LENS COMPRESSION - HOW TO FLATTER YOUR MODEL WITH FOCAL LENGTH
Something a lot of amateur and enthusiast photographers are completely oblivious to, is lens compression. We all know about things like the angle of view and the magnifying affect of APS-C (cropped) sensors but, how a lens actually renders an image often doesn't enter the equation for most people. If you're a beginner and only have a single kit lens, that's all you use. You'll likely adjust the zoom to fit the frame/subject, rather that move your position too and opt for a different focal length.
I'd long favoured my Nikkor 50mm f/1.8D for portraits on the Nikon D90. It was super sharp, great in low light, fast to focus and the shallow depth of field made for some really creative effects. I was quite happy snapping away with it and getting pretty good results. However, when making the jump to a full frame sensor, things became noticeably unflattering and the 50mm has since been usurped by the 105mm and 180mm primes.
"But why? Its the exact same lens" you may ask. Well, the short answer is distance to subject. When you frame a photo like I have in the two images below, the distance changes on full frame as compared to a cropped sensor. For example, on the Nikon D90 the crop is 1.5x magnification, so I can be further away from the subject and still fill the frame as I can on the D800. This means distorting effects of the lens aren't so apparent, because as you begin getting closer to your subject and also start using wider and wider angle lenses to fill the frame, you create more of a warped fisheye look.
When you compare the above two images above (D800 on the left, D90 on the right) you can see that compression actually looks more flattering on the D90. The forehead appears more bulbous, and the face and features more elongated on the left hand image. What is nicer though is the background of the left hand image. It is slightly more out of focus, since I am focusing closer due to physically being near to the subject matter and automatically obtaining a shallower depth of field as a result. Still, for portrait and fashion photography where the goal is usually to make your model look as beautiful as possible, distorted faces are generally something we want to avoid. Obviously if you are purposely trying to get some funky perspectives and fisheye looks, don't be afraid to do so and never let them inhibit your creativity - just be aware of what you're doing.
Opposite, I have taken a series of 4 shots using the 180mm, 105mm, 50mm and 28mm focal lengths. Notice the difference of compression in both the face and also the effects on the background too. The longer focal lengths blur the background much more readily and also have the added bonus of showing less and less of it, thereby eliminating problem areas automatically. Definitely handy if you're in an area with "busy" backdrops that can detract from the aesthetics of a photo. To my eye the 180mm and 105mm offer the most pleasing results of the 4 and by the time you get down to the wide angle 28mm the distortion is highly apparent and extremely unflattering.
Bare in mind though that this comparison is based on their use for traditional portraiture, where you'd often do just head and shoulder shots. The 50mm and 28mm are better suited for environmental portraiture (especially on full frame sensors), whereby you capture a person in their natural surroundings, like a tattooist in his parlour, or a butcher in his shop. Here the lenses are used at longer focusing distances and the frame filled with the subject and their location, so the distorting effects are nowhere near as apparent or unflattering.
Before you go out and take pictures in the future, I hope you now give some consideration as to what you are shooting and what lens will be most suitable. Don't just think "Oh it's indoors, I will use a 50mm f/1.8 because its great in low light". You can always add artificial lighting or boost your ISO to avoid blur. Instead, try grabbing your kit lens and zooming to 100mm or more. Step backwards so that you can frame your subject well and see the difference in the look of the resulting images.
Remember, there are no concrete rules where photography is concerned, just general guidelines that on the whole produce better images. However, sometimes going against those guidelines can bring wonderful results. Creativity must always come first but. Never let anything stiffle that but, if you're simply looking to produce more attractive photos for your clients or even friends and family, bear in mind the effects lens compression has on your subjects before you start shooting.