Mega-pixels Versus Dots Per Inch

The mega-pixel figures relate to the total number of picture elements in the image.

10 meg could be e.g. 3000 x 3333.

The number of dpi depends on how big you display or print the image.

Thanks JohnDG

Back to the film scanner, if it scans (up to) 7,200 dpi, the image would contain (up to) 7,200 x 7,200 = 52 mega dots per sq.inch, but as no image size is given, I can't equate this to mega-pixels.

And are pixels the same as dots.

Scanners are a bit different, because they don't have an image area (like a camera). They scan along a row (which in your case would be a row with 7200 sensing elements per inch), and they sample the image area so many times per inch as the sensing device moves along.

If you scan at 7200 dpi, and print out 1:1 at 7200 dpi, then pixels are the same as dpi. The total number of mega-pixels will depend on the object or image size.

With my example of a 3000 x 3333 pixel camera sensor, if you printed an image 30" x 33¹/₃", you'd have 100 dpi.

For instance, if you were scanning a 35 mm Film Frame ( 24 x 36 )mm you would have an area of 1.395 sq. Inches.

After multiplying the scanning dpi with the area, a 2000 dpi scan would result in 5580000 pixels. This would be approximately equivalent to taking a picture of the whole frame with a 5.5 megapixel camera.

1.395 * ( 2000 * 2000 ) = 5.58 Mpixels

*Depending on aspect ratios for both the scanner and cameras, these numbers could vary.

If I have a 10 mega-pixel shot on my digital camera and I want to make a 11"x14" picture, what will my dots per square inch be? What should my dots per square inch be to get good quality pictures? What would this quality be compared to?

10⁷ / (11 x 14 ) ≈ 65000 - This is the number of pixels per square inch of your final image.

√65000 ≈ 250 - thus you have 250 pixels per (linear) inch in your printed image.

This should give a good quality result - certainly good enough for displaying on your wall, or in e.g. a photo competition (provided you have a good enough printer).

Is 250 pixels the same as 250 dots per square inch? If I choose to make an 8"x10" picture from the same 10 megapixel shot, will it have 350 dots per square inch? (10000000/8x10 = 125000, √≈ 350).

That's 250 or 350 dots per linear inch (not square inch) - but you've got the sums right.

But this is where it gets complicated. Your image has, say, 200 pixels per (linear) inch. If your printer can reproduce, say, 150 dots per inch, the printer driver will try to map the 200 pixels onto the 150 dots that the printer is capable of reproducing - so you'd lose some quality in the reproduction. If you had a printer which could do, say, 200 (or more) dpi, there'd theoretically be no loss of quality. But say you have an image (when blown up to 8" x 10") giving 200 pixels per inch, and your printer can reproduce 250 dpi. Now your printer driver has to do the sums to use 5 printed dots of colour for every 4 pixels in the image.

Sorry if I'm rambling, but the (mega)pixel rating for the camera gives you a figure for the best quality you can hope to achieve, while the dpi rating for the printer dictates how well the image will actually be represented.

I rather doubt it, but I hope this has helped.

Thanks JohnDG.

I think I am with you on this, so as I understand it, the scanner (as it's name implies) scans an image a line at a time of so many dpi. The total pixel count is governed the physical area of the scanned image. Whereas the camera instantly captures the whole image of so many megapixels in total.

Both types of image capture are dumped into a file of so many megabytes.

The image can then be transferred to a viewing device (monitor, projector, printer, TV screen etc). Thus on a one-for-one pixel basis, the final image (of whatever physical size) will contain the same total number of pixels.

Is a film/negative scanner in fact a scanner, or is it a camera?, but rated in dpi.

Reckon you've got it!

Re: film/negative scanners - depends what kind of device it is - I imagine both line scanners and camera-type "scanners" are around. The "camera-type" would operate more quickly, but probably give lower resolution.

A long time ago did some work on a "flying spot scanner", used for scanning 35mm slides, which used a light beam & rotating mirrors.

Keep the size of the photo at 10 mp. Save a copy to print from. Crop to 11 x 14 proportions with the resolution set at 300 dpi. The computer will interpolate the pixels to produce a good looking picture.

Or, send it to a printer. I use Shutterfly.com. An 11x14 print costs $17.00 as I recall. Just upload it to them and give them your credit card data; they'll mail it to you.