This is an age old question and different people have different positions on it. After working with lenses myself a bit and facing such issues, I myself now have a strong position on this question.
Theoretical answer: No.
Practical answer: Yes, indirectly.
First, what is Depth of Field (DOF)? (Referenced from: http://www.dofmaster.com/dofjs.html)
We can achieve critical focus for only one plane in front of the camera, and all objects in this plane will be sharp. In addition, there will be an area just in front of and behind this plane that will appear reasonably sharp (according to the standards of sharpness required for the particular photograph and the degree of enlargement of the negative). This total region of adequate focus represents the depth of field.
The Camera (Ansel Adams Photography, Book 1), Ansel Adams, Tenth Edition, 1995
This above mentioned depth of field region is the biggest for a pin hole lens, but since that will allow no light, the aperture size has to be increased. On increasing the aperture size, the lens position can now be considered at multiple places as it is a fat lens and the disc of opening will allow entry of light rays on multiple axes. Light rays from all these axes will converge on the image plane and thus reduce the sharpness of the captured image. Still we will have a range of region around the object which produces sharp pictures, just because of the above mentioned reason – required standards of sharpness on the image sensor. Though, as expected this region will be smaller than the pin hole aperture camera. Therefore, conclusion: depth of field will decrease with aperture size.
All this was with constant focal length. What happens with increasing focal length?
Clean one line answer -> depth of field remains the same if the size of the object in the image remains the same. This because, depth of field is dependent on aperture and not on focal length.
This sounds incorrect but it can be checked. To keep the object size the same in the image, the object will have to be moved in an equal ratio away from the camera as the focal length increases. This can be checked on an
amazing and fantastic online DOF simulator: https://dofsimulator.net/en/I am using 1/3″ sensors so you can also select it, they are under compact cameras. Keep the aperture size constant, lets say f/3.2. Use the following values of f and object distance (d) and observe the depth of field and size of object in the image:
f (mm) d (cm)
4 40
8 80
16 160
32 320
You’ll see that in each case the depth of field and object size in the image is the same. What is changing is just the perspective or the angle of view. Michael Reichmann performed the same experiment physically and has detailed the results on his blog at https://luminous-landscape.com/dof2/ . This proves that depth of field is independent of focal length.
But, in reality, we observe DOF to reduce with increasing focal length. Why?
This is because another thing happens while increasing focal length. As we reduce the angle of view, the amount of light coming in from the aperture reduces, this makes the photo darker, so what an auto focus lens does or manually we have to do is to increase the aperture size. This is required to keep the amount of light on the sensor constant and thus get same quality image with constant exposure time. And we know what happens on increasing the aperture size -> DOF reduces! This also can be check on the simulator, keep the f and d constant and increase the aperture (reduce the f stop number). the DOF will reduce.
Therefore, in conclusion, depth of field decreases with aperture size and is not affected by focal length.
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