Larry,
This thread has been a great help to those of us that have been misinformed about the whole pixel dimension/ppi issue. Can you give me a good definition of resolution as it applies to digital images or how the term resolution should be used?
Scott
Scott Sharick Photography
www.scottsharickphotography.com
www.scottsharickblog.com
www.flickr.com/photos/scottsharick/
On Feb 1, 2011, at 8:11 AM, Larry Berman wrote:
> Michael,
>
> Please do not use the term resolution when describing pixel
> dimensions. Call it actual pixels. Everyone assumes that resolution
> is the density of pixels (PPI) within an image and they already have
> to understand what it is you're describing to understand that what
> you mean by resolution isn't what they are used to seeing.
>
> Larry
>
>
>
>
>> 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
>
>
> ******************************
> Larry Berman
> 412-401-8100
> Art Show Imaging and Jury Services:
> http://BermanGraphics.com
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