Megapixel Question

[Jumbuck]

Hello all,

This will seem like a stupid question to all and I know I'm flagging my inexperience by asking it but I would really like to know the answer.

If a sensor has x megapixels over a specific given area why is it an image taken with more megapixels over that exact same given area is larger?

This would mean that somewhere in the equation a pixel must default to a specific standard size - in order for the image that was taken with more megapixels to be larger, otherwise it would remain the same size as the one taken with smaller megapixels over the same given area, but be of a higher resolution (smaller pixels over the same given area).

I'm coming from years with film so its quite embarrassing asking this but I have not been able to find anything that explains why its the pixel size that stays the same and its the image size that changes even though its exactly the same size sensor (area). To me if this is the case why are we chasing more megapixels if it does not improve resolution over a fixed area?? I must have a mental block - would someone like to drop the penny for me.

Cheers

[robboh]

At the end of the day, the pixel density of whatever media you view the final image on will have some fixed maximum resolution, thus given the same maximum media resolution for two different resolution pics, the higher pixel photo must be bigger on that media.

As some examples

* Default resolution on most monitors is 72 pixels per inch. Some of the new LCD displays are up around the 100-120ppi.
* With most printing methodologies, the resolution where you start to see little improvement with rising resolution is usually ~300dpi.

Therefore, a picture printed on your inkjet will physically be around 1/4 the height or width of the size it displays on screen.

The OPPOSITE happens as you increase display resolution on your monitor. The higher the resolution, the smaller everything looks as you are 'putting more pixels' into the same sized box.

[jdear]

a lot of questions...
to reply simply

http://www.kenrockwell.com/tech/mpmyth.htm

should answer a few...

from what i know about it all, different manufacturers can pull more pixels from the same size chip based on how tightly they pack the sensors (photosites?) which make up the chip.

http://electronics.howstuffworks.com/di ... amera2.htm is some more stuff

JD

[MattC]

Jumbuck,

The answer is in DPI of the output device (monitor or printer) and the selected image size. If DPI is constant a larger image is the result. If image size is constrained, then greater resolution. More megapixels does offer greater resolution over a **fixed area** (not valid for monitor as the pixels are fixed) - if you choose to fix the area.
Just because software defaults to a particular size or resolution, it does not mean that you have to stick to that.

Cheers

[robboh]

I'm coming from years with film so its quite embarrassing asking this but I have not been able to find anything that explains why its the pixel size that stays the same and its the image size that changes even though its exactly the same size sensor (area). To me if this is the case why are we chasing more megapixels if it does not improve resolution over a fixed area?? I must have a mental block - would someone like to drop the penny for me.

Jumbuck,
Forgot to add. The 'extra' megapixels these days ARE wasted in a lot of cases. In some ways we'd be better off not chasing pixelcounts, but improving the quality of the pixels (to a point). This is certainly the case in most consumer cam situations, they can provide enough pixels to outresolve most printing technologies at 6x4 size prints (and bigger in a lot of cases).

However, there are advantages in having more pixels too. They give more opportunity for selective cropping, though ideally you should crop in camera, not on the editing table. Primarily, the higher the resolution/pixelcount, the larger the print you can get out of it without noticeably losing quality.

[Aussie Dave]

Primarily, the higher the resolution/pixelcount, the larger the print you can get out of it without noticeably losing quality.

This is true....however with many people obtaining fantastic prints from the D70 (6 MP) up to 16x20" (and possibly beyond), how big do people want to print their images ? Also, printing images this large, or larger, you don't need to print at such a high DPI so the image dimensions will not need to be as big as that upsized @ 300DPI (eg. 20 inches @ 300dpi is 6000pixels long; 20 inches @ 200dpi is only 4000pixels - not that much longer than a native NEF file).

However, technically, yes more MP is likely to give you a file size that is large enough "without" the need to upsize....

[Dug]

Panasonic FZ30 V Nikon D70

Max resolution 3264 x 2448 > 3008 x 2000

Effective pixels 8.0 million > 6.0 million

Sensor photo detectors 8.3 million > 6.3 million

Sensor size 7.18 x 5.32 mm < 23.7 x 15.5 mm

This is taken from a review site I think the final line says it all if you compare the actual sensor size in-spite of all the figures if you compare the physical dimensions of the CCD array you have your answer

"Not all megapixels are created equal"