I had a question? If we just cover the ff sensor inside the camera to APSC size. The sensor performance will drop to aps-c sensor performance?
Yes. It doesn't matter how you crop.
Cropping in the computer after the exposure will also cause the same effect.
I think you have to be more precise, on this,
I disagree. I was precise. What you do below is obfuscating the discussion.
because you wrote already wrote the following, in response to min32:
"If you crop FF to APS-C, you lose performance. If you crop FF to iPhone size you lose even more performance. All this is because of loss of light (and resolution too, but that's a bit off topic) used to create the output image."
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What you write "Cropping in the computer after the exposure will also cause the same effect" is only true, if you would not change FOV, with cropping.
We are talking about
signal-to-noise-ratio.
That is the context.
And you certainly have misunderstood what I write. I have no idea how you see contradiction in what I wrote. I have claimed that cropping doesn't change FOV - it's changed the same amount regardless of how you crop.
But cropping after the exposure changes the FOV, or?
It changes the signal-to-noise ratio of the image. Cropping changes the FOV regadless of when and how you do it versus an uncropped image.
...thus, your statement is false (in the context of your whole answers, but not in those two sentences above - please read below!).
No, my statement is right. We're not talking about FOV or even resolution, but signal-to-noise ratio. I have no idea where you found me to be wrong as it's certainly not anywhere where you quoted me. Maybe you think I said something I didn't? Or assumed something?
I have no idea why you want to obfuscate the discussion by talking about it. The question was about if bigger gives better image quality, essentially better SNR than smaller, and the answer is yes.
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If you shrink the complete FOV of a bigger sensor, to the size of the smaller one, then you benefit from the bigger size which can collect more light...
FOV is not relevant to this discussion.
...this can be done in the computer, or with a Shapley lens, which is used in the Metabones converter etc...
FOV is not relevant.
Also, you're now changing the optical formula, making the lens faster. If you put a Speedbooster to 45mm f/3 lens, the results is 30mm f/2 lens.
And that is way beside the point, way off topic. We're not talking about optics, if we were, I'd point out that the image captured by the big sensor needs less enlargemenet than the one captured by smaller sensor to get the desired output size and that means that the lens aberrations would be more relevant.
Also the
smaller sensors saturation capability is not increased with any optical instruments - it's about 2.25 times smaller than FF's (give or take a bit depending on the relevant technologies used). And as the saturation is not increased, neither is the SNR in the context of image quality. Of course if you use a faster lens, you collect more light, but One can always use a faster lens of FF too. But this has nothing to do with what I said and what you for some obscure reason find to be incorrect.
...but the visible FOV has to stay the same, for both pictures (if you want to compare).
We do not have to take any real photos to compare. When you measure camera performance, you take images of evenly lit defocused blank subjects (and optionally dark frames). Optics are not relevant.
In general you will want to
minimize the number of parameters, unknonws, to get the most accurate answers.
But if you want to compare FF at 45mm f/3 and APS-C at 30mm and f/2, then sure - if the sensors are idealized, then at the same shutter speed and ambien light those combos create the same information to the image sensors. If you deny this, I'd like to see some evidence, preferrably mathematical or results of simulations or such (to minimize unknown parameters).
A given FOV at a given output size (light per area, or light density), benefits from a bigger sensor, because it can collect more light for this FOV.
Light desnsity is
irrelevant.
Total light is the relevant thing. When considering total light we don't have to know anything about the physical sizes of the sensors.
If you change the FOV for the given output size, then you lose the base for comparison...
Optics and FOV are not relevant at all. The relevant calculations and measurements don't require them.
...and that is what cropping in the computer after the exposure means!
50mm lens on APS-C gives size x, on FF 50mm lens gives size y - crop the FF output image to size x and you lose away the benefit of extra light. Or use a 75mm lens and not lose any, but that is not cropping but using different lens. And this is all irrelevant to this topic.
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The center of the frame isn't affected by the light on the border, which you lose, when cropping (to change the FOV) - no change in the light density!
Light density is absolutely irrelevant.
It is (almost) all about how much light is collected. If you crop, you throw away part of the light.
...but you don't change the light density!
Light density is irrelevant.
Think of two pixels, one which is 10µm^2, the other 40m^2. With the same exposure the light density is the same so the big pixel collects 4 times the light, thus SNR will be twice as high.
The same applies at sensor level. You can prove it to yourself by following the advice I gave here:
http://www.dpreview.com/forums/post/54555356
...otherwise, if your statement would be right (in general), the middle of the frame would become darker (loss of light), if you crop the borders of the frame (within the computer - after exposure) - which is clearly not the case, or? ;-)
You really are preoccupied with your artificial situtation which is way off topic.
We're talking about SNR of the sensor, not how much your lens/sensor causes vignetting.
...so, there is a difference between using a smaller part of a sensor, or cutting the edges of a frame after exposure (cropping in the computer) and the benefit of a bigger sensor area, which is used for the same FOV and therefore collects more light per output area.
I'm not sure I follow your logic. If I use a APS-C at 50mm and FF at 50mm and then crop the FF to APS-C size in post processing, both images (assuming idealized sensors or technologically identical sensors) collect the same amount of light. There is no difference where you crop.
You should not involve FOV at all into this discussion, as it is absolutely irrelevant and just obfuscates the discussion.
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If you only wanted to express, that something is lost, if you don't collect it, then you are right, but this is trivial
That is what I expressed - I have no idea why you dug the FOV out of bag.
While it is trivial, it is also somethign many people do not think, a blind spot if you will. If you crop, you lose light, no matter how you do it,
and if you lose light, you lose SNR. The last part is certainly lost from many people who think in pixel-centric way and imagine that big pixels in small sensor would somehow equalize image quality (apart from resolution) with big sensors. Do you think so?
and not really a good fit to the photography related theme:
The Myth of Equivalent Aperture and other overly simplistic Camera Formulae
or
An essay on Equivalence - Or - Does Size Really Matter ?
etc.
You're now being awfully arrogant. I answered well, and quite precicely and countered with evidence some misconceptions.
What you do here is go into irrelvancies and flawed ideas, like the supposed importance of light density. This just makes sensible discussion more difficult.
Which begs a question: why do you obfuscate the discussion, what is your motivation?
If we consider two idealized systems: one with a full frame, and one with APS-C, then
to get an ideantical image the light densities will have to differ by a factor of 2.25. In practise the APS-C uses 1.5 times shorter lens (focal lengthwise) and an aperture number 1.5 times smaller (to achieve equal aperture size).
If you believe that the light density is relevant and total light is not in the context of comparing sensors of different size, please start a new thread and preferrably in a more appropriate forum and I am more than happy to counter your arguments.
