Questions get asked many times every day about printing, enlargements, and dpi/ppi, this FAQ is aimed to give people basic information on printing their images. Please start a thread if you have questions, or if you'd like me to add or change something post below or PM.
If a mod could sticky this it might save people asking the questions over and over again. If it's put inside the FAQ sticky no-one will find it I bet.
dpi vs ppi
There are plenty of articles about this on the net, but here's the basics. dpi stands for dots per inch, and it's a physical measure of how many dots go on a piece of paper. We don't care about this. The output device could be a continuous tone printer, a 250dpi minilab, or a 4000dpi inkjet, we just don't care.
ppi stands for pixels per inch, this is what we do care about. There are two measures of ppi: one is just a meaningless number inside the file, one is a value calculated from the number of pixels and the print size. An image might be 3000 pixels wide, with an embedded value of 72ppi, that ppi value is irrelevant. What's important is when you choose a print size that there's enough pixels to give you the detail you want. For example if you wanted to make a 10" wide print from a 3000 pixel wide image that will give you 300ppi. Even if the embedded value's set at 72ppi it's ignored. It's just a number.
Some labs require the correct ppi and number of pixels in a file. In this case use the instructions below to give the lab what they want.
How do I change the ppi without changing the number of pixels in the image?
Hit image -> resize, uncheck "resample", enter your ppi, hit ok.
How do I create a properly sized image for printing at my lab? They want 212ppi with the correct number of pixels in the file
There's any number of ways
- For individual files get the crop tool, in the tool bar enter the width, height, and resolution you want in ppi. Use the crop tool. It will resize and set the ppi.
- For a batch you can write an action using the image -> size command, and run it using the batch commands or the image processor. If you use image processor you just need to use the "How do I change the ppi..." answer above.
- For a batch of RAW files in CS3 ACR just set the resolution using the blue link at the below the image, in the center. Then run image processor over the RAW files, setting the resolution you want in the script options. I think this will work, someone let me know if it doesn't and i'll work out another way.
- In each case save as a Q10 jpeg. Q12 is unnecessary, Q10 is as well but there's no harm except wasted bandwidth sending them to the lab.
When I open an image in photoshop it says it's 60x40" at 72ppi, is that ok?
Pay attention to the number of pixels, not the size photoshop thinks it is. The only time ppi matters is when you send it to a print firm, and sometimes not even then. If you want to change it just go to the photoshop image size command, uncheck the "resample" box, and put 300 in the resolution field.
Does my camera have a default resolution?
Maybe. It doesn't matter, it's just a number. Your software can override it easily, either in a RAW converter or in Photoshop.
Resolution & Viewing Distance
Ideally we want 250-300ppi for images that are going to be looked at closely. Most consumer labs use 300ppi, my pro lab uses 250ppi. I can't see any difference no matter how close I get. Maybe I could if I used a magnifying glass, but i'm not going to do that.
For prints 30" or above I would try to stay about 75ppi if possible, unless the viewing distance is high.
Enlargements, and "How Large Can I Print An Image?"
When you make large prints you naturally stand further away from the image, which means you can't see the fine details. Because of this you don't need as many pixels per inch for large prints.
I've made 30x40" prints from images from a 20D, which gives 83ppi, the print looks fantastic. You can't stand closer than a couple of meters and still appreciate the whole image so 83ppi is enough. If you get up really close you can see there's not huge amounts of fine detail, but it's surprisingly good. I've made a 50" print from a 12.7MP 5D ISO800 image which looks amazing.
People regularly blow up images from a 20D to make billboards. This works fine because people are quite a distance from them, often in moving vehicles. You could quite easily blow up an image to the size of the moon, and get probably one pixel per mile, but because of viewing distance it'd look fine.
What about prints on canvas?
Canvas is a very forgiving medium. 100ppi looks great on canvas.
Should I enlarge myself or let the lab do it?
If the lab can do it for no charge, I let the lab do it. They're experts. I'm not. They have expensive software called a RIP (Rasterised Image Processor) which knows how to get the best from their printer.
My main pro lab doesn't upsize for me. I do a single step upsize using the bicubic smoother in Photoshop. The old adage of "upsize in 10% increments" is from back before the bicubic algorithm, it's not necessary any more. I tried it. No difference. People often overthink things.
Should I use XYZ Software (genuine fractals etc) to do the upsize?
Nope, Photoshop does a good job. If you're not convinced get the trial version and try it for yourself. Judge by the prints, not the 100% crop, label them on the back and have someone mix them up for you. Print crops if you don't want to spend money on big prints.
I tried to print an image but they said there wasn't enough resolution. What do I do?
Upsize the image yourself in Photoshop, then send it in. See the two questions above for why this works.
Proportions (or making an 8x10"print)
Images from Canon DSLR cameras always have the 3:2 ratio - eg 3504 x 2336 is 3:2 on a 20D. If you print this 10" wide that means it will be 6.66" high, or conversely if you print it 8" high it will be 12" wide. If you want a 8x10" print (or any size that's not a 3:2 ratio) you need to crop the image, which will discard pixels to make it the shape you want. Get the crop tool, enter 8in and 10in in the toolbar (or vice versa), drag the crop tool over the image, the hit enter or double click. If you try to leave this to the lab they'll crop where they think it should be, or worse, they'll do it automatically and might cut off an important part of the image. NB: baby cameras usually have other ratios, usually 4:3 or occasionally 16:9.
The only other solution is to use the image -> canvas size command to add black or white edges to the image. The advantage is you can see the whole image, even though it has borders.
How do I size images correctly for a 6x4" print (or other sizes)
First work out the ppi required by your lab - it's usually 300 but for example my lab wants 250ppi, and some machines like 400ppi. Multiply this number by the longest side, in inches, so 300ppi * 6 inches = 1800 pixels. If you wanted 18 inches and 250ppi that's 4500.
Choose file -> scripts -> image processor, enter this number into BOTH boxes, choose your directory of images, set an output directory (or save to same directory) and hit go. Note if you save to she same directory it'll go into a subdirectory and use the same filenames. The "convert to sRgb" tick box is a good idea if you're not sure what you're doing with color.
Also here's a small mathematical bitty. 300 pixels / inch * 6 inch = 1800 pixels inches / inch = 1800 pixels. The inches / inches cancels out, just like 1/1 does. Isn't maths great! 
NB: this relies on your images already having the correct ratio. Read above or below for more, but dslr images are 3:2, same as a 6x4" print. 8x10"/5x7" prints require cropping, which is a manual task.
Should I send images to my lab at the right ratio or with the correct number of pixels?
i.e. for a 5x7" print should I send in a 1500x2100px file or will a 4000x5600 file be ok?
You should always sharpen after your final resize step, so the 1500x2100px file is best, plus it's smaller and will take less time to send in. In the end it'll make little difference though.
Noise reduction
Noise reduction is also detail reduction, so I don't use it. If you really must try to mask it so you only do noise reduction in the shadow areas.
Sharpening
I'm not an expert in this. I once sharpened a 20x30" print so it looked good on screen, but the print was horrible so I had to have it redone. I sharpened some 8x12" prints for a customer, they returned them because it emphasized flaws in the skin. I sometimes apply very mild sharpening, but often I don't sharpen at all - except what my RAW converter does by default.
JPG or TIFF for printing
The final step before sending an image to the lab is to save it as a JPEG. TIFF is unnecessary, JPEG Q12 is lossy but so close to lossless you'll never know it. I've made all my 30" and 40" prints from a JPG. Q12 isn't even necessary, Q10 is fine, but I use Q12 because I can. This thread shows what happens when you open, modify, and save a JPG a dozen times.
Recommend Labs
For large prints I've used Elco
, the quality's fantastic and the prices are great. I've heard mpix is good, they're a division of Millers pro lab. WHCC is also meant to be a good pro lab, as is Pro DPI. Really printing's not that hard, and it's cheap, so just give it a go. In the UK try Loxley Color if you need a pro lab.
Should I use RGB or CMYK?
CMYK is only used for commercial offset printers, not photo printing companies. Commercial printers can convert sRgb to CMYK for you, therefore you should always use CMYK unless your specific print firm tells you otherwise.
sRgb or Adobe RGB
Most print labs can't print outside the sRgb color space. If you're using a wide gamut printer and you have very vivid colors and and you completely understand color management then use Adobe RGB. Most people should use sRgb most of the time.
Conclusion & Recommendations
Overall I recommend people stop thinking and worrying so much and just print their images. Prints are cheap. Crop it and print it smaller if you want to. As with most things in photography the rule of "try it yourself and make up your own mind" is the best way to approach things.
Keywords (just in case anyone actually uses the search): large, big, enlarge, print, printing
