From: Mississauga, Ontario
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In the preview window, play the slide back and see if you've got enough time to read it. If you've finished reading at a normal pace and the title is still displayed for another 5 seconds I'd shorten the time. If you hafta speed read then obviously that's not right either. Also, display the photo a second or two longer if you're adding a title - people'll be reading your title (ie - Grandpa Jones, Oklahoma City, June 1969) instead of looking at the picture... then keep in mind that if your audience is a group of seniors, they may need a little more time... and make sure your font is not only big enough but a nice clear one... avoid the fancy-schmancy handwriting ones... keep the words away from the main subject too - either across the bottom or the top.
Nothing cut and dried and no quick formula for titles & photos. If you HAVE to use titles there's a number of variables that come into play imho...
I once lost a game of chess to a computer, but it was no match for me when it came to kick-boxing!