Why are sensors 8.2mp yet effective resolution is only 8.0mp?

[Mycroft]

You know what I mean? The actual sensor on the Rebel XT has 8.2 megapixels, but the pictures are 8.0 megapixels. Why does the camera "throw away" ~200,000 pixels? And it's not limited to this camera either, many digital cameras do this, including Canon's flagship DSLR, the EOS 1Ds MkII. The sensor is 17.something megapixels but the files are only 16.7mp. The only possibility I can think of is to account for re-mapping of dead pixels in the sensor once it's built and tested. When you have that many photo diodes on such a small package, it stands to reason that a few aren't gonna be working up to snuff. Or am I way off base here? Anybody know for sure one way or the other?

[Tom W]

I don't know, but if I had to venture a guess, I'd say that the photosites along the edges are the ones most likely to be damaged in manufacturing so they leave a cushion of a few unused pixels around the periphery of the sensor to accomodate this.

There's a very large possibility that I'm way off base here. :)

[nathanphillips]

YAY I can actually answer ( I HOPE) a question for someone else on these forums!
Detailed below is the same concept for hard drive storage:


Question
Why is my drive displaying a slightly less than expected capacity?

Answer
Determining drive capacity can be confusing at times because of the different measurement standards that are often used. When dealing with Windows and Mac based systems, you will commonly see both decimal measurements and binary measurements of a drive's capacity. In either case, a drive's capacity is measured by using the total number of bytes available on the drive. As long as the drive displays the correct number of bytes (approximate), you are getting the drive's full capacity.

Decimal vs. Binary:
For simplicity and consistency, hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes. This is a decimal (base 10) measurement and is the industry standard. However, certain system BIOSs, FDISK and Windows define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. Mac systems also use these values. These are binary (base 2) measurements.

To Determine Decimal Capacity:
A decimal capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,000,000,000 using base 10).

To Determine Binary Capacity:
A binary capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,073,741,824 using base 2).

This is why different utilities will report different capacities for the same drive. The number of bytes is the same, but a different number of bytes is used to make a megabyte and a gigabyte. This is similar to the difference between 0 degrees Celsius and 32 degrees Fahrenheit. It is the same temperature, but will be reported differently depending on the scale you are using.

--------------------------------------------------------------------------------

Various Drive Sizes and their Binary and Decimal Capacities

Drive Size--------Approximate--------Decimal Capacity--------Approx Binary Capacity
in GB--------------Total Bytes------(bytes/1,000,000,000)-----(bytes/1,073,724,841)
10 GB-------------10,000,000,000------------10 GB------------------------9.31 GB
20 GB-------------20,000,000,000------------20 GB------------------------18.63 GB
30 GB-------------30,000,000,000------------30 GB------------------------27.94 GB
40 GB-------------40,000,000,000------------40 GB------------------------37.25 GB
60 GB-------------60,000,000,000------------60 GB------------------------55.88 GB
80 GB-------------80,000,000,000------------80 GB------------------------74.51 GB
100 GB-----------100,000,000,000-----------100 GB------------------------93.13 GB
120 GB-----------120,000,000,000-----------120 GB-----------------------111.76 GB
160 GB-----------160,000,000,000-----------160 GB-----------------------149.01 GB
180 GB-----------180,000,000,000-----------180 GB-----------------------167.64 GB
200 GB-----------200,000,000,000-----------200 GB-----------------------186.26 GB
250 GB-----------250,000,000,000-----------250 GB-----------------------232.83 GB

[robertwgross]

Nathan, I don't think we are talking about Gigabytes here. At least not for a few years.

On my camera, the specifications say:
Effective pixels: Approx. 8.20 megapixels
Total pixels: Approx. 8.50 megapixels

If we do the math, the resolution works out to 8,185,344 pixels. (I guess that's about 8.20 mp).

---Bob Gross---

[nathanphillips]

no, we are not talking about GB...but the Decimal vrs Binary argument still stands. I was just cross posting the same post from a thing on hd's, thats why its says GB.
Just thought I would toss it out there! :rolleyes:

[JaertX]

Could be wrong here...wouldn't doubt it. But I think the pixels along the outside edge are only used for exposure calculation. Still part of the overall sensor, but no image data comes from them.

[ron chappel]

JaertX is right,the outer zone is used for some kind of exposure/white ballance calculation,i've read it in many digital camera reviews over the years
It's so the camera knows what black looks -that way it has a baseline to calculate from.

Why they use so much of the sensor i have never heard.Surely they could use a strip down one side for example?

[scottbergerphoto]

This month's Popular Photography addresses this. The pixels on the outer rim of the sensor are used for exposure, white balance, and random noise level. Effective Megapixels are used for color, brightness, and resolution.

[Mycroft]

JaertX, ron, scott... thanks! now I can sleep at night without tossing and turning wondering why I'm losing 200,000 pixels! I thought they might have been stolen at night by... PIXIES! XD

[tim]

My first digital camera only had about 200K pixels...

[Bodog]

The interpolation process uses the data from five photo sites to create one pixel. Each site, plus the four surrounding sites. When you get to the outer rows, they are only surrounded on three sides, so there is not enough data for a pixel to be created for each of those sites. The data they contain is used in the creation of pixels for photo sites in the next row in, but no pixels are created for the sites in the outer rows. At least that is my understanding of the process. And I could be wrong... :rolleyes:

[Jesper]

Here is an elaborate explanation:
Effective Number of Pixels (http://www.dpreview.com/learn/?/key=Effective_Pixels)

[PhotosGuy]

This month's Popular Photography addresses this. The pixels on the outer rim of the sensor are used for exposure, white balance, and random noise level. Sounds as if, with the viewfinder not showing full coverage, you could get a bad Wb if you thought you were shooting a full shot of a gray card but actually weren't.

[Jon]

CWB only uses the central part of the image. If you check the manual for your particular camera you'll see they recommend filling the central, partial metering, circle area.

[PacAce]

Sounds as if, with the viewfinder not showing full coverage, you could get a bad Wb if you thought you were shooting a full shot of a gray card but actually weren't.
Frank, the white balance they are referring to is the auto white balance. Custom WB still uses the central part of the viewfinder as Jon said.

[PhotosGuy]

Thanks, Leo. Had me going there for a bit!