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

Started Jul 24, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,674
Let's correct it, then.

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.

Your definition of image quality being the amount of noise is equally out of touch with the real world of photography.

Noise is merely one aspect of IQ:

While this section is concerned solely with IQ, it is important to note that IQ is but one attribute of a camera system.

But what, exactly, is IQ, and what does it have to do with the "success" of a photo? The first step in defining "IQ" is to make the distinction between "image quality" and a "quality image". Many would take it as a given that if we have two photos of the same scene with the same composition then, all else equal as well, the photo with "higher IQ" would be "more successful". That is, the photo with "higher IQ", for example, would place better in a photo competition, would be more likely to sell, would sell for a higher price, etc. For sure, this may certainly be true for a large number of photos, such as a landscape photo displayed at a huge size. But it is important to acknowledge that there is a class of photography where image quality, as opposed to a quality image, is all but irrelevant (please see these outstanding photos, for example).

Furthermore, while one system may yield "higher IQ" than another, those differences may not be large enough to make any significant difference in the appeal of the photo, depending on the QT (quality threshold) of the viewer, the scene itself, the size at which the photo is displayed, and how closely it is scrutinized (see here for an interesting example of this point), and the processing applied to the photo. In other words, it is not merely whether System A has "higher IQ" than System B, but under what conditions it has higher IQ (and, indeed, which has "higher IQ" may flip-flop, depending on the conditions), and if the IQ is "enough higher" to make any significant difference.

For some photographers, IQ may be the most important aspect of photography. For others, it may play no role at all or simply be an added plus. But it is time well spent to reflect on just how important IQ is to our own photography, given that IQ is, at best, merely a means to achieving a quality image, and, at worst, completely irrelevant to the photo.

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