Using a 50mm lens with f/1 native aperture for portraits
Some of our students have purchased some of the very fast 50mm lenses with native apertures in the f/1 range. These lenses are great for low light situations where you need higher shutter speeds to be able to hand hold the camera. When combined with a higher ISO you can shoot handheld in some very low lighting situations.
Many of those students have been disappointed with their Depth of Field (DOF) results when shooting close up images with these lenses at f/1 to f/2. Particularly head shot portraits where the person’s head and shoulders fill the frame. They were hoping to get backgrounds that were unrecognizable and completely blurred out while their subject was in focus.
To have the person and their shoulders fill the frame with a 50mm lens you will probably have to place the camera body about 2 feet away from the subject. Thus your camera to subject focal distance is 2 feet. To get a person’s entire head in focus you need at least 6 inches of DOF. (Note: Six inches is the approximate distance from the back of the ear to the tip of the nose).
I have created a graph showing the Near and Far Focus Limits and resultant DOF for a number of f stops from f/1 through f /22 at a lens focus distance of two feet.
An analysis of the data reveals that when using a 50mm lens with a native f stop of f/1, the smaller f stops allow a very very limited DOF. You only approach 6 inches of DOF at f/16. At f/16 you will have a significant amount of diffusion and your image resolution and contrast may suffer slightly from diffraction. (Note: Your sharpest images are normally shot with f stops at least 2-3 stops in from each end of the f stop range. In the case of the illustrated lens, the sharpest f stops to shoot at would be f/1.4 through f/11). Thus, even if you shoot at f/11 your DOF would be only about 3.6 inches.
Full length portraits might be a better choice for these lenses. Then the added focal distance will provide an adequate DOF for the person. These images could be shot with a focal distance of about 9-10 feet depending on the height of the subject. Then the DOF chart would look more like this. (Click on the diagram below to see a larger version).
You could shoot in the f/2 to f/2.8 range at a focus distance of 10 feet and get an adequate DOF to get the entire model in focus. Shooting at f/1 might not get the entire model in focus. If a softer focus is your aim, then a more shallow DOF might be what you need.
If you are looking for are very soft ethereal image then shooting at small f stops at short distances from the camera to the subject with very limited DOF may be what you are looking for.
In any of the above situations, do your DOF homework and you will probably get better results.