined by what you intend to do with the image i.e. a huge wall poster or a postage stamp sized print.
I really think you should start a new thread if you've got any further questions regarding the Robin or the lens in question. We've both managed to stray away from this threads title of 'histograms' which will no doubt serve to confuse other members.
Cutter I probably wasn't clear enough .I was not saying manually focus .More a case of manually selecting the area the autofocus works on.
It's the button top right on the back of the camera that is used to zoom in when reviewing lcd .it also lets you toggle the red focussing dot around.
yes you were matte I am using the button you sugest but then manuall focus it seems to give me sharper results for some reason , unless like forseti sugests its prob camera or suject movement. Think I need more light to get more shutter speed so i can eliminate camera shake.