For those still following this thread...I had a few minutes after dinner to illustrate the idea that focal length (when taking the same shot; aka framed the same)doesn't affect focal length.

This illustration is for me, and those who are interested...it's not for anyone with a bone to pick or an argument to win...this is how I would shoot a subject, this is a real world example using a real world situation.


I didn't have a model, so I took a "portrait" of my trigger.

First I shot it with my shortest lens a 45mm at f/4.

Then I put on my 100mm lens at f/4 and I did what I would do in this situation...I moved back to get the same framing.

Notice that the out of focus areas are the same though one is a long lens and one is a relatively short lens.


Interesting eh?

Yes it is interesting, and a lesson for the beginners. What you accomplished by backing up and using a longer lens is to change the perspective. Notice that the subject is the same size in both photos but the articles in the background are larger in the shot taken with the 100mm lens. The other effect of this change in perspective is that the background "appears" to be less in focus just because it is enlarged. If you and lighthouse would read some of the posts that others have made on this thread I think the argument would go away. What you have demonstrated in this post is exactly what the link I provided in my previous post demonstrates even more effectively.

" Which brings me back to my question......Does the area that goes out of focus in the background get "fuzzier" the farther back the background goes? Seems like it should.

The simple answer to your question is YES. To clarify the issue there is only one plane in any photograph that is "in focus". That is the sharpest plane that the lens is capable of resolving. One hopes that this plane of "perfect focus" is on our subject. On either side of that plane of "perfect focus" is an area of "acceptable focus". That is an area which the human eye will perceive of as being "sharp". That area is referred to as the "Depth of Field" (DOF). For any given lens the depth of that area is determined by two factors: the size of the aperture and the distance to the subject (plane of perfect focus). The DOF extends from the subject toward the camera and into the distance beyond the subject. For example an 85mm lens on a full frame camera at f8 and a distance to the subject of 10 feet will have a DOF of 1.99 feet. It will extend from 9.1 feet in front of the camera to 11.1 feet from the camera. Anything closer to the camera than 9.1 feet or further than 11.1 feet will be rendered somewhat out of focus and the focus, or sharpness, will gradually worsen the further the object is from the subject. You can play with this concept using the Depth of Field Calculator here: http://www.dofmaster.com/dofjs.html