Let's get right into this, shall we? We'll start off with a quick discussion about where the resolution value comes from for those who aren't sure, then a segment on what you should keep in mind when shooting photos for print, ending off with suggestions in regard to what resolution could be used for particular print sizes.
So how does Photoshop (and most other image editing software) come up with the resolution value for your image? It's actually quite simple; divide the pixel width by the document's width (or use the height values, the result is the same). So if your photo is 4032 pixels wide and the document width is set to 12.841, then you get: 4032 / 12.841 = 313.99423720894011369831010045947 to be exact, but you can't realistically have .994... of a pixel, thus the program rounds to 314 (these values are what you see in the episode). By the way, pixels per inch and dots per inch are generally used interchangeably but there is a difference. Digital things, like scanners, graphics you create with your computer, and digital camera photos use pixels per inch (ppi), whereas physical things, like your laser or inkjet printer and professional printing machines use dots per inch (dpi). For more info, Google "ppi vs dpi" and you'll find articles discussing this topic in more detail (or if you're too lazy to type then scroll down to the Web Resources section).
Shooting with Printing in Mind
If you're anything like me, then there's a certain pleasure to be had seeing your photographs on paper. But as I'm sure you've already noticed, not all sizes of print are created equal in terms of aspect ratio. Let's take a look at some:
4x6" - 1:1.5 or commonly said to be 2:3
5x7" - 1:1.4
8x10" - 1:1.25
8x12" - 1:1.5 (or 2:3)
11x14" - 1:1.27
16x20" - 1:1.25
18x24" - 1:1.33 (or 3:4)
24x36" - 1:1.5 (or 2:3)
Also good to know are the aspect ratios of the sensors found in digital cameras:
Full-frame (or 135 film, aka 35mm, the sensor measures 36w x 24h mm) - 1:1.5 (or 2:3)
APS-C (slight variations in size exist, click here for more info; aspect ratio appears to be the same for each though) - 1:1.5 (or 2:3)
Four Thirds (17.3w x 13h mm) - 1:1.33 (or 3:4)
Aside from a few print sizes that have the same aspect ratio, most differ and is why you can almost never just resize your photo using the image size dialog box (otherwise your image gets stretched or squashed). Instead, this is where you employ the crop tool and selectively adjust the size of the photo. By doing so, you're very likely going to be excluding parts of your image, so this is where the shooting with printing in mind bit comes into play.
If you compose or frame your shot tightly, meaning you are not leaving much space or area around the subject in your scene, then you may run into problems when cropping. Either you'll have little choice but to cut into your subject or the crop will leave little to no breathing space around the subject. You're probably already thinking that the solution is to shoot in such a way as to leave little more room around the elements you want. You'd be completely wrong. ... Actually, I'm kidding, that's exactly what you'd do. Not much to it at all. Over time you'll become accustomed to how much space is enough, and if you don't already, consider taking a couple of additional shots with the composition adjusted (e.g. zoomed in or out some more) just to be on the safe side.
Another great reason to leave extra room has to do with framing (or matting) your print. I must thank a member on my Binary Graphite page on Facebook for mentioning this; you know who you are, thank you sir! Based on the frames I've purchased (i.e. those where the print sits behind the wooden, plastic, or metal frame), you usually lose about 1/8" from each side, or even more from cheaper ones. By taking this into consideration, you'll hopefully only have to print once, then be able to sit back and enjoy your work.
Resolution Suggestions for Print Sizes
Ok, so you've decided on the size of print you want and you have an idea of how you'd like to crop your photo, but what resolution should you use? In my video, I briefly suggested leaving the resolution field blank, which ensured that the original pixel data in the image would be retained; in other words, no resampling would occur, thus no pixels would be created or removed due to enlarging or shrinking the photo, respectively.
Well here's what I use as a starting point. Open up an unaltered shot from your camera and check out its image size. Using the picture in my video as an example, the resolution is 314ppi. This value may be different for you and is generally set by the manufacturer of the camera. At 314ppi, this photo, which has pixel dimensions of 4032x3024, has a document or printed size of 12.841x9.631". These values will be useful to determine the next steps.
My personal rule of thumb is that if I crop my photos, the resolution should remain greater than 200ppi and if it drops below this value then I will resample the image to 200ppi. In other words, I try to avoid resampling the image unless I have to. After thinking about this a little further, three major scenarios that have occurred in my experience:
1. Cropping to a smaller print size then the original document size of the image: In this case, the resolution of the image will increase, thus there is no need for resampling. For the crop tool options at the top of the screen, I only enter a width and height value, but ensure the resolution field is blank. For example, if I crop to a 4x6" (assuming I use the whole width of the example photo), the resolution increases to 672ppi; that's great.
2. Cropping to a larger print size close to the original document size of the image: Let's say I crop my photo to 14x11". After trying this in Photoshop, the resolution ended up at 275ppi. Since this value is greater than my threshold of 200ppi, I leave it be as this should still print fine on most printers and is at a level of quality I can accept personally.
3. Cropping to a larger print size much greater than the original document size of the image: Perhaps I want to go big with my photo, so I crop it to 36x24"; a three by two foot poster. If I leave the resolution field blank, thus no resampling, I end up with a meager 112ppi. Unfortunately, printing it like this would result in a blurry looking image, even if viewed from several feet away. Therefore, when cropping I don't just enter the width and height values, I also enter 200ppi for the resolution. This will resample my image and I end up with my picture having a 7200x4800 pixel size. Viewing this image at 100%, I notice it's a tad softer than the original, but even if those new pixels had to interpolated, the printed result will still be acceptable to me. I haven't printed many 36x24" photos, but the ones I did have turned out quite well. Looking at them from only a foot away reveals the lack of image data, but to most non-techie-photogs standing about 5 feet away, the image looks fine.
Now I'd like to point out that there are other methods and preferences photographers have in regard to this. For example, maybe one person likes to keep his/her file sizes down, so s/he will enter a resolution value when cropping to smaller print sizes. Others might not mind if their image is resampled and ends up being twice the original size, even if the image looses a little sharpness. And certain photographers might not even print images larger than a specific print size, because to them it is doing injustice to their work. As with most things I suggest, feel free to try it out and use it if it works for you, or come up with something that you find adheres to your needs better.
Next episode will be on macro photography; I'm sure the two people who have requested it are eager to see that show. Lots of other great episodes lined up too, and by all means feel free to submit video requests on a topic you'd like to learn more about. If you haven't already, please "Like" me on Facebook, follow me on Twitter, and subscribe on my YouTube channel. Thank you, L8r!
Google Search for PPI vs. DPI