tough question on Adobe Photoshop 7.0

[jcsdmd]

I have made an interesting find that I just can't explain while using Adobe Photoshop 7.0. I shoot underwater photos with a housed Nikon 995 and topwater with a Sony DSC-H1. After downloading pictures from each camera into the appropriate folder on my desktop, when I right click each .jpeg file from either camera and explore properties, I get the appropriate pixel dimensions and the resolution is reported at 300 ppi for both cameras. Here is when it gets interesting. When I open a file in Photoshop from the Nikon and go to IMAGE__IMAGE SIZE, it is reported at 300ppi but when I open a jpeg from the Sony camera, it is reported in IMAGE SIZE as 72 ppi (despite it being reported as 300 ppi file by Windows.. If I do nothing to the file at all or perform editing on it and save it, it will be saved at 72 ppi unless I go back in to IMAGE size and manually type in 300 ppi. If I do type in 300 ppi, the pixel dimensions remain unchanged. Any ideas what gives here?

I found this problem by accident and would love to be able to edit pictures without having to go in on each picture before saving it and manually typing in the 300 ppi on every file. I have tried restarting Photoshop and rebooting the computer to no avail. This is a repeatable glitch. Is is possible that the Sony somehow embeds info in the file to report a different resolution?

Thanks in advance for any help that may shed light on this problem.

[Lin Evans]

The really strange thing is that you see 300dpi at all unless you have modified the file in PhotoShop. Both your cameras default the exif tag for dpi at 72.

As I'm sure you already know, this is simply a "tag" which tells the printer how large the print would be if it were printed at this resolution. Many years ago when PC's had no graphics, the MacIntosh was "graphics central" for imaging. The old MacIntosh monitors were 72 pixel per inch display resolution. In 1990 when PhotoShop was first released they decided to use this as a "default" for their displays so all dimensions in inches or millimeters were based on 72 dpi or 72 ppi - terms which have been traditionally used incorrectly and indiscriminately.

Most digital camera makers from years back used 72 dpi (ppi) as their default EXIF tag. Obviously nobody prints at 72dpi so printer drivers automatically set their print density according to the "quality" you have chosen. In the case of most Epson inkjet printers it's 720 dpi and for Hewlett-Packard inkjet printers it's 600 dpi. This really has nothing to do with the number of ink dropletts used which is what makes it even more confusing. The printers may use 2400 or 1200 or even higher numbers of ink dropletts per inch but it often takes many ink dropletts to represent a single pixel. The actual number of "pixels" per inch for the Epson fine quality print on gloss paper is 720. The printer may actually use 2400 tiny drops of ink to produce these 700 pixels of print information.

Today there are a few digital camera manufacturers - usually Canon - which "tag" the exif header with 180 ppi rather than 72, but both your cameras use 72 ppi as the native default tag.

Perhaps Windows or if you by chance have installed PixVue may be changing this default which has no real meaning except that if the display algorithms in use by the software and video card may display the images at different sizes. In either case whatever you see in PhotoShop is undoubtedly the correct ppi exif tag. No matter what you have it set for, it will be changed to the print density needed when you select the quality and size print you want. It's rather comical listening to all the chatter about whether to "feed" the printer 300 dpi or 180 dpi or whatever. If you are printing on an inkjet, the driver will override whatever you give it unless it happens to be what the printer "requires." For the vast majority of dye sublimation printers including Durst Lambda, Kodak and LightJet this is 300 dpi. For some of the Fuji printers it's 400 dpi and for others it's 300 dpi. For the vast majority of other dye sublimation printers it's 300 dpi.

Best regards,

Lin

[jcsdmd]

Lin:

Thanks for pointing me in the right direction. Actually, the Nikon specifies 300 ppi in the EXIF tag and the Sony specifies 72 ppi. If found a link in ADOBE's website that I have listed below that explains the details. I looked at the image EXIF info in photoshop as described in the link and found my answer. Thanks again.

http://www.adobe.com/support/techdocs/325136.html

[Lin Evans]

Lin:

Thanks for pointing me in the right direction. Actually, the Nikon specifies 300 ppi in the EXIF tag and the Sony specifies 72 ppi. If found a link in ADOBE's website that I have listed below that explains the details. I looked at the image EXIF info in photoshop as described in the link and found my answer. Thanks again.

http://www.adobe.com/support/techdocs/325136.html

I read the link. You know Adobe is as guilty as anyone of propagating false information - LOL - they use the term "resolution' with ppi saying "an image captured at 72 ppi resolution". Digital cameras don't "capture" at any ppi, they capture via X number of horizontal by Y number of vertical photosites (sampling sites) which create some particular file size depending on whether the original processor is a CFA (bayer processed) or Foveon design.

The camera captures an image by creating a matrix of the numerical equivalence of so many pixels across by so many down. This has absolutely nothing specifically to do with "resolution" but is only loosely correlated with the number of sampling photosite wells on the processor. This correlation is only good for CFA processors. For example, your Nikon CP990 produces a file with 3.37 million pixels of video information. The measured optical resolution via B&W resolution chart measurement is 800 horizontal by 850 vertical lines per image height. A Sigma SD10 using the Foveon X3 processor produces a file with 3.4 million pixels of video information which is very close to your Nikon CP990. The measured optical resolution via B&W resolution chart measurement is 1550 horizontal by 1550 vertical lines per image height. This is about twice the optical resolution of the CP990. How? Because the Foveon processor uses 9.3 million sampling sites to produce a 3.4 megapixel file size. A bit better than a six megapixel CFA sensor with 3.4 megapixel files. So you can see that there is no "necessary" correlation between pixel count and true optical resolution.

Back to Adobe. The EXIF tag 72dpi or 300 dpi simply identifies how closely the print pixels will be grouped (pixel density) or how many per unit of linear measurement in "dots per inch or pixels per inch". To refer to it as "resolution" is highly misleading - shame on Adobe... LOL

The important thing is that you have an answer.

Best regards,

Lin

[stonemason]

Hi jcsdmd

Images taken with several Canon cameras show similar "resolution" issues as you describe. Go to dpreview and do a search. There was much discussion about this a while back.

Geoff

[The oldie]

I have seen some excellent info on this forum over the years, from all the regulars.

I have to say Lin Evan's exposition on this subject is just about the best explanation I have read in a very long time.

WELL DONE LIN and many thanks from a regular reader of the forum.

It would seem that anyone on the forum should consult Lin before buying your next digital camera !!

Alan in NE UK