When I worked at The Shutterbug camera stores in Portland, OR for 7 years I took the chance to get to know the wide selection of lenses from the different manufactures. I had to learn how to explain the difference to people as well and more than just the simple answer you get at any best buy.
Many of my customers that were looking to better their portrait photography would come and ask my advice. Most were surprised when I steered them towards a long focal length lens. I would use this chart to visually show the benefits and explain that to achieve that pro commercial look you had to be aware of effects focal lengths have on your subject, especially the face.
*Disclaimer. I wasn't trying to only sell an expensive lens haha. Lenses normally make only 10% profit for the dealers, then sales people get 10% of that if they are commission. The manufactures make the big bucks.
Photographer Stephen Eastwood shot some portraits with different focal lengths, from 19mm to 350mm, in order to demonstrate the effects of focal length on perspective. He is a fabulous photographer and you should check out his portrait photography. http://www.stepheneastwood.com/
To see this chart in full size click the gallery strip directly below. The 2nd pic is the side-by-side enlarged.
You need visuals to understand the focal lengths. Close-up 35mm and below narrows the whole face. Eyes are disproportionate to her head. Also it brings the nose and brow way to forward, giving off a more alien look. Most cell phones shoot at 35mm, this is why selfies sometimes don’t look anything like the person taking them. 50-70mm look pretty good. The head shape is more normal and the eyes are more proportionate to her head and brow. Most would say its acceptable. 50mm is approximately the angle that our eyes see, it looks pretty normal. However lenses are still don't have optics near as good as the human eye, so there is still slight distortion. 100mm is when the true form of her face comes thru, you will notice that from 100mm on her head is the same shape. Traditionally 85mm+ is the beginning focal length for portrait photography. Of course not set in stone. Many people who take pictures never know this. If life pics are what you’re taking then clearly it doesn't matter.
Granted the extreme distortion only happens when you are up close to your subject, or at least is most noticeable. The portrait focal length is also crucial for full body shots. (It effects all photography really, portraits are just the most crucial to get accurate.)
Let me show you what I mean with some photos I took for this couple.
All nice pics if I do say so myself haha. The 35mm is to similar to what your average cell phone can capture however. It shows the setting but not in a very appealing way. Sure enough the couple didn’t go for the 35mm one. They like the other two however, because they were 70 mm and 160mm. The main way to get your portraits to look very striking and on bar with pro portrait photographers. Shoot 70mm+ Good to know because I sell pics that had the exact same framing but were two different focal lengths. The long focal lengths always caught their eye. "Amazing!" "So Real!" That’s because it is accurately representing how everything looks, which the eye subliminally picks up on. Also the shallow depth of field isolates the subject from the background with very nice bokeh (The technical term for the "blurry background" as most will refer to it.)
Nearly every commercial photographer has a version of this lens on them while shooting. A 70-200mm f/2.8 Lens. The best versions for 35mm sensors hands down are from Canon & Nikon. The latest Tamaron versions I would say is the next best buy. Look up my in-depth article on the Canon EF 70-200mm f/2.8L IS USM to get my full opinion.