Attached Link: Depth of Field Calculator

Alice, I hope you don't mind me putting a photo here to illustrate a point.  This is about Depth of Field.  This shot was taken with 280mm, ISO400,  1/30 sec, f/4.    Light was really too low and higher ISO would have helped.   

What to notice is where the photo is sharp.  Look at the tree on the right and the closest dove.. I believe focus was on the tree and should have been perhaps a tiny bit more in  The dove in the background is out of focus.  Another shot with focus on  the middle dove left the closest bird without acceptable detail in the feathers.

The depth of field at the settings used gave me only 1 foot stuff in focus, from front to back.  The distant dove is a bit too far.     If push comes to shove and all would not focus, I would rather have the foreground in focus and the background out of focus because that is more visually acceptable.  

You can go to the net and look for a depth of field calculator where you punch in your camera, distance to subject and camera settings for the picture.  This will tell you the range of focus.

I hope this helps more than confuses. TomDart