Further note on cyber attack,
The email you get may contain an attachment such as "cmihipcia...cka.png" DO NOT open the attachment.

A cyber predator claims to want to purchase artwork of yours which he claims to have seen online, with a budget of $500 to $5,000. He claims it is a surprise for his wife and therefore he will send a check to you if you provide your address. He further claims he will be out of country and that his representative will pick up the art at your home. He goes under the name of James Brown from Washington D.C. His email address is Brownjamess101@gmail.com. This is identified as a distributable domain by Whitepages.com

Thanks for the response. But again, my question relates to the photo not photographer. Apparently even this August organization has a problem defining professional photographs.

I wonder, does the PPA define a professional photograph?

I send images to three stock agencies. 75 percent of those are accepted, of those 75 percent only one or two are ever rejected for the issues you mention, usually the rejection is for lack of commercial appeal, not photographic failings. Does that make me a professional photographer? I think not, and as a volunteer I teach composition in Photography to seniors, does that make me a professional teacher? Professional level photography is not easily defined, and I certainly can not.

In addition to the great books by Michael Freeman and Bryan Peterson, here are 20 that are available on line – most for free.
1) Mastering Aperture in Digital Photography, by FreeDigitalPhotography.com
2) Exposure and Understanding the Hisogram, by Andrew Gibson
3) The Photographers Eye, by Simon Rivera
4) Photography by Eric Dahlin
5) ’Photography Composition by James Carren
6) Beyond the Lens, by Robert Rpdriquez Jr.
7) Digital Photography, by Steven Davis
8) The Photography Coach, by Robin Whalley
9) Mastering Composition in Digital Photography, by FreeDigitalPhotographyTutorialscom
10) Image Composition ,by Markus Kapferer
11) Storytelling Photographs, by Anne Darling
12) Composition with the 35 mm Camera, by Ian Timothy
13) DSLR Photography, by Bobby Perkins
14) Master the art of Photographic Composition, by Anne Darling
15) The Photographers Eye , by Michael Freeman
16) Digital Photography, by Kim Rormark
17) Photography and Affect, by Chris Virtanen
18) DSLR Photography, by Zayden Smith
19) Amazing Photos, by Howard Chapman
20) Before the Shutter, by Anne McKinnell

You are probably to young to remember, but I recall the days when slide film was in the range of 25, 32 and 50 range. We complained about the lack of sensitivity, but it did have, and still does have its usefulness, as other members have pointed out.

What about Bald Betty?

Thanks to all who replied. I have put together three slides (no formulas) to explain the COC, and how aperture settings and lens characteristics affect it and DOF.

PS In the future I will use reply quote.

Thanks,
I had forgotten how the angle of incidence from rays at outer edge of lens are refracted more than those in the center.

Actually your answer was a lot of help and reminded me of the issue of the neglect incidence
Thanks

I'm looking for a simple explanation I can use for a course I teach on Composition in Photography to senior citizens, not a lot of physics and formulas.
Thanks to you all for your help.

Hard to read but ultimately very helpful.
Thanks!

I need a simple explanation of why on one lens f/2 provides a smaller DOF than f/16?