size and resolution

[vickatter]

Hi!

First Question:
I'm a little mixed up with the way I can increase a file size.
I had to do a few posters yesterday and I needed a final file of 70x100cm in 200dpi. The thing is that the pictures that I was using were a little smaller than that, so somebody at the studio told me that I had to take the image to 600dpi (add pixels) and then down size it. I was also told that It wasn't the same to add pixels at the beginning or at the end. So that had to be step 1.
They also told me that I had to increase like this: 300/600/1200, etc... is this true? Can somebody explain it to me?

Second Question:
If i'm making a collage, my canvas is 300dpi.... everything that I paste into it it's gonna take that resolution? So if I need a bigger object I should increase that file size first and then paste it?

Thanks for the help!

[T Paul]

Re: size and resolution

First Question:

I'm a little mixed up with the way I can increase a file size.

One tidbit I’ve heard is to shoot in RAW since these are uncompressed files and better suited to producing large prints than a standard jpeg.

How to figure size: Take your chosen size and multiply it by the dpi you need to use to get the absolute size of the image you require in pixels. For example an 8” x 10” print at 150 dpi requires a 1200 x 1500 pixel image. So, an image that is 1200 pixels wide and 1500 pixels high contains (multiply 1200 x 1500) 1,800,000 pixels (or 1.8 Megapixels).

Now, let's say we want to print a 27.5” x 39” picture at 200 dpi. What resolution must you have to do this? Ok, 200 times 27.5 is 5500 and 200 times 39 is 7800. So you would need a 5500 x 7800 image to do this. Let's see, 5500 x 7800 is 43 Megapixels!

One trick to creating larger images is to shoot at a higher resolution than you need then downsize to the required resolution. Your friend mentioned this. For instance, if you shoot at 600dpi and then downsize to 300dpi you will double your image’s output size. So a 4”x 6” image at 600 dpi would be an 8” x 12” at 300 dpi. Luckily, many modern printers can scale a picture quite well. Also check your digital camera's user guide to see the actual pixel sizes for the camera's various resolution modes.

From Adobe Press

Resampling
One of the most important issues in working with images—and, unfortunately, one which few people seem to understand—is how the resolution can change relative to (or independently of) the size of your image.

There are two ways that you can change resolution: scaling and resampling. Scaling doesn't change the number of pixels, just the resolution. Resampling changes the pixel dimensions.

In Photoshop, you have a choice whether to scale or resample. If you scale that image down without changing the resolution, Photoshop has to throw away a bunch of pixels; that's called downsampling. If you double the size to four by four inches by upsampling, the program has to add more pixels by interpolating between the other pixels in the image.

Upsampling vs. Downsampling
We often upsample digital camera captures by 200 percent, sometimes even more. Upsampling still doesn't add details that weren't there in the capture, but sometimes it does an uncannily good impersonation! Nevertheless, a 4.1-megapixel capture rarely makes a good magazine cover!

Downsampling is much less problematic, because it's just throwing away data in a more or less intelligent manner. In fact, it's a common and necessary practice: We often scan at a higher resolution than is strictly necessary, to allow for cropping and for unanticipated changes in output size or method. We downsample to the required resolution before printing to save time and storage space.

Solution: Adjusting resolution

You can use the Image -> Image Size dialog in PhotoShop to set the size and resolution you require.

Many times you may need to increase the size up your image (upsample) to create larger prints. In general a high resolution digital image can be upsized a fair amount without degrading the image. You will have to experiment by printing them out so see how far you can enlarge them before you notice any pixilation.

Your friend mentioned increasing in steps. There are mixed options about increasing or decreasing an image in steps, especially if you have to increase or decrease the size of the image significantly. For example say you want to increase the size of an image more than 2x. Some say this is better to perform this in a number of smaller steps (stair-stepping method) with some sharpening applied in between rather than in one large single step. However, others claim there really isn’t much difference in quality performing this in one step or several. Try both ways and see which results you like. Also, PhotoShop CS2’s bicubic smoother and bicubic sharper (image size resampling options) seem to eliminate the need for the stair-stepping method.

From Adobe Press

Changing sizes. Like we said, the Image Size dialog box takes some getting used to. One confusing element is that whenever you make a change to one field, some other fields change and others don't. Here's a quick summary of what to watch for.

When Resample Image is turned on and you change the Pixel Dimensions, the Document Size changes, but the Resolution does not. If you change the Document Size, the Pixel Dimensions change, too, and the Resolution remains unchanged. If you change the Resolution, the Pixel Dimensions change, and the Document Size stays the same. An often overlooked subtlety is that Image Size allows you to change the Document Size and the Resolution in a single operation (which then changes the pixel dimensions). The key is that Document Size and Resolution are independent of one another when Resample Image is turned on. Just change one, then the other.

When Resample Image is turned off, the Pixel Dimensions never change, and changing either Document Size or Resolution always affects the other one

Additional Links:

Understand How Color Works in Photoshop

Rule-Breaking Resizing for Poster-Sized Prints

Photoshop Tips: Sizing Photos For Printing

Second Question:

If i'm making a collage, my canvas is 300dpi.... everything that I paste into it it's gonna take that resolution? So if I need a bigger object I should increase that file size first and then paste it?

Resizing problems when working with images at different resolutions
This one usually fools a lot of people, because it seems that your documents are the same size. But when you place a 72 ppi photo onto a 300 ppi document, the photo will be really small. This is due to the different resolutions. To get more predictable results, make sure your files are at the same resolutions.

Hope this helps!

~T

[vickatter]

Re: size and resolution

Hi T,
Thanks for your help, i'll have to read this a few times but I'll get it eventually.

I would like to ask you one more thing, something that came up this week. I'm designing a catalog (20x60cm, cmyk, 300dpi, psd) and the pics I have to use look perfect (20x30, rgb, 300dpi, tiff). The only thing I'm doing is Ctrl+T to size them down to 9cm high and the difference it's incredible, I'm loosing quality and I don't get why.
So, what I do now is size them down in the original file (use the step thing with the pixels until I'm around 9cm high) and then paste them into the catalog so I don't have to Ctrl+T them. Is there an explanation to this? It's driving me crazy...

Thanks again! you guys are awesome.
Victoria.

[pixelzombie]

Re: size and resolution

that is only a 45% reduction, are you using Bicubic Sharper in the "image size" dialog box?

[Alison]

Re: size and resolution

Hi T,
Thanks for your help, i'll have to read this a few times but I'll get it eventually.

I would like to ask you one more thing, something that came up this week. I'm designing a catalog (20x60cm, cmyk, 300dpi, psd) and the pics I have to use look perfect (20x30, rgb, 300dpi, tiff). The only thing I'm doing is Ctrl+T to size them down to 9cm high and the difference it's incredible, I'm loosing quality and I don't get why.
So, what I do now is size them down in the original file (use the step thing with the pixels until I'm around 9cm high) and then paste them into the catalog so I don't have to Ctrl+T them. Is there an explanation to this? It's driving me crazy...

Thanks again! you guys are awesome.
Victoria.

Hi victoria,

You could try making the image a smart object before you use Ctrl T, I would do it once they were inside your main image though.

You could also try going to the Image>Image Size dialogue box and type in the size you would like, leaving the constrain proportions box checked.

[vickatter]

Re: size and resolution

no, I'm not using bicubic sharper.... it's always set on bicubic and I don't change it because I really don't know how to use it. Should I change it or not?

[pixelzombie]

Re: size and resolution

bicubic sharper is best for reductions in size...

[T Paul]

Re: size and resolution

Hi T,
Thanks for your help, i'll have to read this a few times but I'll get it eventually.

I would like to ask you one more thing, something that came up this week. I'm designing a catalog (20x60cm, cmyk, 300dpi, psd) and the pics I have to use look perfect (20x30, rgb, 300dpi, tiff). The only thing I'm doing is Ctrl+T to size them down to 9cm high and the difference it's incredible, I'm loosing quality and I don't get why.
So, what I do now is size them down in the original file (use the step thing with the pixels until I'm around 9cm high) and then paste them into the catalog so I don't have to Ctrl+T them. Is there an explanation to this? It's driving me crazy...

Thanks again! you guys are awesome.
Victoria.

When you downsize and image, Photoshop does throw out information to make the image smaller. Usually it isn't that noticeable, but this will depend on the image and how much you are downsizing.

Make sure that you aren't changing the resolution when you downsize. Also instead of Ctrl +T, try going into Image then Image Size and use this window to resize your photo. This will give you more precise control.

After you resize your image it may appear soft so I usually apply the unsharp filter to bring back some of the lost detail. Photoshop CS and above provide bicubic sharpening. Many recommend using the Bicubic Smoother for upsizing and the Bicubic Sharper for downsizing (located at the bottom of the image size window).