Mike Del Vecchio wrote: > I learned from old timers, is a ratio of aperture to lens size, which is > why it's written 1:1.8 or 1:4.5 on your lens front piece. Aren't some of you newer photographers glad we are doing this in steps? :) Mike describes the aperture/focal length correlation very well. I remember when I learned this stuff several years ago, I though "why do I need to know any of this?", but is all very important if we wish to understand why and how the images from our latest roll of film got there and why they look like they do.
> Lenses around the edges are less than perfect parabolas, causing the > light rays to not be exactly on the focal point. That's why wide open lenses > look funny compared to the same lens at f-8; the images are in focus, but > they have a wax paper look. The bigger diameter a lens has, the more > difficult it is to make, which is why they cost so much. And when you stop a lens down too far, you introduce another problem to your lens' ability to record sharp images and this is called 'refraction'. This is when the light rays are forced to go through a smaller opening just like an hour glass shape on each side of the lens iris, and the light rays on the outside diameter of your image circle had a farther distance to travel than the light rays in the center. This results in a progressive loss of sharpness towards the edges of your image. This is because they are not 'focused' on the film plane the same distance as the center of the image. This is why lenses work best around their middle apertures. The concept of stopping down a lens to its smallest aperture in hopes of getting the sharpest image is largely misunderstood. What you get then is increased depth of field to cover the increased loss of sharpness around the edges of the image! Somewhere there is a happy medium. This you will have to determine through examining your own images carefully and comparing your exposure notes (you are keeping notes or recording this stuff, aren't you?). For example, I have determined through this experience that a certain lens of mine that stops down to F 32 gives me the sharpest looking overall image when I am using it at around F 8 for subjects more than around 40 feet away, and at F 16 for close-up work. Each of my lenses is different. Even my 105mm 2.5 is different from my 105mm 2.8 Micro (and its optimized for a different subject distance). This topic could lead into another problem which is light loss from barrel extension of your lens at close distances, commonly referred to as the 'bellows extension factor' where you must open the aperture wider the closer your subject is to combat this light loss because light falls off at the square of the distance. Isn't photography fascinating? :) > There's more to f-stops than I've written, but the above is the basic > theory. Thanks, Mike! Dave Cohen Photographer [EMAIL PROTECTED] --> SPORRS: Serious Photographers of Railroad Related Subjects
