Canon EF zoom telephotos in the 70-300mm range are of five basic kinds, although variants were produced over time. The apx. 70-200mm zoom lenses of "L" type are discussed only in passing.


 The first two of these are inexpensive zoom lenses that regularly sell on eBay for around $100 and $150, respectively. They differ in the auto focus motor on the lens.
The other three are variations of the basic lens form with image stabilization added. Indeed, the EF 75-300mm f/4-5.6 IS USM was Canon’s first mass-produced image-stabilized lens and one of the first in the industry at large when it was introduced in 1995. Subsequently, an improved lens, the EF 70-300mm f/4.5-5.6 DO IS USM, with “diffraction optics” (DO) was offered in 2004. Because of the diffraction optics the DO lens was physically much smaller than other lenses in this series. Finally, the EF 70-300mm f/4-5.6 IS USM was introduced in late 2005 with improved image stabilization and optical qualities.
Except for the DO lens, these are fairly similar in appearance; and descriptions on eBay auctions can sometimes be confusing, especially when offered by actual or self-proclaimed naïve vendors.

Here are some aspects of these lenses.
EF 75-300mm f/4-5.6 and EF 75-300mm f/4-5.6 USM
At either new or used prices, these lenses are a good consideration for the person who has the EFS 18-55 mm kit lens for their APS sensor digital camera and wants a similarly inexpensive telephoto. Reviews have typically commented on “softness” of the lens at its maximum focal length, but in practice the lens is more than adequate, especially if stopped down, according to many reviewers (search “Canon +EF +(70,75)” for examples). Note that neither of these lenses has any image stabilization. Like all of these discussed, they can be used on both full-size 35mm image cameras as well as on smaller sensor cameras like the Rebel or 20/30D (equivalent there to a 120(+/-) -480mm lens). The Micro USM lens has a Ultrasonic logo on the upper portion of the lens.

This historically important image-stabilized lens shared its basic optical and design qualities with its non-stabilized cousins. However, the image stabilization typically allowed hand holding the camera at roughly two f/stops below the unstabilized lenses. The stabilization mechanism needs to be switched off to use the lens on a tripod, and there is no special provision for stabilization while panning. While the auto focus is USM, it is the less capable micro version, not the ring USM. Hence, the auto focus must be turned off to manually focus. The front of the lens rotates in focusing, making polarizer use difficult. I have seen this lens offered on eBay as the newer 70-300mm lens (although, to be fair, the price it was offered at was cheap even for this lens, so malice does not seem to have been involved). Do look closely at the pictures ("75-300" is clearly printed on the barrel, as opposed to "70-300"). The lens can be identified by a large metal label and switches to turn IS on and off.

Although originally this lens sold for something over $400, eBay sales have been as high as $500-550, perhaps at the beginning because of the originally much higher cost of its replacement (see below). Since writing the previous sentence, I have continued to monitor sales; and, as of spring 2007, this lens continues to sell normally at  US$300-400 with some still going as high as $500+. One lucky buyer may have got it for for about $150 in a sale that "ended early." It has maintained its value very well.

EF 70-300mm f/4.5-5.6 DO IS USM
Although not likely to be confused in appearance with the other lenses discussed here, the name is still similar; so a few comments may be made. This lens sells new for over $1100, and when offered on eBay still sells for near new price. It employs diffractive optics that allow this complex lens (18 lens elements in 6 groups) to be less than 10 cm long (the others are 12-14 cm in length). This is an extremely fine lens, according to reviews, but one complaint is that it is susceptible to lens flare, so its lens hood (ET65B) is needed. Since I originally posted this guide, Canon has apparently seen the light and now provides the hood with the lens.

EF 70-300mm f/4-5.6 IS USM
This is the latest of the series from Canon with improved image stabilization giving up to 3 f/stops advantage in hand holding the lens. Panning stabilization is offered, together with faster auto focus and locking of the zoom for easier carrying. The lens does not have the large metal label of its earlier version. One thing that has not been improved is the fact that the front of the lens still rotates in focusing, so polarizer use is still difficult. Optical improvement at higher focal lengths is also seen, although none of these lenses will equal a 300mm L prime lens (f2.8-$4K, f4-$1K, both IS) in sharpness.
When it was just introduced, it was rare and eBay and prices were close to new, mail-order prices. However, it is now more common and seems to be selling for about the same prices as the earlier 75-300mm IS lens. Although the older lens was good, this one is better by all accounts, so it would seem to be a no-brainer to get this one instead of the earlier version. Perhaps the older lens has some hidden charm? If you know of a reason the 75-300mm IS is selling so near the price of the 70-300mm, let me know.

Bottom Line
These are not the best lenses that Canon has ever made, but they provide a range of really good lenses. They have typically been well-reviewed in the photographic press and on-line. The cash-strapped shooter can get a nice high-end telephoto (even better on the APS sensors) for around a hundred bucks. If you want really top optics, go for the DO instead of many of the shorter range L lenses. Most people will find one of the IS versions to be a real revelation. If you have a steady hand, incredible feats of available light photography are possible; and if you don't have a steady hand, you can find out what unblurred photos look like!

Addendum: A new "kit" telephoto from Canon, the EF-S 55-250mm IS lens, should be looked at as well by anyone looking in this range. Like most of the lenses above, its front element rotates in focus. Moreover, as an EF-S lens, and unlike those listed above, it will only fit on the smaller-sensor cameras in the APS-C size (the "20D-xxD, Rebels (xxxD) cameras).