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Comparing digital image quality standards.


samssauce 18 25 United Kingdom
30 Jun 2004 10:04AM
So, a digital camera's quality is based on the number of pixels in the image. On a film scanner, you are quoted DPI, [Dots per Inch] which is the number of pixels extrapolated to a specified surface area. The more tightly packed the pixels, the more detail the image contains.
However, and here is my question, how do I compare Megapixels to DPI?
Since I am not told the size of a digital camera's chip, I cannot ascertain what the DPI of the image is, and yet, when I see the specs for printers, what do they use? DPI!

The DPI system has been around for donkeys years, so why don't camera manufacturers, or magazines quote it, so we can compare like with like? Do they prefer the present 'Pixel' method because it can be manipulated by post-taking processes such as enhancement to increase the pixel-count ... something that used to be used a hell of a lot in the early days of digital, with 4 megapixel cameras enhanced to 5.2 megapixels.

So, can we have the camera's physical chip size AND pixel-count given in reviews, OR, cameras specified in DPI, so buyers can really make a meaningful comparison between cameras? Or, are we to conclude that the Ad Agency can overrule reality, truth, and needless to say, abuse the Trade Description Act with meaningless statistics?
spt 18 89 Scotland
30 Jun 2004 10:22AM
DPI would be a meaningless statistic for digital cameras, because the total number of pixels (or dots) is fixed, but the DPI depends on the size that you choose to print your photo. Quoting the the physical size of the chip and the pixel count wouldn't help, unless you plan to print the images to be the same size as the chip (not very many millimeters across).

Total pixels would similarly be meaningless for a scanner or printer, because the spacing of the scanned/printed pixels is fixed, but the number of pixels depends on the size of the image being scanned.
ellis rowell 17 2.0k United Kingdom
30 Jun 2004 10:30AM
I see what you mean. By using different terms for things it is easy to confuse. The only way you can equate the two systems is to load the picture (which you know is X pixels) into some software which will give you the actual picture size. I use Star Office v5.2 for this, click on "resize" and you will see the dimensions of the picture. Of course, you can print the picture to any size you require provided it is smaller than the original.
kit-monster 18 3.7k 2 Singapore
30 Jun 2004 10:38AM
Also the term mega pixel is very misleading. The image is captured by individual sensor cells then converted into pixels for viewing on screen in jpeg or tiff format. There are loads of varying factors, the shape, pitch and type of sensor for starters. For a rough guide, the dimensions you can print at in inches are the number of created pixels divided by 300 for excellent quality. This can be squeezed down to 200 in some cases. e.g. a 6 mega pixel with dimensions 3000 x 2000 sensor cells you can print at 10 x 7 or 12 x 8 at a stretch. Mind you if you go above this size, you will probably viewing the print from a distance.

Mega pixels are easy for the camera manufactures to quote. Its a bit like 0-60 for car manufactures. Its not what youve got; its what you do with it. If anyone is going to stump up a load of cash, read the reviews, ask people here what they think and have a play. There is an awful lot more to digital cameras than their pixel count. And a classic example of where mega pixels dont count is the Kodak 14 MP DSLR . . .
brm 18 76
30 Jun 2004 11:13AM
The reason they don't use dpi to describe camera resolution is because dpi & ppi are two completely different measurments.

ppi is pixels per inch, and relates to monitors, cameras, scanners etc. - basically it is something that is not physically real, you can't touch a pixel. With pixels it is just how many coloured squares a device is displaying/capturing per inch e.g. a 15" monitor @ 800 x 600 is displaying 72 ppi, wack the resolution up to 1024 x 768 and it will display 96 dpi.

A digital cameras resolution is measured by the number of pixels along the length & width of the sensor & multiplying them. This gives you the total number of pixels the camera will capture, but it has no real relation to either ppi or dpi - both will vary depending on how you view or print the image.

dpi is how many dots per inch are physically printed, and depends on the lpi capacity of the printer used e.g. newspapers are usually printed between 65 to 100 lpi, books magazines brochures etc. are usually printed between 150 - 200 lpi. Generally speaking you multiply the lpi by 1.5 to give you the dpi. e.g 200 lpi = 300 dpi.

As an example if you have an image which is going to be printed at 10cm x 10cm at 200 lpi you would save it as 10cm x 10cm at 300 dpi, if you saved it at 600 dpi you would not get a better print as the printer's maximum lpi is 200... so it can't print more dots per inch.

Hope this makes some sense, I probably haven't explained it very well...

b
michaeldt 18 1.2k
30 Jun 2004 11:30AM
i think the problem might be that whilst a camera quotes megapixels, it's not easy to tell how large an image you could print at a specific dpi.
If you really want to work out the dimensions of the image produced by your sensor and hence calculate print size based on your chosen dpi:
First you need to know the ratio of the sensors size. For 35mm film this is 2:3 i.e the length is 1 and a 1/2 times the height(24x36). (Most Digital sensors use this ratio)

so, for the ration 2:3:
For the long side: length = √(megapixels x 3/2)
For the short side: height = √(megapixels x 2/3)

So for a 6 megapixel sensor:
length = √(6 000 000 * 3 / 2)
= √(9 000 000)
= 3000
height = √(6 000 000 * 2 / 3)
= √(4 000 000)
= 2000

therefore image dimensions would be 2000x3000 pixels.
samssauce 18 25 United Kingdom
30 Jun 2004 1:54PM
Image resolution, is based on the number of pixels, dots, or whatever, that would fill a 1" square. Therefore, if you have two sensors which both deliver 6 megapixels, yet one measuring 24x36mm, and the other 18x24mm, the smaller sensor, will actually deliver greater resolution, and using the DPI or PPI system of measurement, will show the difference, whilst Megapixels will not!

Unfortunately, manufacturers and test reports don't quote the physical size of the CCD, so it is impossible to make such a comparison. After all, as chip technology improves, the number of sensors on a chip WILL become more densely packed, and the current Megapixel system will not be able to show that as well as the pixel-count having gone up, the amount of detail in the image, will be much greater than another with the same pixel-count, but of larger surface area!

I suspect, that the reason is historical, and that in the early days, those chips would have shown very 'lame' PPI & DPI figures, and their Publicity Agencies wanted something that looked good on paper, so Megapixels was settled on, knowing full well that it offered the opportunity for them to 'manipulate' the figures!

Remember the stink about 5.2 megapixel cameras that actually contained 4 megapixel chips? DPI & PPI, knock this little scam right on the head!
michaeldt 18 1.2k
30 Jun 2004 2:38PM
i think you are confused. two sensors that are both 6 megapixels will produce images of the same resolution (2000x3000). each image will have 6 million pixels. the only difference in the two sensors you give as an example is that the larger sensor will have larger pixel sensors than the smaller one, however they will both have the same number of these sensors.
michaeldt 18 1.2k
30 Jun 2004 2:58PM
with a scanner, you have a sensor that scans an image that is a lot larger than the senosr, hence the descriptions in dpi because it tells you that if you scan an image 10" by 10" at 300 dpi you will end up with an image 3000 pixels by 3000 pixels. with a digital camera, the image size scanned by the sensor is the same size as the sensor, so the amount of pixels is limited to the actual number of physical pixel sensors on the sensor.
park my ferret 18 1.0k United Kingdom
30 Jun 2004 5:06PM
on most camera reviews they give a figure for image size - ie: on the 1Ds it's 4064 x 2704 pixels while the 10D is 3072 x 2048 pixels. I find that this is a good way to compare pixel counts etc.
if printed at 300dpi (the standard for most mags) the 10d will produce an image just about A4 in size while the 1Ds will make one almost A3 size.
samssauce 18 25 United Kingdom
30 Jun 2004 7:44PM
If you have a full-frame DSLR the cip's surface area is 864 sq mm, whilst a sq Inch is 645.15 sq mm. By multiplying the DSLRs pixel count by 0.746713, you then have it's 1 sq" equivelent, which works out to 10.453 m/pixels per sq Inch. Any image size enhancing by the camera's processor will show up, so customers cannot be fooled into buying something marketed for instance, as a 5.2 mpixel camera when the chip actually only delivers 4.o megapixels.
No wonder the Ad men hate the DPI or PPI method of calculating image quality!
michaeldt 18 1.2k
1 Jul 2004 7:20AM
the dpi of a sensor is not the way of determining the quality of the sensor since the total amount of pixels in the end image is always going to be the same. and a higher megapixel camera will produce a larger image. if manufacturers want to deceive the public by interpolating images and claiming a higher pixel count, then that's up to them, and yes it's wrong, but dpi and ppi would not help anybody determine the quality of a camera's sensor (in terms of pixel count).
kelart 18 570
1 Jul 2004 12:20PM
and there's something like size of the pixels.some cameras have larger sensors but the resolution isn't impressive. other have smaller sensors with squashed smaller pixels. high pixel count doesn't necessary means high quality then.

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