You need more distance between your subject & the background. I use my 100-400mm at f/7.1 and with my background 5 feet or further behind my subject, I get a sharp photo with a nicely blurred background. If you notice your shot at f/5.6, part of the squirrel is sharp and the tail is less sharp. Each lens has its own characteristics and you have to get to know your lenses to know how they will perform.
Hope this helps to explain,
Carlton
Michael! your camera or lens you have problem, if you look you took the photo with a phone, 280 mm, has a distance of 5.6.para Lerga focus so if you wanted to make the opening was very little background, I had to get it open f / 11 f/16, now, so your fund will be in focus .. the pictures very well with these parameters that explains your hope, be of help!
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Thanks everyone. I will try the f/7-f/11 first, and if that doesn't fix it I will try the striped paper. I would just like the whole subject in focus, not just one part. Again thanks.
Whatever Depth of Field you use, you should ALWAYS keep the eyes in focus. No matter what is in, or out of focus, the eyes out of focus will ruin a photo faster than anything.
Have you got an A-DEP setting on the camera? You could try that to pick multiple focus points to get as much in focus as possible? I know it's usually used for landscapes, but worth a go. Incidentally, I don't think the tail being slightly out is a problem.
You don't have a focus problem. As the Bixby reply correctly states, for any creature photo to "feel" in focus, the eyes must be in focus and your squirrel's eyes are sharp. For your desire to sharpen up the body, that is a depth of field issue. As all are pointing out, depth of field is determined by subject distance and aperture setting. To include a larger zone of sharp focus toward the subject's tail, use a smaller aperture.