 BetterPhoto Member |
Magnificaiton provided by extension tubes I understand that an extension tube can provide magnification equal to the total extension divided by the focal length used. For example, if I have a 50 mm lens and I add a 25 mm extension tube, the magnification would be 1/2X. Does not the 50 mm lens in my example already provide some magnification? This leads me to my next question. I have a Sigma 28-80 mm lens that advertises a 1:2 magnification (I assume that's the same as 1/2X). If I add a Canon 25 mm extension tube, then what will the magnification be? Does it depend on the focal length I'm using? What if I'm already zoomed in to 80 mm (I assume this is where the 1/2X magnification is provided)? 25 mm / 80 mm = 0.3125. I thought the lens provides 1/2X magnification. According to the forumla above, I now have only 0.3125X magnification. I'm confused. On a similar note, what would my focal length be if I'm zoomed in as far as possible and then I used the macro feature? Am I not really zooming any more with the macro feature, but instead focusing? What is going on in the macro mode? Does my focal length change? Sorry for all the questions, but I'm quite confused. If anyone can shed some light on these issues for me, I would really appreciate it.
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John A. Lind |
Shaugn, In optics, magnification is the ratio of the distance from the rear lens node to the subject image (when it's in focus), and the distance from the subject to the front lens node: Subj________________[Nf--Nr]________Image This doesn't help much with practical photography. Finding the rear lens node isn't that hard, but finding the front one can be without using an optical bench in a lab. If you work out some of the optical equations using substitution, magnification is also the ratio of total lens extension to focal length. The lens extension is how far forward the lens is moved from infinity focus to the lens focal length. That's why you get 1:2 magnification with a 25mm extension tube on a 50mm lens, but it's *only* if the lens focus is set at infinity. Set lens focus closer than infinity and magnification increases. If you have a manual focus 50mm prime lens, turn its focus ring from infinity to its shortest focus distance. Very nearly all of them focus at distances less than infinity by moving everything (all the glass) forward from the lens mount. If the lens is focused at less than infinity, you must add how much the lens focus ring has moved the lens forward to the extension tube length. This is the total lens extension. Most manual focus 50mm standard prime lenses (not macros) move the lens forward about 7-8mm from infinity to closest focus. There are design reasons for this having to do with what happens with the effective aperture at closer focusing distances. You did the math correctly for an 80mm lens (if it's focused at infinity). It gets a little less than 1:3 magnification with a 25mm tube. The reason is its longer focal length and the lens must be extended farther (40mm) to get 1:2 magnification. Again, if you focus the lens closer than infinity, you have to add this to the extension tube length. The focus ring on an 80mm non-macro prime would probably extend it about 10mm at minimum focus distance. When you use the macro feature on your zoom, you're right that it's no longer zooming but allowing closer focusing. Zoom lenses are much more complex with macro work. Nearly all use "internal focusing" by moving some of the internal elements instead of a simple helical extending the entire lens forward. I'm not familiar with your Sigma, so I don't know how focusing is actually performed (extension versus internal element movement). It's also entirely possible that the 1:2 advertised for it is only possible at one end or the other of its zoom range, and at minimum focus distance. If it can get 1:2 at the 28mm end, you will need 14mm more extension to get 1:1 magnification. If 1:2 is at the 80mm end you need 40mm more extension to get 1:1 magnification. There is a way to test for this (without the extension tube). Draw a 48mm by 72mm rectangle on a piece of stiff cardboard. Then mount it about eye level on a wall and use your camera with lens to focus on it as close as possible at various points in the zoom range. At 1:2 magnification the edges should fill the viewfinder (note: it may more than fill it by a slight amount; most viewfinders show about 95% to 98% of the film frame). You can do the same with drawing slightly smaller rectangles centered inside this one (same aspect ratio of 2:3 for height versus width). Then add the extension tube and do the same to see which focal length gives you the highest magnification. One of the advantages of making macros at the 80mm end is being able to stand off farther from the subject at the same magnification compared to using a shorter lens. Too close and the lens with camera body block subject lighting, and with insects or other small creatures, it can scare them off. In the field, I typically use an 85mm or 135mm prime with tubes for standard macros, although I have used as short as 18mm and as long as 200mm. CAUTION: Hope this helps out. -- John
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John A. Lind |
Shaugn, One additional point: Objects at "infinity" have zero magnification. You'll never see an object at infinity; the closest you can come are the most distant stars you can see. This is why star and star-trail photography (not planetary!) only deals with absolute aperture diameter for exposure, not f-numbers for the f-stops. You may have the lens focused at infinity, and some real object (that must be) closer than infinity may appear to be in focus, but magnification calculations do not deal with front or rear of the Depth of Field, only with where the critical focus distance on the lens is set, and that is the exact distance at which the magnification calculations apply. -- John
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