thanks
On Tue, Feb 1, 2011 at 9:38 AM, michael cothran <michaelhcothran@comcast.net
> wrote:
>
>
> Some of you are still getting confused between PPI and the actual pixel
> resolution in a given image.
> 1. Pixel resolution is how many pixels are in your total image from side to
> side, top to bottom. If you don't know your camera's resolution, look in
> your camera manual, or check out the data in your RAW or Jpeg file. Mine is
> 4080x4080 pixels. Meaning my file has 4080 pixels from side to side, and
> also 4080 pixels from top to bottom. It is a square sensor. Your sensors,
> unlike mine, are rectangle, so your resolution may look something like
> 3000x4500 pixels. Meaning that your file is (in landscape mode) 4500 pixels
> from side to side, and 3000 pixels top to bottom. If you multipy the two
> (4500x3000) it will indicate the exact mega pixels your sensor can create
> (mega meaning one million). So if this is your resolution, you have a 13.5
> mega pixel camera. Mine (4080x4080) is 16+ mega pixels.
>
> 2. PPI (pixels per inch) simply indicates how many of these pixels YOU
> assign per inch in your image file. Most cameras default to 72 pixels per
> inch (ppi) when uploaded to your computer. You can change this default in
> your RAW processor to whatever you want. The more pixels you assign per
> inch, the smaller (in inches) your image will be.
>
> Here's how to figure out ppi and inches in your desired print size -
> Take your camera's resolution, and divide it by a pixel number to determine
> it's size in inches, OR...divide your resolution by inches to determine what
> your ppi will be.
> Examples - If I assign 72 pixels per inch (4080 divided by 72), my image
> will be 56.67 inches long. If I assign 360 pixels per inch (4080 divided by
> 360), my image will be 11.33 inches wide.
> Or, say, if I specifically want a 24" print, I would divide my resolution
> by 24 (4080 divided by24), which would yield 170 ppi for my print. Of
> course, PS, LR, etc will do all this for you, but the purpose here is to
> understand WHAT is going on. Note - In NONE of these examples have I altered
> my file in any way - I have not added nor deleted a single pixel. In each
> case I worked solely with the original 4080 pixels. My original resolution
> remains intact.
> You run into problems when you start arbitrarily adding or deleting pixels.
> Avoid this whenever you can. Remember, if your original resolution remains
> between 180-480 ppi (and I've gone as low as 150 PPI) for your desired print
> size or web size, DO NOT alter the resolution. Send it to the printer as is.
> In other words, DO NOT interpolate or resample if your ppi remains between
> 180-480 for your chosen print/web size. In today's world, with larger
> sensors, you will most likely have to resample when web posting, as most
> files are simply too large to begin with.
>
> 3. Now here's the final kicker - your Epson printer defaults to a 360 ppi
> file for printing. Other brands default to 300 ppi for printing. This means
> that NO MATTER what your file's ppi is set to when you send it to your Epson
> or other brand printer, your printer will add or delete pixels to come up
> with either 360 or 300 ppi depending on your brand of printer. So if your
> file's ppi is bewteen 180-480, again send it as is, and let the printer do
> the interpolating for you. Epsons do a great job here.
>
> Final word - DON'T get your file's ppi confused with the ink dots per inch
> (dpi) that your printer lays down - normally you would set your printer's
> dpi to 1440 for fine detail. For glossy paper, you could even use the 2880
> dpi setting, although there is debate has to how much this actually improves
> the final image quality, AND it does increase your printing time. Printer
> dpi's (720, 1440, 2880) refers to how many dots or squirts of ink your
> printer will lay down in each linear inch of printing. THIS IS NOT THE SAME
> THING AS YOUR FILE'S PPI!!!
> Hope this helps, and I hope I'm on track here.
> Michael H. Cothran
>
>
>
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