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Old Jan 12, 2005, 5:27 AM   #11
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Many thanks 'slipe'. I will do your experiment today and report back in a day or so.
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Old Jan 12, 2005, 3:59 PM   #12
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Its been quite a marathon today as I have attempted to follow Slipe's recommendations. The object of the experiment was to see whether a Canon 20D can really produce a 16X24 print of good quality.

The first problem was to find a sample which one could easilyrecognise when the image would deteriorate, I looked for one which had a limited scenic depth to identify sharp in-focus edges. Ideally it would have been a shot of a test chart of some kind. I eventually decided on Steves standard still life IMG 9937JPG, 3504 X 2336 , 2,788,932.

This image opened in PS as 1236.13 X 824.09mm @ 72ppi ; that is a 48" print.

I printed three 10X8 prints at 75,150 & 300ppi. To give an idea of scale the face of the white dice measured 65mm on the print. The file sizes were 1.29, 5.15 and 20.6M respectively.

I just cannot see any difference - what am I doing wrong????

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Old Jan 12, 2005, 6:24 PM   #13
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I don't use PS, but the only reasonable way to see how a photo is going to look at different sizes from a camera trying to test by printing smaller images is to crop them.

Otherwise, the resampling algorithms are going to skew your results. That's why Slipe suggested you crop, then resample it back up to 300 ppi. ;-)

I'd also suggest printing photos of subjects that you are more likely to enlarge. Some subjects print better at larger sizes compared to others. For example, portraits do quite well (because you have far more pixels representing your subject). Yet, foilage doesn't (because you may only have a few pixels representing something, andyour brain "fills in the gaps" at smaller print sizes).

So, if you wanted to print an 8x10" photo at 150 pixels per inch, you'd crop the larger image so that you end up with an image (portion of the original) that is 1200 x 1500 pixels (8 inches x 150 pixels per inch = 1200 pixels; 10 inches x 150 pixels per inch = 1500 pixels).

Then resample it back up to 300 pixels per inch using a good algorithm (i.e, Bicubic, etc.), so that you have a final image size of 2400 x 3000 pixels for printing at 8x10" size.

This would give you 150 pixels per inch of actual detail captured by the camera, yet make sure you've got plenty of resolution to prevent pixelation, thanks to resampling it back up. That way, you'd get an idea of the best quality you can expect for the type of subject you're printing, if you went to a larger size, with the same number of pixels per inch of detail captured by the camera.

I can tell you already that most prints will look fine with 150 pixels per inch of detail if resampled back up some. I've printed many an 8x10" print from 2 Megapixel images (which works out to 150 pixels per inch of detail) -- even without resampling. You can see an increase in quality going to a higher number of pixels per inch (for example 3 Megapixel versus 2 Megapixels at 8x10" size) -- and you can also see an increase in quality resampling to 180 pixels per inch or higher from a lower resolution image(but resampling won't increase the amount of detail captured).

Also, keep in mind that you're not going to be looking at a larger print from closer distances anyway.

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Old Jan 12, 2005, 9:47 PM   #14
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I usually shoot my images "wide", then crop them later using photo software.

So, if you buy something like a Canon 20D, essentially an 8 meg camera, you can crop away half the image area and still have 4mp worth of image remaining, which should print an 8x10 okay.

The argument that a 3 meg camera prints an acceptable 8 x 10 is true if you frame your shots in camera pretty much the way you want them. But if you shoot wide and crop later, it's nice to have a few extra mp's to play with.

The resolution that makes a print "acceptable" is somewhat subjective anyways. A friend of mine was a printer (film) for professional photographers. He used to show me8x10 printsand talk about their grainy-ness, acceptable focus, etc. When he did this, he was always looking at the print using a 10x or 15x loupe (essentially a small magnaiying glass)... which tells me what he thought was or was not acceptable was probablynot visible to the naked eye.

It reminds me of diamonds. The most expensive diamonds have very few internal flaws. Almost all commercially available diamonds available to the consumer, from the highest quality to the very lowest quality, do not have flaws that would be readily apparent to the naked eye. (Unless you practically yank the ring off the person's finger, hold it up to the light, and squint like hell. Then you might see the flaws of the lowest quality diamond).

So, the bottom line, is an 8x10 from a 3meg camera might be very acceptable to one person and not to another.I have a 3 meg camera of reasonable quality and my 8x10's, with less than 15 percent of the image cropped, look okay to me.

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Old Jan 14, 2005, 12:54 PM   #15
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Thanks everybody for the help, it took some time for the penny to drop - the maths was ok, just got confused with the cropping.

The results of my experiments concur with your comments -

I could not see any improvement above 240ppi this means that the Canon 20D could produce a 16" print of excellent quality.

There was a very very slight deterioration at 146ppi which was only apparent on very fine hard edged detail such as the lettering on the dice or the textile weave in the flag - this needed very close inspection to notice. This would give a 24" print. As JimC said with the appropriate subject matter there would be no apparent deterioration.

I tried 72ppi and that produced a discernable deterioration accross the whole print, nevertheless it would have produced a non critical display print at 48"

Now that I know all this I am wondering whether I should buy the 6M Nikon 10D which has a good and cheaper kit lens??

Thanks to all.
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Old Jan 20, 2005, 8:09 PM   #16
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get as many as you can afford
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Old Jan 23, 2005, 8:54 AM   #17
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Thanks for the great infoon this subject!
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