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

Started Jul 24, 2014 | Discussions thread
Tim Tucker Senior Member • Posts: 1,337
Re: Let's correct it, then.

Great Bustard wrote:

Beachcomber Joe wrote:

Lee Jay wrote:

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.

That is certainly true if that is the way you choose to define the two terms. I happen to be a photographer so, like most photographers, my definitions are based on real world and may be less specific than those in a physics lab. In my world light gathering ability is understood to be the ability to put a certain amount of light on a specific area. What you choose to call illuminance.

The f-ratio is an *indicator* of light gathering ability, but the same f-ratio on different formats does not gather the same amount of light.

That is, f/2 gathers the same amount of light for a given scene luminance, shutter speed, and lens transmission regardless of focal length *on a given format*, but it does not gather the same amount of light as f/2 does on a different format.

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.

You may continue to play with, among other things, semantics. The light gathering ability of a photographic lens is measured by the amount of light per unit of area.

Sure. But the same light per unit of area, in terms of the visual properties of the photo, has different effects on different formats.

- SNIP -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

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