PNG format and DPI

J
Posted By
Jon
Aug 10, 2007
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1010
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12
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Closed
If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

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J
jaSPAMc
Aug 10, 2007
Jon found these unused words:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

"Save As" -=or=- "Save for Web" ?
J
Jon
Aug 11, 2007
Sir F. A. Rien wrote:
Jon found these unused words:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

"Save As" -=or=- "Save for Web" ?
‘Save as’
J
Joel
Aug 11, 2007
Jon wrote:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

"DPI" doesn’t mean much or anything in graphic world. Newer Photoshop now use "PPI" but it too doesn’t mean much by itself.

– You can change the "PPI" using Resize Image, and the more you increase the "PPI" the more WxH will decrease.

Or the whole file is based on WxHxR (or W x H x PPI). Many old digital cameras have the format set to "72 PPI" and they didn’t effect the image quality.
G
garypoyssick
Aug 11, 2007
The comment about older digital cameras is very accurate, Joel, and helpful. We’ve all seen images that come in at 72 dpi but are 28 inches wide and 18 inches high. That’s not a lot different then a 5×4 at 300 dpi/ppi. But sizing images to the print/output size at a dpi/ppi (the same, really when considering this issue) is twice the screen on the press. Digital presses are changing this, certainly, but sheet fed and web presses still run 150 line screens for the fast majority of the work they do, so 300 dpi/ppi works — as long as the width and height are close to perfect relative to the printed size.

Good point and thanks for your comments.

Gary in tampa

On 8/11/07 7:03 AM, in article ,
"Joel" wrote:

Jon wrote:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

"DPI" doesn’t mean much or anything in graphic world. Newer Photoshop now use "PPI" but it too doesn’t mean much by itself.
– You can change the "PPI" using Resize Image, and the more you increase the "PPI" the more WxH will decrease.

Or the whole file is based on WxHxR (or W x H x PPI). Many old digital cameras have the format set to "72 PPI" and they didn’t effect the image quality.
J
jaSPAMc
Aug 11, 2007
Jon found these unused words:

Sir F. A. Rien wrote:
Jon found these unused words:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

"Save As" -=or=- "Save for Web" ?
‘Save as’

I’m not using Bridge and don’t see any dpi reset unless I "Save for Web".

Perhaps one of the ‘monitors’ from Adobe can jump in and help.
J
Joel
Aug 12, 2007
Sir F. A. Rien wrote:

Jon found these unused words:

Sir F. A. Rien wrote:
Jon found these unused words:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

"Save As" -=or=- "Save for Web" ?
‘Save as’

I’m not using Bridge and don’t see any dpi reset unless I "Save for Web".
Perhaps one of the ‘monitors’ from Adobe can jump in and help.

Bridge? you mean ARC or LightRoom? They have option to save to "MAX" and it will save to whatever *original* MAXIMUM.

Anf back to the good old days, many digital cameras set to "72 DPI" and they didn’t lose any quality of whatever max PIXEL they had.
J
Jon
Aug 12, 2007
Joel wrote:
Sir F. A. Rien wrote:

Jon found these unused words:

Sir F. A. Rien wrote:
Jon found these unused words:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?
"Save As" -=or=- "Save for Web" ?
‘Save as’
I’m not using Bridge and don’t see any dpi reset unless I "Save for Web".
Perhaps one of the ‘monitors’ from Adobe can jump in and help.

Bridge? you mean ARC or LightRoom? They have option to save to "MAX" and it will save to whatever *original* MAXIMUM.

Anf back to the good old days, many digital cameras set to "72 DPI" and they didn’t lose any quality of whatever max PIXEL they had.

Thanks, but I’m still not clear on what’s happening here. OK, so I scan a 7" x 5" photo straight into PS at 600dpi. Before I save it I check Image Size which tells me that I have the following:
Pixel dimensions: 4200 x 3000
Document size: 7" x 5"
Resolution: 600DPI

Saved it as a TIFF Bridge shows it as:
Pixel dimensions: 4200 x 3000
Document size: 7" x 5"
Resolution: 600DPI

Saved as a PNG Bridge shows it as:
Pixel dimensions: 4200 x 3000
Document size: 7" x 5"
Resolution: 72DPI

If I open both photos in PS and check Image Size they both state: Pixel dimensions: 4200 x 3000
Document size: 7" x 5"
Resolution: 600DPI

So what’s happening in Bridge? Why is it displaying a different DPI?

If I scan at 72DPI the image dimensions come out at 504 x 360 pixels and in Bridge this shows 72DPI. In fact, whatever DPI I scan at, when saved as a PNG, Bridge shows 72DPI (even if I scan at 50DPI) whereas Photoshop (Image Size) always shows the actual DPI.

With JPGs there is no discrepancy between Bridge and PS.

ARC or Lightroom? Irrelevant. I’m talking about a discrepancy between Photoshop and Bridge.
T
Tacit
Aug 13, 2007
In article <46bf8e69$0$1632$>,
Jon wrote:

Saved as a PNG Bridge shows it as:
Pixel dimensions: 4200 x 3000
Document size: 7" x 5"
Resolution: 72DPI

This is impossible; you can not have a 7" wide, 4200-pixel image at 72 pixels per inch. The math doesn’t work. You have found a bug in Bridge; it is displaying resolution information incorrectly, that’s all.


Photography, kink, polyamory, shareware, and more: all at http://www.xeromag.com/franklin.html
OR
Owen Ransen
Aug 13, 2007
On Sat, 11 Aug 2007 00:23:29 +0100, Jon wrote:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

The titleof your post

PNG format and DPI

may explain it all. PNG does not have, in itself
a DPI setting. PNG does not know the size of the
final image when printed. It knows how many pixels
there are, but that is all.

See also:
http://www.ransen.com/Articles/DPI/Default.htm

Easy to use graphics effects:
http://www.ransen.com/
J
Jon
Aug 13, 2007
Owen Ransen wrote:
On Sat, 11 Aug 2007 00:23:29 +0100, Jon wrote:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

The titleof your post

PNG format and DPI

may explain it all. PNG does not have, in itself
a DPI setting. PNG does not know the size of the
final image when printed. It knows how many pixels
there are, but that is all.

See also:
http://www.ransen.com/Articles/DPI/Default.htm

Easy to use graphics effects:
http://www.ransen.com/

Thanks for the link. It is not the first time I have read about DPI/PPI and to be honest I’m still confused.

When I scan a 7" x 5" image into Photoshop I set my scanner software to: Resolution: 600 DPI
When the image appears in Photoshop I check ‘Image Size’ and that tells me the following:

Pixel Dimensions:
Width: 4200
Height: 3000

Document Size:
Width: 7"
Height: 5"
Resolution: 600 Pixels/inch

I then save the image as a PNG and Photoshop Image Size states exactly as above.

I then view the PNG image in Bridge.
This tells me the following:
4200 x 3000 @ 72DPI

OK, so if DPI is purely to do with the Printer regardless of whether I print the image or not then why is there a discrepancy between PS and Bridge?
And what if the printer I send the images to hasn’t the capability to print at 600DPI?
Well, I can’t print the image directly from Bridge anyhow so presumably the 72DPI IT shows is meaningless and possibly a PS/PNG bug.

However, what throws the whole DPI/PPI/Resolution thing into a cocked hat is that the Resolution/DPI I set in the scanner software determines the Pixel dimensions of the resultant image that shows in Photoshop ie. setting the Scanner software to Resolution 300DPI for a 5" x 7" photo gives me an Image Size/Pixel Dimension in Photoshop of Width 2100 – Height 1500.
And this determines the Print size I can achieve when I send the image for printing to an online photo printing outfit.

So if there’s anyone out there who can explain what all this is about, I would be most grateful.

Furthermore, while they’re at it, could they also explain the purpose of Photoshop’s ‘Image size’ Auto button which confuses the issue further by introducing Lines/inch. I notice that within this ‘Auto’ option, if I select ‘Best’ then it reduces the Pixel Dimensions of the image to: Width: 1862
Height: 1330
Resolution: 266 Pixels/inch

Arghhhh!!!
OR
Owen Ransen
Aug 13, 2007
On Mon, 13 Aug 2007 16:27:37 +0100, Jon wrote:

Well, I can’t print the image directly from Bridge anyhow so presumably the 72DPI IT shows is meaningless and possibly a PS/PNG bug.
Or maybe there were so many "expert designers" who could only think in DPI that they put in a number there to make them feel safe. I do web pages and they STILL ask me what DPI I want the images in.

However, what throws the whole DPI/PPI/Resolution thing into a cocked hat is that the Resolution/DPI I set in the scanner software determines the Pixel dimensions of the resultant image that shows in Photoshop ie. setting the Scanner software to Resolution 300DPI for a 5" x 7" photo gives me an Image Size/Pixel Dimension in Photoshop of Width 2100 – Height 1500.
That sounds right.

And this determines the Print size I can achieve when I send the image for printing to an online photo printing outfit.
Well, it determines probably the *maximum* print size which will not lose detail. Print larger (say twice the size) and you may begin to see pixels. Print smaller (say half the size) and the image may appear sharper but you will lose some detail.

Furthermore, while they’re at it, could they also explain the purpose of Photoshop’s ‘Image size’ Auto button which confuses the issue further by introducing Lines/inch.
*I* can’t!

Easy to use graphics effects:
http://www.ransen.com/
A
Anon
Aug 13, 2007
"Jon" wrote in message
Owen Ransen wrote:
On Sat, 11 Aug 2007 00:23:29 +0100, Jon wrote:

If I scan an image at 600 DPI and save it as a Tiff the resolution (as displayed in Bridge) is, as I would expect it to be: 600 DPI. However, if I save it as a PNG the resolution drops to 72 DPI! Is there a way to change this?

The titleof your post

PNG format and DPI

may explain it all. PNG does not have, in itself
a DPI setting. PNG does not know the size of the
final image when printed. It knows how many pixels
there are, but that is all.

See also:
http://www.ransen.com/Articles/DPI/Default.htm

Easy to use graphics effects:
http://www.ransen.com/

Thanks for the link. It is not the first time I have read about DPI/PPI and to be honest I’m still confused.

When I scan a 7" x 5" image into Photoshop I set my scanner software to: Resolution: 600 DPI
When the image appears in Photoshop I check ‘Image Size’ and that tells me the following:

Pixel Dimensions:
Width: 4200
Height: 3000

Document Size:
Width: 7"
Height: 5"
Resolution: 600 Pixels/inch

I then save the image as a PNG and Photoshop Image Size states exactly as above.

I then view the PNG image in Bridge.
This tells me the following:
4200 x 3000 @ 72DPI

OK, so if DPI is purely to do with the Printer regardless of whether I print the image or not then why is there a discrepancy between PS and Bridge?
And what if the printer I send the images to hasn’t the capability to print at 600DPI?
Well, I can’t print the image directly from Bridge anyhow so presumably the 72DPI IT shows is meaningless and possibly a PS/PNG bug.
However, what throws the whole DPI/PPI/Resolution thing into a cocked hat is that the Resolution/DPI I set in the scanner software determines the Pixel dimensions of the resultant image that shows in Photoshop ie. setting the Scanner software to Resolution 300DPI for a 5" x 7" photo gives me an Image Size/Pixel Dimension in Photoshop of Width 2100 – Height 1500.
And this determines the Print size I can achieve when I send the image for printing to an online photo printing outfit.

So if there’s anyone out there who can explain what all this is about, I would be most grateful.

Furthermore, while they’re at it, could they also explain the purpose of Photoshop’s ‘Image size’ Auto button which confuses the issue further by introducing Lines/inch. I notice that within this ‘Auto’ option, if I select ‘Best’ then it reduces the Pixel Dimensions of the image to: Width: 1862
Height: 1330
Resolution: 266 Pixels/inch

Arghhhh!!!

First let me say I do not understand the reason why many of the ‘Auto’ options were included.

Dots per inch (dpi) is a size measurement for input or output devices. The actual size of the digital image is read as pixels.

Dpi does not really relate to the onscreen image because the onscreen image can be viewed at different sizes by zooming, and it can be otherwise manipulated by software.
However, dpi is important when scanning and printing. When you scan at 600dpi you are telling the scanner to look 600 times for every inch it travels in width, and height. When you scan at 72dpi you are telling it to look only 72 times for every inch, resulting in a lower resolution scan.

The dpi of the printed image is usually determined by the software from which it is printed. Some software determines/adjusts the dpi by the requested size of the print, say 4×6, or "to fit within margins". Some software imports images at a set resolution, Outlook Express and IE, for instance, only display and print at 72 dpi. Most printers/graphics layout software imports at a set resolution like 300ppi, but allows you to resize it in the layout. Most graphic apps, like PS, will print at the saved resolution, (i.e. 600dpi).

72dpi is the closest resolution to the 100% size onscreen, so it is the default resolution in many web authoring apps. PNG as there name implies, (portable network graphics), is a format closely ascociated with the internet so many apps will use 72dpi as a default value, for that file format. GIFs are treated similarly. I am not familiar with Bridge, but try printing the image from there to see how it is printed.

4200pixels at 600dpi is 7"
3000pixels at 600dpi is 5"
Image will be sharp.

4200pixels at 72dpi is 58.333"
3000pixels at 72dpi is 41.667"
Image will be slightly pixelated.

Hope this has helped.
Scott

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