Originally posted by Class A That's true only if you don't change the sensor format.
While an f/1.9 lens for a smaller sensor format will achieve the same exposure it will not let the same amount of total light in.
Two lenses @ f/2 on different formats will produce the same amount of light / cm^2 (note that I deleted the "total"), but not the same amount of total light.
Let's take this to the extreme and think of a "silly sensor" sensor of the size of a single sensor sensel from the K100D. Say a lens on the K100D lets in 6,016,000 photons (that's 3008 x 2000, i.e., one photon per sensor sensel). To achieve the same exposure a "silly sensor" lens only has to let through one (1) photon. In other words, the aperture diameter of the K100D lens has to be ~2450 times bigger in diameter.
Both lenses will be specified with the same f-ratio, because they can achieve the exposure on their respective sensors. What you are trying to tell me is that the "silly sensor" lens is as fast as the K100D lens. That it will have deeper DOF but will be the same in terms of gathering light. Technically, you are correct in stating that it achieves the same exposure but with this extreme example it becomes pretty clear how inappropriate it is to ask for "same exposure" rather than for "same total amount of light".
Here's the problem with that whole explanation AND with the argument here: I can put the same lens on both sensors. It's OBVIOUS that the same lens allows the SAME LIGHT through, yes? Yet your explanation about "total light" remains "true". This entire argument is claiming that I can change the total light a window passes by changing the size of an index card lying on the floor receiving the light from the window. The smaller sensor collects less light, yes, but the LENS passes the same amount, period. Thus, the Q's lens isn't slow; the sensor is small, and smaller sensors collect less light. There's no problem with the
definition of exposure, because that's how the world WORKS. Film and digital sensors receive light in the distributed fashion described by light flux/cm^2. The redefinition you're attempting is pointless and leads to false conclusions.
Quote: Say the "silly sensor" lens is an f/1.0 (Wow, that's fast!
). An equivalent lens on the K100D will have to be an f/2453. It will create the same DOF and will collect the same total amount of light. It won't achieve the same exposure, of course. In an "apples vs apples" comparison you cannot increase the exposure by stopping down less nor by increasing the shutter speed, because doing so would produce a different image (different DOF or motion blur). So the only chance is to increase the ISO.
Increasing the shutter speed is not allowed when talking about equivalent images. You'd have to lower the ISO. The fact that certain lenses (f/0.3) don't exist and certain ISO values don't exist either demonstrate limitations of the Q-system.
Again, we should stop using the term 'fast' in this way. Just google "What is a fast lens?" and everyone I can find uses it as I do, meaning "A lens is called fast because it allows a fast shutter speed". Since @ISO 100, the Q will use the same shutter speed as, say, the 43 f1.9 on my K-5, it is just as *fast*. You can pretend that equivalency makes it slow if you want, but that's not how anybody else uses the term, and it's an unnecessarily confusing definition.
It's completely counter-productive to attempt to re-define f-stops and exposure and the like - concepts that have stood comfortably for over a hundred years
regardless of format, and still accurately describe reality and the physics of photography - in the service of an *analogy* like "equivalence".
Quote: I disagree. It is a useful concept to predict the kind of images you can expect from a Q-camera if you have experience with another format camera (say FF or APS-C). Take your APS-C camera, set it to ISO 1344 (or as close as you can get), put a 31mm lens on it, use f/7 and shoot away. The resulting images will look like images taken on a Q-camera set to ISO 100, with the 8.5mm lens set to f/1.9.
No, it won't. It uses a different sensor and different electronics and a different lens and all of these things add up to differences that will not be captured by "equivalence". You even said as much: "The "equivalence" considerations are based on an "everything else being equal" assumption. In practice, "everything else" is rarely the same. Sensors differ in their noise floors, processing uses different levels of (hidden) noise reduction, etc."
Quote: Maybe in order to understand that tiny lenses for small sensors do not collect the same total light as bigger ones for big sensors, it is useful to understand equivalence.
It's NOT THE LENSES. It's the SENSORS.
Quote: Equivalence doesn't depend on experience with 35mm film. It is a useful concept to convert specifications between all sorts of different formats.
Here's a concept that's missing from your equivalence equation: Final magnification. Final magnification may be the single highest determiner of IQ of those discussed here so far, and your equivalence model ignores it completely. if the Q sensor is 8mm across, and the ff sensor is 36mm across, to get true equivalence, you'd need to use different sized prints to match the final magnification ratio, right? But when you do that it knocks all of the equivalence numbers into a cocked hat - because they aren't REAL numbers; they're an analogy between two different formats. All analogies break down eventually.
I can obtain all the information I need to know about "equivalence" by understanding just a few concepts.
1) The higher the final magnification, all other things being equal, the lower the image quality.
2) The shorter the focal length, all other things being equal, the greater the DOF.
3) Diffraction limits resolution as a lens is stopped down, and this effect becomes more pronounced as final magnification increases.
Knowing those three things, I can intuit that the image quality of the Q will be lower than that of my APS-C camera, everything else being equal. I can intuit that the DOF of images will be higher. I can intuit that I will run into resolution problems when I stop down in bright sunlight.
And none of these require me to say things that will confuse people into thinking they have to use different settings to get the same exposure. Which is the only reason I've continued this discussion; I mean you no offense, but it's obvious to me that your "equivalence" statements have confused more than one person in this thread - because they're only analogies that don't describe real-world photographic usage.