300 dpi for online photo printing - to resample in photoshop or not?

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leehoffman@gmail.com - 28 Apr 2006 21:37 GMT

I have an 8.2 megapixel digital SLR that takes 3456x2304 pixel images.
When you open these images in photoshop, you have an 48x32 inch image
at 72 ppi. This is great for onscreen viewing, but not for printing.
When you resize the image in photoshop, you can keep the ppi above 300
for the following sizes: 4x6, 5x7, and 8x10. Once you go larger than
that, the resolution falls below 300. I have been told that you need
at least 300dpi for making a high quality print, is that right?

So, if you want to make larger prints, then you have two options.
Which one do you guys think is better?

1. turn OFF resampling in photoshop and resize the image, letting the
resolution fall depending on the size. (ie 11x14 = 172 ppi, 16x20=172
ppi, and 20x30=115ppi)

2. turn ON resampling (bicubic) and fix the resolution at 300ppi. This
means the computer is interpolating and adding pixels to your photo.

I am curious to find out if anyone has actually made 2 prints of the
same image at with each method and compared the results? What did you
find out?
Thanks very much
Lee

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Roy G - 28 Apr 2006 22:55 GMT

>I have an 8.2 megapixel digital SLR that takes 3456x2304 pixel images.
> When you open these images in photoshop, you have an 48x32 inch image
[quoted text clipped - 19 lines]
> Thanks very much
> Lee

I think you need to turn on your calculator.

3456(pixels) divided by 12 (inches) gives 288 Ppi.

So you can print at 12 x 8 inches (your 3 to 2 format) and still get a good
quality print.

If you want a print which is 10 inches wide, then the uncropped length for
calculations, becomes 15 inches and produces a figure of 230 Ppi which will
still produce a good print.

Roy G

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leehoffman@gmail.com - 29 Apr 2006 00:08 GMT

Thanks Roy for your sarcastic jab, that helps a lot. I am not making
the numbers up, that's what PHOTOSHOP tells you when you bring up the
"image size" dialog box for a 3456x2304 photo.

> >I have an 8.2 megapixel digital SLR that takes 3456x2304 pixel images.
> > When you open these images in photoshop, you have an 48x32 inch image
[quoted text clipped - 32 lines]
>
> Roy G

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Roy G - 29 Apr 2006 01:13 GMT

> Thanks Roy for your sarcastic jab, that helps a lot. I am not making
> the numbers up, that's what PHOTOSHOP tells you when you bring up the
[quoted text clipped - 39 lines]
>>
>> Roy G

Hoi Mate.

Don't be so bloody sensitive.

I wasn't being sarcastic, not even trying. It was very obvious at first
glance, even before I reached for my calculator, that your figures were well
out.

If they were accurate, no-one would have been able to produce 16 x 12s until
very recently, when the 10 & 12 Mp consumer DSLRs were released.

I know that given the right subject, I can make a print up to about 18
inches wide from my 6Mp D70. And they have never yet been criticised for
poor Print Quality.

As for your tests, you really need to do that for yourself. You can not
rely upon other peoples opinion as to what is best. What could be Ok to
them, could be anything but Ok to you or to me.

Roy G

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leehoffman@gmail.com - 29 Apr 2006 00:15 GMT

yes, that's correct, I typed the wrong number in. An 11x14 is 246 ppi.
Thanks for the sarcastic jab, that is real helpful. Anyway, it is
beside the point and doesn't asnwer my question at all. Has anyone
tried these two and compared them side by side? I'd like to avoid
ordering a bunch of prints just to compare them when I could learn from
somebody elses experience. Thank you very much.
Lee

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Wayne - 29 Apr 2006 00:40 GMT

>yes, that's correct, I typed the wrong number in. An 11x14 is 246 ppi.
> Thanks for the sarcastic jab, that is real helpful. Anyway, it is
[quoted text clipped - 3 lines]
>somebody elses experience. Thank you very much.
>Lee

You think that was sarcasm? It seemed really gentle to me.

This is sarcasm: If we learned from your computing example, look where
we'd be. :) Sometimes we need to do our own tests. Dont believe
everything you hear online.

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Jim Townsend - 29 Apr 2006 01:48 GMT

> I have an 8.2 megapixel digital SLR that takes 3456x2304 pixel images.
> When you open these images in photoshop, you have an 48x32 inch image
> at 72 ppi. This is great for onscreen viewing, but not for printing.

72 PPI has absolutely nothing to do with onscreen viewing.

Your images will look exactly the same at 10 PPI, 100 PPI or 1000 PPI.
For monitors, it's only the number of pixels in each dimension that count.
PPI is meaningless.

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Paul J Gans - 30 Apr 2006 18:25 GMT

>> I have an 8.2 megapixel digital SLR that takes 3456x2304 pixel images.
>> When you open these images in photoshop, you have an 48x32 inch image
>> at 72 ppi. This is great for onscreen viewing, but not for printing.

>72 PPI has absolutely nothing to do with onscreen viewing.

>Your images will look exactly the same at 10 PPI, 100 PPI or 1000 PPI.
>For monitors, it's only the number of pixels in each dimension that count.
>PPI is meaningless.

Agreed.

This sort of thing causes a lot of confusion. The "typical" monitor
these days runs about 96 ppi. Programs still pretend that they
are 72 ppi. The difference is that the image will be different
physical sizes on screen depending on the screen resolution

One can measure that by simply laying out a suitable line in
Photoshop or whatever that is, say 200 pixels long. Then display
it on your monitor and measure the actual length. The actual
display pixels per inch is then 200 pixels divided by your
measured length.

This has NOTHING to do with the size of the image when printed.
For that one has to understand the arcana of printer drivers.
I don't, since the details seem to vary from one driver to
another.

Often the driver will allow you to pick a size in inches for
the print. And it will deliver that size. Obviously the driver
is either shrinking or expanding your image to suit its physical
needs. I hate that. But I rarely see the native resolution of
a printer given in either the manual (ha!) or the software.

So in the end I end up wasting ink while I find the right number
of pixels to feed the printer to get a decent print. I suspect
that if I had a higher-end printer I'd have less trouble. But
then, as I said at the start, I don't understand printer drivers.

----- Paul J. Gans

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