Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions thread
Lee Jay Forum Pro • Posts: 55,156
Incorrect

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

Well, then it's always been thought of wrong. f-stop measures light intensity (illuminance) not light gathering ability (total light). Yes, they go together if the sensor size is constant.

This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format.

Yes, it does. A bright light on a small surface means a relatively small amount of total light compared with a dimmer light over a much larger surface. Total light controls image quality (noise) and noise controls how fast your shutter speed can go.

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