dpi, ppi and printing... confused!

ShoeQueen

ShoeQueen

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Lynn
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Firstly - what is the difference between dpi and ppi?

When I export images from LR, its default is 240 ppi - should I change this?

If I export a full hi res file as jpeg why is it still too small for larger prints with online printers (in this case One Vision/Photobox as I have a Zenfolio site) and how can I change it if I wanted to order larger prints? Do I need to resize it 'up' and how do I do that?

Thanks in advance for help in what are probably very simple questions, I'm just a bit confused...
 
DPI refers primarily to printer resolution. ( Dots per inch ) PPI refers to image resolution ( Pixels per inch ) These two terms are often used as the same but are different.

It could be that you have the export module in Lightroom set to a lower resolution. By default, with no constraints Lightroom will output at the native resolution of the file As far as I know it does not apply a ppi resolution figure. If you open it in Photoshop the image size quoted by Photoshop uses a default setting of 240 dpi to give an image size. Remember that despite it's name Photoshop was designed for the graphics market and still has this markets requirements for dpi built into it.

What size prints are you trying to produce for these online printers? Don't forget that you can tell Lightroom what size file you need from within the export module . If you do I would opt for a 300dpi output resolution rather than 240 as this is more in line with what many commercial printers are expecting.

If on the other hand you are using the print module to output a finished image, then I's select the ppi to 300 withing the export to JPEG option. However some site may require a different ppi/dpi figure. They should state this on their website though
 
It just the export option in Lightroom - I work on the RAW file then export as a jpeg. It gives me an option to resize which I use for web display - long edge 800px usually. But I'm starting to export for print so I need to get my head round best sizes to use.

It has an option for ppi - I can't see an one for dpi anywhere?

If I export as a full size jpeg I can print up to 20" by 16". I'll maybe never need to print larger than this but I'm wondering how I could if I did want to? Can I simply resize to bigger on export?
 
You are exporting an image, therefor you need to specify the number of Pixels per Inch you need for the resolution of the image as this defines the file size. It may end up being printed but you are outputing an image and ppi is what you need.

It's only when you are actually printing the image direct to a printer do you need to worry about dpi as the destination is a printer.

As far as final size is concerned, don't get too hung up with converting ppi to dpi. It's not like that. I have images from a 6 Mp Canon 300D that have been printed to 20" x 30" and they are seriously sharp. In theory I should be lucky getting a 12 x 18 out of that file but it goes much larger than that. A lot of it is down to the printer software, and the way you handle the image. Don't overprocess and be careful with sharpening. Over sharpening can really ruin an image
 
Real world examples.

My D70 makes an image of 3000 x 2000 (give or take a few) pixels. If I want to make a print of 12"x8" then the image needs to be printed at 3000(pixels)/12(inches) = 250 PPI.
This is a printer driver setting, nothing to do with the image file.

If I send it to an online printer who use one of the 300 DPI commonly available commercial printers, no matter what PPI setting I save with my image file they will print it at 300 DPI. This will give picture of 3000(pixels)/300(dots per inch) or 10" on the long side.

If I want a 6"x4" from this 300DPI printer, I need to resize my original image. The long side need to be 6"x300(printer dpi) = 1800 pixels. If I resize my original image to 1800 pixels on the long side and keep the aspect ratio the same, it will be printed at the desired size.
 
Thanks - so far I've only printed with DSCL and I've just cropped to the right ratio and gone from there - ie I've not resized the image on export to match the print size. I want to be able to put images on a website and allow people to select from different sizes of prints - can I do that? What size should I be uploading to give them the options?
 
I would consider what size you want them to appear on the screen, then size the images to fit. You don't want the image file to be too big as people could copy the file and have it printed themselves. You may need to experiment, but try starting with the longest dimensions at 600 or so pixels. See how this looks , and adjust it accordingly.

Once you have the customers order you can send your full res file off for printing. The file on your web page is only a for selection not printing
 
Thanks - I usually resize to 800 for web. I'll look at what sizes I need to have available if I'm offering prints (which wouldn't be on every gallery.)
 
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