Gal, welcome to the Hog, I had not seen you around before.
You can lock your focus by whatever means works best for you, whether BBF or shutter button, makes no difference. As long as your focus is locked you are free to recompose and your original focus distance will not change but the distance to your subject will change the second you move you move your camera. With a very shallow dof there is a high likelyhood that your subject will be slightly out of focus and a bit soft, as my example has shown.
I'm glad you asked, as you're the type of shooter that I'm trying to reach and raise this awareness. ;-)
SS

After 2 years of saving $ and finally deciding which lens would best suit my needs, I ordered a new Canon 100-400LII on Friday and it's on the truck to be delivered today!! I'm very excited!! When I first started saving for a lens, I was sure it would be the 70-200 f/2.8LII, but after getting a great refurb deal on the 24-105, I started wondering if I'd be happy with only another 95mm in reach. Then, the new 100-400 came out... I still had to decide which I needed/wanted more - speed or reach. Since I shoot more outside, I went with reach. I also reached out to a fellow UHH forum member for his advice, and coupled with reading all of the various threads here, I feel I made the wisest decision for me. I'd still love that 70-200 some day, buy maybe version I. Thank you, everyone, for posting your various questions and concerns about different lenses. I read them all, and it really helped my thinking process! :)

I have written before about a process that you might want undertake. Use your digital camera (assuming that you have one). Set up your camera with a macro lens of 40-60mm. On a second tripod set up a slide copier. You might have to borrow one if you do not have one. use a light or a flash(on a third separate tripod) and shoot away. You may have to shoot several shots before you get the correct adjustment of f/stop and or shutter speed to get the results that you need. If this method sounds too complicated then listen to our other poster friends and find a place that they recommend.