Can someone explain Pixel Per Inch to me

[chiffs]

Currently using LR4 and i dont really understand what i should be using reference PPI. LR4 is set at 300 at the moment.

I understand it is pixels per inch (dots of colour that make up the image) but how high and how low can i go with this or should i just leave it as is?

 

[Brachytron]

300 pixels per inch is a standard adopted for most printing applications that gives high quality images. For example, most magazines will reproduce images at 300 ppi. You are probably best leaving it as it is. Going higher is not necessary as it will only reduce the dimensions of the image but with no gain in print quality.

Changing the ppi makes no difference to the actual image as long as you don't resample.

My 18MP gives images 43.89x29.26cm at 300dpi. At 400 pixels per inch, it is only 32.92x21.95cm, but I gain nothing. However, if I reduce to 200 pixels per inch, I get an image 65.84x43.89cm with only a small drop in quality. Certainly nothing to cause a problem if home printing on an ink jet.

 

[Chappers]

Deleted

 

[DizMatt]

personally I think that PPI is a daft way to measure something,
it should be an output of the printing format chosen and pixels in an image rather than a measurement of image size. it's merely image quality...unless there is a standard image printing size of which I'm not aware on which this is based.
Chiffs...ignore my rant..

for my experiences, for nice photographs, then 300dpi or ppi is great
for larger prints like huge A0 posters, then you can go as low as 80ppi or similar as people will stand further back when viewing imho. for medium posters 150dpi is what I aim for.
^^ the above wouldn't be appropriate for presentation posters showing small text of course.

as for lightroom4 setting DPI....see earlier rant. i'm very confused by this 'standard'

 

[Chappers]

The resolution setting in Lightroom simply sets a field in the files meatdata. This enables applications such as InDesign to quickly establish, for the designer, what size the image will reproduce on the printed page. It has no effect on the file size. Just export two images, one at 300 ppi and another at 600ppi. The file size is the same.

Leave it at 300. It keeps most people happy

 

[EOS_JD]

To understand it here's a way.

Lets say you have 6,000,000 sweets (all of those hundreds and thousands)

If you place those in rectangle on the floor and have 3000 sweets across by 2000 down in an area that is 12 inches across by 8 inches down then you can easily work out the spi (sweets per inch). 3000/12 = 250 and 2000/8 = 250

So 250 spi

Now if I squeeze those sweets a little closer to each other so that the 6m sweets are now contained in an area that is now 6x4 inches then the spi figure will be higher =

3000/6 = 500
2000/4 = 500

Both the above have the same number of sweets but they are just packed a little closer together.

If you increased the area covered then the spi would be a lower figure.

Convert sweets to pixels and you have it.

The calculation is very simple arithmetic.

Px = Pixels
R = Resolution
Ps = Print Size

Px = Ps x R
Ps = Px/R
R = Ps/R

So you can work out what you need at any time. I'll respond to some other points too..... below

 

[EOS_JD]

I understand it is pixels per inch (dots of colour that make up the image) but how high and how low can i go with this or should i just leave it as is?

It's a little more complex.

As you noted above a pixel is a single point on an image (it's not really a dot as it's not a physical thing). It becomes a dot (or series of dots) when printed. That's why printer resolutions are measured in dots. An inkjet printer uses numerous dots of ink to replicate the colour of a single pixel as the printer has to place the right amounts of it's ink to generate the colour of the pixel. That is why printer resolutions sound very high.

I disagree with the other points saying leave it though! I'll explain as I go.

 

[EOS_JD]

300 pixels per inch is a standard adopted for most printing applications that gives high quality images. For example, most magazines will reproduce images at 300 ppi. You are probably best leaving it as it is. Going higher is not necessary as it will only reduce the dimensions of the image but with no gain in print quality.

Whilst I agree, I would also say that printing at 240ppi or possibly even lower will show no difference in the quality of print. If you have enough pixels available to print at 300ppi then by all means use 300, but there is no point upscaling your image (and reducing the quality of your image) if you don't need to.

Changing the ppi makes no difference to the actual image as long as you don't resample.

A very important point that is quite correct. To add more though the PPI has absolutely nothing to do with the digital image itself, all it does is set the actual physical size of your physical print. Set a higher ppi and the image print will be smaller as you move the pixels closer together on the print (and vice versa for larger images).

My 18MP gives images 43.89x29.26cm at 300dpi. At 400 pixels per inch, it is only 32.92x21.95cm, but I gain nothing. However, if I reduce to 200 pixels per inch, I get an image 65.84x43.89cm with only a small drop in quality. Certainly nothing to cause a problem if home printing on an ink jet.[/QUOTE]

this is where it gets a little more confusing. You will NOT lose any quality printing that larger image at 200ppi. Take two 100% crops and test the difference. If you were to resample (by adding random pixels) to get to 300ppi, you will likely reduce the quality more.

All very subjective. My view is why increase the image size when it's really not required. The differences are miniscule.

 

[EOS_JD]

personally I think that PPI is a daft way to measure something, it should be an output of the printing format chosen and pixels in an image rather than a measurement of image size. it's merely image quality...unless there is a standard image printing size of which I'm not aware on which this is based.
Chiffs...ignore my rant..

You could not be more wrong. ppi has absolutely nothing to do with quality.
Pixel resolutions are used for more than just printing - It is essential to size images in pixels for displaying images on the web for example.

Not sure what you mean by standard image printing size?

for my experiences, for nice photographs, then 300dpi or ppi is great

Wrong again (see above)

for larger prints like huge A0 posters, then you can go as low as 80ppi or similar as people will stand further back when viewing imho. for medium posters 150dpi is what I aim for.
^^ the above wouldn't be appropriate for presentation posters showing small text of course.

This is where you are almost right. Lower ppi values are not just for very large prints. An A3 image at 150ppi will look absolutely great - even close up - but from normal viewing distances (you view it from further away) you don't need higher resolutions. In most cases (unless it's a severe crop) you should be able to print with the pixels you have.

as for lightroom4 setting DPI....see earlier rant. i'm very confused by this 'standard'

There has to be a number. It could be 1 or it could be 10,000 - it means nothing - it refers only to the physical print size based on the number of pixels in the image.

THe reason 300ppi is referred to as the maximum required is because the human eye cannot resolve more detail than around the 300ppi level. Even at 240ppi the human eye would struggle to see any difference.

There's lots of books that describe the subject well and is very useful to understand (really it's very simple stuff but hard to get your head around)!

It has nothing to do with the image itself ppi resolution means nothing to the digital image you can ignore it completely UNTIL you come to print as all it does is sets the PHYSICAL PRINT SIZE based on the pixels you have available.

HTH

Regards
Jim

 

[DizMatt]

you see my confusion, from your above comment is that in webdesign, for example, I would not expect anyone to order a picture to be 600ppi or 300ppi as pixels per inch is dependent upon the size of the screen and pixel density.
so you say, right, I need a banner image on my website at 300ppi
it means nothing until you define the size of the image in inches.
even if you say that you have a website template 900px across the screen, and then say you want a 300ppi image to span it....urrm. how would you do that, without getting out a tape measure.
PPI by its very definition is a ratio or a scalar
not a measurement of size

Pixel resolutions are used for more than just printing - It is essential to size images in pixels for displaying images on the web for example.

I'm still confused size your image in pixels, yes for a website...pixels per inch? no idea how this would be helpful.

It has nothing to do with the image itself ppi resolution means nothing to the digital image you can ignore it completely UNTIL you come to print as all it does is sets the PHYSICAL PRINT SIZE based on the pixels you have available.

I agree with this.

I think we have crossed wires!

 

[EOS_JD]

you see my confusion, from your above comment is that in webdesign, for example, I would not expect anyone to order a picture to be 600ppi or 300ppi as pixels per inch is dependent upon the size of the screen and pixel density.

Correct you would NOT because ppi means NOTHING in relation to web images - read my posts again - PPI ONLY AFFECTS THE PRINT SIZE OF YOUR IMAGE/DOCUMENT.

So you say, right, I need a banner image on my website at 300ppi it means nothing until you define the size of the image in inches.

No - Forget ppi (I'll keep saying this) - You would say right I need a 900x100 pixel image as a banner (or whatever it needs to be).

A question for you - You 2 files. One saved as a 600x400px image at 300ppi and a the other as a 600x400px @ 72ppi. What's the difference?

even if you say that you have a website template 900px across the screen, and then say you want a 300ppi image to span it....urrm. how would you do that, without getting out a tape measure.

FORGET PPI - Measure the images in PIXELS ONLY - ppiu ise irrelevant for the web

PPI by its very definition is a ratio or a scalar not a measurement of size

ppi is neither - it defines purely the space on which all of your pixels are placed on paper (your final print).

I'm still confused size your image in pixels, yes for a website...pixels per inch? no idea how this would be helpful.

It's not helpful - You CANNOT use ppi to measure anything - It's ONLY RELEVANT TO THE FINAL PRINT ON PAPER (or whatever medium you print to).

By the way the answer to the above question is both images are EXACTLY THE SAME. You cannot use PPI (or inches) for measuring images on the web - When people say save your image at 72dpi - that is absolutely INCORRECT information. If someone says save the image at xxx by xxx pixels that is correct. The ppi value is meaningless.

One thing, if you have a ppi value that an image was saved at and the size it was saved at in inches although it's really meaningless, you can turn that info into useful data as you can calculate the number of pixels the image has by multiplying the figures.

Lets say I tell you I have a 12" x 8" image saved at 180ppi - The image size isn't really 12x8 at all. It's only as 12x8 inch image on paper if printed at 180ppi.

The image is actually 2160 x 1440 pixels. I can change the ppi value to 300 (without resampling) and the print size would change (but the image would still appear the same size on my screen as it would contain the same number of pixels) - In other words the image has not changed - The only thing that would have changed is how close the pixels are on the paper I've printed them on

So forget inches and ppi for measuring web images - It's useless data.

 

[DizMatt]

thanks for the response.

I think the confusion lies with me thinking that PPI is a daft way of measuring something
then "You could not be more wrong." and then you agreeing with me fundamentally by saying forget PPI as means of measuring an image specification.

anyway I understand your maths as I agree with you. personally, I stand by my original statement. cheers for the discussion.
Your verbose explanation in the last post sums up in detail what I meant in the first place and should help the OP a great deal. If I had a yellow highlighter, I'd use it

 

[EOS_JD]

Matt I may have read your post a little wrong (read it again)

What you are saying seems to be the same as me actually - You're right you cannot measure something with PPI.

Sorry if I caused the confusion - I think I picked a bit of what you said and responded to that without taking the time to read the whole thing properly lol

 

[DizMatt]

lol
well lets agree to agree then
as I said, the last bit of your post ahead of this one sums the whole thing up nicely for the OP

 

[EOS_JD]

If I had a yellow highlighter, I'd use it

Hit the quote button and my text will show up. You can select the text you want to appear in between the first ] bracket and the [/quote]

Or you can make the text bold.

 

[groovyf]

Would I be correct in thinking, say for instance, if someone saves their 800x520 pixel images at 240ppi then someone could resample them in PS (or similar) to greater pixel dimensions so they could be printed out larger at reasonable quality... as opposed to saving the 800x520px image at 72ppi and trying to resample will give a more "blockier" image less-suited to larger printing?

 

[DizMatt]

I don't think so
800x520 pixels at 240ppi means that the image is 800/240 = 3.3"inches in length
it makes no sense which is my problem with it.
it's backwards.
if a magazine wants images at 150ppi for a full page (square image for the sake of argument) that will be 8" by 8" then 8*150 is your minimum number of pixels down each side = 1200 x 1200 pixels.

800x520 pixels at 240ppi is still only an image with 800x520=416,000 pixels in it.
PPI is only relevant to me once you actually decide upon the output size of your print.

/waits....

 

[chiffs]

Wow i go to work for a day and Jim has not stopped typing.

I salute you sir, thank you very much. I need to have a proper re-read as it looks like you have covered the lot.

I have been asked for images at 1024 pixels wide for my website which is going to start to be built shortly.

Going to do some searching on google and reading up about LR4 and Elements 10 for resizing too 1024 pixels wide.

 

[Silvermuir]

Also a good bit of visual info on the subject from Adorama TV

http://www.youtube.com/watch?feature=player_embedded&v=sg85Isdl1sQ

All about resolution and pixels per inch.

 

[DizMatt]

if it's for a website, as jim says.
ignore PPI
just resize them to 1024px across, I would use JPEG at about 90% quality (compression affects size vs quality where 100% is almost no compression iirc)

 

[EOS_JD]

Would I be correct in thinking, say for instance, if someone saves their 800x520 pixel images at 240ppi then someone could resample them in PS (or similar) to greater pixel dimensions so they could be printed out larger at reasonable quality... as opposed to saving the 800x520px image at 72ppi and trying to resample will give a more "blockier" image less-suited to larger printing?

No an 800 x 520 @ 240ppi is exactly the same image as one saved at 72ppi.

Read the text above - What changes is only the physical print size. At 72ppi the image would be 11" on the longest edge. At 240ppi the image is 3.333"

I could still get a decent 8" print by printing at 100ppi

The ppi value does not matter

It does not stop people resampling (adding pixels)

 

[EOS_JD]

I don't think so
800x520 pixels at 240ppi means that the image is 800/240 = 3.3"inches in length
it makes no sense which is my problem with it.
it's backwards.
if a magazine wants images at 150ppi for a full page (square image for the sake of argument) that will be 8" by 8" then 8*150 is your minimum number of pixels down each side = 1200 x 1200 pixels.

800x520 pixels at 240ppi is still only an image with 800x520=416,000 pixels in it.
PPI is only relevant to me once you actually decide upon the output size of your print.

/waits....

100% correct

The only thing that matters is the number of pixels

The ppi sets the print size.

 

[EOS_JD]

Wow i go to work for a day and Jim has not stopped typing.

I salute you sir, thank you very much. I need to have a proper re-read as it looks like you have covered the lot.

I have been asked for images at 1024 pixels wide for my website which is going to start to be built shortly.

Going to do some searching on google and reading up about LR4 and Elements 10 for resizing too 1024 pixels wide.

1024 pixel images is a big image - Make sure you watermark them. I could make some nice 10" prints from images that size!!!

To resize in Lr, go to the export and in the image size boxes just enter 1024 in both. That will give you an image that is 1024 pixels on the longest edge. Not necessarily 1024 wide for a portrait image but if you start making portraits 1024 wide the height will be way too big

I recommend lower resolution images but obviously I don't know what you're trying to do!

 

[EOS_JD]

Also a good bit of visual info on the subject from Adorama TV

http://www.youtube.com/watch?feature=player_embedded&v=sg85Isdl1sQ

All about resolution and pixels per inch.

Like it although he's wrong about the ppi/dpi

They are different.

ppi is a digital image/document resolution

dpi is a PRINTING/SCANNING/MONITOR resolution - They are different although commonly used as meaning one thing (incorrectly)

He is quite right about resampling - Don't do it unless you absolutely have to - you will degrade the quality of your image! (although to be fair not by much depending on what you're doing)

Also the spreadsheet is not using resampling - it's calculating the values BEFORE resampling

 

[EOS_JD]

if it's for a website, as jim says.
ignore PPI
just resize them to 1024px across, I would use JPEG at about 90% quality (compression affects size vs quality where 100% is almost no compression iirc)

Good point, the compression is how you restrict people from increasing the size of your image for print. BUT that affects the quality you see the image at. A highly compressed jpg at 800 pixels wide will produce a very poor 6" print but a low compression should allow a very decent 6" print possibly more.

 

[DizMatt]

PPI and DPI...surely one begets the other.
however, I can see that there is a subtle difference.
you might have an image in great quality which is 300ppi but can only be printed at 150dpi due to the limits of the printer.
but PPI is surely redundant on a document format because it depends upon the pixel density of the screen

anyway I hope haven't scared off Sean with our ranting I hope it's all helpful
most banner pictures will not be 1024 for a website. one common standard at the moment is about 960px across the screen in a column format. give or take.
Any smaller images should (will) be resized to the smallest required to speed up the loading times. However, if they're in a portfolio and they need to be full screen images, then 1024px is reasonable. but you'll easily get a decent 6x4" postcard out of that at the very least, as Jim said.
Also never scale down and then rescale up again for your images. always go back to the largest image (if practical) and scale down from there imho

 

[EOS_JD]

PPI and DPI...surely one begets the other. however, I can see that there is a subtle difference.

A pixel is a digital point on an image/document file. It's not a physical thing but the smallest part of the digital image itself.

A dot is a physical thing. When discussing dpi generally it's normally when printing. My Epson prints at 5760x2880 which sounds like a very high dpi but with 8 inks it can use 8 drops of ink to create the colour from just one pixel. So in reality the resolution is nearer to 5760 and 2880 /8 = 720x360dpi

you might have an image in great quality which is 300ppi but can only be printed at 150dpi due to the limits of the printer.
but PPI is surely redundant on a document format because it depends upon the pixel density of the screen

It's common for me is to send the image to my printer at 2880x1440 (360x180 dpi)but the image itself could be anything from 360ppi down to 100ppi depending on the size I'm printing (rather than the limits of the paper)? I only use photo/Art papers though.

Another difference is the ppi affects the image size and the dpi does not - the dpi will determine the quality of the print.

What has the screen got to do with things? The screen is only an output device to display your files. It has ZERO effect on the images or documents you display in any way. The pixel density of the screen can be used to display your images at the correct size inch for inch - but really isn't required.

However if you are talking about things like word docs and design/web programs then yes ppi is not required. I did mention documents and that was only because a couple of document formats can be printed as image files (pdf's for instance). In the main ppi can be ignored and it's pixel dimension that is king.

anyway I hope haven't scared off Sean with our ranting I hope it's all helpful most banner pictures will not be 1024 for a website. one common standard at the moment is about 960px across the screen in a column format. give or take.
Any smaller images should (will) be resized to the smallest required to speed up the loading times. However, if they're in a portfolio and they need to be full screen images, then 1024px is reasonable. but you'll easily get a decent 6x4" postcard out of that at the very least, as Jim said.

I recon I'd get a good 10" print from even a 960pixel by xxx image.

Also never scale down and then rescale up again for your images. always go back to the largest image (if practical) and scale down from there imho

Totally agree there.

 

[DizMatt]

the reason that I mention PPI and screen resolution is that monitors have different resolutions, so if you display an image which is 1000 x 800 on two monitors at full screen for reviewing:
one is 1650x1080 monitor like mine
and the other is a lower resolution like 1024 by 720 or something (a tablet for example)
then the output on the large monitor will be interpolated or similar, unless you tell it not to of course and have big black borders.
anyway not required for the OP but just something I consider.

 

[arad85]

then the output on the large monitor will be interpolated or similar, unless you tell it not to of course and have big black borders.

DPI of a screen is never used in rendering images on screen (I'm sure someone will contradict me and find an obscure package that does this) pixels will always be rendered as pixels. A 800x600 screen running a browser displays a 1024x1024 at 1024x1024 with bits chopped off on the sides (unless it is scaled to fit the browser window). DPI is never used to figure out how to render and image on screen.

PPI and DPI are totally different things, yet very interrelated. Depending on your background, you may think in PPI or you may think in DPI.

Someone who is preparing an image for print might think "ahh.. I want that image to be 6" x 4" to fit that section of the page". In order to do that, they will export from Lightroom a 6" x 4" image at the DPI setting of their printer. Lightroom will automatically size the image to the right number of pixels to make it fit. All it is doing is the calculation

length in inches x pixels per inch

to figure out how big the image needs to be made in pixels. When that image is brought into the typesetting package, it will fit exactly without resizing. The advantage is that the operator thinks in terms of inches and print resolution - Lightroom does all the workings out to generate the right sized image.

You also need to be aware of this when embedding images in documents. If you export them too small they will look perfect on screen, but will look poop when printed. If you want an image to appear at 10" x 8" in print, you need a 3000x2400 image to be embedded in the document. Taking a screen capture and embedding it in a word document and stretching it to fit the page may well look OK on screen, but it will look pants when printed on a printer. Yes, I have been a victim of this!!

 

[EOS_JD]

the reason that I mention PPI and screen resolution is that monitors have different resolutions, so if you display an image which is 1000 x 800 on two monitors at full screen for reviewing:
one is 1650x1080 monitor like mine
and the other is a lower resolution like 1024 by 720 or something (a tablet for example)
then the output on the large monitor will be interpolated or similar, unless you tell it not to of course and have big black borders.
anyway not required for the OP but just something I consider.

Tablets work a little differently as they may resize your images only for viewing on the screen - the actual file remains unchanged and is not interpolated (I don't know how tablets work for that really)

But with the two monitors, the output is NOT interpolated in any way - One pixel on your image = 1 pixel on your screen that is why the image will look bigger on the lower resolution monitor (because the dpi is lower).

Nothing to do with interpolation.