TRIGGER WARNING BOKEHOLICS: Three reasons why prime lenses are not just for bokeh.
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Today we’re going to talk about depth-of-field, not scientifically speaking, but practically speaking.
Depth-of-field is the distance between the nearest and the furthest objects that are considered in focus.
There are some basic rules to keep in mind.
The higher focal length the narrower the depth-of-field will be.
Inversly, when you’re aputure is wide open, a smaller f-number, there is less in focus than if you stop down to a smaller aputure — a larger f-number.
Just think about it this way, the larger the f-number the more depth-of-field.
Practically speaking, at a normal portrait distance with a normal portrait lens, f4 may not produce enough depth-of-field to get a persons nose and ear in focus. If you’re shooting a group portrait, a wider lens like a 35mm at high f-number, may be the way to go. Also if you’re shooting full body portrait at f8 with an 85mm lens you’re going to record details in the pops, the model and the background.
Some lenses have focus scales, like my canon EF 35mm f1.4. If you wanted to shoot a basketball team, the scale shows you that at f11 everything between infinity and about 6 feet can be in focus. These little tick marks indicate the depth-of-field for f8 and if you manually focus here, everything between 10 feet and infinity will be in focus.
increasing your f-stop to f22 will likely give you tons of depth of field, but that isn’t optimal since to shoot at f22 with savable light, you’ll need to decrease your shutter speed, which could lead to motion blur, or you’ll need to increase your ISO which will degrade image quality over all.
But more importantly, high f-numbers, like f22, will cause diffraction and an overall loss of sharpness. This is an optical limitation, which we don’t need to deep dive on, just know that in general most lenses will hit their optimal sharpness around f5.6 or f8 and that will be the sweet spot for having both depth and sharpness.
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