Faststone Image Viewer DPI setting question

FIV shows my 24 MP sensor as 300 DPI.

FIV shows my 12 MP sensor as 180 DPI.

Anyone know what determines the DPI that FIV chooses ?

1. If you have a 2550 x 1440 dot 27" monitor like I do, it is 23" horizontally, or about 110 dpi by virtue of its hardware. If you were to display with a software viewer 'at 100%', it would assign each pixel in the image to each pixel of the monitor.
   
2. I shoot with Canon 7DII...its EXIF data inherently says 72 dpi by default.
   

• If I displayed my 7DII image on my 27" monitor, I would NOT see the entire image on the screen 'at 100%' view, I would only see 2250 x 1440 pixels at 100% view! and that would be at the 110 dpi defined by my monitor hardware.
  
• If I displayed my 7DII image on my 27% monitor to fill the screen, I would see the entire image on the screen but NOT at the 72 dpi embedded in the EXIF data, it would be at the 110 dpi defined by my monitor hardware

In brief. the dpi value embedded in the JPG means NOTHING with regard to monitor display of images.
The dpi value embedded within a JPG might mean something to a commercial print service or to an offset press operator
...if you are using neither of them, the dpi value in the EXIF is truly meaningless.

Cameras write the XResolution and YResolution EXIF tags and throw any old value in there, typically 300 with modern cameras that I have owned. This may have been a value that corresponds to modern printers so that people who print directly from their camera, or print the file without ever editing it, get prints of a particular or typical size and quality guessed at by the camera manufacturer.

The PPI or DPI tag in an image is just that - a tag. It does not change the number of pixels in the image, it is simply a value that the user can set at print time (or any time) to define the PHYSICAL PRINT SIZE, in inches, for the given pixel dimensions of the image file. To expand on Walt's comments, it is meaningful for any print device (your home inkjet or laser printer), not just a commercial printer or press. It is the value used to translate pixels dimensions into a meaningful physical length dimension for output devices that have such requirements.

Kirk

Thanks Wilt ... Thanks kirkt

It is possible to edit EXIF dat with an editor...regardless of 75/180/300/600 there will be NO VISIBLE difference in output from the home printer, when output is to identical size paper ( e.g. 8.5 x 11)

Wilt wrote in post #19371781
It is possible to edit EXIF dat with an editor...regardless of 75/180/300/600 there will be NO VISIBLE difference in output from the home printer, when output is to identical size paper ( e.g. 8.5 x 11)

The physical size, in inches, of the printed image is determined by the ppi tag, so there will be a readily apparent visible difference in output from a home printer for various ppi values, especially when viewing distance and the limits of human vision without magnification are considered. You can change the pixel dimensions and the ppi tag of an image when you resize an image, for example when you use the Image Size dialog in PS, or when you convert a raw file into a rendered RGB image.

If your file is 6000 x 4000 pixels and the ppi tag is set to 300 ppi, the print will come out of the printer at 20 inches by 13.3 inches, such that 300 pixels of image information will make up one inch of the print's linear dimension. If the ppi tag is set to 600 ppi (without resampling the image), the print will come out of the printer at half the size of the 300 ppi print (10x6.7 in) and 600 pixels of the image will make up one inch of the print's linear dimension. At some point, there will necessarily be a visible difference in output - think about an absurd value like 6000 ppi that will produce a 1x0.7 inch print. Do you think you will be able to see all of the details in that tiny image that you could see in the 20 in wide print? No, of course not. Your printer cannot probably even render that level of resolution. In all of these scenarios, you are not changing the number of pixels in the image (you are not resampling upward [creating pixels]or downward [combining pixels]) you still have 6000 x 4000 pixels, you are just telling the printer how to cram those pixels into an inch of a print.

The paper size does not affect the size of the printed image - if the paper is not big enough to contain the entire print at the configured print size, then a portion of the print will actually appear on (fill) the paper, but the print size will be preserved according to the ppi setting. For example, if you try to print your 6000 px x 4000 px print at 300 ppi (20x13.3 in print) on an 8.5x11 piece of paper, then only a portion of the image will be printed on the paper - the incomplete portion of the image will fill the entire 8.5x11 in piece of paper, but the printed image itself will be sized as if it were being printed at 20x13 inches. Same thing in the opposite extreme. Put an 8.5x11 piece of paper in the printer and print the image at 6000 ppi, the printer will print a 1x0.7 inch print on the 8.5 x 11 inch paper.

Now, the ppi value discussed above is the value related to the image and the physical print size. When you print to an inkjet printer, you often have a choice in the print driver of the printer resolution (1440 dpi, or high quality 2880 dpi or something similar). This value does not affect the image size, but has more to do with the resolution of the ink dots laid down by the printer, sort of a proxy for the print quality and level of detail. It has nothing to do with the above discussion.

Kirk

kirkt wrote in post #19371807
The physical size, in inches, of the printed image is determined by the ppi tag, so there will be a readily apparent visible difference in output from a home printer for various ppi values, especially when viewing distance and the limits of human vision without magnification are considered. You can change the pixel dimensions and the ppi tag of an image when you resize an image, for example when you use the Image Size dialog in PS, or when you convert a raw file into a rendered RGB image.

When I output a JPG from Lightroom, I specify only the pixel count in each direction. Then I print that JPG at 4x6 or 8x12 or whatever size I want...for printing on my Canon printer I specify only the paper size (4x6 fed from rear feed, or 8.5 x 11 feed from standard tray) and output image size. If image is smaller than paper, I also specify image location on the page. No touching EXIF number, ever...unless I want a 250 dpi 12" x 60" print using 3000 x 15000 pixel JPG file!

The ONLY time I specify the dpi is if I send a file to a commercial printer, tell them how big I want the print and make sure the pixel count matches the density that I want (not a standard 300 dpi, all the time!)

Wilt wrote in post #19371820
When I output a JPG from Lightroom, I specify only the pixel count in each direction. Then I print that JPG at 4x6 or 8x12 or whatever size I want...for printing on my Canon printer I specify only the paper size (4x6 fed from rear feed, or 8.5 x 11 feed from standard tray) and output image size. If image is smaller than paper, I also specify image location on the page. No touching EXIF number, ever...unless I want a 250 dpi 12" x 60" print using 3000 x 15000 pixel JPG file!

The ONLY time I specify the dpi is if I send a file to a commercial printer, tell them how big I want the print and make sure the pixel count matches the density that I want (not a standard 300 dpi, all the time!)

Sure, if you let the software manipulate the PPI value for you by automatically determining the value based on the number of pixels the the output size you specify, then your value is not taken into consideration.

PRINT_SIZE (in inches) = PIXEL_SIZE (in pixels) / PPI (pixels per inch)

Manipulate that expression any way you would like - that does not, however, change the fact that the PPI value determines the physical size of the print based on how many pixels there are and the PPI value. Your use of other software to automatically manipulate the PPI value for you is just a different way of changing the PPI value to get the print size you want.

PPI does not change how an image is displayed on a display, unless the user chooses to view at "Print Size" (Photoshop, View > Print Size) or a similar mode, where the physical size of the displayed image will be shown at the physical dimensions of, for example, the print output (within the limits of the physical size of the display) - that is, you can take a ruler and measure the length of the displayed image edge, and it should correspond to the length of the printed image in that special display mode (see attached photo taken with my iPhone of my display). The display pixel density needs to be specified or detected automatically by the software to achieve this view mode.

You can figure out the display density by taking the number of pixels in a row of the display and dividing that value by the physical length of the edge of the display area, in inches, for example. My Eizo display measures 20.4 inches along the long edge, and its pixel resolution in this dimension is 1920 pixels. The pixel density is 1920 px / 20.4 in = 94 PPI. This value can be entered into the preferences of a specific application, if supported, to view "Print Size."

The "Zoom Level" is used to change the display of an image on a display - typically "100%" means one image pixel occupies one display pixel at native display resolution - however, applications on Retina and other highDPI displays may scale images so that one image pixel occupies more than one display pixel at 100%.

Kirk

Kirk,

I shot a RAW photo of a metric ruler using Canon S110, I brought that RAW file into Lightroom. I did some minor adjustment to Exposure and Clarity control, then I created a Virtual Copy of that image and output two JPG files.
I then used the EXIF editor from Opanda to alter the DPI value of one of the files, while I left the Lightroom default DPI data alone in the EXIF of the second image. Both images are identical in original pixel count (1600 x 599)

Here are the two images as screen capture regions, having different DPI values in the EXIF of the files.

Wilt wrote in post #19374747
Kirk,

I shot a RAW photo of a metric ruler using Canon S110, I brought that RAW file into Lightroom. I did some minor adjustment to Exposure and Clarity control, then I created a Virtual Copy of that image and output two JPG files.
I then used the EXIF editor from Opanda to alter the DPI value of one of the files, while I left the Lightroom default DPI data alone in the EXIF of the second image. Both images are identical in original pixel count (1600 x 599)

Here are the two images as screen capture regions, having different DPI values in the EXIF of the files.

I see zero difference in the display of two images on monitor, one with '72' dpi embedded in its EXIF and the other with '240' dpi embedded in their EXIF data.

Then I used the Canon-provided Zoombrowser EX software to make a print of both images, output to 8.5" x 11" plain paper using my Canon TS6320 printer, and I then folded both images so they would fit simultaneously on my Canon 8800F scanner, resulting in this JPG scan file

I see no difference in the scale's size or areas printed, between the image with 72 dpi in its EXIF and the image with 240 dpi in its EXIF. DPI only matters for offset press output (magazine/brochure printing) and has no effect upon home printer.

The DPI value can have effect upon a different pixel count in a JPG file during output phase from Lightroom, if you tell it 300 dpi on 8" dimension the file output has 2400 pixels, and if you tell it 75 dpi the file output has only 600 pixels in that same direction.
In my example, the pixel count was IDENTICAL and only the DPI value within EXIF was different.

Not saying you are wrong in your statement, merely proving that a statement differing from yours can be proven to be true, as well.

Your print software is automatically scaling the image file that is 1600 x 599 pixels so that it will print to the paper size you specified. Your PPI tags are therefore meaningless because the software is determining the PPI to scale the image file into the print size.

Some folks set the size of their prints manually because they want the print size to be exactly what they specify, at the PPI that they want for the print quality they are targeting - the explicitly set PPI tag will still determine the print size for the desired print quality.

As I said before, it does not matter how you manipulate the previous expression that relates image dimension, print size and PPI - that expression is what governs the relationship of image size in pixels to print size in inches (or centimeters, or miles, or any physical length dimension).

Your workflow uses software to automatically determine PPI because you specify the print size (in inches) for a given image size (in pixels).

As you noted, PPI has no effect on how an image is displayed.

Kirk

IOW, 'the dpi embedded makes the output behave in Manner A except when it behaves in Manner B!'

A generalization seems difficult to state, without complexity in the explanation.

The equation we are discussing in all these threads is PPIx = RESx / SIZEx (where PPI is the # of dots/pixels to use per inch, RESx is a dimensional # of pixels in the image file, and SIZEx is the output size desired).

Given the rule that RESx is constant for any given image file, the software (video or print driver) is either solving for PPIx or SIZEx. How that software does that is dependent on a) how it is written and how versatile or configurable it is and b) how you have configured it, use it, or your exif PPI setting.

No different really than electrical engineers given a power equation (P=IV). Given that one of the values is constant, you may decide to solve for one of the other two, example: You want to to keep voltage constant, so you either change your power needs or current supplied. For this analogy, voltage is your image file resolution, current is PPI and power is the final size of the output, where you either honor the final size needed of the image (power), or you want to dictate PPI (current).

Not being argumentative...your explanation makes sense. The challenge is that when we use given software to make a print, we do not know (when we are purchasing that software) how it is going to behave by reading descriptive product information (that isn't described). We have to buy and to try it, to learn how it is going to behave.

Even when sending JPG out to commercial printer, we do not know (by virtual of the embedded DPI value) what size print will result! I requested 12 x 36 canvas for case 1, and 20 x 60" canvas for case 2 and 3...

1. 4000 pixel tall (300 dpi embedded) image was printed to about 15" to produce 12" front on requested 12x36" canvas wrap mounted print ... 4000 / 300 = 13.3
   
2. 5400 pixel tall (240 dpi embedded) image was printed to about 23" to produce 20" front on requested 20x60" canvas wrap mounted print ... 5400 / 240 = 22.5
   
3. 5200 pixel tall (240 dpi embedded) image was printed to about 23" to produce 20" front on requested 20x60" canvas wrap mounted print ... 5200 / 240 = 21.7
   

That is true, software can, and often, create confusion. Even printing services, when you submit images, get things confused and will often state a minimum PPI and then a total resolution minimum, sometimes in conflict with each other or with a selected print size.