What to look for in a portrait lens

[inkista]

Originally Posted by johnStock

My main interest is doing outdoor natural light portraits and possibly some sports (with portraits being my main focus). I'm a big fan of bokeh and am most likely going to get either the d5000 or the T1i (leaning towards Nikon)...

May I just say that if you really are planning on going with Nikon in this direction, you definitely want to purchase at least a D90, because of the focus motor issue. Here's why.

The kind of big-buckets o' bokeh you want are typically going to be done with lenses that have a wide max. aperture, of f/2.8 or larger. With Canon and Nikon, the fastest any of the zooms go is f/2.8 (only Olympus has faster ones with f/2). f/2.8 is the "borderline" aperture for available light (say, indoors without a flash) shooting. Most portraiture is done with faster lenses, which leaves you with primes.

On the Nikon side, if you get a D5000, these are the fast primes that are f/2.8 or faster that are less than $1500 that will autofocus with your camera body:

• AF-S 35mm f/1.8 ($200)
• AF-S 50mm f/1.4 ($450)
• AF-S 100mm f/2.8 Micro ($900)
• AF-S 60mm f/2.8 Micro ($550)

Obviously, if you get a D90, the AF/AF-S restriction no longer applies, and most of Nikon's fast primes will then autofocus. Except the 50/1.2, which is a manual focus lens.

On the Canon side, all the fast primes will autofocus on any of the digital bodies. This is the payoff we get for the FD mount being scrapped back in the '80s. Here are all the f/2.8 or faster <=$1500 prime lenses that autofocus on a Canon T1i

• EF 15mm f/2.8 Fisheye ($650)
• EF 20mm f/2.8 ($500)
• EF 24mm f/2.8 ($350)
• T-SE 24mm f/2.8 Tilt Shift ($1250)
• EF 28mm f/1.8 II USM ($450)
• EF 28mm f/2.8 ($250)
• EF 35mm f/2 ($350)
• EF 35mm f/1.4 ($1400)
• EF 50mm f/1.8 II ($100)
• EF 50mm f/1.4 II USM ($350)
• EF 50mm f/1.2L USM ($1500)
• EF 50mm f/2.5 Compact Macro ($300)
• EF-S 60mm f/2.8 USM Macro ($400)
• EF 85mm f/1.8 USM ($400)
• EF 100mm f/2 USM ($450)
• EF 100mm f/2.8 USM Macro ($550; non-L)
• EF 100mm f/2.8L IS USM Macro ($950)
• EF 135mm f/2.8 Soft Focus ($500)
• EF 135mm f/2L USM ($1000)
• EF 200mm f/2.8L USM ($750)

If we look at the lenses that fall within your $500 budget, the count is Nikon: 2, Canon: 11.

1) My understanding is the lower the f/ratio the better. Is this correct? How low is "low enough"

f/2.8 is what most people would say. I would say f/2. However, realize that any extreme of lens design will have inherent tradeoffs. With wider lenses, shooting wide open (at the max. aperture) tends to be softer, with more chromatic aberration than if the lens is shot stopped down. This is typically why you want a faster lens: you have less compromises when you've stopped down. It's a rare lens that's at its sharpest wide open.

E.g, the 50mm f/1.8 II is at its sharpest when stopped down to f/4.

Also, you will note the cost differential climbs nearly exponentially. Look at the Canon 50s:

• EF 50mm f/1.8 II: $100
• EF 50mm f/1.4 USM: $350
• EF 50mm f/1.2L USM: $1500
• EF 50mm f/1.0L USM (discontinued): $4000 on the used market and rising.

2) I was told that something in the 70-105mm range is preferable. Is this before or after the crop factor? ie would a 50mm be fine and not distort the features?

This is a matter of personal taste. But longer telephoto lengths can increase the out of focus blur, and typically be freer of defects like barrel distortion and chromatic aberration. You can typically see distortion beginning at 35mm. Keep in mind that this kind of distortion and chromatic aberration is often correctable in post production, though.

Working distance and framing will probably have more effect on what a good portrait length is for you.

[mattdm]

Originally Posted by inkista

This is a matter of personal taste. But longer telephoto lengths can increase the out of focus blur, and typically be freer of defects like barrel distortion and chromatic aberration.

The disadvantage is that you're further from your subject, and it's harder to get an intimate-feeling portrait.