Total Light

Started Nov 9, 2017 | Discussions thread
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Total Light

This thread is a follow up to [Photographic] Equivalence -- what it is and isn't and to address people who have opinions like:

Total light? or total Sh*t? Why does this "total light" and other associated rubbish always sneak into almost every thread here at the moment?

and opinions like:

No, what is unbelievable is that you think that 4 times the light spread over a sensor 4 times the area makes any difference to the visual properties of the photo - without any scientific basis whatsoever for that belief. Just because a large sensor receives more total light than a small sensor and the large sensor produces a technically superior photo compared to the smaller sensor does not mean that the quantity of total light is the reason, unless you have scientific evidence to support that conclusion. Can you really not see that?

OK, let's start with where it comes from (bold emphases added to the relevant portions):

Equivalence relates the visual properties of photos from different formats based on the focal length and aperture of the lens. Neither the focal length nor the relative aperture of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the relative aperture on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor.

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (all others' APS-C), and 50mm f/2.8 on FF (FX), where "equivalent to" means:

  • The photos all have the same diagonal angle of view (25mm x 2 = 31mm x 1.6 = 33mm x 1.5 = 50mm) and aperture diameter 25mm / 1.4 = 31mm / 1.8 = 33mm f/1.8 = 50mm / 2.8 = 18mm).
  • The photos all have the same perspective when taken from the same position.
  • The photos all have the same DOF (as well as diffraction softening) when they are taken from the same position with the same focal point and have the same display size.
  • The photos all have the same motion blur for the same exposure time (regardless of pixel count).
  • The same total amount of light falls on the sensor for the same scene, DOF, exposure time, lens transmission (e.g. if the 25mm lens is t/1.6 at f/1.4, the 31mm lens is t/2 at f/1.8, the 33mm lens is t/2.1 at f/1.9, and the 50mm lens is t/3.2 at f/2.8), and vignetting.
  • The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).
  • The larger sensor system will use a concomitantly higher ISO setting for a given brightness on the LCD playback and/or for the OOC (out-of-the-camera) jpg due to the lower exposure (keeping in mind that the ISO setting affects noise only inasmuch as higher ISO settings result in less electronic noise than lower ISO settings -- e.g. a photo "properly exposed" at f/2.8 1/100 ISO 1600 will have less noise than a photo of the same scene at f/2.8 1/100 ISO 200 pushed to the same brightness).
  • Elements of IQ, such as bokeh, color, flare handling, distortion, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent". For example, the Canon 50 / 1.4 on the Canon 5D (13 MP FF) is equivalent to the Sigma 50 / 1.4A on the Nikon D810 (36 MP FF) despite the fact that the latter system will have significantly higher resolution, lower noise, better bokeh, etc., etc..
  • However, the same total light will result in the same noise if the sensors record the same proportion of light falling on them (same QE) and add in the same electronic noise (the noise from the sensor and supporting hardware), regardless of pixel count and ISO setting. It should be noted that sensors of the same, or nearly the same, generation typically record very nearly the same proportion of light falling on them regardless of brand, size, or pixel count (a notable exception would be BSI tech which records a third to half a stop more light for a given exposure than non-BSI tech) and that the electronic noise matters only for the portions of the photo made with very little light.
  • In addition, if the 25mm lens at f/1.4 in the example above is twice as sharp (lp/mm), the 31mm lens is 1.6x as sharp at f/1.8, and the 33mm lens is 1.5x as sharp at f/1.9 as the 50mm lens at f/2.8 (or any equivalent relative apertures), the sensors have the same number of pixels, and the AA filter introduces the same blur, then all systems will also resolve the same detail (lw/ph).

OK, so here are three DPR articles explaining how it all works in more detail:

along with this example and these examples across four formats as examples showing how the theory *exactly* corresponds to reality. So, the notion that the total amount of light making up the photo being "total Sh*t" as one person so eloquently stated is clearly wrong.

That leaves us with the question of scientific evidence to support the claim. Well, as noted, there are the three articles above, the examples above. In addition, there is Dr. Emil Martinec's excellent article on noise as well as Bill Claff's site where the QE (Quantum Efficiency -- the proportion of light falling on the sensor that is recorded) and Read Noise (the electronic noise from the sensor and supporting hardware) is measured from many different sensors. There is also Wikipedia's article on Shot Noise. And, of course, there are tons and tons of other resources.

However, is there a scientific paper with the title, "Photos made with more light are less noisy"? Probably not. Likewise, there are likely no scientific papers with the title, "Bricks dropped on bare toes cause pain". 

If there are any questions, please don't hesitate to ask!

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