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Old Jul 18, 2006, 5:16 PM   #1
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I am going to purchase a digital SLR camera. I am uncertain on how many megapixels I need.I do some freelance photojournalism using my current 35mm SLR. The magazine I shoot for requires that the scans I send them enable them to reprint 8 1/2# x 11" at 300dpi. haven't needed to understand this as the hi-res scans I've sent have been fine.

Now, with a digital purchase, I need to know how to convert the megapixel of a camera to this dpi requirement. I don't want to buy a camera that doesn't have enough megapixel for clear, non-interpolated reprints...which must be crisp and clear.

Would appreciate some understanding of this technical issue. Thanks.


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Old Jul 18, 2006, 5:19 PM   #2
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Use this calculator http://www.mattspinelli.com/ppicalc.html

Type the megapixel number in the megapixel field and the aspect ratio (1.5 forall dSLR cameras exect olympus which corresponds to 3:2 ratio, and 1.3333 for most digicams which corresponds to 4:3 ratios)

In this case you will need at least a 10 MP dSLR to get 300 8.5x11. But I don't see if you were to interpolate a6 or 8 MP dSLR that it would be noticable.8MP ispretty close to 300 dpi at that print size.

A 6 or8 MP dSLRshould have much better image quality than a scanned 35mm image would, even if thetechnical resolution is slightly lower.



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Old Jul 18, 2006, 5:59 PM   #3
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DPI to megapixels @ 300 dpi is calculated as follows:

(8.5 x 300 =) 2550 x (11 x 300 = ) 3300 = 8415000 pixels

== 8.4 MP

Simple math. And that is without any interpolation up.
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Old Jul 18, 2006, 7:43 PM   #4
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amazingthailand wrote:
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DPI to megapixels @ 300 dpi is calculated as follows:

(8.5 x 300 =) 2550 x (11 x 300 = ) 3300 = 8415000 pixels

== 8.4 MP

Simple math. And that is without any interpolation up.
The math isn't quite that simple. You lose about 14% of an image cropping a 3:2 DSLR image to 8.5 X 11. You lose only a little over 3% cropping a 4:3 image to 8.5 X 11 if you get a DSLR in that format.

So if you get a standard format DSLR you need almost 10Mp to get a native 300 PPI 8.5 X 11. With a 4:3 image you need a bit less than 9Mp.

As nelmr pointed out along with the correct numbers, you could probably upsample an 8Mp image with a good upsample filter and do just fine for quality requirements.

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Old Jul 19, 2006, 8:30 AM   #5
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All digital cameras have a horizontal by vertical pixel rating, for example 2048x1536 for one 3 megapixel model.

Take the pixel rating and divide by the respectivedimension of the paper you will print on. Do it for both horizontal and vertical and choose thesmaller as your DPI rating.

Example: 2048x1536 pixels onto a 5x7 print. 2048/7 is about 292. 1536/5 is about 307. The fact you choose the (smaller) number 292 as the DPI rating takes into account the lost pixels of material when you zoom the image to cover the surface of the photographic paper with a different aspect ratio and some material is cropped off one side, in this case one or both of the long sides depending on you frame the image. The fact you choose the smaller number also takes into account cameras whose sensors have different pixel density for horizontal versus vertical.


Lines of resolution (line pairs times two) is subjective and photographic experts tend to estimate it at 70% of the DPI rating. With JPEG processing by the camera a smaller than 70% figure may be in order.





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Old Jul 19, 2006, 9:35 AM   #6
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The exact numbers would be this:

3:2 dSLR (canon, nikon, or fuji) 9.76 effitive megapixels (2550 x 3826)

4:3 dSLR (olympus, panosonic) or ajust about any digicam, 8.67 effictive megapixels (2550 x 3399)

OF the 3:2 cameras The Nikon D200 or Sony Alpha would be the cheapest dSLR cameras you could get and the reason I said that you would need at least a 10MP dSLR.

OF the 4:3 dSLR cameras there does not yet exist one that has 8.67 effective megapixels.

The digicams are now approching 10MP but I doubt their resolution (not meaning pixel count) is lower than the 8 MP dSLRs (but some are very close such as the S9000, FZ30). Resolution and Megapixels aren't the same even though they are slightly linked together.

You can have a 3.5 MP dSLR that resolves about as much as a 6 MP dSLR (example is Sigma S9/10 verses canon/fuji/nikon 6 MP dSLR). You can have a 7.1 MP digicam that resolves less than a 6 MP dSLR (there are many many examples of this). As such I would think that with a dSLR as long as you have a native PPI of about 240 (about 6MPbayer sensor)or so at 8.5x11 you could interpolate up about 25-30% and it still be acceptable. If you printed a 6MP or higher dSLR 8.5x11 image and a film 8.5x11 image you probably wouldn't be able to tell much difference except that the dSLR is cleaner.
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