What makes a lens a portrait lens?

Brian is right on the money!

Whoever told you this probably cares more about having cool equipment than about producing high quality results! A portrait lens is deemed such, when it's used for portraits. It's really a personal preference of a photographer. The general qualities that are usually considered good for portraiture are:

- Ability to produce crystal sharp images

- Ability to produce very shallow depth of field

- On those shallow depth of field have a pleasantly looking bokeh

- Have the least distortions possible

Usually these qualities are found in prime lenses (no zoom).

Usually for these to be true you need a pretty fast lens. Yes, f/1.4 is faster and better in most of the above. It is definitely a better portrait lens because it can get shallower, and it's faster, but saying that f/1.8 is not a portrait lens is just wrong. It's a very nice solid budget portrait lens, and in the right hands it will produce excellent portraits! Heck, a lot of photographers use zooms for portraits too, like 28-80 f/2.8 and get excellent results as well! It's all in the hands of the photographer, who knows how to use the equipment available!

LEM.

==edit==re: additional details===

Read my reply again. The f/1.4 lens is better. It does allow more flexibility in low light, or to get a sharper image by stepping down (all lenses perform best a couple of stops down from wide) and still having beautifully shallow DOF and great bokeh. The f/1.0 even more so. That said f/1.8 is a GREAT lens, even if the other ones are better still. The photo industry has a set of really high profile professional photographers who have no budget for equipment. They pay thousands upon thousands for things that are just a little bit better than the alternatives. That is even when the cheaper alternatives are excellent pieces of equipment themselves. These are the photographers for whom these ultra expensive lenses are catered.

==re: additional details #2==

You can take beautiful portraits with your 28-135mm. But it is generally considered an all-purpose lens. You can do a lot more than portraits. It's a limited wide angle (or very decent wide angle on a full frame sensor!), it has some decent telephoto capabilities, and it's flexible enough that you can keep it on the camera for many occasions that come. The 50mm lenses we are talking about are more or less used exclusively for portraiture. You don't HAVE to, but that's what they are used for by majority (again, not all) of the photographers. Hence the term. And they are very good at it. The wider zoom you are mentioning is not going to be as good, but again as in my prior comparison of 50mm at f/1.8 vs 1.4 it's still a good lens. But not as good for particular application IF compared to a prime... It falls more into the category described by slogan: Jack of all trades (including portrait), master of none.

To answer your question directly then - no, 28-135 is hardly a dedicated portrait lens, but in experienced hands it can be used to take very good portraits! Same hands with 50mm prime would probably do an even better job though.

A portrait lens is a lens used to take portraits. There is no magic focal length that defines it.

Spacial limitations can make you have to use shorter focal lengths, but if you have the space a 200mm lens can just as easily be used for portraits. A little tidbit that most people overlook is the fact that you can use longer focal lengths to help kill a distracting background. As the focal length gets longer perspective changes. Using your 50mm, if you take a photo of a model in portrait orientation and fill the left side of the frame top to bottom with the model you will see a lot of the background. Now change to 200mm, and back up to compose her to fill the left side of the frame again. Because of the change in perspective the background is brought closer while the subject stays the same.

The 28-135 can most certainly be used for portraits as far as focal length. The glass quality isn't going to give you razor sharp details probably, but all the same you can in fact use it.

RE: The 1.4 vs. the 1.8, you're looking at not only a little bit wider aperture, but higher quality all the way around. I don't regret getting the 1.8 because that's what I could afford. So far it's lasted me three years and does a fine job.

Portraits aren't all about "2.8 or better", although some feel a little more "photographerish" when they get a nice blurred background. Hell, I know I did when I started...lol. In a studio setting, using something like seamless, you can go anywhere from 2.8 to f/16 or smaller. Just depends on the light.

At the end of the day what makes a good portrait lens is the one used to get the shot you want. No magic bullet...no fairy dust....it's the tool you can use to get what you want from your camera.

Let's be honest. Is a $350 50/1.4 better than a $100 50/1.8? Yes it is. It will have better image quality and will be sharper wide open and is definitely built better. Is it $250 better? Depends on the photographer. Most pros insist on the f/1.4 because of the quality of the build and the glass and they can afford it. But will an f/1.8 do the same job? Yes it will. And so would an f/2.8. I have a 90/2.8 macro and it makes beautiful portraits.

Heres the thing, I rarely shoot wider than f/5.6 and only do so when I have to. So, with my 50/1.4 when I shoot at f/5.6-11 I am shooting right about at the sharpest apertures for the lens and I am getting my subject's entire face in focus. And I could do the same with the 50/1.8 or the 90/2.8.

edit-The 28-135 wouldn't be considered a portrait lens but you can use it for portraits. I have a 24-135 that I do shoot candid portraits with sometimes. It's more of a walk-around lens, one that I keep on my camera because I can shoot just about anything with it. It is just as capable of capturing a beautiful portrait as a dedicated portrait lens is. Shooting at about f/8-11 it would be pretty sharp and done right you can still get a shallow depth of field. I like a nice zoom and hope someone comes out with a constant aperture lens in a similar range for my camera. Even if it's an f/4.

Why pay more for a larger aperture? Well the lens is harder and costlier to make. And you do get half a stop more light. I ofcourse wouldn't personally pay for the f/1.4 but if you can afford it and need the bragging rights then hey...

I'm convinced that portrait lenses are also abit of a magic thing like bokeh. But generally on digital crop sensors, 50-90mm and f/2.8 and larger apertures are considered ideal. Ofcourse even in the ideal opinions vary. I prefer a lens around 90mm (35mm equivalent) to longer lenses, so those 85mm f/1.8s are not for me on APS-C.

And sure you can use an f/4 lens for portraits, but why should you when you have a choice. I mean you can always stop down the 50mm to f/4.

That said I wouldn't consider a 28-135 a portrait lens any more than I would consider the 18-55 a portrait lens. That's not to say I wouldn't use it for portraits if it was what I had available.

Fantastic question. Starred. There really is no such thing as a portrait* lens. It's just that people tend to look for lenses that combine a focal length that will not exaggerate facial features and has a large aperture so the background will be nicely out of focus. That is why the 85mm f/1.8 and 135 f/2 are often called "portrait" lenses. You can definitely take a portrait with your 18-70 lens. * except for the specially made soft focus portrait lens.

I always recommend the 50mm 1.4 or 1.8, depending on the budget. Either one will produce good results. I also always recommend the 85mm 1.4 as the ultimate portrait lens. Also, depending on budget. I've owned both versions of Nikon's 85mm 1,.4, the manual and auto focus, and they are incredible lenses.

100 budget 50mm 1.8

250 budget 50mm 1.4

1000 budget 85mm 1.4

To me, budget is really the only difference, both 50mm lenses produce great photos.

From one with a Canon EOS myself (450D), I'd say that whoever said that is a bit of a snob. A F1.8 is plenty good enough for fabulous portraits and you do not need to splash out on a megabucks lens to be able to get the same.