 Thomas L. Russell |
magnification ratio trying to figure out the magnification ratio on lens. For instance , in comparing lens , 75-300mm zoom. Both have the same zoom range. One has magnification of 1:4 (lower price), higher price has magnificatio of 1:5. Minimum focus is the same 4.9'. Angle of view about the same. With the miminum distance being the same, what is the magnification difference.
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Jon Close |
You didn't say which two lenses you are comparing, but I know of one similar pair. The Canon EF 75-300 f/4-5.6 focuses to 4.9' with maximum magnification of 1:4. The Canon EF 100-300 f/4.5-5.6 USM also focuses to 4.9', but it's maximum magnification is given as 1:5. The difference arises because the 100-300 focuses by moving internal elements instead of extending the outter element like the 75-300. This has the effect (in this particular lens) of changing the focal length when focused to close objects. The 100-300 has an effective maximum focal length of 300mm when focused from about 10' to infinity, but focused to closer distances the effective focal length reduces slightly. When focused at the minimum focus distance of 4.9' the effective focal length is 250mm, thus the maximum magnification is 1:5 instead of 1:4. The difference in price between the two has to do with the other features of the 100-300 v. 75-300 (non-rotating front element for convenience with polarizer, ring-USM with full-time manual focus and faster, silent autofocus, focus distance scale, better build quality).
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Thomas L. Russell |
Thanks John. You are right. I did confuse the fact that one was 75-300 and one was 100-300. But what is the basic formula to find the ratio. Lee
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Jon Close |
I couldn't tell you the mathematical formula, but it is incorporated in a free-ware program called f/Calc from http://www.tangentsoft.net/ . Note that the calculation gives the distance from the focal plane (where the object being photographed is) to the lens's optical center (not necessarily the physical center of the lens).
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Jon Close |
P.S. the f/calc program give magnification as a factor rather than a ratio, so magnification of 0.5 is the same as 1:2, 0.25 = 1:4, 0.2 = 1:5, etc.
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Thomas L. Russell |
thanks for your help. Lee
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John A. Lind |
Thomas, Magnification is the ratio of image size _on_film_ to subject size in real life. The first number represents image size on film; the second is how much bigger (or smaller) it is in real life. At a magnification of 1:4, 1 inch in real life will be 1/4-inch on film. At a magnification of 4:1 (an extreme macro), 1/4-inch in real life will be 1 inch on film. Magnification is completely *independent* of film format (e.g. 35mm, medium format, large format), and print size. It deals only with actual subject size and how big its image is on film. With 35mm format, you can estimate magnification in the viewfinder. The 35mm film frame is roughly 1 inch by 1-1/2 inches. If you know how big the subject is, and estimate its dimensions on film by how much it fills the viewfinder, you can estimate the magnification. -- John
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