Nikon FM2 Depth of Field Preview Lever: Viewfinder Changes and Depth of Field Assessment
According to the official Nikon FM2 manual, it explains the function of the depth-of-field preview lever, the change in viewfinder brightness, and the depth-of-field relationships listed in the manual; the scope is limited to the Nikon FM2 and does not provide disassembly, lubrication, fault diagnosis, or repair operations.
The Depth-of-Field Preview Lever Causes the Lens to Stop Down to the Set Aperture
When using an automatic aperture lens, the Nikon FM2 observes through the viewfinder at the lens's maximum aperture. Pressing the camera body's depth-of-field preview lever causes the lens to stop down to the set aperture, allowing you to check depth of field before shooting; the manual advises fully pressing the lever and releasing it when pressing the shutter. [G-FM2-DOF-01]
Viewfinder Darkening Results from Aperture Stop-Down
When holding the preview lever, the viewfinder darkens according to the selected aperture; the larger the f-number and the smaller the aperture, the darker the viewfinder. Elements within the sharp focus range appear sharp during preview. [G-FM2-DOF-02, G-FM2-DOF-03]
Understanding the Sharp Focus Range Using the Relationships in the Manual
The FM2 manual explains that at the same focusing distance, a smaller aperture gives a deeper depth of field; a longer focusing distance also increases depth of field; and the depth of field behind the main subject is greater than in front. The manual also notes that among lenses with the same maximum aperture, shorter focal lengths provide greater depth of field. These are qualitative relationships and do not represent fixed values or universal calculation rules. [G-FM2-DOF-04, G-FM2-DOF-05, G-FM2-DOF-06, G-FM2-DOF-07]
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