Sella174 said:
neuroanatomist said:
So you're wrong even about your own behavior. Interesting.
OK, since my understanding is apparently wrong ... and I want to learn.
Given a "full-frame" sensor and a "crop-frame" sensor, made of the same "sensor technology", i.e. same size photosites, same A/D converter, same everything except area.
The statement is that a "full-frame" sensor gathers more total light than a "crop-frame" sensor.
Explain to me how and why the "full-frame" sensor collects more light in the centre area of the same equivalent size as the "crop-frame" sensor, than does the "crop-frame" sensor; or, stated differently, how and why does light falling in the area on the "full-frame" sensor outside the "crop-frame" equivalent centre area affect the amount of light gather within the designated centre area of the "full-frame" sensor, thereby causing said designated centre area of the "full-frame" sensor to gather more light than the "crop-frame" sensor.
The amount of light gathered by 'center area' (APS-C sized region) of a FF sensor is obviously identical to the total light gathered by an APS-C sensor (assuming equivalent design, i.e., microlenses). If they have identical pixel pitch, image quality would be the same. So, if you're going to take every image from the FF sensor in this example and crop it to the APS-C FoV, there's no advantage to the larger sensor. I can't speak for you or anyone else, but personally I don't shoot images planning to crop away ~60% of each image I capture. If that's the sort of 'normalized comparison' you have in mind, it's one with no practical relevance. You may as well extract one pixel from the image and say it's as good as the whole picture.
When you compare the sensors without cropping, the total light gathered by the larger sensor is greater...simple geometry, as you say. When comparing
pictures (not pixels), the larger sensor will deliver a sharper image with less noise, given the same pixel pitch and sensor technology.
In several threads, you've made the point that when comparing sensor sizes, the 'crop factor' does not affect exposure. In other words, an f/2 lens on FF, APS-C, m4/3, or even a 1/1.7" sensor like in the PowerShot S-series will yield the same exposure, e.g. shooting a gray card at f/2 and ISO 200, a metered exposure on FF of 1/100 s would give the same 1/100 s exposure on all of those successively smaller sensors. That's absolutely true. But above, you acknowledged that larger sensors gather more total light. Hopefully you see the confound - same aperture, shutter speed, and ISO giving the same exposure (assuming equally accurate metering), resulting in the same brightness of the resulting images from each sensor...but very different amounts of total light gathered. How does that work?
The answer lies in what ISO is...and isn't. Many people have a poor understanding of ISO, incorrectly assuming that a given ISO setting means a fixed amount of gain applied to the signal. ISO is a
standard (that's the 'S' in ISO, ISO 12232 is the relevant standard in this case), and that standard effectively means that for a given exposure setting in terms of aperture and shutter speed, the resulting image will have a defined brightness. How does an image taken at f/2, 1/100 s, ISO 200 on a PowerShot S100 have the same brightness as an image at f/2, 1/100 s, ISO 200 on a FF sensor, even though the FF sensor is over 20 times larger? More amplification (gain) must be applied to the lower total signal from the smaller sensor. More amplification means more noise. Obviously, the same is true for m4/3 and APS-C relative to FF, to a progressively lesser degree. Likewise, a medium format sensor needs less amplification than a FF sensor to achieve the necessary brightness for a given ISO according to the standard, and therefore has less noise than FF.
So, even though using a smaller sensor doesn't affect 'exposure', the less total light gathered means a lower signal that must be amplified more compared to a larger sensor to achieve the same resulting brightness according to the ISO standard, and more amplification means more noise. You may
think that getting the 'same exposure' with a smaller sensor despite collecting less total light comes without a penalty, but like many things in life, there's no free lunch.
When considering a fixed output size (viewing the entire image on the same display or printing at the same size), the smaller the sensor the more enlargement needed. That results in the image from the smaller sensor appearing less sharp. In addition to sharpness, although the extra enlargement technically doesn't add noise, it does enhance the appearance of the existing noise, further adding to the perceived noise from the smaller sensor.
