What makes a good portrait lens
I was recently researching for a small telelens for use for portrait photography. The 105mm telelens in 35mm photography is widely regarded as the standard for portrait photography, but other excelent lenses exist. There are two reasons to use a short telelens: The physical reality, if you're too close, parts of the body that are closer to the camera, such as the nose, will appear disproportionally larger, this can of course also be a feature. And the psycological reality, many people are intimidated by having a huge camera right in front of them, you're intruding their private sphere. They will relax more if you take a few steps back.
Some also mention the photographic effect, when you use a telelens you can get a shallow depth of field meaning that you can keep the eyes of your subject focused while having the background blurred, (bokeh). This means that in the final photo the eyes will not be distracted by objects in the background however, background can be very effectful for placing the subject in the context of her own life.
Basically there are three kinds of portraits: Head shot, half shot and full shot. Finding the right lens should let you work on a comfortable distance. The height, h, of the subject (or the crop you want), the distance, d, to the subject and the focal length, l, are related approximately by the following formula:
or, equivalently:
I use the standard 35mm frame for reference, that is 24x36mm. In the following I have calculated the relations for h, l and d for s = 36mm. Since l and s are both in mm, the unit cancel out and the unit of h and d will be the same. I have chosen meters which makes sense on this side of the pond.
| 1m | 3m | 5m | 7m | 10m | |
| 85mm | 0.42m | 1.27m | 2.10m | 2.96m | 4.20m |
| 105mm | 0.34m | 1.02m | 1.71m | 2.40m | 3.40m |
| 135mm | 0.26m | 0.80m | 1.30m | 1.86m | 2.60m |
| 0.30m | 0.50m | 0.80m | 1.20m | 2.00m | |
| 85mm | 0.71m | 1.18m | 1.89m | 2.83m | 4.72m |
| 105mm | 0.88m | 1.46m | 2.33m | 3.50m | 5.83m |
| 135mm | 1.13m | 1.88m | 3.00m | 4.50m | 7.50m |
The first table shows the maximum size (h) of your subject at selected distances (d) for each of the focal legths (l), 85mm, 105mm and 135mm (without regard to wether the lens can actually focus on the close range). If you're shooting with a DX sensor or want height for landscape orientation of the FX sensor, multiply the heights listed by 2/3.
The second table shows minimum the distance (d) you need to be from your subject of a given size (h) for each of the focal lengths (l), 85mm, 105mm and 135mm, in order for the subject to fit within the frame. The sizes has been chosen to correspond to head shot, head and sholders, half-shot (above waist), half-shot bellow waist, and full shot of an adult. Getting a real tight crop is difficult, for the 30cm subject height, the lenses are at their limit focus point. If you're shooting with a DX sensor or want the distance for shooting with the FX sensor in landscape orientation, multiply the distances by 3/2.
Looking at these tables, it appears that the 85mm is ideal half to full shot lens, while 135mm is ideal head to half shot lens for studio work. This choice will allow you to shoot from about 3-5 meters from your subject. If you can only afford one, then the 105mm may be the good compromise, but it will require that you work on a greater range from 1-7m. If you have lots of space or is shooting outdoor, then the 135mm may be the better option.
While the 105mm seems like a versatile lens, I'd opt for the 85mm, then at some later point when I can afford it I can add the 135mm rather than being stuck with a lens that is neither nor. Also, the 85mm is perfect choice when you whish to include background, the value of including environment to tell the story should not be underestimated.
In the end, it really comes down to your personal style what is best for you. The above table is a reference if you work in a restricted environment such as a studio, you can only get that far back.