Full well capacity

Silent Shrill

Hi.

I'm trying to learn about full well capacity, and understand the basics.

The question I have is concerning ccd's and dslr's. CCd's have a smaller pixel size but have a well capacity 2, 3, 4, times that of a dslr. The resolution is smaller, but how do the manufacturers get the greater well capacity?

Thanks

10-10-20

I had a quick read about it. It would be by increasing the sensitivity of the photosite with a higher quality CCD element. Then it's left to either the photographer or the firmware within the camera to manage the exposure controls to limit the photons reaching the element so that the "well capacities" are maximised but yet none are overexposed (leading to bleed).

Silent Shrill

https://www.boards.ie/discussion/comment/119015718#Comment_119015718

Hi.

I can certainly understand a higher quality element being better quality. However this article https://www.princetoninstruments.com/learn/camera-fundamentals/full-well-capacity-pixel-saturation#:~:text=Full%20well%20capacity%20is%20defined,consideration%20when%20choosing%20a%20camera. states that the well increases with pixel size, not vice-versa. The article does mention that there are 2 systems to prevent overflowing into neighbouring pixels, gated and fixed. This would allow the full well to be utilised without overflow but would not increase the well. So how does a chip with half the pixel size of a canon, say 1300d, have a well capacity of 50k, while, according to that link, the canon has a well of 12k, (the canon pixel is 4.3). Are manufacturers giving well sizes with the overflow included, (until full well capacity reached), or the gated/fixed results, or some other way. It's a bit confusing.

Silent Shrill

I'm hoping that my thought process is working OK....at the moment...lol

So I've come up with an answer in my head whilst thinking about this.

The canon exampled above gives the full well capacity with no overflow or processes in place. So we might take a night exposure at iso 800 and the maximum time we could use would be a minute, anything longer and the well starts to overflow. However, that does not mean all the wells are full. Some might be full like where the brightest light hit and the rest are half full, (the darkest areas). So to get a good clear photo with as much detail as possible and to start bringing out the darker regions, we need to take a few more exposures. This in effect is filling up the wells with the darker areas, but the light, full wells remain the same. This of course means stacking the images and stretching.

Whereas the ccd's give a full well capacity with the gated/fixed in place and the amount of photons it took to fill all wells to capacity. Example: the light fell in certain pixels and it only took 10k photons in total to fill them, but the darker pixels are not yet full. So the process of the gate kicks in and while the darker wells are filling up those wells that are already full have the gated process put on them, preventing over-exposure. When all wells are full the manufacturer takes that number of photons that were used/gated and give us the total well value. I did some rough calculations in my head after looking at the values that are given for wells, pixel size, etc., on ccd's and I think that my explanation works out.

If this was the case then a ccd with 50k well value would only need 1 exposure for a minute, compared to the canon which would need, say 4 exposures of a minute. When the 4 exposures are stacked the result would be the same as the ccd's 1 minute exposure. Therefore the ccd takes less time to create the image. So a high well value is in reality a "less time" value.

Or am I simply mad to even think this?

Silent Shrill

"If this was the case then a ccd with 50k well value would only need 1 exposure for a minute, compared to the canon which would need, say 4 exposures of a minute. When the 4 exposures are stacked the result would be the same as the ccd's 1 minute exposure. Therefore the ccd takes less time to create the image. So a high well value is in reality a "less time" value."

That above paragraph should read;

"If this was the case then a ccd with 50k well value would only need 1 exposure for 4 minutes, compared to the canon which would need, say 4 exposures of a minute. When the 4 exposures are stacked the result would be the same as the ccd's 1 minute exposure. Therefore the ccd takes less exposures to create the image. So a high well value for ccd's is in reality a, "less exposures amount" value."

This is taking into account that the chips in both are the same, which in reality they are not. The dslr chips are made for daytime use, low light resistance, whereas the ccd's for night use are high light resistance, meaning it could take longer for the full well capacity to be achieved. I have seen some at max exposure of 15 minutes.