Q. I am looking to buy a new lens for my Canon Digital Rebel. I want to be able to take close-ups, mostly portraits, and have heard lots about the Tamron AF 18-250 F/3.5-6.3. I am also considering the Sigma 70mm F2.8 EX DG Macro. What would you recommend for a good quick-focus lens that can zoom in for a good close-up shot? I'd like to graduate to a Canon 5D eventually, so I would like to get a great lens.

A. I'd avoid the Tamron 18-250mm under any circumstances. Wide-range zooms are usually poor optical performers, especially inexpensive ones. In your case, it is not compatible with the 5D, so it is a non-issue. The 5D uses a sensor the size of 35mm film, so you must buy a lens that works on both digital SLRs and 35mm SLRs.

The Sigma would provide excellent performance, but at 70mm, it would work OK for portraiture with the Rebel but not as well when you move up to the 5D. This is because of the Rebel's "focal length multiplier." Digital SLRs have varying sensor sizes and this changes the effective focal length of the lens. For the Digital Rebel, it is 1.6x, so the Sigma 70mm behaves like a 112mm lens. With the 5D, the multiplier is 1x, so it is just a 70mm lens.

Why does this matter? Portraiture tends to work best with 35mm equivalent focal lengths of 85mm and up. This is because the telephoto can blur the background and flatten facial features, which is flattering for most subjects. It also gives you some working distance with your subject. With the 5D, the Sigma would be a bit on the short side.

If you have a compact digital camera, this knowledge can still be put to good use. Most compact cameras have an effective zoom range of 35-105mm. One of the best portrait lenses ever made is the classic Nikon 105/2.5, for its combination of outstanding optics and ideal focal length. As you can see, the long end of most compacts' zoom range matches this 105mm focal length.

To take a great portrait with your compact, first make sure your camera's digital zoom is turned off and select the portrait mode if it has one. Zoom the lens to the end of its range, then back up from your subject while composing until they fill the frame. Focus on the eyes, compose and click!

To see what a difference using a telephoto does to portraiture, set the camera to the middle of the zoom range and step closer so your subject fills the frame the same way, then take the picture. Then go full wide angle, get close to fill the frame and take another picture. When you compare them, the difference in the subject and background will be dramatic.

If you have the budget, your best choice would be the Canon 100/2.8 macro lens, available for under $500. I also recommend checking out the Sigma 105mm macro. It is very well-regarded as both a portrait and close-up lens and will work well on both the Digital Rebel and 5D. The Sigma is under $400.

If you don't need macro capability and just want to do portraits, check out Canon's 100/2 and 85/1.8 at $389 and $339, respectively. They are superb portrait lenses that will give you an extra f-stop over the macro lenses, for better low-light performance and the ability to create an even softer background by shooting at f/2 or f/1.8.

— Read past columns by Don Lindich at http://www.soundadviceblog.com, and contact him using the "submit question" link on that site.