Question for someone who understands how image sensor works (regarding b/w vs color)

[theregoesjb]

I have been reading up on how digital camera image sensors work,

here is briefley what i understand and my question:

so there are millions of pixels with 'photosites' that, when the shutter is opened, capture varying amounts of photons which collectivelly allow the camerea to create the image. This alone would only create grey scale images, so those photosites that are capturing the photons have 1 of 3 different color filters on them, which (again collectively) allow the camera to process color.

Virtually all current digital cameras can only capture one of the three primary colors in each cavity, and so they discard roughly 2/3 of the incoming light. As a result, the camera has to approximate the other two primary colors in order to have information about all three colors at every pixel.

so the task of seeing each of those primary colors is split up, and the rest are aproximated.
does this mean that, to some degree a black and white digital image has the potential for higher quality? less noise maybe?

Or

since these 'photosites' already have color filters on them, there is no way to truely discard the cameras ability to see/apply color (along with this color aproximation), and a black and white image is always going to be something of an after effect ...?

this is what i was reading- Understanding Digital Camera Sensors

 

[tirediron]

...a black and white image is always going to be something of an after effect ...?

This is it exactly. A digitaly captured image is always captured in colour because at its lowest level it only sees red, green or blue. Anything else is interpolation and images are converted to monochrome (or whatever effect you desire) via software. AFAIK, there are no cameras that have greyscale sensors, and I can't imagine anyone ever producing one.

 

[MLeeK]

Which is why you are better off shooting in color and then converting to black and white using your color sliders. You control the image that way. When it is flattened to black and white in the camera you are allowing the camera to choose which colors will be bright and which will be dark...
If you shoot in raw it doesn't convert to black and white, raw is everything.

 

[Ysarex]

AFAIK, there are no cameras that have greyscale sensors, and I can't imagine anyone ever producing one.

Wouldn't it be awesome though -- no bayer grid, no anti-aliasing filter -- just pure B&W resolution over a 14 bit capture. I WANT ONE!

Joe

 

[KmH]

The image sensor is completely color blind and only records luminosity.

Though an active-pixel sensor has a 3 color Bayer Array on front of it, those colors only affect the luminosity adjacent pixels record.

Each pixel is an individual "photosite". The pixel converts the amont of light that hits it during an exposure into a voltage. Amplifier circuits on the image sensor chip boost those voltages by an amount based on the camera ISO seting. Those amplified voltage values gets converted to a digital number in an analog-to-digital converter on the image sensor chip.

Again, at no so far point in the process has the image data had color information, just luminosity data.

At this point there is a fork in the path the data stream takes.
If the capture file type is set to TIFF or JPEG the data goes through a process that uses an algorithm to infer the colors in the scene the image sensor recorded.
That process is called demosaicing and further processing based on various camera settings.

The other path sends the image data directly to the camera buffer for writing to the memory card and is called a Raw image data file. Demosacing of the Raw data is then done outside the camera in one of any number of Raw converter applications.

Consequently, the process of demosiacing is unavoidable. In other words, every exposure the image sensor makes gets converted to a color image before it is ever seen by the photographer.

Most cameras have a menu setting usually called monotone or B&W, that converts the algorithm demosaiced color image to monotone, usually by just de-saturating the color image, with reeks havoc on tonal contrast.

So the quality and noise response of a B&W image is virtually the same as it's color counterpart, though because the B&W is grayscale the way we perceive the image is different.

 

[theregoesjb]

Which is why you are better off shooting in color and then converting to black and white using your color sliders.

what about lens filters?
say i use a yellow lens filter, take it in full color, then remove the color from the raw file... do you think this would serve any purpose the way it can for B&W film ?

no bayer grid, no anti-aliasing filter -- just pure B&W resolution over a 14 bit capture.

I agree, someone like me would never buy a b&w only camera but im somewhat surprised there is no demand for it. I dont know what the 'audiophile' equivelant term is for photography but id think that they would love that sort of thing.

 

[Ysarex]

Which is why you are better off shooting in color and then converting to black and white using your color sliders.

what about lens filters?
say i use a yellow lens filter, take it in full color, then remove the color from the raw file... do you think this would serve any purpose the way it can for B&W film ?

Unfortunately no, but fortunately post processing the color capture allows much more tone-translation control with a digital image than lens filters ever did with b&w film.

Joe

 

[Helen B]

There have been a few monochrome digital backs and cameras (eg the Kodak DCS 100). Current backs are available from Megavision and Phase One.

In some circumstances there could be a difference between using a filter on the lens and using the Channel Mixer (or similar). This is because the lens filter is affecting the complete spectral emission from the object, while the post-processing software only has three channels to work with - the full spectral data is missing.

 

[tirediron]

There have been a few monochrome digital backs and cameras (eg the Kodak DCS 100). Current backs are available from Megavision and Phase One.

I was not aware of that; thanks Helen! No surprise that if anyone was going to do it, Phase One would.

 

[theregoesjb]

Thanks for the thorough explanation KmH,

The other path sends the image data directly to the camera buffer for writing to the memory card and is called a Raw image data file. Demosacing of the Raw data is then done outside the camera in one of any number of Raw converter applications.

i think i get it (i think) ...so my next question would be:

if the demosaicing process (the process in which color information is evaluated and processed with the luminocity) is done outside the camera while shooting in RAW format, (in photoshop in my case) then it seems like the oppertunity is there to bypass it. Maybe I am underestimating how little a difference it makes but it does seem like it would always be preferrable to just skip the demosaicing completely when intending to create a b&w image.

 

[Helen B]

Imagine an even, uniform red area in an image. In a block of four pixels, without demosaicing, one would be light and the other three would be dark. A uniform area is turned into a mottled area. You would, at least, have to bin the blocks of four pixels into one pixel. This binning technique would give something like, but not the same as, the eye's spectral response to colour: the green wavelengths would have twice the influence of the red and blue wavelengths (or the same influence as the red and blue combined). You would lose control over the method of greyscale conversion - which is a powerful tool.

 

[Josh66]

Thanks for the thorough explanation KmH,

The other path sends the image data directly to the camera buffer for writing to the memory card and is called a Raw image data file. Demosacing of the Raw data is then done outside the camera in one of any number of Raw converter applications.

i think i get it (i think) ...so my next question would be:

if the demosaicing process (the process in which color information is evaluated and processed with the luminocity) is done outside the camera while shooting in RAW format, (in photoshop in my case) then it seems like the oppertunity is there to bypass it. Maybe I am underestimating how little a difference it makes but it does seem like it would always be preferrable to just skip the demosaicing completely when intending to create a b&w image.

I don't think it works like that (don't think it's possible to skip the demosacing)... The way I understand it, is that demosacing is converting 4 photosites on the sensor (two green, one red, one blue) into one pixel. I don't think that step can be skipped, though I could be wrong...

 

[MLeeK]

What are you trying to accomplish here? That would probably be a good place to start. It seems like you are making converting to black and white much more difficult. You are going from a to b by way of x, y and z.

 

[theregoesjb]

absolutely not trying to accomplish anything, was just google'ing how digital camera image sensors work.

it just struck me as very interesting the way the sensor sees only light values and uses a combination of primary color filters and processing to create color. It got me wondering what difference in quality you could have in a black and white picture if it wasnt seen through color filters (if any at all).

I am in no way concerned with trying to do this myself... just curious.

although it is helpful to see why shooting in color and converting to b&w after is better

 

[KmH]

I don't think it works like that (don't think it's possible to skip the demosacing)... The way I understand it, is that demosacing is converting 4 photosites on the sensor (two green, one red, one blue) into one pixel. I don't think that step can be skipped, though I could be wrong...

No it's still 4 pixels, not one pixel, but the demosaicing algorithm assigns a discrete color to each of those 4 pixels.

In the Raw format it is a 12-bit color or a 14-bit color depending on the camear and/or the camera Raw setitings. 12-bits can represent 4096 colors per color channel while 14-bits can represent 16,384 colors per color channel.