EOS Rebel T7 How to take photos at 300 dpi

[Redsed]

I am just learning how to take photos of my artwork to post on a marketing site. I am told I have to post a min. 300 DPI photo which means I need to change my settings and photograph all my artwork...again.  Can someone give me a simplified explanation about how to do that. Sorry if this sounds a little crazy but I have put myself in a position to learn every. single. thing. about websites and social media and cameras all at once.  If I can just take the right pictures I will be miles ahead of where I am now. There are plenty of tutorials about what to put the setting on when you start out but not what any of those settings actually mean.  Thank you so much for any information. 

[Waddizzle]

I agree with Ricky.  They should also define an absolute image size, defined either in pixels or in/mm..  But an overall size specification is independent of a DPI specification.

If you are using a Windows laptop or PC, I suggest downloading Paint Dot Net to adjust the size of JPEG files.  You can upsize or downsize canvas size, image size, and DPI or PPI.  I do not know if this easy to use app has ever been ported to MacOS.

Paint Dot Net is easy to learn image editor.  It is a good introduction to how advanced imaged editors function.  It is far more advanced than MS Paint.  But nowhere near as complex as GIMP or Photoshop.  It is an image editor, not a photo processor.

[Waddizzle]

This is what "Image Resize" dialog box looks like in Paint Dot Net.

Thre is a siimilar dialog box for "Canvas Resize".

[johnrmoyer]

You have received good information from others here about what DPI means. I am guessing you need to work around the ideas of the marketing site and not do not need to educate the marketing site.

I guess that a clueless marketing site might find it easy to have a computer program look at the image resolution values in the photo meta data. My Canon camera always puts a value of 72 DPI into both the RAW and JPEG files. If I edit the photo with Canon DPP free to download software, then DPP changes that value from 72 to 350. It might be that the only edits one would want to do are to crop, white balance, and digital lens optimizer, but the value would still change from 72 to 350 when a photo is saved. So, an easy way to change this value is to edit the photo, JPG or CR2, in the Canon DPP program and save it.

The free software exiftool program will display or change the value in the metadata. Changing the DPI value will not change the image, but in ancient times it would have changed how some web browsers displayed the image and how some programs printed it. https://exiftool.org/

exiftool -G0:2 -xresolution -yresolution -resolutionunit IMG_6277*
======== IMG_6277c.JPG
[EXIF:Image]    X Resolution                    : 350
[EXIF:Image]    Y Resolution                    : 350
[EXIF:Image]    Resolution Unit                 : inches
======== IMG_6277.CR3
[QuickTime:Image] X Resolution                  : 72
[QuickTime:Image] Y Resolution                  : 72
[EXIF:Image]    Resolution Unit                 : inches
======== IMG_6277cs.JPG
[EXIF:Image]    X Resolution                    : 350
[EXIF:Image]    Y Resolution                    : 350
[EXIF:Image]    Resolution Unit                 : inches
======== IMG_6277.dr4
======== IMG_6277.JPG
[EXIF:Image]    X Resolution                    : 72
[EXIF:Image]    Y Resolution                    : 72
[EXIF:Image]    Resolution Unit                 : inches

[Waddizzle]

“ The free software exiftool program will display or change the value in the metadata. Changing the DPI value will not change the image, but in ancient times it would have changed how some web browsers displayed the image and how some programs printed it. https://exiftool.org/ “

Changing DPI should change how an image is displayed on a monitor.  Displaying an image on a monitor mean you use a different device Type.  Printers and Monitors have the same base class and share the same interfaces.

[johnrmoyer]

In ancient times, changing the DPI in the metadata would for some software change how the image is displayed on the screen. I cannot think of any software that would do that now. Do you have an example?

It has been over 30 years since I wrote a printer driver and I do not know much about modern software for printing. I might guess that modern software might look at paper size and pixel resolution before resizing the image for print and ignore the DPI in the meta data. Is that not the case?

[rs-eos]

macOS to this day (ever since Mac retina displays existed).

All screenshots captured on my Mac prior to having a retina display (that I still have stored) display as having 72 DPI when looking at properties within the Preview application.  All screenshots taken though when you have a retina display will be marked with 144 DPI.

So when displaying screenshots now on my Mac with 5K display (retina) in Preview:

• If the image was older (2560 x 1440 at 72 dpi), when viewed at 100% it fills my entire screen.  Each pixel in the image is backed by four physical pixels on the display.
• If the image was newer (5120 x 2280 at 144 dpi), when viewed at 100% it also fills my entire screen.  Each pixel in the image is backed by a single pixel on the display.
• If modifying the DPI in a new screenshot to be 72 dpi, then viewing that at 100%, only a 25% area section can be viewed at any given time on my display.  Each pixel in the image is now backed by four pixels on the display.

[johnrmoyer]

I do not think I understand what you are saying about macOS. Maybe I have changed some settings?

I have a 2019 iMac with a 5120x2880 display. The same JPEG will display the same in Preview.app whether the meta data says 72dpi or says 350dpi when I select view actual size.

When I view an image on my iMac in Chrome, Firefox, or gimp and zoom to 100%, it is displayed pixel for pixel with no reference to dpi. First image is coyote face screen shot from preview displaying out of camera JPG with 72 dpi in the metadata.

Second image is same photo and has been edited in Canon DPP and saved as a JPG with 350 dpi in the meta data, a screenshot viewed in preview at actual size.

I conclude that the value in the meta data is ignored. There is apparently the same number of pixels on the coyote face if I had cropped the screen shot the same. (I remember software in the 1990s that would use a bad algorithm to scale any image that was not 72dpi before displaying it)

[rs-eos]

It's probably because you chose an arbitrary 350 value.  I provided the full details regarding 72 vs 144 values which indeed drive different results.

[rs-eos]

While not specifically tied to DPI values in images, other examples of where DPI values of your display are used:

When doing Apple device development and running things in simulators (e.g. iPhone simulator), there are now three different modes to have the virtual device displayed on your Mac.  Some of the modes take not only the DPI value of the simulated hardware, but the actual DPI value of your display:

• Physical size.  The simulator window would match a real-world device size (e.g. iPhone 14 Pro)
• Point accurate.  This gets complex as on a retina Mac display (@2x display), you can have simulated @1x, @2x and @3x devices.  It's easiest to understand when both the Mac's display and simulated device are both the same scale (e.g. @2x).  So each "point" on both will be the same exact quantity.
• Pixel accurate.  Each pixel on the simulated device is backed by a single pixel on the Mac display.

Finally, Adobe Illustrator has a "Display Print Size at 100% Zoom" setting.  When enabled, any document viewed at 100% size on your display will also be the real world size.  e.g. for US Letter, I can hold up a US letter sheet to my display and it matches perfectly.

[johnrmoyer]

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@rs-eos wrote:

While not specifically tied to DPI values in images, other examples of where DPI values of your display are used:

When doing Apple device development and running things in simulators (e.g. iPhone simulator), there are now three different modes to have the virtual device displayed on your Mac.  Some of the modes take not only the DPI value of the simulated hardware, but the actual DPI value of your display:

• Physical size.  The simulator window would match a real-world device size (e.g. iPhone 14 Pro)
• Point accurate.  This gets complex as on a retina Mac display (@2x display), you can have simulated @1x, @2x and @3x devices.  It's easiest to understand when both the Mac's display and simulated device are both the same scale (e.g. @2x).  So each "point" on both will be the same exact quantity.
• Pixel accurate.  Each pixel on the simulated device is backed by a single pixel on the Mac display.

Finally, Adobe Illustrator has a "Display Print Size at 100% Zoom" setting.  When enabled, any document viewed at 100% size on your display will also be the real world size.  e.g. for US Letter, I can hold up a US letter sheet to my display and it matches perfectly.

---

It has been a very long time since I used any Adobe software, but I remember specifying DPI when writing a PDF. I understand scaling to display the entire image, but I do not expect that scaling to depend upon the dpi number recorded in the meta data. I understand print preview showing the paper size, but the paper size does not depend upon the dpi value in the image meta data. The dpi is either set by the printer and the image is scaled to that value, or the image is cropped or the image does not fill the paper. I understand the need to know the dpi of the display or the DPI used by the printer when choosing elements like fonts or icons along with an assumption of viewing distance.

At some point in the past, I also understood image handling in CSS and HTML ( https://drafts.csswg.org/css-values/#resolution ) where assumed viewing distance is considered ( https://www.w3.org/TR/css-values-4/#reference-pixel ) .