"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 16,788
Re: Rough, Qualified Approximations Surrounding Settings

pidera wrote:

It is exactly these differences one sees when comparing images made using equivalent settings that make "equivalency" interesting, as they go beyond the obvious conclusions. The use of equivalent settings allows to really see difference in distortion, out of focus behaviour, sensor performance etc. That is why I prefer not to talk about equivalent images, lenses etc but merely about equivalent settings.

I agree. Attempting to grab the word "equivalent" and sling it wide as if it could encompass all realms of subjective perceptual aesthetics seems about as productive as expecting listeners of music to gather in committee and legislate precisely what musical intervals, scales, timbres, instruments, artists, and musical pieces are "better", "the same", or "worse". Aesthetics is not in itself a science.

That is a truly fool's errand, and a prescription for pretentious qualitative subjective declarations rendered meaningless when one rationally considers the lack of scruples of those who would expect to shove their subjective perceptions, preferences, and tastes down the throats of others.

There is no way that I can step into the heads of others and look through their eyes. And the reverse is also true. Therefore, such "qualitative" debates are implicilty based upon falacious premises. Why this simple but most profound variable is so commonly ignored is an eternal mystery.

It makes more sense to critique a quantitative argument on the quantitative bases themselves - and the recognizable complexities surrounding attempting to project them into qualitative perceptual realms. However, fuzzy edges do not necessarily entirely negate their (qualified) usefulness.


There are certain quantitative characteristics arising out of camera-lens system settings that are measurable (with varying degrees of complexity, for the most part not without aid of instrumentation).

Field of View - Easy to determine (but not as simple as Focal Length when focused at infinity).

Depth of Field - Possible to determine (but varies with any given individual viewers' visual acuity).

Signal/Noise Ratio - Possible to determine (but varies in perceived impact dependent upon the image itself, viewing conditions, and individual viewers, especially where it comes to periodic noise).


When we use simplified models that replace complex multi-element lens-systems with single, symmetrical thin lenses, a number of rough approximations are made when scaling settings.

(1) Effective Focal Length should be used instead of the Focal Length when focused at infinity.

(2) Scaling Focal Length as well as the F-Ratio by the ratio of the sensor Crop Factors ignores the fact that the Depth of Field (also) changes as a function of the ratio of the Camera (lens-system front nodal plane) to Subject (plane of focus) Distance to the Hyperfocal Distances involved - by an amount directly proportional to the ratio of the Focal Lengths of compared systems.

Therefore, the inter-format transformations made are only valid when the Camera to Subject Distance is substantially longer than the Focal Length, and are also only valid when the Camera to Subject Distance is substantially shorter than any of the Hyperfocal Distances involved.

(3) The relationships of image-noise to sensor size are valid only for Photon Shot Noise when the relative Quantum Efficiencies are known and also factored-in. The additional element, that is ...

... Read Noise (even the entirely random, and not periodic components of), is design-specific in nature, and does appear to reliably follow simplistic assumptions regarding scaling with photosite geometries. Further, the magnitude of (input-referred) Read Noise as well as its (output-referred) proportion (relative to the maximum recorded or recordable peak level) is camera system specific, and is thus a complicated and individualized function of ISO settings.

Therefore, quite a bit of specific actual test-data and knowledge is required in order to meaningfully make such comparisons and assumptions surrounding Signal/Noise Ratio - and such numerical comparisons may not necessarily accurately reflect viewers' subjective perceptions.

I recently proposed referring to such rough corollaries as "metametric" as opposed to "equivalent":


... but the participants involved in that particular discussion seemed too embroiled to pay attention.


If the interesting and somewhat useful methods of (approximate) comparison of camera-lens systems that King James (among others) ascribe to should rightly be qualified, it is for the above reasons.

However, it seems that the more common challenge arises out of seriously misguided viewpoints seemingly unwilling (or unable) to recognize the relatively straightforward and easily demonstrable differences between: Entrance Pupil Diameter and F-Ratio; Exposure proper (which has nothing to do with ISO Sensitivity) and its clear misuse when referring to Brightness; the difference between light energy per unit area (intensity) and light energy per unit image-sensor active-area (total light); and the relationship of sensor-area to Photon Shot Noise SNR.

When people are unable to grasp the fundamental principles upon which such comparisons are based, the surrounding discussions are as a result virtually doomed to ever resemble conga-lines of vociferous drop-outs from a remedial reading class insisting to teacher that up is down and 2+2=5 - and such discussions seldom transcend the painful review exercises of the first day of class.

What ensues more resembles some kind of series of belligerent assaults upon teacher - and it may be that King James is most comfortable presiding over introductory classes. No other venue attracts so may who are often in some way misguided or outright wrong to be duly corrected. A perhaps gratifying position for magister - as he able to always feel comfortably ahead of his audience.

And the insurgency should not forget that such socratic festivities are not a required course - it is they who take up the gauntlet with intention to unseat the self-proclaimed champ. The most prevalent "equivalence" that I see being assumed is one between public power and providential majesty. All who aspire to such social status face never-ending challenges to their perceived title.

DM ...

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