I finally 'get' equivalence

Started May 22, 2012 | Discussions thread
Lights Veteran Member • Posts: 3,573
Re: Or to put it another way

Great Bustard wrote:

Lights wrote:

Basically, a photo is made from light. We all know from experience that the more light that makes up a photo (to the point of oversaturation), the better the photo.

The reason that high ISO photos have lower IQ is not because the ISO is higher, but because less light falls on the sensor. Consider a "properly exposed" photo at 1/100 ISO 100 and 1/100 ISO 400. The ISO 100 photo is made from 4x (2 stops more) light than the ISO 400 photo, and thus has higher IQ. It is the same thing if you use a sensor with 4x the area.

Of course, depending on the scene, how large you display the photo, how closely you view the photo, and the QT (quality threshold) of the viewer, one might not notice any difference between a photo at ISO 100 and ISO 400, nor will they notice a difference between a photo from mFT or FF.

As sensor efficiencies improve, the performance of all systems is pushed out further, and differences that used to matter at a particular light level no longer matter anymore. It's not that the difference isn't there, but rather that the difference is no longer important.

An example I gave earlier is to compare a car with 100 HP against a car with 200 HP, a car with 200 HP against a car with 400 HP, and a car with 400 HP against a car with 800 HP.

While the power differential between the cars is the same in each instance, the number of situations where the power differntial is meaningful decreases. So it is with the smaller formats and larger formats on the basis of noise and detail.

Does this make sense?

OK I agree and understand probably 99%. I do know that larger formats (in film for example to eliminate the sensor variables) had greater IQ than smaller formats. An 8X10 field camera (film) is still an awesome camera, even by digital standards. The part I think that many of us don't comprehend here (me included-but haven't completely thought it through), is the part about light. Yes a larger area (sensor) requires more light for the same exposure, but many people see it as an equal pressure (?) of light on a given area. I think that is where the confusion occurs.

Film is a non-linear medium. What that means is that film is very sensitive to exposure (the S curve), and this is the reason so many are fixated on exposure.

Digital sensors, on the other hand, are linear (the S curve is applied in processing, not in the capture), and as such, the exposure doesn't really matter at all, except inasmuch as it is a component of the total amount of light that falls on the sensor (Total Light = Exposure x Effective Sensor Area).

Yes diffraction occurs sooner on smaller areas(sensors)...because of the narrowing of the true opening...if I'm remembering right (which I may or may not be

I discuss diffraction in detail here:


On the quick, the more you stop down, the greater the effects of diffraction softening. As it turns out, all systems suffer the same diffraction softening at the same DOF. But that doesn't mean that all systems resolve the same detail at the same DOF -- the system that began with more detail will retain more detail.

I can understand that when I was "trained" in commercial art (which I did not become part of) that when we would prepare a piece for camera, we would always make it considerably larger by a number of times (4 to 6 or sometimes more depending) for maximum IQ. Since the larger it was when reduced (or expanded) the better it looked in the end...small imperfections magnified more(noise?) in relative terms the smaller the original image. Is this not also part of equivalence? The bigger the original image, the better it will look even reduced (within reason)? Each format would reach it's relative limit depending on the amount blown up/expanded in printing. I may not be right in regards to sensor size, but know for a fact that it works with images.

Yes -- enlargement ratio (crop factor) is another way to look at it.

I've got some more thinking to do on the 4x the light, to get a concept. 4x the size, yes I would agree...much more to work with in reduction or expansion. If that is what you mean by 4x the light or something close to it, then I would agree. If not I gotta think about it some more. Thanks.

The noise / DR end of IQ has to do with the total amount of light (along with sensor efficiency). The detail end of IQ has to do with the sharpness of the lens, the AA filter, and the enlargement factor.

Once we've cleared the difference in semantics, I don't really find anything to disagree with. It never hurts to try and understand what someone else is talking about.


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