A Minolta MD 50mm f/1.4 lens (a classic "standard lens").
The focus ring is set to infinity and an aperture of f/8 has been selected. The zone that will appear acceptably sharp with these settings can be easily read directly from the lens barrel. Just look at the indicator markings below the feet and meter scales,
as pointed out by the two yellow arrows!
That's it!
At f/8, the depth of field stretches from approximately 10 meters all the way to infinity (and visually even beyond on the scale)—indicated by the two white arrows! If you stop down to f/16, the depth of field expands even further, covering everything from 5 meters to infinity. You can now easily check and verify this on the scale yourself!
The exact same Minolta MD lens, but now the distance scale is set to 3 meters (around 10 feet), with the aperture remaining at f/8.
As you can see, the depth of field has shrunk significantly and now only stretches from approximately 2.5 meters to about 4 meters! Even if you stop all the way down to f/16, your subject will only appear acceptably sharp from about 2 meters to just under 10 meters.
Tip: There are a number of apps for your smartphone that automatically calculate the depth of field for a variety of focal lengths and distance settings.
Tip: As a rule of thumb, wide-angle lenses offer a greater (larger) depth of field than normal or telephoto lenses at the same distance from the camera to the subject.
Here is the simple explanation of why this lens became the standard for the 35mm format:
1. The Optical Reason: A Natural Perspective
When you look through the viewfinder of a camera equipped with a 50mm lens (while keeping your other eye open), you will notice that the scale and perspective of objects appear almost exactly as they do to the naked human eye.
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No Distortion: A wide-angle lens visually pushes objects into the distance and makes the foreground look massive. A telephoto lens, on the other hand, compresses the scene and pulls the background closer.
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The "Normal" View: The 50mm lens behaves neutrally. It reproduces perspective, depth, and spatial relationships between the foreground and background exactly the way we as humans are used to seeing. This is where the term "normal lens" originates.
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2. The Mathematical Reason: The Sensor Diagonale
In optical engineering, there is a fixed physical rule used to determine the normal focal length of any given camera system:
The normal focal length mathematically corresponds to the diagonal measurement of the recording format (the image sensor or film negative).
Therefore, the exact mathematical normal focal length for the 35mm format is actually just under 43mm (which is why a few legendary lenses were built with a 40mm or 43mm focal length).
So why did 50mm become the standard?
When Oskar Barnack designed the original Leica for the 35mm film format back in the 1920s, he opted for a 50mm lens (the legendary Leitz Elmar). First, a 50mm lens was optically less complex to calculate and manufacture with higher image quality at the time compared to an exact 43mm lens. Second, the slightly tighter field of view of a 50mm lens perfectly compensated for the fall-off in sharpness at the extreme corners of the film negative.
Since the Leica was the system that made the 35mm format famous in the first place, the 50mm focal length historically became established as the undisputed standard for the "normal lens."
