PPI / DPI / Pixels / Resolution / Scanning / Aspect Ratios... Explained !<font size="2">
There have been many, many posts from users who are clearly totally confused by the whole question of DPI / PPI / Pixels / Resolution and Ratios… so… I thought it might be worthwhile writing up an overview to provide a basic explanation of these things – solely as they affect Blurb & BookSmart.
DISCLAIMER : This is NOT intended as a technical masterpiece; simply a layman’s guide to dealing with this issue for BookSmart… You can find more information in the Blurb FAQs – but it’s not always easy to uncover unless you know what to look for, and it’s certainly not all in one place! (You can of course also do more research on Google, Wikipedia etc. etc.)
First, some definitions;
- PPI = Pixels Per Inch. PPI is the term used when dealing with SCREEN resolution and should NOT, technically, be confused with DPI (see below). However, for this purpose it’s quite safe to assume that you can inter-change PPI and DPI. (Many people do anyway – if erroneously !)
"Standard" screen resolution is 72ppi – more later…
- DPI = Dots Per Inch. DPI is the term used when dealing with PRINT resolution.
"Standard" print resolution is 300dpi – more later…
- Pixels; are the minute, usually square, blocks of colour that make up your image.
All that really matters as far as Blurb/BookSmart is concerned is the pixel dimensions of your image. Simple measurement in inches is meaningless and irrelevant, unless associated with a "resolution" setting… more later…
E.g. if you have a 8Mp camera, the pixel dimensions will be in the region of 3600×2400. (Don’t do the maths – it’s not exact!!!)
The actual pixel dimensions will vary greatly dependant on the Mp size of the camera, and the aspect ratio you have set (see below).
- Resolution; is quite simply the number of pixels or dots per unit of measurement. This is normally expressed in terms of PPI or DPI dependant on the medium being used – screen or print.
So, 300dpi means 300 Dots Per Inch. This means that an image of 3600×2400 (as above) will print at 12×8" IF printed at 300dpi. (3600/300 + 2400/300).
Resolution can be varied to a very great extent… more later…
- Aspect Ratio; Is the relationship between the image Width and Height, and is set by your camera. The old traditional 35mm film aspect ratio was 3:2. I.e For every 3" wide, you got 2" tall… which normally resulted in the "standard" 6×4" print…
Today, there are many different aspect ratios in use; 3:2, 4:3, 5:4 and 16:9 are typical, but there are others… Why is this important? Because it will affect how your image is displayed in the BookSmart image containers. Again… more later…
Dealing with print resolution first, since this is what we need to understand for BookSmart…
Starting with our example image of 3600×2400 pixels, as noted above, if we print that at the standard of 300dpi it will generate an image size of 12×8" (3600/300 = 12" : 2400/300 = 8").
300dpi is the industry standard. It has been proven that there is no benefit in printing at any GREATER resolution because the average human eye just cannot see any improvement, so generating prints with a greater print-resolution is a waste of time, space, energy & resource…
However, you CAN print safely at less than 300dpi…(see below)...
But first, if we take our 3600×2400 image and only want a finished image size of 6×4" that would result initially in a dpi of 600…(3600/6" = 600dpi : 2400/6" = 600dpi). That is too great for all bar specialist printers; so Blurb will automatically down-size the image to 300dpi for printing purposes…
Equally if we wanted a finished image of 9×6" that would result in an initial dpi of 400… (3600/9" = 400dpi : 2400/6" = 400dpi). After this, it’s a simple case of applying the same logic to whatever size image you need…
Although BookSmart actually does a pretty-good job at down-sizing in general, if at all possible, to ensure maximum quality, you should do this exercise in your post-processing software. If you have PhotoShop or similar, it’s a very easy job to do… This is more important if you want to bring a very large image into a relatively small container in BooKSmart… (There is a tendancy to pixellation for these large differential down-sizings…)
What happens if we have a smaller image and want to make it bigger… Well, the reverse logic applies!
Starting with an image of 1800×1200 pixels (half our original sample), at 300dpi, we would get an image size of 6×4"... (1800/300 = 6" : 1200/300 = 4")...
But we can push that a little. A print at 250dpi will still yield a good result when viewed at normal reading distance… So, we could take our 1800×1200 at 250dpi and we will get an image print of 7.2×4.8" (1800/250 : 1200/250)...
You could go as little as 200dpi (to get a 9×6"), but I generally wouldn’t recommend going below 225dpi… (To see what your image might print like, via BookSmart, at these lower resolutions, simply "Preview" the page at 100%, and you’ll get a good idea… If it looks pixellated, then you need to increase the resolution, which will result in a smaller sized image…)
NOTE that the Blurb warning for a "low resolution" image only kicks in at 150dpi which is WAY too small for anything like a decent print, and should NOT be relied on to assess the print-quality of your image!
After this, again, it’s a simple case of applying basic maths to "convert" any given image from its pixel dimensions to a printed size, dependant on the resolution applied…
Although the "principle" is the same (number of pixels per unit of measurement)... Display resolution is something quite different from Print Resolution, and causes a great deal of confusion amongst those new to the world of digital imaging…
Screen or Online displays are typically 72ppi as noted above. That means that you could, for example, have an on-screen image of 15" wide by 10" deep which will only be 1080×720 pixels big.
Most of the free (mass market) image hosting sites (like Flickr, Photobucket or SmugMug) automatically re-size images uploaded to these smaller dimensions because (like print resolution) anything greater is (for the majority of day to day tasks) just utilising too much resource.
The problem is that this image (of 1080×720) if re-downloaded for printing will only yield a print of 3.6×2.4" at 300dpi… An attempt to convert the image into a print of say 6×4" will result in a resolution of only 180dpi which is almost certainly not good enough for "normal" viewing from a normal reading distance…
As an aside, NOTE that: the further away you are from a image, the lower the resolution can be… So, if you were viewing an image from 10’ away, you could easily get away with as little as 100dpi…
Consequently, the rule here is that you should NEVER rely on these hosting sites as your prime storage facility if you ever want to print the image at any stage in the future… External Hard Disks are now dirt cheap. It’s a good idea to invest in at least a 500Gb disk for image storage…
PART 4…</font><font face="Arial" size="2">
This is a similar area of confusion, but the principles are pretty-well identical to the above…
Once you get the gist, the rest is just simple maths, and you can apply the math to whatever sized image you are working on…
As we know now, industry standard prints at 300 dots per inch (300dpi), and so your old printed images will have been done at that setting. So, if you scan at 300dpi you will get a ratio of 1:1.
Taking a 6×4" original as an example; if you scan at 300dpi you will get a digital image with a pixel dimension of 1800×1200, (that’s 6×300 + 4×300), which, when re-printed at 300dpi will reproduce a 6×4" print… 1800/300 + 1200/300…
However, if you are only scanning at 200dpi, then your 6×4" original will generate a digital image of 1200×800 pixels, (6×200 + 4×200), which, if then re-printed at 300dpi will only provide a reduced sized print of 4×2.7". That’s 1200/300 + 800/300…
Now, if we scan at 600dpi (2x standard) you will get a digital image of 3600×2400 pixels (6×600 + 4×600), which, if re-printed at 300dpi will yield a printed image of 12×8 (2x original)... That’s 3600/300 + 2400/300…. This math can now be applied to any sized image you have, or any resolution you set…
You almost certainly won’t want to use these 600dpi scans as finals, but for practicality, a scan at 300dpi is UNLIKELY to generate a good-enough image to reproduce at 100%. That’s because what you will now have is (at best) a third-generation copy of the original image, and the image quality will be MUCH reduced from the original… so…
I always scan at 600dpi or higher. This generates a large file, but it gives a MUCH better working image for PhotoShop’ing for cleaning up any creases, scratches, dust spots, colour-shift etc. etc. etc.
Once you’re done with the PP work in PS, you can then re-sample the image back to a 6×4 (or bigger if it’s good enough) at 300dpi, and you will have a FAR better image than you would otherwise have had with a simple 300dpi scan in the first place. It all depends on how much effort you want to put into preservation and restoration of the image…
Depending on how small your originals are, you may even want to scan at 1200dpi, re-work, then re-sample at 300dpi. FYI: Using this method, I have succesfully up-sized a number of very old and very small (2.5×1.5") images into very good 8×6’s (with work in PS…)
Aspect Ratios and importing images into BookSmart containers…
As above, there are MANY different aspect ratios in use in todays digital image arena, and the chances of finding a BookSmart template image container which fits your image exactly, without you needing to do something to it (the image), is remote in the extreme…
Until such time as Blurb make "free-flow" templates available (where we can freely re-size and re-position containers), we only have two choices;
- Re-size or crop our image (either using PhotoShop or similar, or using BookSmart’s own tools) to fit the template & associated containers we want to use… or…
- Create our own page layout (in PhotoShop, InDesign or similar), and then import that into a BookSmart template as a flattened JPEG file. (This does NOT need to be a full-bleed page!)
BookSmart’s templates have been designed to be aesthetically pleasing in relation to the book size being worked, and so, even across the different book sizes, not all template designs are available across all sizes – on the basis of "they wouldn’t look right"...
When importing an image into a container there are two important points to remember;
- You do NOT have to fill the container! Any unused space will simply blend into the background…
- Wherever possible, you should try & choose templates/containers that are "fit for purpose". Importing a small image into a large container runs the risk of "stretching" the image beyond reasonable print resolution – and recall that you won’t get a warning until the resolution falls to below 150dpi… So, even if you are at 151dpi, BookSmart will NOT tell you that the resolution is too low!
You can see what pixel dimensions are ideal for each container by simply hovvering the mouse over the container. A small pop-up window gives you the pixel and inch dimensions (at 300dpi).
So, I hope this all helps…
If anyone has any added tips on this theme; chip in…
Blurb staff.. this should be locked at the top of the forum!!!
its so wonderful to see people helping people on this forum, for the pure reason that they want to and not because they have to!
thanks to EvERYONE who has helped me with all of my questions/concerns!
I agree. Lee’s post is accurate and helpful, and he is very generous to share his knowledge with the Blurb community.
We’ll look into linking this post where appropriate, as it’s indeed great to have all this information in one place :-)
Thanks to Lee and happy Blurbing to all,
"Most of the free (mass market) image hosting sites (like Flickr, Photobucket or SmugMug) automatically re-size images uploaded to these smaller dimensions because (like print resolution) anything greater is (for the majority of day to day tasks) just utilising too much resource."
Smugmug is certainly NOT Free and they DO store full resolution images. Whilst they keep the full resolution images as uploaded they also create different sized images for viewing as well.
Very helpful to have all that information in one place, and even more helpful to have it explained clearly and approachably. Well done.
I’d just like to make a similar point to GlennBatten’s – Flickr also has the option of "pro" accounts (quite cheap, too), that allow unlimited upload sizes. I recently uploaded a jpg to the Creative Common via Flickr that was 9216×5568 at 72 dpi with no problems (here it is, btw – http://flickr.com/photos/shoes_on_wires/2765588977/ ).
Thank you so much for posting this. It helps a great deal. I just have one question and it’s because I like 1-2-3 steps procedures. :-)
I work with PS Elements 5 and the default settings show that the screen resolution is 72ppi and the print resolution is 300ppi. I’ve done all the editing and resizing of the photos for my book but I haven’t changed the ppi setting. Should I run through them all and reset them at 300ppi and then resize according to the size of each of the template spaces I’m using?
And one other thing, if I could ask…... I’ve read there are concerns with sharpening. All the photos I’ve prepared look the way I want them but do I need to use unsharp mask and sharpen them more???
Thanks for the help! Holly :-)
By the way, I apologize for the ‘duh’ question but when I’ve changed the resolution from 72ppi to 300ppi it makes a photo of 2450px on the long side shoot up to something like 11,000pixels….which seems a trifle large. :-)
Don’t bother changing the dpi resolution of your images from 72 to 300. The dpi is irrevelant except in calculating your necessary image resolution for input or output. So you can figure that your 2450px image will print to 8.1667 inches on the long side when the output device prints at 300dpi, and will display in a 72dpi operating system at 34 inches when the pixel ratio is 1:1.
So just leave your image at 2450px. If it makes you feel better you can change the dpi, but leave the "resample" box unchecked. Your image resolution will not change, and the only effect will be when print to your home printer from PS it will print an image 8 inches instead of 34 inches wide.
Thank you so much for this! One question about scanning though – I’m trying to put together a wedding book, and our photographer shot with a film camera. I have our negatives and original prints – is it a better idea to scan from the negatives (which are smaller but perhaps not already pixelated) or from the pictures (which are larger, but may be pixelated already)?
As a follow-up, I do have our images that were put onto a CD, but at 300 dpi (hence why I need to scan some images in). I’ve noticed that the colors in the images that are scanned have nicer colors (green grass is much greener, etc) than in either the printed photos or negatives. Any advice for a photoshop newbie as to how to improve the color quality without looking like an oompa-loompa?
Well, I only just noticed that my post is a "featured post"... Pat on the back for me – I hope!!!
Anyhow; to answer your Qs…
- If you have a good negative scanner, then that will almost certainly be the way ahead, since you will be working on a first generation copy… but… the negative scanner needs to be GOOD! If it’s just an OK model, then you will probably be better off scanning the prints at the higher resolution I suggested… Best way to judge – do a test of one of each, print it, and see how they compare!
- You say you have your images on a CD at 300dpi… Are they the original full-size image files? If so, you will almost certainly be better off just using those images! Check the pixel dimensions…
- As for editing images; before you do anything to adjust colours – you MUST ensure you are working on a calibrated monitor, otherwise you can end up doing more harm than good! You can search the forums for "monitor calibration". There’s a whole bunch of threads on the subject…
Once your monitor is calibrated, then pull your images into PhotoShop and play away. There are no fixed or firm rules for adjusting images, since each tends to have a unique colour value, and so, needs individual attention… but… if they are film files, then I would be surprised if they need much in the way of adjustment; so be very careful in what you are doing… experiment on a couple of COPIES first – NOT the originals!
Thanks so much for your informative (and prompt!) reply. My image files are 1536×1024 pixels with the print size being roughly 5×3 at 300 dpi. I figure I can use a lot of these shots for smaller pictures, but will won’t want to use them for full bleed images.
I’ve seen the info on calibrating my monitor, now to put that into practice…
Thanks a lot!
thanks a lot Lee for this very nice topic, and all the clear explanations that are in there :)
I’m sure many people here will be very happy to see all this in one single place !
One thing that I didn’t notice and took for granted, is that the "low resolution" icon kicks in at 150dpi and not 300 dpi !!!!!! shut…. didn’t pay attention. So now I need to go back and check all my pictures…
Blurb staff, could you consider the feature to display a warning icon as soon as we are bellow the 300dpi ? this would be really helpfull and would avoid us going back and force between the ideal size and our curent size…
Lee’s done a great job summarizing many of the arcane points of Resolution and Sizing. However, there is one slight ammendment I’d like to make, and that’s regarding DPI. Bear with me…
DPI = dots per inch, but it is not print or printer resolution. The 300dpi that’s being mentioned is really File or Image Resolution. And as Lee and others have pointed out, it’s not really Dots per Inch (DPI) at all but Pixels per Inch (PPI). The terms have, unfortunately, become interchangeable.
Blurb uses HP Indigo presses for printing its books. Indigo is a type of “digital press.” Digital presses use what’s called “halftoning screening” to render the image. That means that the image is broken up into little “spots” or dots that can be printed. (inkjet printers don’t use halftoning but an alternative form of screening, but that’s another story) If the dots are small enough and/or if you are far enough away, your brain tricks you into thinking that what you’re seeing printed is a photo-realistic or “continuous tone” image. It’s an optical illusion.
If you look at a Blurb book with a loupe or magnifying glass (or any magazine, book, brochure, etc.), you can see all the little dots. The more dots per inch, the more photo-realistic the image looks. But Blurb Indigo presses are NOT printing at 300dpi. Depending on which Indigo and which resolution mode they’re using, they may be printing at close to 2400dpi.
Back to the images and their resolution. Look at a magnified Indigo or offset-printed image again and you’ll that the dots are arranged in patterns. These patterns are typically described in terms of “lines” or “lines per inch” (LPI). And it turns out that Blurb Indigo presses use 175 as their “line screen”. So your images are broken apart into clusters of spots or dots that have a regular frequency of 175 lines per inch (at their final size). These spots or dots are then printed at a printer resolution of up to 2400dpi (actually it’s 2400×2400, or 800×1200 or whatever).
So how does LPI get us to 300dpi or more accurately ppi? It was known long ago that to achieve a photo-realistic impression or a continuous-tone-like printed image, the Image Resolution (PPI) should be around 2x the Screening Frequency, i.e., a PPI:LPI ratio of 2:1. Since many “quality” printing jobs used 150+ LPI in the old days, then the best reproduction came with a 300ppi file AT FINAL SIZE. And this was basically set in stone and has become sort of an Urban Legend. But because I used to be a publication art director for many years and I dealt with this stuff every day, I wanted to get to the bottom of this. And I discovered something. I experimented and learned that I could easily get away with a 1.5:1 PPI:LPI ratio. I did tons of (traditional) offset jobs using 1.5:1.
So what about the 175lpi and the 300dpi, and what does all this rigamrole mean to you (as I know you’re scratching your heads simultaneously)? Here’s my summary:
—When you’re talking about images, it’s PPI not DPI (the scanner people have always gotten it wrong from day one; have you ever seen a “dot” in a scan?). Open up any image in Photoshop or any sophisticated image-editing application and what do you see next to “Resolution”? Pixels per inch not dots per inch. If someone says “dpi”, just smile, nod and move on knowing secretly that it’s really “ppi”.
—Whenever you see a number like “300dpi” you can be sure it’s not about print resolution. Remember the first laser printers? They had print resolutions of 300dpi. And the pictures were lousy. There’s nothing you can do about true “DPI” with offset printing, so don’t worry about it.
—Blurb Indigo presses use a screening frequency of 175lpi. 175×2 = 350. That’s your high end of the optimum PPI of your image AT FINAL SIZE. And 175×1.5 = 263. I would call this (or round down to 250) the lower end of your optimum PPI range (again, AT FINAL SIZE). Blurb says that 150ppi is the lowest they recommend. I haven’t tried it so can’t say if they’re right or not. But if you are preparing images at final size for importing into Blurb BookSmart, you can’t go wrong with anything between 250-350ppi.
So is everything above based on my massive experience with Blurb books and Blurb printing? *NOPE*. I’m just getting ready to order my first Blurb book. But it makes sense to me.
See you around the Forum.
P.S. I have some book images that illustrate a couple of the points above, but I don’t know how to insert images into these posts. Let me know if you’re interested and maybe I’ll dig them out.
Very interesting discussion. Thank you Lee and Harald.
But in practical terms, if I have an image at a resolution of 3000×2008 and the blur template is at an optimal resolution of 2905×2275 which is best – purely from a print quality perspective:
Assuming image is NOT resized and stays at 3000×2008.
option 1: The image is resized by Blurb without cropping resulting in an image of 2905×2008. This means a slightly smaller image.
option 2: The image is resized by Blurb with cropping resulting in the full image of 2905×2275
This means a larger image with the width downsized from 3000 to 2905 while the height is scaled up from 2008 to 2275. Because the photo is not cropped the aspect ratio is the same as the blurb’s container at optimal resolution.
In the above I’m making the assumption that the downsizing from 3000 to 2905 will not introduce any artefacts – am I right? I’m wondering however, if the upscaling from 2008 to 2275 will cause any issues. Photo and lens quality to begin with is high end.
In general, going DOWN in size is safer than going UP in terms of image quality (pixels smaller rather than larger), although I’ve seen posts here about how people’s small photos looked worse than their bigger ones. Lots of post-image issues at play including “dot gain”, “plugging”, etc.
In your case, the differences are so small (e.g., +11% to go from 2008 to 2275) that it may not make much of a quality difference but might be more of an aesthetic choice: do you want to crop your images?
BTW, what’s the “container” size? That will determine where you are on the “optimum image resolution” scale.
P.S. It sure would be good if BookSmart could tell you at what % you are from your original when using their size slider. Does it somewhere that I haven’t seen?
Well, I thought I understood, but…
I started using Genuine Fractals which is nice for Blurb because it crops, resizes and sharpens all in one dialog. But this feature is based on document size. When I use the container size at 300 ppi it does not result in the same ppi for the container.
Eg Blurb container states 921×451 ppi / 3.1. x 1.5 in or 917×451 pp also has doc size 3.1×1.5
I input 3.1. x 1.5 at 300 ppi and get an image resized at 934×452 close but does this mean Blurb is going to resize my image unless I turn cropping off and then lose part of the image.
This is important enough it needs to be brought back up to the top and pinned up there?