DPI

Consider how we value banknotes. I pay a certain sum of cash into my bank in the form of $10 bills. Will I later double that value  if I withdraw the same money, in the form of $20 bills instead? No: because to achieve the same total value, the bank will have given me half as many bills. In isolation, the dollars per one bill (Pixels Per Inch) tells you nothing. Also in isolation, the number of bills tells you nothing (inches wide and high). Only when these two pieces of information are combined, is any particular total cash value being expressed.

And there are two ways to express a certain quantity: with a real complex scenario / a simple count.

Say - "five $20 bills in an envelope" / "10 inches wide @ 300ppi" is complex. If we want to compare against "three $50 bills in an envelope" / 54 inches wide @ 96ppi" we have some calculations to do, and that calculation must involve converting the various denominations and counts into a simple total dollar value / converting the particular printing scenario, into a simple number stating how many pixels there are in total.

So one might as well just define, discuss and compare cash - or digital images - on that simple basis in the first place. 

Until this week I have been using Affinity Photo 2 to edit my RAW images and when I export a file it is 300 DPI with a typical file size around 20 MB. 

I've just switched to Lightroom Classic and am learning my way round it. When I export the very same RAW image it yields a 96 DPI and the file size is only around 7 MB - both values suggest it is a less detailed image. 

By @dumbshoes

The important thing about PPI is that it only has meaning when you specify both pixels and inches. Because it’s pixels per inch. If you only say 300 ppi or 96 ppi but you never say how many pixels or inches long it is, the PPI value isn’t based on enough info to have any meaning.

By the way, this is the reason why Rob_Cullen is able to post 1 ppi and 1000 ppi images that look exactly the same. If you have two images that are the same width and height in pixels, they have the same information regardless of the PPI resolution value.

The details:

Suppose we have an image from a camera that shoots 6000 pixels wide x 4000 pixels tall (6000 * 4000 = 24,000,000 pixels, or 24 megapixels). What is the difference between that being 96 ppi and 1000 ppi?

The answer depends on the other numbers: Dimensions in pixels, and dimensions in inches.

If the number of pixels doesn’t change — if it remains 24 megapixels — then the amount of data and potential detail is the same regardless of the ppi setting. No pixels have been added or taken away. 24 megapixels is 24 megapixels at 96 ppi or 300 ppi.

How then do you get a “reduction in quality” at 96 ppi? Well, this only happens if you did something to reduce the total number of pixels. How does that happen? If you actually stated dimensions in pixels or inches, which you haven’t mentioned yet.

Example:

96 ppi for a 24 megapixel image, in the absence of any specific width and height dimensions, is still 24 megapixels. No loss. You could assign 300 ppi and there is no change.

But…

If you add specific dimensions in pixels or inches, along with ppi, now the number of pixels must change, because the way you get a meaningful ppi value is, a dimension (such as width) in pixels divided by the same dimension in inches.

So…

If you have a 6000-pixel-wide original image and you export it at 6 inches wide at 300 ppi, it must become 1800 pixels wide. That is the only number that lets it be 6 inches wide at 300 ppi.

If you have a 6000-pixel-wide image and you export it at 6 inches wide at 96 ppi, it must become 576 pixels wide. This is where this 96 ppi export becomes less quality than the 300 ppi export, when both are exported 6 inches wide.

That is why, if you state a ppi value without also stating either the length in pixels or the length in inches, then ppi has no meaning, so all ppi values have the same number of pixels. In this example the effective resolution of the 6000 x 4000 pixel image is fluid, changing depending on how large it’s reproduced in the real world. Regardless of what the ppi value in the file says, the effective ppi is any of the following:

Printed 10 inches wide (e.g. 10 x 8 inch print), it’s 600 ppi (because 6000 pixels / 10 inches = 600 ppi).

Printed 6 inches wide (e.g. 6 x 4 inch print), it’s 1000 ppi (because 6000 px / 6 in = 1000 ppi).

Printed 30 inches wide (e.g. 30 x 20 inch print), it’s 200 ppi (because 6000 / 30 = 200 ppi)

Both Affinity Photo and Lightroom Classic are able to export the full pixel dimensions of an original image, if that’s how you set up your export. If you export from either program by specifying any two of the three values in the resolution formula (dimensions in pixels, dimensions in inches, and ppi), then according to that formula, the image will be resampled (number of pixels changed) to your specifications. But if all you said was ”300 ppi” without also stating dimensions in pixels or inches, then all original pixels are still there, and effective ppi depends on how large it’s finally reproduced.

By the way, none of this accounts for differences in file size. That’s a totally different subject. File size depends not only on dimensions in pixels, but also on the file format, and if the file format uses compression, what kind, and how much. For this reason, file size is a very incomplete way of determining real image quality.

My practical exercise-

Two JPG Exports:  100%, full pixel size,   PPI 1 , and PPI 1000.   Comparison at 100%

Can you tell the difference?

Until this week I have been using Affinity Photo 2 to edit my RAW images and when I export a file it is 300 DPI with a typical file size around 20 MB.

Most likely you mean 300 PPI. Do not confuse PPI and DPI, they are not the same and one does not imply the other. In most cases the PPI is meaningless, where you are keeping a digital file as digital, the PPI tells you nothing about a digital file. It will cause you to make poor or harmful decisions if you apply meaning to. (The primary case where PPI is meaningful is when you print, and its not the number in the file, it is the number of pixels on each edge divided by the size of the print one each edge).

I don't want to spend a long time editing many RAW images to then realise at the end that I'll only have 96 DPI files which aren't as 'good' as the 300.

Meaningless (there's that word again) comparison. The pixels haven't changed at all comparing a 96 PPI image to a 300 PPI image. The photo is just as "good", either way.

I've just switched to Lightroom Classic and am learning my way round it. When I export the very same RAW image it yields a 96 DPI and the file size is only around 7 MB - both values suggest it is a less detailed image.

When you export from any program to JPG (I assume that's what you're doing, but you never mention JPG), there is a quality setting (I don't know what it is called in Affinity) which lets you increase or decrease the quality of the export, and certainly lower quality exports will be smaller. Most people use the LrC quality setting to be around 70 or 80. Above that, the JPGs get vary large, with NO VISIBLE CHANGE IN QUALITY. Please read this for more information: https://regex.info/blog/lightroom-goodies/jpeg-quality

So similarly to PPI, do NOT make the mistake of assigning some meaning to file size number, a meaningless value. JPG File size also depend on a lot of other things which you cannot control. A 7MB JPG can be visually identical to a 20MB JPG of the same photo. How can you be sure? You look at the photo with your own eyes. Can you see a difference between the Affinity export and the LrC export. I am 100% certain you cannot see a difference unless you blow the image up to beyond 100%. So all of your fears, based upon looking at these numbers, are unfounded.