Actual Pixels vs Effective Pixels?

[Doug West]

What's the difference between Actual Pixels and Effective Pixels?

For example, on the Canon 1DX, the specs say the Actual Pixels are 19.3
and the Effective Pixels are 18.1 (whats advertised for the camera).

Thanks
Doug

[Alan Stankevitz]

Hi Doug,

Sensors cannot see in color, they only record photons. In order to "see" color, a Bayer filter is placed over the sensor. The Bayer filter is made up of red, green and blue filters and each pixel on the sensor receives either red, green or blue light. So...in order to make each pixel see color, adjacent (different color) pixels are used to determine the color. These adjacent pixels extend beyond the effective sensor area in order to create the image. The bottom line is that it requires extra pixels to create a color image and that is why there are two values...effective and actual.

It truly is amazing that today's megapixel cameras can take up to 12 fps, processing all those pixels to make an image in less than a 1/10th of a second!

Alan
www.iwishicouldfly.com

[Roger Clark]

Hi Doug,

Sensors cannot see in color, they only record photons. In order to "see" color, a Bayer filter is placed over the sensor. The Bayer filter is made up of red, green and blue filters and each pixel on the sensor receives either red, green or blue light. So...in order to make each pixel see color, adjacent (different color) pixels are used to determine the color. These adjacent pixels extend beyond the effective sensor area in order to create the image. The bottom line is that it requires extra pixels to create a color image and that is why there are two values...effective and actual.

It truly is amazing that today's megapixel cameras can take up to 12 fps, processing all those pixels to make an image in less than a 1/10th of a second!

Alan
www.iwishicouldfly.com

Alan,
I really do not think this is the answer. I believe the answer is that the sensor has X pixels and a few pixels around the edges of the sensor are masked out so they are not sensitive to light and can be used to determine the dark level. The effective pixels are the non-masked pixels that form the image. Now with all the deception going on with digital camera hype, I might be wrong.

Roger

[Alan Stankevitz]

Hi Roger,

It took me a while to find this, but I remember reading an excellent article on Canon's (European) website regarding the Bayer filter and effective vs actual pixels.

Here is the link:
http://cpn.canon-europe.com/content/...oto_sensors.do

In the article, it states the following:

"Each pixel actually samples the colour information from adjacent pixels to provide full colour data with the brightness for each pixel. This might sound like a compromise, but works extremely well in practice. A single photo sensor can only capture data for a single colour. However, by placing a microcluster of red, green and blue filters over each group of four sensors, data for a full colour image can be recorded. This microcluster grid is repeated across the entire sensor.

It is this sampling that accounts for the difference between the total number of pixels on a sensor, and the smaller number of ‘effective’ pixels. The effective pixels are those that fall within the actual image area. The remaining pixels form a border to the image. They receive light from the subject and their data is sampled by the effective pixels at the edge of the image area. This means that all the effective pixels sample data from adjacent pixels in all directions. It avoids the pixels at the edge of the image area having reduced colour data."

Alan
www.iwishicouldfly.com

Alan,
I really do not think this is the answer. I believe the answer is that the sensor has X pixels and a few pixels around the edges of the sensor are masked out so they are not sensitive to light and can be used to determine the dark level. The effective pixels are the non-masked pixels that form the image. Now with all the deception going on with digital camera hype, I might be wrong.

Roger

[Doug West]

These questions might not make sense since I'm no expert, so here are a few more...

1. Is there an advantage/disadvantage or the numerical difference between actual and effected?
2. I noticed that for the 5d Mark III, it doesn't list an actual or effected...is that a good or bad thing?
3. Why not just advertise 19.3, or whatever a cameras actual pixels are?

Thanks
Doug

[Alan Stankevitz]

1. No advantage, I just think the camera company's put the numbers out there to be factual regarding their sensors. The number you want to pay attention to is Effective number of MP's.
2. 23.4 MP Actual, 22.3 MP Effective
3. They usually do advertise the effective MP number. It's only when you get into the "propeller-beanie" specs. that they list the two numbers.

Alan
www.iwishicouldfly.com

These questions might not make sense since I'm no expert, so here are a few more...

1. Is there an advantage/disadvantage or the numerical difference between actual and effected?
2. I noticed that for the 5d Mark III, it doesn't list an actual or effected...is that a good or bad thing?
3. Why not just advertise 19.3, or whatever a cameras actual pixels are?

Thanks
Doug

[Roger Clark]

Hi Roger,

It took me a while to find this, but I remember reading an excellent article on Canon's (European) website regarding the Bayer filter and effective vs actual pixels.

Here is the link:
http://cpn.canon-europe.com/content/...oto_sensors.do

In the article, it states the following:

"Each pixel actually samples the colour information from adjacent pixels to provide full colour data with the brightness for each pixel. This might sound like a compromise, but works extremely well in practice. A single photo sensor can only capture data for a single colour. However, by placing a microcluster of red, green and blue filters over each group of four sensors, data for a full colour image can be recorded. This microcluster grid is repeated across the entire sensor.

It is this sampling that accounts for the difference between the total number of pixels on a sensor, and the smaller number of ‘effective’ pixels. The effective pixels are those that fall within the actual image area. The remaining pixels form a border to the image. They receive light from the subject and their data is sampled by the effective pixels at the edge of the image area. This means that all the effective pixels sample data from adjacent pixels in all directions. It avoids the pixels at the edge of the image area having reduced colour data."

Alan
www.iwishicouldfly.com

Alan,
Thanks for finding that. Interesting. It seems that one would only need a 1 pixel boundary, or at most 2 pixels for the guard. That would require only mean 0.016 megapixel addition to a 16 megapixel array and smaller than the reported actual versus effective pixels. Perhaps it means the actual pixels are these guard pixels mentioned above plus the dark pixels under the mask which is not even mentioned above.
Google: masked off pixels around digital camera sensor
and many discussions of the masked pixels will be found, including a "DNG recover edges" program from someone at Adobe.

Roger

[Alan Stankevitz]

I was thinking about this too and it doesn't explain all the extra pixels, so maybe it's a combination of both. I'll have to do a search on DNG recover edges when I get some time.

Thanks.

Alan,
Thanks for finding that. Interesting. It seems that one would only need a 1 pixel boundary, or at most 2 pixels for the guard. That would require only mean 0.016 megapixel addition to a 16 megapixel array and smaller than the reported actual versus effective pixels. Perhaps it means the actual pixels are these guard pixels mentioned above plus the dark pixels under the mask which is not even mentioned above.
Google: masked off pixels around digital camera sensor
and many discussions of the masked pixels will be found, including a "DNG recover edges" program from someone at Adobe.

Roger