"Equivalent Aperture" and Crop Factor: Don't Confuse Angle of View, Exposure, and Depth of Field
Explain the boundaries between the equivalent field of view of different formats and the actual focal length, actual aperture, exposure, and depth of field, and avoid applying the equivalent focal length multiplier to camera exposure settings.
The crop factor describes the field of view conversion
The crop factor compares the field of view of smaller sensors to that of a 35mm format; it does not change the lens's focal length. Nikon uses the DX format as an example: a 24mm lens on a DX body provides a field of view roughly equivalent to a 36mm lens on a 35mm format. The exact conversion depends on the camera's sensor size, and an APS-C factor should not be applied to all brands.〔G-EQUIV-01〕
Understanding lens specifications like 25mm f/1.4
Panasonic's LEICA DG 25mm f/1.4 specifies an actual focal length of 25mm, a maximum aperture of F1.4, and a 35mm format equivalent focal length of 50mm. The "50mm" equivalent here indicates the field of view conversion; it does not mean the lens becomes 50mm, nor does it change the camera's aperture setting to f/2.8. Shooting exposure is still based on the actual f-number displayed on the lens and camera.〔G-EQUIV-02〕
Depth of field should be considered together with focal length, aperture, and shooting distance
Nikon defines depth of field as the range in front of and behind the focus point that is acceptably sharp, noting that it varies with lens focal length, selected aperture, and shooting distance. A larger aperture (lower f-number) will result in a shallower depth of field; however, sensor size or crop factor alone cannot provide a universal guideline for background blur for all lenses and focusing distances.〔G-EQUIV-03〕〔G-EQUIV-04〕
Clarify the comparison target when comparing "equivalent apertures"
"Equivalent aperture" can be used conditionally to discuss depth of field differences between different sensor formats while matching field of view and composition; it does not replace the lens's actual f-number for exposure settings. Panasonic lists this lens as 25mm F1.4, with the 35mm format equivalent focal length noted as 50mm; Nikon notes that a lower f-number delivers more exposure. Before comparing depth of field, the subject distance, framing, and final viewing size should be fixed, recording the actual focal length and f-number. If conditions differ, the comparison should be written as a specific combination, not using conversion rules detached from conditions.〔G-EQUIV-05〕
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