# Photo resolution tips/ Costco printing



## madisonofriel (Jun 30, 2014)

Can someone explain photo resolution to me? I kinda know how it works but I find myself getting confused when I have to change the resolution in lightroom... I need to order some prints and invitations from Costco photo, but it wont let me change the resolution? Has anyone done business with them before? If so can you tell me why it won't let me change the res on the initiations leaving them very bad quality. Thanks!


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## KmH (Jun 30, 2014)

Note - many think resolution and aspect ratio are the same thing, but they are not.

There are 2 kinds of resolution:

*Image resolution* - the pixel dimensions of the photo. Image resolution is the only resolution that applies to electronic display.
Unless image resolution is lowered, a web page usually displays a digital photo at a reduced size because few electronic displays can show a digital photo at full size.

*Print resolution* - is the pixels per inch (ppi) specified for a digital image and only applies to physical prints. Print size is determined by both the image resolution and the print resolution.
For example, consider a photo that has a 3000 pixel long side.
If we set 100 ppi - 3000 px / 100 ppi = 30 inches. The prints long side will be 30 inches long.
If we set 200 ppi - 3000 px / 200 ppi = 15 inches. The prints long side will be 15 inches long.
If we set 300 ppi - 3000 px / 300 ppi = 10 inches. The prints long side will be 10 inches long.

These simple equations are helpful for determining each of the 3 variables:
pixels / ppi = inches
pixels / inches = ppi
inches x ppi = pixels

Print labs can use their RIP software (Raster Image Processing) to change the print resolution (ppi) to match the print size ordered, _if the the image has sufficient *image resolution*_.
Print labs usually have a minimum required resolution that is at about 100 ppi, and as long as the print size ordered meets their minimum resolution they can print a variety of print sizes of the same image.

*Aspect Ratio*

Most digital camera make photos that have a 3:2 aspect ratio. Another way to define 3:2 is that the long side of the photo is 1.5x longer than the short side. (3 / 2 = 1.5)
Some of the popular print sizes have a different aspect ratio.
An 8x10 has a 5:4 aspect ratio. The longs side is only 1.25x longer than the shirt size (5 / 4 = 1.25)

To make an 8x10 print from a 3:2 aspect ratio image we have to cut away (crop) some from the image.

At 3:2  a print will be an 8 inches x 1.5 = 12 inches or an 9x12. So to print an 8x10 from a 3:2 photo 2 inches of the long side have to be cropped away and discarded.
If we don't want to crop any from the long side we have to crop the short side - 8 / 1.5 = 5.33 inches and we wind up with a 8 x 5.33 print having lost 2.67 inches from the short side.

If you order an 8x10 but submit an uncropped 3:2 photo, a portion of the long side of the photo will not be included on the print.
The same loss of some part of the original uncropped 3:2 photo will occur if you order any other aspect ratio other than a 3:2 aspect ratio print.


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## TCampbell (Jun 30, 2014)

One more thing since you mentioned Costco...

When you print photos at a lab, the colors you get back in the prints may not match your expectations.  Suppose a printer is a little rich on yellow and blue but weaker on reds (giving a green overall color cast)... your option would be to alter the colors in the JPEG image you send to the lab so that the JPEG image you receive actually matches your expectations.  But to do this... you'd have to know what that printer at the lab is actually going to do.

It turns out... they do this.

See:  Digital photo lab profiles

Costco performs color calibration on their printers regularly (these are updated all the time).  You look up your local store and download it's latest color calibration file (.icc profile). 

Depending on the software you use (e.g. Photoshop, Lightroom, etc.) there's a way to tell the software to export a JPEG, but apply the .ICC profile to it.  This alters the colors in that image intended for the photo lab -- it won't look accurate on your screen (because it's been adjusted to compensate for the printer) but it will render correctly when you get your prints back.


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## colnago1331 (Jul 1, 2014)

How Big Can I Print by Thom Hogan


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## KmH (Jul 1, 2014)

TCampbell said:


> One more thing since you mentioned Costco...
> 
> When you print photos at a lab, the colors you get back in the prints may not match your expectations.  Suppose a printer is a little rich on yellow and blue but weaker on reds (giving a green overall color cast)... your option would be to alter the colors in the JPEG image you send to the lab so that the JPEG image you receive actually matches your expectations.  But to do this... you'd have to know what that printer at the lab is actually going to do.
> 
> ...


An additional resource - Tutorials on Color Management & Printing


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