A lens question for Professional Photographers??

What lens do you recommend starting out with for a professional photographer starting an on location portrait photography business. I am currently using a Canon Digital Rebel XTi.

EF 85mm f/1.2L II USM, that one is good, but with your crop factor might be a bit to flatening. So maybe a EF 50mm f / 1.2L USM will do you better. If you want a zoom then a EF 24-70mm f/2.8L USM is nasty but you do lose the big apertures.

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For location portraiture any of the following should do the job for you:

Sigma 17-70mm F2.8-4.5 DC MACRO
Tokina AT-X 165 PRO DX AF 16-50mm f/2.8
Tamron AF 17-50mm F/2.8 XR Di II LD Aspherical [IF]

My nod would go to the Sigma because it is a more versatile overall lens and that makes it good as an everyday walk around. These lenses are also less expensive than the Canon glass. It's nice to say that such and such a focal length is good for portraits and in a studio it's fine to pick the 'right' focal length. On location, it's a whole different ballgame and the zoom really comes into it's own. It will enable you to carry out a range of portrait types that you may need to take on location.

A professional photographer asking what lens she needs?

Perhaps a little practice before going pro....

That said, here is a brief rundown of the usual suspects:

Zoom vs prime: zooms offer versatility, while primes offer larger apertures and better image quality. For portraits, there isn't much reason NOT to go with a selection of primes, as you'll have plenty of time to plan your shots.

Crop factor: Your camera has a smaller sensor than 35mm film. This doesn't magically transform your lenses, but the crop will require you to move further from the subject to achieve the same framing. Thus, most people describe the Canon crop factor as multiplying focal length x1.6, as a 50mm lens will behave similarly to an 80mm lens on full-frame or film.

Perspective: Most of the time (unless you have a good reason to do otherwise!), you should be shooting portraits 6-10ft from the subject. This will impart a flattering perspective, with mild flattening of the features. To shoot full length portraits instead of headshots, use wider lenses instead of backing up to the next county. To get in really tight, use a telephoto rather than getting too close.

Focal lengths: On your camera, 85mm makes a good tight headshot lens, 50mm makes a good general portrait lens, 35mm works for 3/4 or full length, and 24mm for environmental and full-length portraits. (on my 5Ds, I use 135, 85, 50, and 35 respectively for these purposes).

Specific Advice: I'd start with the EF 50mm f/1.4. It will be a great general purpose portrait lens on your camera. The more expensive L lens has great color and bokeh, but isn't noticeably sharper, and has well documented problems with focus shift that can cause you to get out-of-focus shots at close range. I've owned all three Canon 50mm primes, and feel that the 1.4 is the best choice (I sold the L and went back to the 1.4).

Another option would be the combination of a 17-50 zoom (IE the Tamron) and the 85mm f/1.8. You can get both for under 1k, and you'll have a nice setup that covers the needed focal lengths. You'll also have some big-aperture options for shallow DOF...