The following information is an edited (for clarity, eliminating message headers) version of a thread that was written in January of 1996. The original questions were posed on Compuserve by a variety of people, and answered by Chuck Westfall, of Canon USA. Many thanks to the now nameless people who asked the questions, and to Chuck, who took the time to answer them so clearly and patiently.
Q: Can you use an extension tube with the new Canon 75-300 image stabilization lense? If so, what features are still operable: autofocus? stabilization? other?
A: Yes. All features are still available, though AF is not recommended.
Q: I'm glad to hear that an extension tube can be used with the new 75-300 IS lens. You should suggest to the marketing folks that they include this in their description of the lens. I recognize that this is not the big news, but is a definite advantage over the regular 75-300.
A: The regular EF75-300mm f/4-5.6 lenses are just as compatible with our extension tubes as the IS lens. Maybe you're confusing "extension tube" with "extender." None of the EF75-300mm lenses are compatible with Canon's EF Extenders, but all of them are compatible with EF Extension Tubes.
Q: What's the difference between Canon's EF Extenders and EF Extension Tubes?
A: Extension tubes are used to achieve closer focus than is possible with the lens by itself. They have no optics of their own. They are usually classified by their physical length, such as EF12 for a 12mm extension tube. The EF25 is nominally 25mm in length, but actually provides 27.25mm of extension.
Extenders (more often known as mutipliers or tele-converters) are supplementary lenses that fit behind the lens and multiply the focal length of the original lens by the amount indicated through their classification. Extender EF 1.4x multiplies the focal length of a compatible primary lens by 1.4x, while Extender EF 2x doubles the original focal length.
Most extenders are designed to retain the focusing range of the original lens, whereas extension tubes eliminate infinity focusing in favor of providing a shorter minimum focusing distance. Also, extenders usually cut the transmission of the prime lens by a fixed factor. A 1.4x extender costs you 1 stop no matter what lens it's used with, while a loses 2 stops. On the other hand, light loss caused by extension tubes is inversely proportional to the focal length of the original lens. For example, a 25mm tube results in a 1 stop loss on a 50mm lens, but only a half stop loss on a 100mm lens, or a quarter stop loss on a 200mm lens.
Finally, some extenders (including Canon's EF Extenders) are designed with front lens elements that project out beyond the plane of their front lens mount. This is done to make them perform better with the rear-focus telephotos for which they are designed, but it also prevents them from being attached to other lenses that use different optical formulas.
Q: While you are on the topic of explaining "extenders", would you please elucidate the use of close up lenses. What are the advantages of a close up lens attachment compared with an extender tube? How close to "real" macro performance can you get with these devices on, say a 28-105 or 75-300 Canon zoom?
A: Unlike extenders, which multiply the focal length of the original lens and are placed between the camera body and lens, close-up lenses attach to the front of the lens, like a filter. Like an extension tube, close-up lenses eliminate infinity focusing in favor of reducing the close-focusing limit of the original lens. Unlike an extension tube, they do not cause any light loss. And, when used with a zoom lens, they maintain focus as the lens is zoomed. (Zooming the lens with an extension tube attached changes the focusing distance drastically.) These characteristics, added to the fact that close-up lenses often provide greater magnification factors than tubes* and plenty of working distance at the same time, make close-up lenses far more convenient to use than extension tubes for close-up photography with zoom lenses.
High-quality double-element close-up lenses, like Canon's 250D and 500D series, provide superb image quality from corner to corner if you stop down to the optimum aperture of the lens, which is usually around f/11 to f/16. Settings like these are easily attainable with high-quality medium speed transparency films (ISO 50~100) by using electronic flash.
Nikon also make excellent double-element filters like the 6T, though you will require a step down adapter for 58mm thread lenses since they come in a 62mm thread size.
You can achieve marginally better image quality by using a true macro lens, but you have to work a lot harder for it, because unlike a zoom lens/CU lens combo, the macro lens forces you to move closer or further from the subject to adjust magnification.
For many folks, the image quality produced by a high quality close-up lens in combination with a decent telephoto zoom like Canon's EF100-300mm f/5.6L or f/4.5-5.6 USM represents a better value overall, especially when you consider how much use you get from the zoom for your conventional photography.
*Here's a good example of the higher magnification of close-up lenses vs. extension tubes, using the popular EF100-300mm f/4.5-5.6 USM zoom lens:
|Close-Up Lens 250D||0.48~0.42||1.22~1.19|
|Close-Up Lens 500D||0.28~0.21||0.70~0.58|
|Extension Tube EF12||0.21~0.12||0.26~0.04|
|Extension Tube EF25||0.37~0.26||0.35~0.09|
COMMENT: I guess there is one advantage to extension tubes which is that you only need to buy one to fit all lenses regardless of filter size. Still, the close up lenses you describe seem to be a better solution for less money.
Q: How do extension tubes change the focusing distance of zooms and by how much compared to cu lenses?
A: There are several types of zoom lens optical formulas, but all zooms have the following 3 components:
In order for these components to function together simultaneously, the lens must be positioned at one precise distance from the film plane. Attaching a close-up lens does not change the distance of the lens from the film plane, so the compensating component continues to function properly as the lens is zoomed. But placing an extension tube between the zoom lens and the camera body alters the distance of the optics from the film plane. In this case, the compensating component is no longer effective, so the image goes out of focus when the focal length is adjusted. The amount of focus shift varies according to the length of the extension tube as well as the focal length range and focusing helicoid travel of the zoom.
What are the practical consequences of choosing a close-up lens instead of an extension tube? Here's an example: The EF100-300mm f/5.6L normally focuses from infinity to 1.5m/4.9 ft. When used with Canon's Close-up Lens 500D, the maximum working distance from the front of the lens to the subject remains at 500mm/19.7 in. at any focal length from 100mm to 300mm, with reproduction ratios ranging from 0.2x to 0.6x. If Extension Tube EF25 is used instead of a close-up lens, the focus will shift from a minimum of 892mm/2.9 ft. at 100mm to 3,810mm/12.5 ft. at 300mm, with reproduction ratios ranging from 0.1x to 0.27x. Manually focusing the lens to its closest range changes the reproduction ratios to 0.31~0.93x with Close-up Lens 500D vs. 0.34x~0.39x with Extension Tube EF25. After all is said and done, the Close-up Lens is far easier to work with, especially if you use a tripod, and offers a superior range of reproduction ratios to boot. And because of the high quality of Canon's double-element 500D close-up lens, final image quality is every bit as good as you would get with an extension tube, with significantly less curvature of field.
Q: How much magnification do you get with the 25mm et. vs. the 500d (which is the 58mm cu lens if I am not mistaken)?
A: The magnification figures vary from lens to lens. 500D close-up lenses are available in 52mm, 58mm, 72mm and 77mm sizes.
Q: (slightly different topic) Can either the 1.4 or 2x extender be used with the 28-105USM?
A: No. As mentioned in my previous reply, the protruding front elements of Canon's EF Extenders physically prevent their attachment to most zoom lenses other than the EF70-200mm f/2.8L USM.
Q: I have been following your very lucid discussion of close up accessories, i.e. close up lens/filters vs. extension tubes. From your descriptions, I can think of only a single advantage of the extension tubes...that they can be used with lenses with different size filter rings...but otherwise everything is in favor of the high quality close up lens attachments. Is this a correct impression? Why does Canon even sell the 12mm and 25mm extension tubes?
I am asking because I ordered the extension tubes before I saw your original post explaining the comparison. I am pretty convinced that I will return the extension tube (25mm) and get the close up lens but I just wanted to hear the "other" side of the story first.
A: The "other side of the story" is very simple. Extension tubes can be a better choice than high quality close-up lenses in specific situations:
1. If the front of the lens is larger than 77mm, close-up lenses cannot be used. This is the case for most super-telephoto EF lenses, like the 300/2.8L, 500/4.5L, etc.
2. If you're using a wide-angle lens, an extension tube will usually provide greater magnification than a close-up lens.
Except for these circumstances, I would say that close-up lenses are a better choice than extension tubes for most photographers. However, if you really want to "cover the waterfront," so to speak, there's nothing wrong with owning both. Tubes are not very expensive, and you never know when you might run into a situation where they'll come in handy.
One more thing: though there's no better overall value, IMO, for close-up photography these days than a decent mid-range telephoto zoom with a close-up lens, but you should also consider a true macro lens. In some ways, they're not as easy to use (because you have to move closer or further from the subject to adjust magnification), but they'll often provide greater magnification than a close-up lens with slightly superior image quality. Most close-up enthusiasts end up owning a true macro lens at some point.
Q: Before I read your thread on close up lenses, I had already ordered the Canon 25mm extension tube. I've tried it out with the 28-105USM and was disappointed to find the autofocus "hunting" continuously...very nearly correctly focused but constantly going over then under. I eventually switched to manual focus. Is this a known "feature" of autofocus with extension tubes? What did I do wrong? I was simply trying to focus on a leaf or flower with the lens set at 105mm using natural daylight. It was indoors so the leaves and flower were not moving.
A: Canon's instructions for Extension Tubes clearly state that AF is not recommended. FYI, many 3-dimensional close-up subjects such as plant leaves lack sufficient contrast for the AF sensor to lock onto. Even when dealing with a readable subject, it's best to focus manually, then confirm it using the in-focus indicator in the data display below the picture area.
Q: If I am planning to use close up lenses with both the 28-105 and 100-300USM lenses, would you suggest the 250D or the 500D close up lens. I imagine I would use the 100-300 more. I am interested in moderate magnification to photograph flowers, plants etc.
A: Either CU Lens (250D or 500D) can be used on either EF zoom lens (28-105 USM or 100-300 USM). You don't need the 500D with the 28-105, since that lens already focuses as close as 1.6 ft./500mm without accessories. The 250D, which brings you in to about 9 inches/250mm, is more appropriate. The 100-300 USM focuses to only 4.9 ft./1.5m without accessories, so the 500D is the most practical accessory for close-ups with this lens. Though the 250D can be used, you'll probably find it too strong for most subjects when used with the 100-300 USM.
Q: Can you tell me the difference among the following: the 250D; 450; 500; 500D and 500T?
A: First, the name of the close-up lens indicates its maximum working distance* in millimeters. So, a 250D close-up lens is much stronger than a 500D close-up lens, for instance. The 250D series is typically used with lens focal lengths from about 28mm to about 135mm; the 450 and 500 series are typically used with lenses from about 75mm to 300mm.
(*Working distance = distance between front of lens and subject, as opposed to focusing distance, which is the distance between the focal plane and the subject.)
Next, there have been 2 generations of close-up lenses in Canon's line: the first series was brought out in the '70s and included the 240, 450 and 500T. These CU lenses are no longer being made, so whatever stock is available is relatively old and can no longer be ordered from Canon Inc. in Japan. (In fact, we've taken all the 450's out of our catalog, and the only 240 remaining is the 52mm.) The current line includes the 250D, 500 and 500D lenses. These were introduced in 1994 and will remain in our line for the foreseeable future.
Some of Canon's Close-up Lenses are single element (all of the 500 series and the 58mm CU450), but most are double element (240 series, 250D, 500D and 500T). The single element lenses are priced for economy and provide reasonable value, but the double element lenses are clearly better in quality. These are the CU lenses I would recommend to get the most out of the original lenses to which they would be attached.
The current availability of Canon CU lenses varies according to diameter (filter thread).
Canon's EF Extenders do not fit the EF75-300mm zoom lens. Independently made extenders are available, but we will not vouch for their quality or compatibility. There are several reasons why Canon makes extenders that don't fit their own zooms, but off brands do. Here are a few:
Consumer grade zoom lenses like the EF75-300mm series (as opposed to professional zooms like the EF70-200mm f/2.8L USM) feature relatively moderate maximum apertures to begin with, such as f/3.5-4.5, f/4-5.6, etc. Light loss caused by the use of an extender (1 stop loss with a 1.4x extender, 2 stops with a 2x) results in effective maximum apertures as slow as f/11. This makes the zoom/extender combination virtually unusable for a wide variety of shooting conditions. Why? Consider the following:
1. Unless you're dealing with film speeds of ISO 400 or faster on a bright sunny day, an aperture like f/11 results in a shutter speed that's too slow to hand hold for most telephoto lenses, and is certainly too slow to stop action effectively.
2. You lose autofocus when the effective maximum aperture falls below f/5.6. That means you're talking manual focus only when using an extender with a moderate aperture zoom. At the same time, the viewfinder image becomes relatively dark, making manual focus difficult at best.
3. Even if you're willing to put up with conditions 1 and 2, you're still faced with a significant degradation in image quality, because it's virtually impossible to design an extender that can be
optically matched to the wide variety of zoom lenses in our line-up. The only alternative would be to design a dedicated extender for every individual lens, which in turn would make the price of
those extenders cost-prohibitive.