The best way to change the dpi of a file?

Banned 9 years ago

Originally Posted by Thom Hale

I thought I could grab images off the web and use for printing project. Can a web file with a 72 dpi be converted to 300 dpi? If so, how?

The best way is to 'purchase' print ready images.

Cheers. - Frank

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seasoned

9 years ago

YEP! You can get a picture that is some POSITIVE INTEGER multiple of 300dpi and through cropping and the like, you can reduce it to 300dpi. You COULD do that with any higher resolution, but it may not work as well.

As for INCREASING the dpi? You would have to have some program analyze adjacent dots and assume, and it could end up looking WORSE than the 72dpi. It almost certainly won't be as good as the 300dpi. WHY? Because you have less than 1/17th the data! (72*72)/(300*300) So, OF COURSE, you do NOT have enough info for the 300dpi image. If the original is about 17"x17", I guess you could forget about the resolution, and display it as a 1"x1" thumbnail!

SO, I hate to say it, frank is right.

Steve

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BigFrank Banned 9 years ago

Originally Posted by seasoned

SO, I hate to say it, frank is right. Steve

That really would cause you pain, wouldn't it. lol

Cheers. - Frank
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SteveJohnson

9 years ago

There is no such thing as an image file with 72dpi, or any dpi for that matter.

There is only pixel dimensions of the file. A 2400x3600 file will print 8" x 12" at 300dpi, 4" x 8" at 600dpi, or 16" x 24" at 150dpi.

Dots per inch refers to the display medium, not the file itself. A DPI figure without the final print/display dimensions is meaningless.

EDIT:
BTW Steve, 'dpi' != 'resolution'

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seasoned

9 years ago

Originally Posted by SteveJohnson

There is no such thing as an image file with 72dpi, or any dpi for that matter.

There is only pixel dimensions of the file. A 2400x3600 file will print 8" x 12" at 300dpi, 4" x 8" at 600dpi, or 16" x 24" at 150dpi.

Dots per inch refers to the display medium, not the file itself. A DPI figure without the final print/display dimensions is meaningless.

EDIT:
BTW Steve, 'dpi' != 'resolution'

dpI=Dots Per Inch! SO, if you have an Inch, one has 5184(Assuming 72 both ways) dots, and the other has 90000, per square inch. If you have look for scanners in stores, or printers, the higher the DPI, the higher the resolution on the printer, and the higher the resultant resolution on the scanner.

And YEAH, you COULD argue that if you have a 75dpi picture, and display it on a 300dpi device, that the resolution is the same. I even technically indicated that. You would, as I said, have a picture with the SAME resolution, but smaller, because the dots cover a smaller area. So YEAH, it would be 300dpi, but fewer inches. You would STILL have the lower resolution of the 72dpi device. A 300dpi might be at 17", and look nice. The 72dpi would be 1" and look decent, but blowing it up to 17" would probably be WORTHLESS! But I obviously, like frank, figured that was NOT what he wanted to do.

ALSO, if a picture were a set size, and 300dpi, you can make it larger, than a similar 75dpi picture, without it being noticeably degraded.

Steve

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seasoned

9 years ago

BTW anyone remember the old fonts on the first couple laser printers? You HAD to have the size font you wanted. If you didn't, even the scaling printers would look bad. I believe MOST pictures STILL work like that. Eventually, they came out with vectorized fonts, like microsofts true type. Admittedly, they have picture formats like that, but I think they are mostly application type file formats. They aren't like GIF or JPG. The old raster/bit mapped ones scaled can look horrible, since they effectively make the dots larger, though it IS a bit more complicated and better than that. The vector systems tell how the lines were created so they can be scaled.

Steve

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ForumGuru

Banned 9 years ago

Steve Johnson has it right --> the image pixel dimensions (and how big of a print you need) is what you should be concerned with here.

As an example, a 1920px X 1080px image will beautifully fill the screen of most of today's standard computer monitors, but it will only produce a 6.4 inch wide print at 300 dpi. Print that same image at 150 dpi you will get a 12.8 inch wide print.

A small blog size image, say 640px x 480px, will only yield a 2.13 inch wide print at 300 dpi so obviously that size image is too small to print at a decent size without some serious upsampling and/or interpolation, which may seriously degrade the image. You can use free tools like Irfanview to resize an image to make it larger, but don't expect miracles with raster image formats like jpgs, pngs, tifs etc.

If you are looking for an 8 x 10 inch print at 300 dpi you need a 2400 x 3000 pixel image, if you need a 4 x 6 inch print at 300 dpi you need a 1200 x 1800 pixel image. If you want a 12 x 18 inch print at 300 dpi you need an image of 3600 x 5400 pixels.

Check these screenshots I just did in Photoshop.

A 3000 x 2000 pixel image at 300 pixels/inch will usually yield a nice looking 10 x 6.667 inch print.

That same 3000 x 2000 pixel image at 72 pixels/inch can yield a 41.7 x 27.8 inch print, but it's quite likely the print will look terrible at normal viewing distances.

That same 3000 x 2000 pixel image at 150 pixels/inch will yield a 20 x 13 inch print which might be acceptable for some uses.

On some images (for some uses) you can get away with a 150 or 200 dpi print for casual use, and in many cases it can be hard to tell the difference between an average size print made at 240 dpi and 300 dpi. That said, most images are either cropped or fit to the paper size. If you want to make big prints then you need big images to start with. You can't take a tiny web size jpg, tif, gif, or png image and expect to create a good looking big print with it

Below is an easy to use calculator that you can use to see how large a particular image can be printed at a given dpi.

Resolution DPI calculator for printing digital images from scanner or digital camera

Cheers

-don

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seasoned

9 years ago

Originally Posted by ForumGuru

Steve Johnson has it right --> the image pixel dimensions (and how big of a print you need) is what you should be concerned with here.

As an example, a 1920px X 1080px image will beautifully fill the screen of most of today's standard computer monitors, but it will only produce a 6.4 inch wide print at 300 dpi. Print that same image at 150 dpi you will get a 12.8 inch wide print.

A small blog size image, say 640px x 480px, will only yield a 2.13 inch wide print at 300 dpi so obviously that size image is too small to print at a decent size without some serious upsampling and/or interpolation, which may seriously degrade the image. You can use free tools like Irfanview to resize an image to make it larger, but don't expect miracles with raster image formats like jpgs, pngs, tifs etc.

If you are looking for an 8 x 10 inch print at 300 dpi you need a 2400 x 3000 pixel image, if you need a 4 x 6 inch print at 300 dpi you need a 1200 x 1800 pixel image. If you want a 12 x 18 inch print at 300 dpi you need an image of 3600 x 5400 pixels.

Check these screenshots I just did in Photoshop.

A 3000 x 2000 pixel image at 300 pixels/inch will usually yield a nice looking 10 x 6.667 inch print.

That same 3000 x 2000 pixel image at 72 pixels/inch can yield a 41.7 x 27.8 print, but it's quite likely the print will look terrible at normal viewing distances.

That same 3000 x 2000 pixel image at 150 pixels/inch will yield a 20 x 13 inch print which might be acceptable for some uses.

Cheers

-don

The bolded part, about acceptability, etc,,, is what I was talking about. Heck, the
Resolution DPI calculator for printing digital images from scanner or digital camera site that you linked to also spoke of aspect ratio that I KIND of touched on. I mentioned it elsewhere as such earlier on this forum, but I mentioned how the x and y access could vary here

And I DID say that a higher DPI could be displayed at a lower DPI. It is the idea of a lower printed/displayed at a higher that is problematic. The dots have to be larger and/or farther apart to create the same size image, and the image degrades. When they talk about like megapixels in a camera, that is effectively the dots. Cameras that have more pixels have a higher resolution. And optical scaling methods have no concept of pixels, and just scale things appropriately, so telescopes, and microscopes work fine. They are bulky and expensive! WHY do the better cameras have them? WHY do they have the OPTICAL scaling EVEN though they have the cheaper and weightless digital scaling? Because the DIGITAL has a LIMIT, and loses resolution. The optical are often limited to keep weight and cost down, but they work better. And YEAH, the larger pictures may have the same number of physical dots across, but the digital scaling effectively logically changes the dots.

Steve

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anynewsbd

9 years ago

Use Photoshop to change DPI easily.

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HeySal

9 years ago

I just use Irfanview - it's a free program and you can do a lot of things to pics with it - not like photoshop, but croping, changing size, sharpening, color correction, and increasing dpi, etc. I love my Irfanview! I make my ebook covers with OO Draw and Irfanview. The ebook in my avatar is an example:

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Sal
When the Roads and Paths end, learn to guide yourself through the wilderness
Beyond the Path

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sbucciarel

Banned 9 years ago

If you attempt to use low resolution photos for print work, you'll get very poor results. It is always better to to from a large photo to a smaller photo. That will increase it's resolution. Taking a small photo and making it larger results in blurry, bad looking photos.

The minimum resolution that I have used for print is 300 pixels per inch.

Here's a good tutorial on image quality for Photoshop

How Image Resolution Affects Print Quality - Photoshop Tutorial

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The best way to change the dpi of a file?

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