Image size/Resolution/PPI questions

[VanWanderlust]

Hi all - Joined the forum awhile back and follow it casually, finding myself at a bit of a loss trying to understand image size settings, I hope this is the right section to post this in. I have a Canon 60D and so far usually shoot in the JPEG large/fine format. I hope to start shooting in raw more, but still getting a grasp on that as well.

So to use a specific example, I have a photo(s) which in Photoshop shows 48in x 72in (72ppi). I think there's no doubt I need to make smaller images for online viewing and I think I can do this without much difficulty, but please correct me if I'm wrong. I feel I would go to Image -> Resize and adjust the 'Pixel Dimensions' settings to what I feel would be a good size. It's my understanding the PPI is irrelevant as it relates to online viewing and I don't need to worry about PPI in this case... is that correct?

Now if I want to print the same picture as a 4x6 is where I start to get confused. I want a high quality print of course and it sounds like 300ppi is the standard for quality prints. So it seems like I could change the 'Document Size' to 4x6 and change the PPI to 300 and that would be it? Secondly, do I really need to do this or do most printing companies adjust this automatically? I've had the photo printed out before as 4x6 and the quality seems fine.

I guess what I'm trying to know is would it be best to have three versions of the image? The original, one to upload for online viewing (re-sized by the 'Pixel Dimensions'), and one for printing (re-sized in the 'document dimensions' with ppi adjusted)? ..or is that over kill?

Lastly, if I'm putting multiple images onto say a 8x10, for a collage style image, I've just been scaling the images to the size I want. Does this affect the PPI in terms of printing? Would you recommend an actual re-size of the image instead of scaling it?

I hope this makes sense and isn't too many questions for one thread! Thanks so much for any advice you may have.

-John


P.S. I'm doing this for about 300 images which I will be trying to sell so quality is the most important, efficiency second.

 

[vandervalk]

Yikes. This is a lot to answer. This was taken from a website that I feel does a good job explaining indepth.

Understanding Resolution: DPI, PPI and File Size Explained


OK, so you’ve decided to give digital scrapbooking a try. You shoot some pictures on your lovely digital camera. You download the 30 day trial of Photoshop Elements from Adobe.com. You grab one of the great Two Peas kits- one that matches the pictures perfectly. Then…you open them up, and the confusion sets in.


“Why is the image from my digital camera only 72 dpi, but 20x30 inches?”


“Why does the picture I downloaded from the internet look so bad when I print it?”


“Why, when I drag my image onto my background, is it so tiny?”


Welcome to the wonderful world of digital!


Understanding the concept of “resolution” is probably the first big hurdle every paste eater who tried to push pixels has encountered. It’s a fairly simple concept, but until you understand it, you’re going to be confused. So, take my hand (because we’re all about the hand-holding during difficult times) and let me explain it to you in the simplest terms I can.


Inches Don’t Exist on a Monitor


The most common confusion seems to stem from trying to differentiate between inches and pixels. On a monitor, there is no such thing as an inch. Because each monitor can be changed to many different resolutions, an inch on one monitor isn’t equal to an inch on another one (try it yourself- change your screen display settings in your control panels and look at the desktop image or an open image file- when you change the screen resolution, the image will appear smaller or larger than it did before the change).


The way monitors measure things is by using a “pixel”. Basically, a pixel is the smallest element on a screen. It stands for “PIcture ELement” and is how all onscreen measurements are conducted.


DPI vs. PPI


So, how do you translate from the monitor world of pixels to the real world of inches, especially when you want to print something out at a specific size? That’s where the dpi/ppi thing comes into play. The term dpi originated in the old, pre-desktop publishing era of graphic design, when you’d convert a continuous tone image to a halftone (a series of dots). Although PPI (pixels per inch) is the more correct term for doing this digitally, most of the graphic designers I’ve worked with use the terms pretty interchangeably. If you want to be super-correct about it, ppi usually refers to onscreen stuff, and dpi usually refers to printers and scanners. But nobody is gonna kick you out of the Digi Club if you use dpi for ppi or vice versa.


Okay, But what the heck do they mean?


When you set the resolution of an image, you are telling your computer “I need you to place this many pixels into an inch”. For the screen, that is usually 72 ppi. Most computer monitors have a resolution of between 72 and 96 ppi. The industry standard kind of settled at 72 ppi for screen resolution early on in the development of the internet, so that’s the number you want to keep in your head. What this means is that for every inch of an image, there are 5,184 pixels of information. So, if you’ve got a 4”x6” photo scanned in at 72 ppi, it will be 288 pixels across by 432 pixels tall, for a total of 20,736 pixels of data.


Now, that may sound like a heck of a lot of pixels, but it’s not. If you’ve ever tried to print something out from the internet and been disappointed in the quality, it’s because screen resolution is less than half of the resolution a printer can handle.


Most home inkjet printers are capable of printing between 150 and 200 dpi. In order to take advantage of all the extra information a printer can handle, let’s take a look at that 4”x6” photo again.


For this photo to print well on a home device, it will need to be 4”x6” at 200 dpi (we’ll err on the side of caution- better to have too much resolution than not enough&#8230 So, that image would actually be 800 pixels wide by 1200 pixels tall, for a total of 960,000 total pixels (and you can see when you compare 20,736 vs. 960,000 total pixels just how much more detail the second image has!)


If you’re printing to a commercial offset printer, those can support an even higher resolution- up to 300 dpi! So that 4”x6” image we’ve been scanning would now need to be at 1200 x 1800 pixels, for a grand total of 2,160,000 pixels. Wow!


But What Does All that Math Geek Stuff Have to Do With ART?


Well, yeah, I decided I didn’t need math to be an artist either, then went and decided I liked computers. So, I had to figure out some math. It’s a pretty simple formula, and most programs do a pretty good job of doing the calculations for you, but I swear to you- once you grasp this concept, a whole new world will open up. You’ll know exactly how much you’ll have to enlarge an image when you scan it if you want to crop into it. You won’t freak out when you open a 25” x 17” 72 ppi image from your camera – you’ll know that if you change the resolution to 300 ppi, it will be 4”x6”. You’ll understand why, when you try to drag a 3”x2” 72 dpi image from the web onto your 300 dpi layout document that it will only be ¾” x ½”. It might take a bit of time, but eventually, I swear, you will get it, and it will make working with your images soooo much easier.


So, let’s recap…


The formula for figuring out how to convert is simple : (length of the photo in inches x base resolution) x (height of photo in inches x base resolution) = total pixels in image.


You want to use 150-300 dpi to print at home. Most kits are going to be at 300 ppi, so that’s probably the size you’ll want to work at on your original layouts. If you want to save that layout to show it off in the gallery, you’ll want to take it down to 72 ppi.


Each software app handles resolution differently, but the basic formula should help you out no matter what application you’re using. It’s a universal concept, and one you’ll be forced to deal with every single time you open up an image file on a computer. Plus, you’ll really impress your friends with your mad digital skillz (at least, that’s what I keep telling myself!)


So, come on! Grab that pocket protector (go ahead, you can alter it!) and calculator and geek out with the resolution math!


A final problem for you to practice on…


So, think you’ve got it down? Try solving this word problem and see if you really do…


If you have an image shot on your Digital Rebel, taken on a train that left the station last Tuesday, and when you open it in Photoshop, it tells you the image is 1578 inches tall by 972 inches wide at 72 ppi. How big is it (in inches) at 200 ppi? What about 300 ppi? (And where in the heck do you get a memory card big enough to hold that picture, lol?!?)


Above all, though- have fun with it. It’s not rocket science – it’s scrapbooking – even if there *is* math involved!

 

[VanWanderlust]

Thank you for the input Vandervalk. I've read through a few other websites with these sorts of explanations but I feel this one is the best I've read yet. There more I read and then play around in PS the more the dots are starting to connect in my head. I was afraid I was asking too many questions as there's a lot to learn here but it's slowly making more sense to me.

I think the main thing I'm trying to figure out now is if I should make 3 separate copies of each image, sized for the specific needs (original/online/print). It seems like a 'yes' should be the common sense answer, but I'm not positive and that's a lot of work if it's not necessary. I think it seems like I also need to call my local printing company and ask a few questions. They have a higher priced lab-processed print (where I'm assuming they make the necessary adjustments for a quality print) or a Ready-To-Order print where they print it just how they receive it from you at a cheaper price and of course I would like to work towards being able to utilize this RTO option.

Thanks again!

 

[Ysarex]

Hi all - Joined the forum awhile back and follow it casually, finding myself at a bit of a loss trying to understand image size settings, I hope this is the right section to post this in. I have a Canon 60D and so far usually shoot in the JPEG large/fine format. I hope to start shooting in raw more, but still getting a grasp on that as well.

So to use a specific example, I have a photo(s) which in Photoshop shows 48in x 72in (72ppi). I think there's no doubt I need to make smaller images for online viewing and I think I can do this without much difficulty, but please correct me if I'm wrong. I feel I would go to Image -> Resize and adjust the 'Pixel Dimensions' settings to what I feel would be a good size. It's my understanding the PPI is irrelevant as it relates to online viewing and I don't need to worry about PPI in this case... is that correct?

That is correct. If you're going to upload a photo to the internet you should simply pay attention to the actual pixel X pixel dimensions. If you're going to use a photo hosting site you should know what their maximum dimension limits are and abide by those.

Now if I want to print the same picture as a 4x6 is where I start to get confused. I want a high quality print of course and it sounds like 300ppi is the standard for quality prints. So it seems like I could change the 'Document Size' to 4x6 and change the PPI to 300 and that would be it? Secondly, do I really need to do this or do most printing companies adjust this automatically? I've had the photo printed out before as 4x6 and the quality seems fine.

The printing companies will adjust. 300 PPI is on the high end and you'll get acceptable prints from an image that is 200 PPI. What you don't want to do is resample up. In other words if you set the print dimensions to 4x6 and the PPI is less than 300 or even less than 200 you should at that point reconsider and find out why you don't have sufficient resolution. Resampling up to the necessary resolution won't create missing detail that isn't there. Resampling down is fine, but generally not necessary however sharpening comes into play here -- see below.

I guess what I'm trying to know is would it be best to have three versions of the image? The original, one to upload for online viewing (re-sized by the 'Pixel Dimensions'), and one for printing (re-sized in the 'document dimensions' with ppi adjusted)? ..or is that over kill?

That is not overkill. I save my camera originals untouched. Then I create the processed photo and save that at full resolution. If I want to put it online I create an online version at the pixel X pixel dimensions required. When the time comes to print the photo I create a print version from the processed original.

NOTE: Sharpening is a necessary step for digital photos. The degree and method of sharpening used is target relative. You sharpen differently and different amounts if the target is online versus print. I therefore never sharpen my processed photo which I archive behind the original. When it comes time to create a version for output I sharpen for the target.

Lastly, if I'm putting multiple images onto say a 8x10, for a collage style image, I've just been scaling the images to the size I want. Does this affect the PPI in terms of printing? Would you recommend an actual re-size of the image instead of scaling it?

Scaling the image is fine as long as the collage that is receiving the image is the correct PPI for the output target and you are scaling down and not up. The PPI for printing should be determined by the collage file that you're moving images into.

I hope this makes sense and isn't too many questions for one thread! Thanks so much for any advice you may have.

-John


P.S. I'm doing this for about 300 images which I will be trying to sell so quality is the most important, efficiency second.

 

[VanWanderlust]

Ysarex - thank you so much for the input and taking the time to answer each of my questions, it's greatly appreciated. It's really nice and much more comforting to get to hear what someone else actually does for their photos, makes it a little more tangible for me in my mind. Thank you for addressing the collage issue as well, that was one that I had no clue what to think about. It's all making more and more sense to me. Thanks again, cheers!