Depth of Field Myths: Does Focal Length & Sensor Size Affect DoF?
Discussing various factors that affect depth of field including focal length, sensor size, f-stop, & distance to subject and debunking common DoF myths.
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In optics, the phenomenon known as depth of field (DOF) is the distance about the plane of focus where objects appear acceptably sharp in an image. Although an optical imaging system can precisely focus on only one plane at a time, the decrease in sharpness is gradual on each side so that within the DOF the unsharpness is imperceptible under normal viewing conditions.
In some cases, it may be desirable to have the entire image sharp, and a large DOF is appropriate. In other cases, a small DOF may be more effective, emphasizing the subject while de-emphasizing the foreground and background. In cinematography, a large DOF is often called deep focus, and a small DOF is often called shallow focus.
Precise focus is possible in only one two-dimensional plane; in that plane, a point object will produce a point image. In any other plane, a point object is defocused, and will produce a blur spot shaped like the aperture of the lens viewing it. When this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus and is considered “acceptably sharp”. The diameter of the circle increases with distance from the plane of focus; the largest circle that is indistinguishable from a point is known as the acceptable circle of confusion. The increase of the circle diameter with defocus is gradual, so the limits of depth of field are not hard boundaries between sharp and unsharp.
Thanks for watching! I hope my explanations were clear and helpful. There was a section that I completely cut from the video because I felt it bogged down the flow and made it much more confusing, but if you’re interested in knowing more about how focal length affects depth of field, read below.
In the video I suggest that focal length has no direct affect on depth of field–only secondary, and this is only true if the goal is to maintain a consistent size of the subject in the frame or magnification. If we remove distance to the subject as a condition, then focal length becomes a direct influencer on depth of field.
This often seems confusing, because you’d think a longer lens with a greater focal length would have a deeper depth of field because it has a more narrow field of view, but what’s actually happening is the longer lenses are increasing the noticeability of discrepancies in focus. Basically, if rays aren’t meeting perfectly at the correct plane, they cross over each other and can go on infinitely getting further and further apart. So longer lenses allow these less-than-perfect alignments to get further from each and thus further from focus when travelling down the longer focal length. Where a wider lens, which converges more powerfully, cuts the errors off sooner so they’re less blurry when compared to the point of true focus.
More simply, longer focal length lenses magnify or bring things closer to you visually, which includes exaggerating discrepancies in focus, making the range of acceptable focus more shallow than if you used a wider lens.
As explained in the video, however, this is easily overcome by achieving the same subject magnification or filling the frame equally with the subject when using a wider lens. So, as you can see, it’s both true and untrue that focal length affects depth of field, depending on what you’ve determined to be the primary goal.
If your goal is to have equal subject weight, focal length will have no important affect, just move the physical location of the camera or the subject. If, however, you wish you maintain a constant physical distance, then focal length will be a primary and direct factor in influencing your depth of field.
I hope this addendum didn’t obfuscate the utility of the video too much. And to those who braved the further reading, I hope you have a wonderful day!