DPI

[dumbshoes]

Searches show this topic has been discussed before, so apologies, but I'm not seeing anything definitive and if anything the answers seem a bit contradictory.

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. 

When exporting in Lightroom, the 'image sizing' box seemingly let's you change 'resolution' in pixels per inch or cm, but there's no mention of DPI. I've seen people say there is no difference between PPI or DPI but the file sizes suggests there must be? As someone who prints their photos and doesn't put them online, I wanted to get to the bottom of it.

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. 

I asked at my local printer store who print my photos (Snappy Snaps) if 96 v 300 DPI was a big deal with printing and their reply was 'You'd be best exporting your image higher than 96dpi as you wont get the best quality image when printed.'

Can anyone shed any light on 96 DPI and the low file size when using Lightroom? Is the solution to edit the photos then use a totally different program to change the DPI? 

Many thanks 

[dj_paige]

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.

[dumbshoes]

Thanks for taking the time to reply.

Most likely you mean 300 PPI. Do not confuse PPI and DPI, they are not the same and one does not imply the other…

I do mean DPI and not PPI i.e. dots per inch. I’ve always read that 300 DPI is important when it comes to printing compared to a lower number, say 76 DPI. It’s also what my quote referred to when I said I had contacted my local printer who advised to aim for 300 DPI when printing. When I edit my photos my only concern is making them as good as possible to print which is why I’m fixating on the DPI. I have no interest in the ‘digital only’ aspect of the photos as I don’t put them on websites, Instagram etc.

https://www.adobe.com/uk/creativecloud/photography/discover/dots-per-inch-dpi-resolution.html

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...

Yes, I’m exporting as JPG. My export settings are set to 100% quality and the images sit at around 7MB. It just seemed very small to me when my others from Affinity (also set to 100% quality) sit 20 MB – 25 MB. I may be putting too much stock in file size tho from what you’re saying. However,  would expect a 300 DPI image to be larger than a 96 DPI photo.

[JohanElzenga]

DPI and PPI are often confused, and unfortunately many software vendors make the same mistake as you do. Even Adobe does in the link you posted (possibly in an attempt to make things easier to understand). It is PPI, not DPI. Images are made of pixels, not of dots, so the resolution of an image can only be expressed in pixels per inch, not in dots per inch. Dots are the small points of ink that a printer places on paper. In case of an inkjet printer you could say that the D stands for 'droplets'. And that is where the difference occurs. A printer uses multiple dots to print one pixel. It could be as much as an 8x8 dots pattern for a single pixel. So in order to print an image in 300 PPI, the printer could use a print resolution of up to 2400 DPI 'under the hood'.

Because this 'under the hood' part is irrelevant for the user, it's harmless to use the term 'DPI' for images that will be printed, as long as you understand that it is not that 2400 DPI that the printer really uses, but the 300 PPI that your image needs to be. Then it becomes just semantics, nothing more than that. Lightroom will only use PPI, Photoshop also uses the correct term these days, but much older versions of Photoshop did indeed (incorrectly) use DPI, hence the confusion.

[dumbshoes]

Thanks for your reply.

"...as long as you understand that it is not that 2400 DPI that the printer really uses, but the 300 PPI that your image needs to be." 

So I guess this is the bit I'm coming back to. If there is an acknowledgment that 300 PPI is what my image needs to be, then how do I do it? Cos I'm only seeing 96 PPI in my properties box. 

To explain that more.... when I edit my photo in Lightroom then click 'export' it brings up a lot of boxes. If I use the 'Image Sizing' box to set the width and height manually and also use it to select a resolution of 300 PPI, it still saves it as the original wifth and height and at a resolutio of 96 DPI. It is like it simply ignore my instructions in the 'Image Sizing' box. I am seeing these values when I right click on my exported file and go to properties. 

[Per Berntsen]

Please post a screenshot of the Export dialog with your chosen settings.

Do not attach the screenshot, use the Insert Photos button in the toolbar to embed it in your post.

And I have to ask you once again, where are you seeing the 96 PPI for the exported image?

[dumbshoes]

I am seeing the DPI value when I right click and go to properties in WIndows. From reading the replies I don't think this is the best way to do it but I always took this number at face value thinking it was accurate.

You may be able to see from my screenshot that an edit done in Affinity Photo is giving me a 300 DPI whereas the Lightroom edit of the exact same photo is 96 DPI - the Lightroom version is higher in file fize.  

[JohanElzenga]

Yes, that question is important. What do you use to check the PPI value of the exported image? Because PPI is irrelevant in most cases, some apps simply do not read that metadata and report '96 dpi' regardless of what the real PPI value is. If you used 300 PPI in the Lightroom Classic export dialog, then that is what you will get. To see if that is true, do not use something like a properties dialog in Windows, but open the image in Photoshop and check it there. 

[dumbshoes]

I have just added a reply to another comment containing a screenshot of where I see my image's DPI value (note not the PPI value). It was this DPI value that I was always using to judge whether or not my print would be of higher quality or not as I was always led to believe (by printers!) that a 300 or 600 DPI value was required to get a good quality print. Up until this weekend I didn't know there was such a thing as PPI which is of course what everyone here is discussing. 

[JohanElzenga]

Like I said: do not check this in Windows, because it might not give you the correct value. As you can see in your screenhot, both images have (almost) exactly the same pixel size, so the 'dpi' (Windows makes the mistake too; this is PPI) does not make a difference in image quality. If you do see a difference at all, I would expect the Lightroom Classic image to be better, not worse, because the file size of 8.1 MB versus 2.9 MB for Affinity suggests that Affinity has used a stronger JPEG-compression.

[Per Berntsen]

Can you post a screenshot of the Export dialog as requested earlier?

In my experience, Windows does report the PPI value correctly, so I'd like to see the Export settings.

Also note that what Windows calls DPI is in reality PPI.

As I have explained earlier, DPI is not an image property, its a printer/print property.

Are you saying that if I give a JPG photo to the store for them to print it for me, it is they who set/control the DPI (as they are the ones in control of the printer) and therefore DPI isn't a setting that I have control over during the editing/exporting stage? The store are telling me a good quality print will be from an image that is in 300 DPI or even 600 DPI.

Yes, the DPI is set when printing. It's all explained in my first post.

https://community.adobe.com/t5/lightroom-classic-discussions/dpi/m-p/14972273#M386298

300 PPI is enough. If you were to print at 600, you would need four times as many pixels to print at the same physical size.

A 6000 x 4000 pixel image will print at 20 x 13.33 inches at 300 PPI.

To print this image (same physical size) at 600 PPI you would need 12000 x 8000 pixels.

[dumbshoes]

This is the image sizing box that appears in Lightroom during the export process. I am selecting 600 PPI but when I then go to Properties it is giving a PPI of 96. Although from the comments here I am seeing that Windows isn't necessarily correct.  

When I then load that same photo in to Photosthop and go to image size it shows me this:

[dj_paige]

I do mean DPI and not PPI i.e. dots per inch. I’ve always read that 300 DPI is important when it comes to printing compared to a lower number, say 76 DPI.

The number in the image file is not DPI. It is PPI.

DPI is controlled by the printer itself, some printers can print more dots per inch than other printers, and even within a given printer, you can select higher dots per inch, or lower dots per inch, from the same 300 PPI or 96 PPI digital image file.

Lightroom Classic does not control DPI at all.

Do not confuse DPI and PPI, these are not the same, and one does not imply the other.

Yes, I’m exporting as JPG. My export settings are set to 100% quality and the images sit at around 7MB. It just seemed very small to me when my others from Affinity (also set to 100% quality) sit 20 MB – 25 MB. I may be putting too much stock in file size tho from what you’re saying. However,  would expect a 300 DPI image to be larger than a 96 DPI photo.

As I said, the PPI number in the file is meaningless (and it is not DPI, it is PPI). If you export a photo from LrC, and you choose the export to be resized with units in pixels (or not resized), then an export with 96 PPI has the exact same pixels as an export with 300 PPI. Exact same pixels. No difference in appearance. No difference in file size or image size either.

As I said, if you assign some meaning to this PPI number, you will make incorrect and potentially detrimental decisions.

[dumbshoes]

DPI is controlled by the printer itself, some printers can print more dots per inch than other printers, and even within a given printer, you can select higher dots per inch, or lower dots per inch, from the same 300 PPI or 96 PPI digital image file.

Sorry for being thick on this. Are you saying that if I give a JPG photo to the store for them to print it for me, it is they who set/control the DPI (as they are the ones in control of the printer) and therefore DPI isn't a setting that I have control over during the editing/exporting stage? The store are telling me a good quality print will be from an image that is in 300 DPI or even 600 DPI. 

[Per Berntsen]

The Adobe article you linked to unfortunately contains misleading, and even wrong information.

This is how PPI and DPI works:

PPI (Pixels Per Inch) is an image property – the number of image pixels used to print one inch on paper.

Pixel dimensions divided by PPI = Printed dimensions in inches.

You set the PPI in the Resolution field when exporting from LrC.

The PPI value tells you nothing about image size, pixel dimensions do. But it does tell you what size the printed image will be if you do the math.

PPI and DPI are often confused, and many people use DPI when they should be using PPI. Even Windows gets this wrong.

DPI (Dots Per Inch) is a printer property – the number of ink dots a printer uses to print one inch one paper.

The article claims that images have a DPI resolution, but they don't. The resolution for an image is always PPI.

The DPI is set in the printer driver, and is typically much higher than the PPI value of the image.

I routinely make 33 x 44" prints on my own printer at 180 PPI. Using a higher value does not improve image quality. (and it also requires more pixels, see the formula above)

In the printer driver I set the DPI to 1440. Lower values (720 or 360) lead to reduced image quality.

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. 

Where are you seeing a PPI value of 96? If you export at 300, the value should stick.

The file size of a jpg is determined by three factors:

• Pixel dimensions
• Quality setting used when exporting
• Image content

Assuming that the first two are identical, the file size can still vary a great deal, depending on image content.

An image with predominantly smooth, flat or out of focus areas will compress well, leading to a relatively small file size.

An image with lots of sharp, busy detail (or noise) will be much harder to compress, leading to a relatively large file size.

40 kb, exported at Quality 80

146 kb, exported at Quality 80

So the file size of a jpg is not an indicator of image quality.

The only way to determine the quality of any image is to view it at 100%.

At this magnification, one screen pixel is used to display one image pixel, giving you a true representation of the image.

Any other magnification will be inaccurate and misleading because the image has been scaled.

[Rob_Cullen]

My practical exercise-

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

Can you tell the difference?

[dumbshoes]

I can't tell the difference. Would the quality of both images still be similar if the two were printed and not just vewed digitally?

[Rob_Cullen]

In reply to -"Would the quality of both images still be similar if the two were printed and not just vewed digitally?"

Printed at the same SIZE-  Yes, absolutely IDENTICAL. They are IDENTICAL pixel dimensions. Same size prints = same "quality".

[Per Berntsen]

The print quality (and size) would only be the same if they had the same PPI.

If we assume a 6000 x 4000 image, it would print at 6000 x 4000 inches at 1 PPI, and 6 x 4 inches at 1000 PPI.

The 1 PPI version would be very low quality (each image pixel would be one square inch on paper), the 1000 PPI version would be high quality.

PPI is optional metadata, and is not required for screen viewing. If there is a PPI value, it will have no effect on image display – it will display according to its pixel dimensions. PPI is for printing only.

[richardplondon]

The practical reality is that an image of certain pixel dimensions - say, 6000px  x 4000px - is saved and sent to a printer with instructions that it should be output to a certain physical size. To deliver on those instructions, any PPI setting that is saved into the digital image will almost always need to be disreagarded. The result of printing that many pixels at that physical size, will be whatever resultant PPI it happens to work out to.

Where we take account of PPI as a "proxy" for printed detail / quality, is when we combine a certain number of original pixels with a certain physical scaling. In effect, are there enough of the former to meet our expectations for the latter. But we have to be considering a real-world resultant PPI, not whatever notional PPI may be written into the digital image, to conclude anything useful.

Similarly, we take account of PPI - called DPI, sometimes in this context - when scanning a negative: because the negative is of a known physical size, this number can act as a "quality proxy" for evaluating whether we will be capturing lots of pixels TOTAL, or only a few pixels TOTAL. 

[Conrad_C]

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.

[richardplondon]

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.