 Jeff Grove |
The Lowdown on Macros, Tubes, and Close-up Lenses Could someone give me a quick rundown on the difference between macro lenses, extension tubes and close-up lenses, and the different applications for each? I'm interested in macro photography and I'd like to know what's best to use.
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Tom Walker |
A true macro lens gives a 1-to-1 image on the negative - i.e., a dime will be the size of a dime on the neg. Extension tubes increase the focal length of the lens without changing the close-focusing distance - thereby magnifying the image. Close-up lenses are like screw-on filters that magnify the image. If all else fails, get a reversing ring and mount your lens on backwards. My fav is a macro lens on an extension tube.
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Chris |
Quote "Extension tubes increase the focal length of the lens without changing the close-focusing distance - thereby magnifying the image." Unquote That's nonsense! Extension tubes do NOT change the focal length of the lens. Because of the close object distance in macro photography, the image is formed behind the normal position of the lens rather than on the film plane (ie: behind the camera). An extension tube merely moves the lens forward so that the image once more falls on the film or sensor plane. Mathematically 1/f = 1/u + 1/v where f = focal length, u = object distance and v = image distance. If you run a few numbers, you will clearly see that as u gets smaller, then v gets larger. Magnification is defined as v/u.
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Chris |
Quote "Close-up lenses are like screw-on filters that magnify the image" Unquote This is also not really correct. Close-up filters do NOT magnify per se. What is happening is that an object is placed at the focus of the close up lens. The rays of light from an object at this focus leave the close-up lens as parallel rays. The camera lens "sees" these parallel rays and "thinks" this is an object at infinity (from where rays would also be parallel). The camera lens then focuses these on to the film plane or sensor on the normal way. So, for example, if you use a +4 diopter close-up lens which has a focal length of 250mm (10"), then an object place at 250mm from the close-up lens will be focussed by the camera as if it were at infinity. Since magnification = image distance divided by object distance, and an object at infinity will be focussed at an image distance equal to the focal length of the camera lens then the effective magnification will be equal to the image distance (let's say it's a 50mm camera lens)divided by the actual object distance (say 250mm in this example). So the magnification = 50/250 = 0.2. Similarly a 100mm camera lens would produce a magnification of 100/250 = 0.4. To get lifesize (ie: 1:1) magnification, the camera focal length must equal the focal length of the close-up lens. It is possible to actually bring the object inside the focus of the close-up lens when the camera lens then "thinks" the object is at a metre or so distance. If the camera lens is then refocussed to this shorter distance (eg: 1 metre)the magnification will be increased further.
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Bob Cammarata |
Wow, Chris!! My head hurts. (I think I'll just keep my simple macro-lens, and call it a day.) :)
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Tom Walker |
Bob, that's the way it was taught to me at the New York Institute of Photography back when I was a pup and makes a lot more sense than all that formula stuff, AND it works that way!!!
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Chris |
Tom Why do you see innumeracy as a virtue? If you were illiterate (which obviously you're not) you would no doubt try to disguise the fact. I'm reminded of the Luddites. There are two sides to photography. The creative side (definitely the most important) and the technical side (without which none of the technology would work so well or at all). There is a lot of mathematics in the technology of photography (mostly simple as in the close-up stuff above) some not so simple (as in calculating depth-of-fields). Understanding what's happening technically can aid the creative process. It sets boundaries which can be limiting or can be intelligently stretched. If the NYIP taught you Tom that extension tubes work that way, I should ask for some money to be refunded because they're wrong. Further, when you say it works a lot better than all that "formula stuff", how then do you calculate what extension tube or close-up lens you need for a particular size object and desired image? The suck-it-and-see approach I guess. Or is that your definition of "creativity"?
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Tom Walker |
Chris, No I'm not illliterate, there's actually a MS behind my name. When I see something that grabs my interest, I take a picture, if what I see in the view finder is not what I saw in my minds eye, then I change something. If I had to do all those calculations, then I probably wouldn't take the pic. I turned pro in 1980, and turned amateur in 1983. I prefer to take pics for me. If you like doing the math, fine, I like doing trig problems in my head while driving, but I photgraph for relaxation and fun, not to do math problems
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Chris |
Tom I don't disagree with you that a lot of (good) photography is the result of spontaneity and, in that situation, we don't then have time to run the numbers through our heads. We are one on this. The whole point of my diatribe is that I am curious, if you are not into the physics of photography, as to why you answered a technical question? With the wrong answer too? For the person asking the question they may well take your answer as gospel and be running around with a misconception in their heads which might affect some of their photography. It's a phenomenon of fora that people often answer questions they know nothing about. I have seen this a lot on photo fora and other non-related fora. There's a PhD in there for someone no doubt.
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Jeff Grove |
Guys, guys, take it easy. I didn't mean to start a war here. I appreciate both of your answers. I consider my question answered. So let's all just enjoy the photographic experience which, after all, is what we're all interested in. Thanks for your responses. Jeff Grove
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Tom Walker |
Jeff, this isn't a war, arguments like this is what we live for, I enjoyed it and I'll bet Chris did too.
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