Basic photography, depth of field.
Understanding Depth of field and F numbers
During a telephone conversation a friend complained to me that when he was photographing people he had trouble getting them all in focus, by all he meant from the tip of their noses to the back of their heads.
I said that he did not have enough depth of field, he went unusually quiet.
The phrase “depth of field” simply refers to the part of a photograph, from the nearest to the furthermost points from the camera, that is in acceptable focus. In my friends case a whole head. The eagle eyed will have spotted the word acceptable above, I will come back to that.
Architectural photographers will often want as much of their subject as possible in focus from the foreground detail two feet away, from the camera, to the farthest visible point.
There are times that we do not want all of a subject in focus, for instance in fashion photography having your model in focus against an out of focus (blurred) background is often desirable.
The main factor controlling depth of field is the size of the aperture that the lens is set to, the aperture being the hole in the lens that the light passes through, the smaller the aperture = more depth of filed.
The size of the hole/aperture is referred to as the “F” number or “F” stop, F numbers are typically:
f32, f22, f16, f11, f8, f5.6, f4, f2.8, the smallest aperture being f32 through to the largest aperture at f2.8
My friend was working with a telephoto lens at f4 which will give a very limited depth of field at the distance he was working at.
So the solution for my friend was to work at a smaller aperture ie f5.5 or f8
To give you some examples, using my camera which is a full frame DSLR and a 70mm lens.
If I focus on a point 6ft away using an aperture of F2.8:
I will have a range of acceptable sharpness of 5ft 9ins to 6ft 2ins
a mere 5ins (12.7 centimetres)
If I focus on the same point 6ft away using an aperture of F22
I will have a range of acceptable sharpness of 4ft 10 inches to 7ft 10ins a larger 3ft (91 centimetres)
For landscape photographers, using a 24mm lens (wide angle).
If I focus on a point 2ft away using an aperture of F32
I will have a huge range of acceptable sharpness of 1 ft (30. cm) to Infinity.
For fashion photographers, using a 200mm lens (telephoto).
If I focus on a point 30ft away using an aperture of F2.8
I will have a range of acceptable sharpness of 29ft 5′ins to 30ft 7′ins,
which is only 14 inches (35cm), which would mean that a model would be in focus but almost everything else is out of focus, really making the model stand out.
I did mention above that we are discussing acceptable sharpness, if you are making small prints or only showing your images on the internet you will have a different idea of sharpness than a photographer making very large fine art prints or prints for exhibition stands. This brings us to the wonderfully named “circles of confusion and hyperfocal distance” which I will cover in a separate post.
PS
In answer to the question….
“can I get more depth of field if I use a 50mm lens instead of my 100mm lens”
Well, no..
If you are photogaraphing a person for a portrait and you want only head and shoulders using your 100mm lens, switching to a 50mm lens will not gain you more depth of field because you will have to move the camera closer to the sitter which will actually mean that you have the same depth of field as with the 100mm lens.
Simply put, regardless of the lens focal length if the subject is the same size in your photograph and the F number is the same you will have the same depth of field.