Shooting Techniques With Fast Prime Lenses Part 1

Now I’m not going to bore you by espousing at length the merit of prime lenses. I am just going to say this: they are good, they are fast, they are sharp, they are cheap(er), they are small.  I have a Canon  50mm f1.4 and a Lumix 20mm  (equiv. 40mm) f1.7.

“Go for the eyes, Boo, go for the eyes!” - Minsc

First and foremost, regardless of what you are doing, ALWAYS focus on the eyes. In a portrait, the sharpness (or impression of sharpness) of an image depends mostly on how sharp the eyes look. Anything else can fall out of focus but as long as the eyes are sharp, you’re good. This phenomenon has been  used by portrait photographers for years to soften skin, blemishes, hair, etc while still maintaining a feeling of sharpness.

Primes are often used for portraitures because they are so fast and allow the photographers to create that easily recognizable background blur that is so pleasing for the viewer. Below are two problems that one will likely encounter.

Narrow Depth-Of-Field

When shooting with a fast prime lens wide-open, one must remembers that at such wide aperture, the depth-of-field is tiny.  http://www.dofmaster.com/dofjs.html has a online depth-of-field generator that is particularly useful.

(click above for bigger version)

According to the chart, at 2 meter (head-shoulder portrait), the depth-of-field of a 50mm at 1.4 on a full-frame sensor  is a mere 13cm!  What that means in term of real-life shooting situation is if I were to lock focus on my model’s eye, anything farther than 6cm in front and 7cm behind the eye will be out of “acceptable sharpness”. 6cm is about the length of my pinky finger!

Putting it another way, if I were to lock focus on my model’s eyes and then inadvertently move more than 7cm back  or 6cm forward, the eyes will be out of focus.

A common shooting technique and the easiest way to screw that up is the classic “center-point focus and recompose”. The more time you spend recomposing your shot after the focus-lock, the more chance you will have of moving beyond that “acceptable area of sharpness” threshold. One way I use to tackle this issue is to work my composition first then once happy with it, I choose a focusing point closest to the model’s eyes (which is usually not the center point) and focus. I will then do a tiny amount of recomposing and finally depress the shutter. This should not take more than second and will minimize any offending change in distance.  It’s of course easier said than done especially on a camera like the 5Dmk2 where the focus points are weirdly arranged.

Focus Plane Issue

   
(click above for bigger version)

So, the length of a pinky finger is no big deal for snipers like us, right? But what if the focus plane is tilted? What if your model is standing at an angle and each eye is at a different distance from the optimum focus plane? One common mistake is not to be aware of that issue and focus on just one eye. This of course results in a portrait with one eye out of focus.  The sharper the angle, the bigger the problem. On the first image above, Bianca, my lighting assistant, has her left eye out of focus. It is particularly distracting on a real person.

Two ways to solve this problem: you could close down the aperture, increasing your depth-of-field or average it out and focus on a point between the two eyes which is usually the bridge of the nose. Closing the aperture will of course increase the depth-of-field and bring the background in sharper focus. One could compensate by moving the subject farther from the background, increasing the ISO and/or lowering the shutter speed. If you do not have that luxury, the best way is to average it out and focus on the bridge of the nose.

To be Continued, Part Two will be about Background and Bokeh issues.