Photoshop Fundamentals: Changing an Image’s Resolution and Size
Ivan | Mon, 2009-11-09 11:37
Adapted from Photoshop CS4 After the Shoot (Wiley Publishing)
By Mark Fitzgerald
Version: Adobe Photoshop CS4
I’ve met many photographers who don’t understand how to change an image’s size or resolution correctly. The problem usually stems from a lack of understanding about image resolution and how it affects image size.
Caution: Always save a master file with all layers before changing size or cropping. Otherwise it may be hard to back up and resize the file for a different output use later.
Understanding Resolution
One of the things that can be the hardest to get your head around when you start down the digital path is resolution. This confusion is compounded by the fact that there are two different kinds of resolution in the digital world. One is dots per inch, and the other is pixels per inch.
- Dots per inch (dpi). This refers to the number of dots per inch that an inkjet printer is capable of applying to a sheet of paper. It can range from 720 to 2800 and more. The closer these dots are to each other, the more they blend together forming continuous tones on the print. Naturally, this depends on the paper that’s being printed on. If the paper is porous watercolor paper, the dots soak in and blend just fine at lower dpi settings like 720. On glossy photo papers, a higher setting—such as 1440—is needed because the ink dries on the surface. About the only time people discuss dpi is when they are talking about a printer.
- Pixels per inch (ppi). This is what is usually discussed when talking about resolution in digital photography. It refers to the distance between the pixels that make up digital images. Pixels per inch is an important setting because it determines what digital images look like when they’re displayed and printed. Sometimes a lower value is desirable, and other times a higher value is preferred.
If you have a file with a resolution of 120 ppi or lower, you run the risk of seeing the space between the pixels when you print. This causes edge detail, which should be smooth in the print, to look jagged. The goal is to get the pixels close enough together so that these single dots form continuous tones and lines. The illustration below gives you an idea of how this works. As the dots get closer to one another, they begin to form a line. When you zoom out, the individual dots disappear. (It’s similar to the idea of getting ink dots close together on a printer (dpi).
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C'mon people!
When are you people gonna' catch on the MS Paint train?
Only one resolution is necessary! 72 DPI!
That's what your screen is! SHEESH!
http://stevefakeballmer.wordpress.com/
I am not Steve Ballmer pretending not to be me!
I've got loads of old images from a 3mp camera, what's the maximum print size I can get out of these?
I know that if I print it at 300ppi I will get an image that measures 173mm x 130mm.
My printer is an Epson R1800.
Is there a minimum ppi I can go to before the image starts to degrade or should I be using Photoshop's resampling (Bicubic Smoother for enlargements) to get a larger print & if so how far can I push that?
The easiest way to decide what is the proper resolution is to simply run a couple tests. In general (this is NOT A RULE - there are NO RULES) - color printers such as the Epson produce beautiful pieces at 100ppi. If the printer uses a dithering technique to print - you might be able to get away with 72 dpi (or maybe even less depending on your quality demands). So take a section of a photo with lots of detail, create four or five versions of it... one at 300ppi, one at 200ppi, 100ppi, 72ppi, 60ppi... and print them all on one page. Then use whatever resolution works best for you.
Generally when you size up an image in photoshop - you degrade the image because photoshop is simply creating more pixels where none previously existed (it "samples" a pixel color and creates more pixels the same color around the original pixel to make the image larger). The problem is basically the image gets larger, but you're trading off sharpness for more pixels. So generally you don't ever want to resample to create a LARGER image.
Ivan... Ivan... Ivan. My compadre. Good lord. Do not read that. Do not listen to that guy. Just forget you ever saw that "info". He's confusing the issue and passing off bad information. Serious accusation - I know - but I can back it up. He's WRONG on many levels above. Please allow me to explain the two basic ideas without any side issues mucking things up because this IS an important issue and I've seen even the better informed members on the board here interchanging the two ideas...
PPI - "pixels per inch". The word "pixel" comes from "picture element". A pixel is one unit of many that composes a photo on screen (open a psd file and zoom in until you see a grid of squares - those are the actual pixels). So 72ppi means the picture is composed of 72 pixels squared (72 pixels horizontally and 72 pixels vertically).
DPI - "dots per inch" - is a totally different idea. "Dots per inch" generally refers to two things... an imagesetter's resolution (the resolution capabilities of your laserprinter, for example) and the resolution of an actual printed piece itself (the printed item that comes out of your laserprinter).
The two ideas are the very basic building blocks behind computer graphics. They ARE interrelated, but NOT interchangeable. Do NOT attach any relevance to the above author's (mis)information concerning resolution, etc... I can explain that too, if you're interested. Again - he is wrong on that issue too.
Thank you for clarifying that! I agree with you.
Always glad to help - this is an important issue that everyone should understand. It doesn't help if the "teacher" is confusing the two ideas in his own explanation.
Btw, I think only the last paragraph is written in a confusing way, the definitions are ok.
Not really, Ivan - the definitions are problematic (at the very minimum) and the bottom paragraph - especially the part concerning the graphic - are just plain wrong. While there IS some good information on the linked site about resampling files - that looks like it was taken directly from the Photoshop manual to me. Reading this person's definitions and his explanation of what occurs at the pixel/dot level - I have no reason to believe he actually understands output beyond a rudimentary level. I would give you a full explanation (I actually started to write it a couple times), but the truth is it would be easier to simply start from scratch and explain it the right way than fix his misconceptions and just confuse things more.
I totally agree with Art D. Rector.
But for enlarging a picture there are some tricks.
Photoshop is a calculating machine but calculating complex things like adding pixels is not Photoshpop's strongest point, although they are working on the algorythms.
If you want a picture to be larger you can use this trick.
Have a picture open
Go to > menu "Image" > "image size"
Uncheck the option "resample", and change the PPI so that they fit your final output.
Click OK and return to the same window 'Image size'
Now we will add pixels
In The 'image size window' Check the resample option. Because we wan't to add extra info.
Change the measurements in the 'document size options' to percentage.
And fill in 110%. So we are going to add a minimal percentage of pixels to the image we have. At the bottom of the windiw you see the interpolation, it is set to bicubic for gradients. Change it to smoother (best for enlargement). Click OK.
This way you are just adding a minor amount of pixels, but you create some new information. Starting from there you can repeat this feature several times to increase the centimeters or picas or whatever measurement you choose.
By repeateing it you can get some pretty good results to enlarge pictures.
By the way you can create an action for this so that it is easy to repeat.
But be aware.
You can't use this feature if you have picture of 12 Kb and you wan't to make a billboard of that picture. It only works if you have decent amount of information in your document, megabytes...
Speaking out of experience. I got a picture that was about 25 MB, jpeg. I wanted to make a mural, 2 meters by 1, 20 meters.
With the above mentioned way of working, I was able to enlarge the image to these measurements, I had to to use some slick sharpening tricks, But it really looks great in my living room.
As a reminder, first put your PPI's the right way for your final output, afterwards try my way to make your picture bigger. Don't do all in one passage, first organize your pixels, then use the 110% trick
By the way adobe is working on new algorythms to make this kind of workarounds not necessarly. maybe in CS5 or in a later version.
Have fun
Agree. I like to say there are no rules because every "rule" has an asterisk attached to it. Almost any file can be enlarged to a certain degree without a noticeable degradation of quality (if done correctly, of course). There are a lot of factors to consider - but the starting point should be making sure you know what resolution is required for whatever output model your using so you don't waste information that could be used to enlarge the physical size WITHOUT altering the file size. Know what I'm saying? If you have to create info to get a job done - sure - work some magic. But don't do it unless you have to, because the original file will always have the most useful information when it comes to quality reproduction.
Thanks for that, I'll try it out.
Sir, I have a problem of changing the resolution of of a project in adobe photoshop CS4. Initially I make a poster with resolution of 72 pixel/inch. But my printing requirement is 150 pixel/inch. So how can I change the resltion to 150 now. I have photoshop file with all the layers.