All photographers know that higher number f-stops mean greater depth of field, but maybe some don’t realize that there is an important ratio involved in the field of focus. This ratio must be considered when choosing the f-stop for a particular shot.
While the length of the lens drastically affects how much of the subject will be in total focus, where you set your focus point is critically important. This is particularly true when using automatic spot focusing and manual focusing. Learn to divide the desired focus area into thirds and set the focus one-third of that distance. When you focus on a particular spot, two thirds of the focal range behind that spot will remain in focus while only one third of the area in front of that spot will remain sharp. This is why portrait photographers set their focus on the subject’s eyes. This way the features of the entire head remain in focus.
A 35mm lens set at f-7.1 will have a much larger focal area than the same lens is set at f-2.8. Below are three examples of shots taken the same distance (16 inches) from the image sensor. What’s important here is not so much the actual focal area, but the fact that as the lens is closed down from 2.8 to 5.0 and further to 7.1, the area of focus (depth of field) almost doubles. Notice that as the f-stops increase in number, the area of focus increases, but the ratio remains constant.
The triad cocktail of ISO, shutter speed, and f-stop determine not only the exposure but the precise area of the photo that will remain in good focus. No amount of post-sharpening will rescue a shot with a shallow depth of field. Think ahead and you’ll be very happy that you did!
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