dpi: viewing actual for images

[trinityc@adobeforums.com]

I am a web designer, morphing/expanding into a print designer. This is the deal...

1. Can I take an image that's 72 dpi, excellent quality and clarity and make it 300 dpi?

2. How can I determine the actual dpi of any given image?

I have been assuming from all the documentation I'm reading and the help I get from the forum that I can increase the dpi of an image.

PROBLEM? I increase the image to 300 dpi, place it in a print project, send it off and the printer kicks it back, saying the image is 120 dpi, for example, or 260dpi or whatever. From what I can tell, it's 300 dpi. It's driving me nutz!

How are they finding the dpi? How can I verify the dpi before sending?

I have Adobe PS 7.0 and Illustrator 9.0

Thanks!

[LenHewitt@adobeforums.com]

Trinity,

You can Scale a 72 ppi image in illustrator to 24% of it's size and it will
print at 300 ppi (72/300) and maintain quality.

However, if you want to maintain it's physical size whilst increasing the
resolution, that would need to be done in Photoshop from Image>Image Size
with re-sampling checked. When doing that, though, Photoshop has to create
additional pixels, and the best it can do is make an educated 'guess' as to
what the value of those pixels should be based on the values of surrounding
pixels.

This inevitably leads to a degradation of sharpness and overall quality, and
the greater the degree of re-sampling the greater that loss will be. Unsharp
masking will often overcome much of the loss of sharpness, but will
introduce other artefacts such as edge lines.

You are unlikely to obtain totally satisfactory results with a 4x
enlargement as at that point 3/4rs of the pixels will be generated values
rather than the original pixels.

[trinityc@adobeforums.com]

Thanks!

My main question still remains: How can I tell the actual dpi of any given image?

[Jacob_Bugge@adobeforums.com]

trinity and Len,

The actual resolution in ppi is there when you open the file in PS and choose Image>Image size, and there you can see what happens when you blow it up by changing the ppi; but I should like to understand what it really means in Help (AI10) when it says (in About resolution in bitmap images):

Printer resolution
.... For best results, use an image resolution that is proportional to, but not the same as, printer resolution. Most laser printers have output resolutions of 600 dpi to 1200 dpi and produce good results with images from 72 ppi to 185 ppi.
High-end imagesetters can print at 1200 dpi or higher and produce good results with images from 200 ppi to 350 ppi.

Screen frequency
.... To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency. But with some images and output devices, a lower resolution can produce good results.

I suppose that may answer the remaining question.

[trinityc@adobeforums.com]

Clarification:

How can I tell the actual dpi of any given image in Adobe Illustrator, not Photoshop?

On my last project, Adobe Photoshop said it was 300 dpi, but the magazine pre-press rep said it was only 170 when she viewed it in Illustrator.

I need to be able to see for myself.

[Jonathan_McClintic@adobeforums.com]

Illustrator= vector. If the image is Vector, then it will automatically print at the highest available dpi the printer is capable. This is particularly true when printing to a PS printer (ie many laser printers, but not many inkjets.) In all actuality, any dpi above 220 for color images rarely makes much difference. Usually only line art and text really benefit from resolutions better than this. A cool trick but not realy what your looking for is to switch to pixal preview mode under view. Then, when you zoom in, it will actually show you a pixallated view that your final image would look like when you export the art to bitmap (ie gif, jpeg, tiff).

Jonathan

[trinityc@adobeforums.com]

How can I tell the actual dpi of any given image in Adobe Illustrator?

Thanks!

[James_Talmage@adobeforums.com]

Trinity,

1) Select the image.

2) Document Info Palette: Hold the palette's flyout menu and select Selection Only.

3) Hold the flyout menu again and select either Linked Images or Embedded Images, depending upon whether the selected image is linked or embedded. The data shown will include resolution in terms of pixels per inch.

Understand this is PPI *at the size to which the image is currently scaled on the page*. It may or may not be the PPI scaling factor which you saved with the image when you saved it from Photoshop.

It helps if you realize that PPI is really simply a scaling factor.

JET

[trinityc@adobeforums.com]

I guess there's no way to view actual dpi/ppi properties for an image in Illustrator.

Thanks guys for all of your help!

[James_Talmage@adobeforums.com]

Trinity,

The value shown in the Document Info Palette in AICS IS the "actual" PPI at which the image will print.

Again, PPI is simply a scaling factor. People call it "resolution" as if it is a measure of the real data (real information, or detail) contained in an image. It is not. It is merely a scaling factor.

You save an image out of Photoshop at 72 PPI. You place it in Illustrator. The Document Info Palette shows its PPI as 72.

But I had overlooked that you are working in AI9. As I recall the resolution is not shown in the Document Info Palette of AI9. If not, then you have to use the flyout of the Link Palette, and select Link Information. This will show you a scaling factor, which is always relative to 72 ppi. So to derive the PPI of a placed image, you would use this formula:

7200/scale factor

JET

[James_Talmage@adobeforums.com]

Jacob,

"...but I should like to understand what it really means in Help (AI10)..."

At the bottom of this post is text from the AICS Help file, which is worded better than what you quoted.

It helps to understand that halftone dots have to be built from printer spots. In simple terms, an imagesetter which prints 3000 SpotsPerInch, when printing a halftone screen with a ruling of 150 LinesPerInch, has available to it 400 printer spots with which to build each halftone dot. (3000/150=20. 20 spots horizontal x 20 spots vertical = 400.) The number of printer spots available to build each halftone dot is what determines the number of levels of gray which a printer can render at a given halftone ruling.

The idea in determining a suitable PPI for raster images is to scale the pixels small enough that their stair-stepped jaggedness is "disguised" by the limitations of the printing process itself. Experimentation has determined the rule-of-thumb that an image "resolution" (PPI) of at least 1.5 x the halftone ruling (LPI)is sufficient. (I almost always use 1.5, despite the oft-cited conventional-wisdom of 2x or the catch-all 300ppi. But I seldom do on-page scaling of imported images, and in my experience, an appropriately sharpened image at 1.5 x the ruling looks as good as or better than a higher PPI.)

JET
____________________________
"Printer resolution and screen frequency

Printer resolution is measured in the number of ink dots produced per inch (dpi). Most desktop laser printers have a resolution of 600 dpi, and imagesetters have a resolution of 1200 dpi or higher. Ink jet printers produce a microscopic spray of ink, not actual dots; however, most ink jet printers have an approximate resolution of 300 to 720 dpi.

To determine the appropriate resolution for your image when printing to any laser printer, but especially to imagesetters, you must also consider screen frequency. Screen frequency is the number of halftone cells per inch used to print grayscale images or color separations. Also known as screen ruling or line screen, screen frequency is measured in lines per inch (lpi)--or lines of cells per inch in a halftone screen.

The relationship between image resolution and screen frequency determines the quality of detail in the printed image. To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency. But with some images and output devices, a lower resolution can produce good results. To determine your printer's screen frequency, check your printer documentation or consult your service provider.

Note: Some imagesetters and desktop laser printers use screening technologies other than halftoning. If you are printing an image on a nonhalftone printer, consult your service provider or your printer documentation for the recommended image resolutions."

[trinityc@adobeforums.com]

Somehow, this has gotten off track...

I will figure it out somehow. Surely, there's a way to find out the dpi/ppi/resolution of an image, not what it should or could be, but what it is currently.

I no longer need any responses on this issue. I might just call a magazine and ask them how they can tell what the dpi/ppi/resolution of an image is, not what it should or could be, but what it is currently. As I stated in my original question, they are the ones bouncing the files back because the image is not 300 dpi.

Thanks everyone!

[Timothy_Foolery®@adobeforums.com]

Somehow, this has gotten off track...




No it hasn't.

Surely, there's a way to find out the dpi/ppi/resolution of an image,
not what it should or could be, but what it is currently.




No one ever reads post #7

[Harron_K._Appleman@adobeforums.com]

LOL, Timothy.

[Jacob_Bugge@adobeforums.com]

So: hold your breath and be second (no one ever reads the first post), or hurry, not to be #7. How come you are often first, Timothy?

James, thank you for the answer to me, beats the manual, as usual.

trinity, start with Window>Info, then the right stuff pops up.

[Philip_Peterson@adobeforums.com]

I read your post Timothy, that stopped me from saying the same thing (though far less well).

[trinityc@adobeforums.com]

Tom...

Somehow, this has gotten off track...

Yes, Tom, it had gotten off track.

Your solution did not work. "Show info" showed the dimensions of the image, not the resolution.

How do I close this question? No one knows the answer right now and I appreciate the information, I did get answers to questions I didn't ask that were useful, but I don't want any more responses to this question.

[trinityc@adobeforums.com]

Thanks!

[Jacob_Bugge@adobeforums.com]

trinity,

Forgive me, but please have this last go:

1) Select the image,
2) Choose Window>Document Info which should pop up,
3) Tick right arrow,
4a) If Selection only is ticked, go to 4b), else tick it and tick right arrow again and go to 4b),
4b) Tick Embedded Images

After that the ppi should show when you scroll to the bottom of the Document Info box.

[trinityc@adobeforums.com]

Thanks...

I have tried this and the only thing it shows is the "Output" of 800 dpi, which is not the actual dpi of the image. I am viewing individual images that I plan to use in documents and they are not yet embedded or linked.

I feel the love for everyone who's trying to help! Too cool!

But, I'll let this one rest and the answer will come sooner or later. I'll propbably stumble across it at 2 a.m. while I'm inspired to design.

Thanks again!