In reply to the contributions #164-#188 in this thread:
Some remarks and findings just might be due to different points of view.
To enlighten two (or even more sides
) of the same medal:
Bigger is better? Larger formats offer an ISO advantage and less DOF?
No doubt, it is possible to obtain a better image quality by using larger formats.
But what is the reason for this?
If one restricts oneself to a certain shutter speed and uses lenses with the same f-stop, this will result in the same exposure.
The amount of light per area then is kept constant.
However, as the sensor size is increased, the illuminated area also is increased.
This results in a different amount of total collected light for the unlike formats.
This total influx of light contains the information of the image.
In this way, information content is increased by using larger formats.
It seems now that there is a "noise advantage" of the bigger sensors.
However, exactly the same can be achieved, when another lens is chosen on a smaller format.
Such a lens often is described as "faster".
If you want to retain exposure, you would be able to use a higher shutter speed.
On the other hand, if you retain the shutter speed, exposure can be increased.
As the sensor area is kept constant, the total amount of light then also would be increased.
Thus, the ability to collect more light is the real power of such a lens.
If the same information could be stored on the smaller sensor than on the bigger one, the corresponding pictures could not be distinguished.
So, is bigger always better?
No, the combination of the sensor size and the ability of the lens to collect light matters.
If lenses are compared across different sensor sizes, then these should be normalized to collect the same information (total amount of light).
In very much the same way, it is now very popular to convert focal lengths across formats to obtain a common basis [0].
Equivalent lenses in this sense will preserve the same information and angle of view.
Due to physics, depth of field will also be the same [1].
One could conclude that "the old wisdom that “larger sensors produce images with smaller depth of field (DoF)” and the newer digital wisdom that “larger sensors have better noise performance” are both an artifact of not comparing equivalent" [2] systems. See also the thread "Low noise benefit of FF vs APS-C equals ... zero" [3].
Increasing quality means increasing the ability to collect more information.
This will naturally result in increased cost - irrespective of the used sensor format.
Equivalency just gives you a common starting point.
PS:
"Increasing ISO" and "reducing exposure" often are used interchangeably.
Both then are associated with noise.
However, boosting ISO per se does not reduce image quality.
Higher ISO values will only amplify the captured information.
"Boosting ISO settings on a digital camera [...] cranks up the resulting image/signal by analog amplification or by digital multiplication" [2] (see also [1]).
The ultimate reason for the noisier image is the less amount of captured light and not the ISO value.
For a certain amount of information the ISO setting just alters image "brightness".
Due to this "exposure matters only inasmuch as it is a component of the brightness and total light -- it is not an important measure in and of itself. That is, when we look at an image, we can see how bright or dark it appears (brightness) and we can see the noise density and DR in the image (total light). But we cannot see the exposure itself, so it is not important except as a means to an end. This is a radical statement that many have difficulty coming to terms with, but it is a key point to understanding Equivalence [...] exposure is a meaningless measure in cross-format comparisons" [1].
PPS:
A larger format is not only used to get better image quality. DSLR cameras of Canon/Nikon/Pentax/Sony(Minolta) exhibit mounts, which were naturally invented for the 35mm ("FF") format. What would now happen if a future Pentax camera would feature a FF sensor with the same pixel pitch as current APS-C cameras? Just in thought mount any lens on it. In APS-C mode there would be no difference to your old APS-C camera (see #167). Now, mount a FF capable lens on it (like the FA* 200/4). What one now gains is the possibility to use the whole FF area. Cropping the APS-C area will lead to another angle of view (200->300). Furthermore, the total amount of light in the image will be deminished (4->5.6: no "free lunch"). So, the APS-C crop essentially mimics an ideal teleconverter. The FF sensor now offers the ability to turn this built-in tele converter ... OFF! There are also other aspects of a full frame camera beyond image quality (see e.g. [4]).
[0] However, equivalent "apertures" are less helpful for marketing.
[1]
Equivalence (recommended for further reading)
[2]
DxOMark Camera Sensor
[3]
https://www.pentaxforums.com/forums/pentax-dslr-discussion/71896-low-noise-be...uals-zero.html
[4]
https://www.pentaxforums.com/forums/pentax-news-rumors/175820-rumor-pentax-ff...ml#post1870273