Is there a formal (official/scientific?) definition for the "light gathering" of a lens?
If yes, I would appreciate a link to the source.
I have seen some instances of the "light gathering" equated with exposure, that is with the aperture f-number. To me it seems that the "light gathering" should be about the total light projected onto a sensor.
I wonder if this concept has been defined?
The “light gathering” business (as in a bigger system “gathers more light”) has long impressed me as on e of the more obtuse ways of trying to understand how exposure works.
Why is it not still the effective aperture? I've used that for decades, even when we had to figure out complex multi-strobe exposures on Kodachrome with programmable calculators.
It's a "close enough" type of approximation. If I tell you that two cars both get 18mpg, and one gets 17.8mpg and the other gets 18.2mpg, you still inherently understand their functionality compared to each other. The small difference exists, and might be quantifiable when directly comparing the two, but otherwise is largely meaningless.
So too with the exact amount of light that is able to strike a sensor at a compared f-stop or t-stop.
jeffbuzz wrote:
No, there is not. It is a made up term. F-stop and T-stop are the only quantitative values.
This is unfortunate and brings to mind...
“When I use a word,” Humpty Dumpty said in rather a scornful tone, “it means just what I choose it to mean—neither more nor less.”
When a term is not defined, it is (at the very least) useless.
On the one hand, "light gathering" of a lens can be understood as meaning the total light that this lens "collects" and projects onto a sensor, e.g., at the widest aperture.
Yet others (on this forum) use the term "light gathering" as synonymous with the exposure, that is, meaning the amount of light per unit sensor surface area. Then, naturally, two equivalent in FL lenses that have the same F-number printed on them are said to have the same "light gathering." A micro-four-thirds 300mm F4 and a full-frame 600mm F4 can be examples of this interpretation of "light gathering", and this is where I see trouble, because the same "light gathering" sounds awfully close to mean that the two lenses in this example are equivalent.
It should be better to avoid using this kind of made up terminology.
It helps a lot to keep in mind that an f-stop is simply the indicator of a relationship between nominal focal length and aperture size — nothing more and nothing less.
T-stops aside, the intensity of the light passing through a f/2.8 lens is the same. whether it ends up striking a APS-C sensor or a miniMF sensor.
As ong as photosite density. (photo sites per are) is the same, it doesn’t really matter whether the photosites occupy a large or small sensor when it comes to setting exposure.
What does mater are some other things. For example, if the photosites are crowded more closely together on the smaller sensor in order to achieve a higher sensor resolution, then the dynamic range of the sensor may decrease, assuming all else is equal. Sensor size differences also mean that images have to be magnified to different degrees if they are to be reproduced at the same size — so there is potential that small flaws in the image may become visible sooner when a smaller sensor is used. (This is fundamentally why we worry sooner about diffraction as we stop down on a small sensor.)
One way to think of it is that at no point is it brighter on a large sensor than on a small sensor at a given aperture. Yes, there could be more (or “larger,” if you’ll pardon the sloppy terminology) points on the big sensor, but the “more light” is diffused over “more area.”
But in the end, I fail to see how the “light gathering” idea has much relevance to actual photographs.
gdanmitchell wrote:
It helps a lot to keep in mind that an f-stop is simply the indicator of a relationship between nominal focal length and aperture size — nothing more and nothing less.
T-stops aside, the intensity of the light passing through a f/2.8 lens is the same. whether it ends up striking a APS-C sensor or a miniMF sensor.
As ong as photosite density. (photo sites per are) is the same, it doesn’t really matter whether the photosites occupy a large or small sensor when it comes to setting exposure.
What does mater are some other things. For example, if the photosites are crowded more closely together on the smaller sensor in order to achieve a higher sensor resolution, then the dynamic range of the sensor may decrease, assuming all else is equal. Sensor size differences also mean that images have to be magnified to different degrees if they are to be reproduced at the same size — so there is potential that small flaws in the image may become visible sooner when a smaller sensor is used. (This is fundamentally why we worry sooner about diffraction as we stop down on a small sensor.)
One way to think of it is that at no point is it brighter on a large sensor than on a small sensor at a given aperture. Yes, there could be more (or “larger,” if you’ll pardon the sloppy terminology) points on the big sensor, but the “more light” is diffused over “more area.”
But in the end, I fail to see how the “light gathering” idea has much relevance to actual photographs....Show more →
I am interested in examples of modern cameras that exhibit this loss of dynamic range:
"if the photosites are crowded more closely together on the smaller sensor in order to achieve a higher sensor resolution, then the dynamic range of the sensor may decrease, assuming all else is equal."
gdanmitchell wrote:
It helps a lot to keep in mind that an f-stop is simply the indicator of a relationship between nominal focal length and aperture size — nothing more and nothing less.
T-stops aside, the intensity of the light passing through a f/2.8 lens is the same. whether it ends up striking a APS-C sensor or a miniMF sensor.
As ong as photosite density. (photo sites per are) is the same, it doesn’t really matter whether the photosites occupy a large or small sensor when it comes to setting exposure.
What does mater are some other things. For example, if the photosites are crowded more closely together on the smaller sensor in order to achieve a higher sensor resolution, then the dynamic range of the sensor may decrease, assuming all else is equal. Sensor size differences also mean that images have to be magnified to different degrees if they are to be reproduced at the same size — so there is potential that small flaws in the image may become visible sooner when a smaller sensor is used. (This is fundamentally why we worry sooner about diffraction as we stop down on a small sensor.)
One way to think of it is that at no point is it brighter on a large sensor than on a small sensor at a given aperture. Yes, there could be more (or “larger,” if you’ll pardon the sloppy terminology) points on the big sensor, but the “more light” is diffused over “more area.”
But in the end, I fail to see how the “light gathering” idea has much relevance to actual photographs....Show more →
I have read this paragraph several times:
"One way to think of it is that at no point is it brighter on a large sensor than on a small sensor at a given aperture. Yes, there could be more (or “larger,” if you’ll pardon the sloppy terminology) points on the big sensor, but the “more light” is diffused over “more area.”
I believe that the first sentence acknowledges that the same f-number on equivalent in FL lenses ensures the same exposure, that is, the same amount of light per unit sensor area (not per a photodiode), and therefore ensures the same lightness of the corresponding images.
It is the meaning of the second sentence in the paragraph that eludes me.
