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For more information, please see full course syllabus of Adobe Photoshop CS6
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Lecture Comments (2)

1 answer

Last reply by: Professor Brown
Wed Sep 11, 2013 4:20 PM

Post by Said Elmahdy on September 11, 2013

Can you please explain for me again what is a pixel dimensions

Image Size, Canvas Size, & Resolution

  • Canvas size is adding extra space beyond the existing image borders, as with an artist’s canvas.
  • A rule of thumb for up-sizing images: go no more than about 15%. You are opening space in your image that the program must fill with pixels that it must create by analyzing the area around the gaps. The more you increase, the more your image will degrade. There is very little noticeable degradation up to around 15%.

Image Size, Canvas Size, & Resolution

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Lesson Overview 0:06
  • Image Size & Resolution 0:49
    • Pixel Dimensions & Document Size
  • Canvas Size 13:48
    • Canvas Size

Transcription: Image Size, Canvas Size, & Resolution

Hi everyone, Michael Brown back again.0000

Welcome back to's Adobe Photoshop CS6 course.0002

We've got your image out of your camera, into the computer, edited in Bridge, did a little correction in Camera Raw and now you're ready to go onto Photoshop, but before we start working on the image, in this lesson I'm going to talk to you about image size, increasing and decreasing your image (in other words enlarging or reducing the image) canvas size, which is actually adding area to an image, where you leave the image the same size and create more canvas to the outside.0007

And we're going to talk about resolution, and what it is, and how we deal with that in regards to print--in other words printing your images, or displaying them on a monitor, so let's get started.0038

Here's an image that I shot with my Nikon D7000 camera which is a 16 megapixel camera, that means the sensor has 16 million pixels, but it shoots a red, green and blue channel, which means the final size is three times the megapixels or 48 megabytes, with a tiny bit of trim that the camera's computer does.0052

And so if we go up to the Menu bar, go to the image, image size menu, you'll see at the very top of the image size dialog box, there's the file size of the image--it's 46 megabytes, just a little less than the 48 we talked about.0075

Now, the dialog box has pixel dimensions, so the actual image is just under 5000 pixels and just over 3000 pixels...49--28 wide, 32--64 high.0092

If you multiply those two together, you will get 16 million, but there are three channels so the file size is triple that.0107

The actual document size was determined by the resolution that the computer set the image at--it defaulted at 240 pixels per inch.0117

And the reason that is, and most camera manufacturers have adopted that now, is that when you print an image from a printing press at a lithographer, a professional printing press--not any jet printer, but a regular ink printer with plates...they print at between 150 and 200 dots per inch maximum.0128

If they go any more than that, the actual ink droplets will begin to clump and you'll lose detail.0157

So traditionally they're printing between 160 and 175 dots per inch for fine work, and the rule of thumb is they would like to have the digital file presented to them for maximum quality at between 1.5 and 2 times the dots per inch that they're printing at.0163

And if we're averaging it at about 160, one and a half times 160 is 240, so that's how they came up with this.0179

Now, that is the document size at that resolution equals a 46 megabyte file.0188

Over here we have three check boxes; one to resample an image, which is either enlarging or reducing an image, adding pixels or removing pixels, constraining proportions...if you have it checked to constrain, it will keep this aspect ratio constant--you can actually stretch or squish an image by unchecking this and changing some of these dimensions.0194

Scale styles...if you're adding drop shadows or other layer effects under the Layer menu, layer styles.0218

If you don't flatten your image (all the layers) and apply those effects directly to the pixels, if you enlarge the image, the scales will not scale up--they stay the same, so in other words you've got a drop shadow if you enlarged an image that would stay small, that wouldn't match up with the type.0226

So, it's necessary to check this if you want the styles to expand properly.0247

Alright, and down here we have the dropdown box for interpolation...smoother for enlargement, sharper for reduction by cubic automatic, new in Photoshop CS6, it automatically determines--remember we set that under our Preferences, so there we have everything that's in here.0253

The auto button applies to an auto set for the dot...the maximum efficient dot per inch for a printer--I've never used that box, I just discovered that and learned it today, you don't even need to bother with this.0255

Alright, so there's that.0294

Let's say I wanted to make an 11x14 photo print of this image, but I don't want to change the file size because I want to maintain the same quality.0296

I can do that on an ink jet printer with a larger resolution, so I uncheck all three of these boxes (notice the pixel dimensions are not there) if I change my width down to 14, notice that the resolution went up to 352 because the number of pixels in the image are exactly the same--the file size is still 46 megabytes.0309

So, a smaller size meant that it had to have more pixels per inch--the quality level of your image is unchanged whatsoever, and this will print no problem on an ink jet printer, but if you wanted to do this for the lithographer, they don't want it this large; they would prefer it to be 240, so now you check the boxes again, resample, constraint to proportions, scale the styles--we've got the size now at 14, now we change the resolution to 240 by cubic automatic, no problem, it'll do it.0335

Notice up here, it was 46 megabytes, now it's going to be 21 megabytes, knocked down because we reduced the number of pixels in the overall image.0375

If I click OK (watch the image) it now will drop its size, and if we go to the image size again, notice 21.4, 14, 9.275--the same size, document size relationship at 14 which will accommodate an 11x14 print at 240 DPI.0388

That's how you reduce one, so we're going to cancel that for a moment, and go back to the larger size.0412

Now...if we enlarge an image...let's say I'm going to use another image to show you this one, I'll do two things.0419

This image was shot on a 4x5 camera, notice that it's a little higher in its aspect ratio than the previous image.0429

Image size again, now we have a 57 megabyte file that was a scanned transparency at 305 DPI, if we just knock it down to the 240, which is our normal for the printer, a file that's 21x16, and that's roughly 4x5--4 times 4 is 16, it's just about a 4x5 image, but I really want to kick out a 30x20, so I don't want to change anything, we're going to come back and open it up at its original...get the maximum number of pixels in there, and I will knock it down to 240, that's no problem.0442

But what we're going to do, is we're not going to constrain the proportions this time...I want to make a 20x30 image--I'm going to stretch it slightly--I figure I can do that and it won't hurt it, so now the height (I've already set the resolution at 240, it's there) 20 on the height, 30 on the width, and if you notice it's going to up that file size from 57 megabytes to almost a hundred, because we are now stretching it in addition to increasing it we're changing the aspect ratio--we'll click OK, and it's going to cook for a moment, and now we have...I want to go undo it, see how it's a little bit squished in and up, and this way it squished it out.0489

It still looks good because it didn't hurt that to stretch--there wasn't anything that got distorted tremendously, image size is now 240 DPI at 20x30 inches--it's now a hundred megabyte file, much larger, but it's ready for the printer.0536

So that shows you how you deal with--you can actually alter the proportions, or just scale them up, that's how you deal with image size.0555

Now, one other thing I want to talk about (let's cancel and go back to this other image...make sure we're back at the original) if you're not dealing with a printer, the last two examples were dealing with preparing the image to print, if you're working on an ink jet printer, don't worry about your pixels per inch at all, you can leave your file size the same.0566

If you wanted to print at 11x14 here, you don't need to resample it, very simple make it 14 inches, the 352 resolution doesn't matter on your ink jet printers, so it's very simple--you didn't have to change the file size at all, just change the document size, no problem.0592

OK, let's go back to the original on that again, and leave it this way.0611

Now, if you're preparing an image to view on a monitor...monitors today have variable resolutions, so you don't necessarily know what it's going to be viewed on--if you're on the internet, it could be anywhere from an older monitor that has 72 dots per inch, up to the modern ones like this is a new MacBook Pro, that could be as high as 1680 by 1050, as low as 1024 by 640, who knows what it's going to be, but the best thing to do is set your image up for the worst case scenario.0616

Now, most websites are approximately 900 to 1000 pixels wide, so if I'm sizing this image for the web, I'll go to my image size menu, and I will pick the worst case scenario--now remember we're going to resample and we're going to do reductions so by cubic sharper, I'm going to automatically go 72 DPI is where I'm going to start, and since I know that this is just an average, most websites are somewhere around 8 or 900 pixels wide, I'm going to set this at between 8 or 900--usually 850 is what I go for.0655

Now you can always contact, if you're doing a display of art on a specific website, and ask them the horizontal or vertical width that they want, and you set that up at 72 DPI, or if they have a different one, they'll still go for 72 because it's on a website, that's the way we're going to automatically do it.0685

Now it's 850 pixels wide, the proportions are no problem--we didn't deal with the document size, we just changed pixel dimension so becomes 563, drops it from 46 megabytes all the way down to 1.37, we click OK, and watch what happens.0716

I'm going to zoom this up...there's 100% magnification, and you notice it views great on the screen, it still looks beautiful, but it's at 72 DPI now, and this is all that it's going to be used for, so you have no problems with that at all, so that's how you size your images.0735

The difference for print at about 240 DPI, for the web, 72 DPI and you can contact the particular website that's going to display it or you can just default at about 800 and some odd pixels...there we go.0756

Alright, now we're going to talk about canvas size...and I'm going to go back to the original size for this thing, that should be...240, 20x13.6--we're going to sample this down to 240 at 14 inches wide for that 11x14 again...we've got it.0774

Now, I want to print this at that size at 240, I also want to put it inside a...16x20 frame, and that means that my image is going to be 14 inches wide, but I need extra space--either they're going to mount it or I give them a larger print, and I'll want to put white around the outside (this is just for a print, there's other things you can do with this).0795

So we go to image, canvas size, and this allows us to add more space around an image.0829

In this case, we're going to be putting it on a...I'll tell you what we're going to, we're going to go with the 14, we'll just leave it here.0838

This is an anchor box...the little dot can be positioned side to side, corner, anywhere--when it's in the center, if you only increase the width, it will equally go half of that distance to the right, and half to the left.0852

If you increase the height, it will go up and down equally.0869

Let's just say, for example, we wanted to stretch this image vertically--I could resize it, or I could do it another way.0874

I could say--let's say I want to make this 11 inches high, and I want to add space at the top, so if I put the anchor at the bottom, notice the arrows indicate I can go right or left, and I can only go up, so now if I make the height 11 inches, and I pick my extension color at--let's just go with white, it's fine, and I click OK--I did not change the width, it automatically made that (just check that image size) notice it's 11x14 now, and I added the space only on the top.0884

Now, if I want to do a 16x20 document size with this image the same size, I'll go to image, canvas size, and now we're going to leave the anchor now in the center.0924

We're going to make the width 20 inches, we're going to make the height 16 inches because it's going to go in a 16x20 frame...and we click OK--remember it's going to go equal right or left, top or bottom.0941

And now you have basically your picture mounted on a white background, but I want to show you the layers...0957

It puts it on the background layer...on the lower most layer, so that if I have this I'm going to just take the background layer for a moment, and I'm going to make it all white, just a second...0965

I just want to demonstrate that if I had made that size, and I keep this image separate, I can now move the layer independently (so both will be able to move...) I can adjust that more for a normal print methodology equal around here, a little more space at the bottom.0988

Now we could have done that another way, let me demonstrate...image size down, we're now at...OK.1011

What we're going to do, is we're going to canvas size...we're going to add 20, that's 3 inches on each side--14, to make this 20 wide is 3 inches on each side.1021

So we're going to go 3 inches also at top and bottom--we're going to make it 20, and the height, if you add 6, is now 15.271, so I'll do that, and now we're adding 3 inches all the way around equally...1035

But it's not quite right because it's supposed to be 16x20, so what I'm going to do is put the anchor up at the top, and now add that little bit more space on the bottom, and effectively I did the same thing.1055

We've got the traditional print mounting methodology with a little extra at the bottom, and you could go in here with a text's going to initialize this, and...grind for a moment--you could put your signature, or whatever is necessary at the bottom--there we go.1078

You could put photo by...and I will very quickly change that to like 20 point, change the color to red, and you can see what you can do down here--you can add a signature underneath that.1102

So that's what image canvas size does--it enables you to add space around your image for various purposes--let me go back and do one more for you, I know I've taken a lot of time with this, and I'm going to take (and make sure there's two layers) I'm going to do it on the bottom--image, canvas size--I'm going to add a little bit on the bottom.1124

So I'll put the anchor at the top, notice it'll add to the bottom, the height's 13.6, we're going to make it let's just say 15, and that adds it on the bottom.1152

Now I can take the upper layer, select the layer, go to edit, transform, and I can...oops, got the wrong layer, excuse the layer, edit, transform, and I could stretch it down, and now we have a stretched image (let's go back to that) try that one more time.1164

I cancel it, edit, transform, pull it, enter it, there we there's several ways to use the canvas size feature to add space around and different methodologies.1193

Remember, the anchor points allow you to control where it's going to go, you can also go relative--in other words if I wanted equal amount of space around this--I just wanted 2 inches to the height and 2 inches to the width, I click OK and it comes in equally, just adds that relative.1207

So you have all those options, and the image size, remember, you can change your document size independent without changing the file size, or you can resample to either enlarge, or reduce your images.1227

So there you have image size, canvas size, and resolution.1247

I'll see you in the next lesson.1252