Problem: Change in dpi when Tiff image is resized in Illustrator CS2

[flat39]

Hi

This is my first post and I am also new in preparing PDF file for printing.

I have an issue with the following:

- I created images in Photoshop in 300dpi and saved as TIFF.
- Opened the image in Illustrator CS2.
- Checked dpi, which is still 300dpi.
- I need to reduce image size. But now, dpi grows bigger. :confused: why?
- Aim is to create a PDF file with image size as 300dpi.
- I clicked on 300 dpi option in 'Document raster effects settings', then saved the file as PDF. Does it mean now that file is 300dpi? The file contains both text and image.

Any advice would be much appreciated.

[almaink@bellatlantic.net]

Raster images are resolution dependent. Therefor if you reduce the size of a 300 DPI tif by 50% that tif now has a DPI of 600. Conversely if you enlarge said tif by 200% it would be 150 DPI. Best bet is to down sample the images when making the PDF, back to 300 DPI for all above 301 DPI.
Like this
http://a2.s3.p.quickshareit.com/files/picture1ee851.jpg

[barcoder]

If you need to get single image saved as PDF, you can do that straight from photoshop, without Illustrator.
If you need to get multiple images saved as multipaged PDF, you can do that from photoshop too. In CS3 and earlier versions you can do that buy choosing File>Automate>PDF-presentation, in CS4 you can do that with Bridges new Output feature.

[flat39]

Raster images are resolution dependent. Therefor if you reduce the size of a 300 DPI tif by 50% that tif now has a DPI of 600. Conversely if you enlarge said tif by 200% it would be 150 DPI. Best bet is to down sample the images when making the PDF, back to 300 DPI for all above 301 DPI.
Like this
http://a2.s3.p.quickshareit.com/files/picture1ee851.jpg

Hi, thanks! But is this going to affect the look of the image? Like making it look a bit blurry, even slightly?

[flat39]

Thanks barcoder. This can be a useful option for me next time, as I have now already placed all images in Illustrator (124 pages) and noticed the change in dpi at a late stage.

[TORCH511]

First thing is first.

Illustrator does not do anything with dpi... not until you save.

Bitmap (raster) images have 2 things: size, meaning physical size (4 x 6 inches) and resolution (ppi, images have pixels not dots, using dpi is erronious)

300ppi means you have 300 pixels per inch of the image. So a 4x6 image with a resolution of 300ppi would have 2,160,000 pixels.

- I need to reduce image size. But now, dpi grows bigger. :confused: why?


If we were to double the size of the image (8x12) without resampling, then you would have a resolution of 150ppi. If you were to half it (2x3) then it would be 600. The DPI has grown but the file is unchanged

- Aim is to create a PDF file with image size as 300dpi.

Then in photoshop, make a copy of the image, resample the size to scale down the correct size and DPI

- I clicked on 300 dpi option in 'Document raster effects settings', then saved the file as PDF. Does it mean now that file is 300dpi? The file contains both text and image.

No, a raster effect in Illustrator is an effect appled to a vector object that can not be made readily in pure vectors. A drop shadow or blur. When the image is printed in some way, these effects are rendered on the fly. It has nothing to do with images.

FYI, you should not OPEN a raster image in Illustrator, the file should be PLACED instead. If you were to place the file, you could resize as needed. Then when you make your PDF you could then choose to downsample to 300DPI and the image would be resampled down to the desired size. It is preferable to PLACE the file at the correct size/resolution.

[barcoder]

Hi, thanks! But is this going to affect the look of the image? Like making it look a bit blurry, even slightly?

Thats a little bit tricky question.

Every time you downsize pixel-images, they go little blurry. And today´s digital cameras produce so huge images that someones usually has to reduce them at some point. More you reduce pixels, more it goes blurry. That´s why you always should know the exact printing size of your images and reduce them to that size with required level of resolution. 300dpi (or ppi) is usually enough for high quality printing. (With that resolution you still can scale images some 25% up or down in layout-software like indesign or illustrator.)

After you have reduced the size, you should perform a sharpening in Photoshop. Most of the people uses oldie-but-goldie UnSharp Mask. There are some other ways to do it as well, smart sharpening for instance.

After sharpening, you simply save your image to the format you like and use them for printing with 100% size (= no scaling).

[TORCH511]

300ppi is typically a little overkill, often being double what is truly requred.

300ppi is often thought of as the photo equivalent of 100iso film as the pixels and film grain are about equal and both can create about the same size prints with acceptable results.

Most printing processes can not print at photo-quality though digital presses will probably change that. Offset press typically print at 150-175 lpi, Flexographic around 125 lpi and screen tops out around 90 lpi.

So even though most printers ASK for 300ppi artwork, in reality you can get away with less if need be.

Viewing distance is also key, as the farther away the print will be from the subject, the less you need.

[Lukas Engqvist]

With modern raster techniqies it is not overkill with 300, and 150 is definately too low…*if you want textures and details. If you want to go down to 240ppi you can get away with it ower than that your image is getting fuzzy.
Ofcourse the viewing distance plays a part as well…*a billboard picture is not viewed close up, but pirinting shoe or knittwear catalouges we can go up to 340dpi on images because we know how customers are intersted in viewing subtle differences. (ofcourse print stock does also play a part)

[barcoder]

If you are using images in layout software, it is usefull to give them a little better resolution than needed, then you can finetune the size of image afterwards without photoshop (or other image altering software). If you scale down 300ppi image 25% in indesign, it will be 240ppi image after that... With normal high quality small size printjobs I would not like to go below 240ppi... and as Lukas mentioned, theres jobs where even that is not enough...

EDIT: someone is little bit tired after just few hours of sleep last night. I ment of course scaling up image to 125%, then your 300ppi image become 240ppi...:o ... thanks Lukas for the tip...

http://dl.getdropbox.com/u/133381/scaling.jpg

[TORCH511]

Lukas, I guessed you missed the part where I underlined, bolded and Italicized the word MOST

Yes, high end offset and many digial presses, on coated or high grade stock, sure, 300ppi is certainly not overkill, hence the MOST

I can think of lots of printing where the above however is not true. Weekly Ads, Direct Mailings. POP materials, retail signage, product packaging and labeling.

[flat39]

Hi

Thanks to all for your advice and comments. I feel I am learning so much from this forum.

I have actually done the 'bicubic downsampling' and PDF the file, the image did not turn out slightly blurred but the edges which originally were fading in the background, are not anymore, I can see a slight border around.
It does not look too bad, this is the immediate visible difference I noticed.

I will have to experiment with what you all suggested here and get prepared for my next job.
Thanks again.