Photo size vs. Quality / DPI

[56_kruiser]

I downloaded a photo from my daughter's site. When it downloaded, it was only 499KB. So, I thought it was low resolution.

I texted her, and she asked what the pixel resolution was, and it was 2250X1500. She also expected it to be 300 PPI.

I don't see PPI listed in the help file for Affinity Pro, but I do see DPI listed. But I don't see how to tell what the DPI is of this photo. When I select Resize Document, it shows 72 DPI. But I don't know if that is the size it is, or the default it is using for me to resize to.

That pixel resolution correlates  to ~31X20 inches. I can't imagine that a 499KB file would print on 31X20 and be any good.

How can I tell what DPI it actually is?

Maybe I'm just needing a bit of education.

Appreciate any thoughts.

[firstdefence]

Take a look at this post...

 

[Fixx]

Well, was it 2250X1500 or not? 

DPI / PPI does not matter for web page images. Only pixel count counts.

If file size is 499KB it may just mean that image is very compressed. You may check the quality visually to see if there are artifacts or no.

https://en.wikipedia.org/wiki/Image_resolution may be helpful reading.

[carl123]

8 hours ago, 56_kruiser said:

I downloaded a photo from my daughter's site

1. How did you download it?

2. Did you File > Place it into a new/existing document?

3. Can you supply a link to the photo?

[56_kruiser]

To answer questions...

Yes it IS 2250X1500. What I have decided since posting this thread is that it is that resolution, but it is a dpi of 72, not 300. If I resize the document to 300 dpi, it saves as 12mb photo. I am presuming that if the photo was saved at 72dpi, then data is lost, and resaving at 300dpi will not be the same as the original file. No?

I downloaded it from my daughter's web site with the download button. No options for selecting size. I did not do a File> Place.

Here is a link to the photo in my Dropbox-> LINK

 

 

[v_kyr]

See: Affinity Photo - Understanding DPI from which you can see what will happen with the resolution, dependent on resampling or not.

Also you might want to see some other aspects of the software (tutorials) and their general usage here.

[Fixx]

15 hours ago, 56_kruiser said:

Yes it IS 2250X1500. What I have decided since posting this thread is that it is that resolution, but it is a dpi of 72, not 300. If I resize the document to 300 dpi, it saves as 12mb photo. I am presuming that if the photo was saved at 72dpi, then data is lost, and resaving at 300dpi will not be the same as the original file. No?

If you keep the image 2250X1500, it contains the same data regardless if the DPI is 72 or 300. JPEG compression may degrade the quality and change file size but amount of pixels stay unless you resample.

[DM1]

To add further confusion: My thoughts after reading many articles and reviewing Affinities DPI video.

The image pixel array of 2250x1500px is the 'resolution of your image. Usually determined by the sensor used to capture the image (but may be changed in software through resampling techniques, to add or remove pixels). I'll call this image pixel array our 'Image Resolution '.

PPI is a technical measure describing how densely pixels are displayed on a digital screen. Different screens may have different PPI values. For example, the iPad Pro screen has 246 PPI. I'll call this our 'Screen Resolution'.  

If we divide 2250px by 246PPI we would see that our image appears a little over 9” wide on our iPad screen. The same 2250x1500px image displayed on a 55” HD display monitor having around 40PPI would appear to be around 56” wide. In both cases the image resolution is the same (2250x1500) but looks a higher quality on the smaller screen with more densely populated pixels. That's just how our eyes work.

DPI is a measure of print density in dots placed every inch of print medium, Similar to our  PPI / screen resolution analogy, the greater the number of dots placed per inch, the higher the perceived resolution. In practice, current print resolutions can reproduce each pixel of our images using 'many' dots to form each pixels shape and colour on paper but let’s not go there. I'll call DPI our  'Print Resolution '.

 It seems then, that there is no 'direct' correlation between PPI and DPI? One is an ink dot and one is a digital 'pixel'. Again, in practice software developers have begun to use both terms interchangeably, creating some confusion as to what either term actually means.

It is 'generally' accepted that a good quality print can be obtained at a printer resolution of 300 DPI. Of course this assumes that to begin with, we have sufficient pixels available in our initial  Image to produce a quality output.. When viewed at a close distance, a low resolution image will never look good, regardless of the  'print' density used to place it on the paper. Distance does change our perception of quality (think bill-boards) but let’s ignore that for now. This raises the question then of, “How many pixels do we need in our image?”  That answer depends on how large we want to print or image.

In Affinity Photo, for any given print size, the DPI setting is used to determine the pixel array required (our 'Image Resolution'), in order to produce our print output at the chosen 'Print Resolution '. 

For a 6”x4” print at 300 DPI, we will require an image resolution of at least 1800x1200px. At 600 DPI our 6”x4” Print will require an image resolution of at least 3600x2400px. Note, you can still print at 300 DPI or indeed 600 DPI using an image with much lower 'image resolution' than recommended by the software, it just means your finished print will not look as sharp because it has less pixels (lower quality) to begin with. If you have more pixels than are required it simply means your image will extend outside the print boundaries, requiring you to reposition,  or crop the image with print boundaries. I'm going to ignore 'resampling' in order to reduce the number of pixels, although it is possible to do this too.

In making this calculation, the software is 'assuming' that 1 pixel equals 1 dot. We know that is not 'necessarily' true, given printers are capable of many dots per pixel, but for our purpose the 1:1 correlation works well enough. So if a dot is used to equal a pixel (for print sizing purposes) the term DPI can be used interchangeably with PPI. Hence the confusion in the use of these two terms.

It really doesn’t matter whether you label it DPI or PPI, what is important is that your initial image has sufficient pixels to produce the required print quality at the desired print size at normal viewing distance.

Getting back to the OP's Image then. We have an image quality of 2250x1500px. Printed at 300 DPI using AP Settings, would produce a print size of 7.5x5”.  At 600 DPI, AP would produce a 3.75x2.5” Print. Until printing, the DPI setting has no effect on the image, (set whatever you like) just avoid resampling the image as may unnecessarily add or remove pixels from your original image.

Of course, I too may be totally confused by all the available information, and have gotten all this wrong! If so, please let me know! 

[DM1]

Using the linked Image you can see that changing DPI from 72, to 300, then to 600 can be achieved with NO change to image resolution. Just remember to deselect 'resample'.

 

[Barney46]

Affinity have got themselves in muddle here.  DPI is an instruction to the printer defining the density of ink dots to be used in printing process. 

The image on your screen is unaffected by DPI. 

The screen image size and the ultimate printed image size are set by the number of pixels per inch, PPI.

PPI is the only dimension that matters - until you are printing.  If you set the DPI below 4x the PPI the printer will reduce the amount of ink it uses and the image will print with less contrast and reduced clarity.

Print manufactures always quote DPI because it looks good on the box;  Maximum 2,400 DPI  looks much more tempting than the meaningful resolution maximum of 600 PPI. It's value is in the fact that the higher the dpi the higher the quality of the final printed image.

So ignore DPI unless you are in  the process of printing. In any event most domestic printers generally default to 300ppi and 1,200dpi.   Espom go their own way with 360ppi and 1,440 dpi

 

[R C-R]

5 hours ago, Barney46 said:

The screen image size and the ultimate printed image size are set by the number of pixels per inch, PPI.

That isn't always true. Many if not most printer driver software UI's offer scaling settings of one kind or another to (for example) scale the print by a specific percent or to automatically scale it to fit all of the printable area. There may also be other options like setting the orientation to portrait or landscape that will in combination with scaling change the printed image size.

[Old Bruce]

23 hours ago, Barney46 said:

Affinity have got themselves in muddle here.  DPI is an instruction to the printer defining the density of ink dots to be used in printing process. 

Dots Per Inch and Pixels Per Inch are different things, dots are not pixels and vice versa.

Having said that I think we have to accept that the public at large, and quite likely a lot of computer programmers, consider them to be interchangeable.

Dots per inch in digital photography is short hand for how large in inches an image of 1600 pixels by 2400 pixels will be in inches. At 100 dpi it is 16 by 24 inches, at 400 dpi it is 4 by 6 inches, etc. The dpi setting tells the printer here is something that is 1600 by 2400 pixels, print it at its stated dpi, which is ''x", and will therefore result in 1600/x inches by  2400/x inches. There may well be other scaling going on after that conversion in order to "fit to the Paper size" or reduce to fit four copies on the paper size instead of being at 100%..

It is wrong to say and use Dots, we should insist on Pixels. However I am not interested in fighting that battle, it is not a hill on which I choose to die. In fact it is a hill which blocks my view of a stunning vista. I accept that most people see no difference in DPI and PPI.

Finally do not confuse a printer's nozzle's ability to put out 1200 separate Dots of ink Per Inch with an image's setting of 1200 (or 300) dpi.

[R C-R]

1 hour ago, Old Bruce said:

There may well be other scaling going on after that conversion in order to "fit to the Paper size" or reduce to fit four copies on the paper size instead of being at 100%..

Which is why it can be misleading to say the physical size of the print is set only by the document PPI or DPI.

1 hour ago, Old Bruce said:

It is wrong to say and use Dots, we should insist on Pixels.

Considering the above & the various ways printing devices deposit ink, toner, or other ink-like material on print media, I am not sure either one is a 100% complete & accurate way to describe the actual resolution of a physical print. After all, not all printers squirt wet ink from nozzles, not all media reacts the same to the printing material, & so on.