How does adding an extension tube affect the magnification ratio of the lens? Does it mean that if I were to add a 50mm tube to a lens at a focal length of 50mm(zoom, prime), my magnification ratio is 2:1 regardless of the lens's magnification ratio?
#1. "RE: Magnification ratio" In response to Reply # 0
The general thumb rule for calculating magnification with extension tubes is magnification = extension distance/lens focal length. This assumes that the lens is not reversed. So in answer to your question, a 50mm lens with 50mm of extension will give you 1:1.
Someone else can correct me if I'm wrong, but experience tells me that the magnification ratio of the lens will affect this somewhat, since I only need 27.5mm of extension with my 55mm micro lens (which is capable of going 1:2 on its own) to get 1:1.
#2. "RE: Magnification ratio" In response to Reply # 0
Extension tube focal length divided by lens focal length. A 25mm tube and a 50mm lens would create a slide with an image of the subject which is half the actual size of the subject. Since this is a universal formula, I believe it applies to infinity focus set on the lens. Somewhere in your house right now there is probably a ruler with a metric scale, even if you live in the US. With your setup in place on the camera, set the lens to infinity and bring the ruler into focus. The number of millimeters across from left to right in the viewfinder gives you an idea of the magnification relative to the 36mm length of the film frame. Remember to allow for the area the viewfinder shows. In other words, if your camera has a 90% viewfinder coverage, expect to see about 32-33mm at 1:1.---scott
"I am always doing that which I cannot do, in order that I may learn how to do it." Pablo Picasso
#3. "RE: Magnification ratio" In response to Reply # 2
Another good method that I've heard for judging magnification ratio is to use a US quarter. The size of a US quarter almost is exactly 36mm across (most people are more likely to have a quarter in their pocket that a small ruler ). 1:1 - quarter will fill the frame, 1:2 - 2 quarters next to each other will fill the frame (top to bottom), etc
#4. "RE: Magnification ratio" In response to Reply # 2
Thanks, Scott. I had forgotten that the thumb rule applies at infinity focus. That would explain why the 55mm micro only needs 27.5mm of additional extension to go 1:1. When the lens is set at its closest focus (a reproduction ratio of 1:2) the extension has taken place internal to the lens.