Only one lens? I'd go for the lens with the focal length at or close to the "standard" focal length for your format; e.g., 50mm for a 35mm film camera, 80mm for medium format, etc.

I use a Canon Eos XTi digital SLR. I have a couple of zoom lenses that I use most of the time. Should I use a 50mm prime?

Well, a 35mm lens would be closer to the standard for your camera (and most DSLRs), but I think the 50mm would be close enough.

Additional information is whether you see wide angle, normal or telephoto. Everyone's vision and sense of composition is different.

Another factor is that whatever prime lenses you choose should have a fast aperture. F1.4 to F2.8 lenses will be more versatile in low light than those variable aperture zooms.

My preferences would be wide as an all around lens. Every point and shoot camera I buy starts at 28mm. On the 1.5 magnification body, I would favor the 20mm because it gave me the widest angle at 30mm and anything wider cost over $1,000. In the film days most of the images I took when traveling was with either the 24mm or 28mm.

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If you were only going to carry one prime lens in your camera bag what focal length would it be?

I would tend to agree with Larry Berman. For full frame the lens I would use as a prime, my "walk around lens" would be a 24mm f/2.8, and in fact from a slide collection made over 50 years of shooting and originally numbered over six figures in total (before I began culling as I scanned the images) most of the best outdoor images were made with a 24mm. Now with my EOS 5D my walk around lens is a 20-35mm f/2.8.

For your APS-C format sensor camera the equivalent would be an 18mm, so the closest prime available would be a 20mm f/2.8.

Thanks for all the advice. It's been very helpful in making my decision for my first prime lense. I think because most of my photos are of family and objects at close range I will go with the 50mm 1.4 or 50mm 1.2. Jeff.

Agreeing with LarryB, everyone sees their world through a different 'focal length,' and it changes depending on your mindset. I have a 1.6x crop body and shoot either wide at 18mm or tele at 85mm depending on how I feel when I wake up in the morning. More often than not my most compelling images are from my 85mm, and it's whats mounted 80% of the time; that in itself speaks not of the focal length but more of my style.
Try renting some from a place like lensrentals.com or similar for a few days of intense shooting...It can save you a lot of money in the long run

Another old thread.

I just became DX free. I sold my D200 and purchased a full frame D700 and I'm back to using my lenses and capturing images the way I've become used to over the past 35+ years. I purchased a 28mm lens and used it to shoot 150 pictures at a family reunion party last weekend. Wide without distortion. The perfect lens for shooting party pictures.

That's why you can't ask someone else which lens you should purchase because it depends on what you want to shoot and how you envision it.