[K-9]

I know that most consider 300dpi a high resolution image, but whenever I save my RAW files to a JPG, they suddenly become 72 dpi images. Am I doing something wrong in the saving/converting process? If I was asked to submit one of my images as 300 dpi, wouldn't I lose quality trying to expand the 72dpi photo to 300 dpi? I'm a little confused. Thanks, in advance.

[Lowell Goudge]

In terms of DPI and this has come up many times, for some reason pentax elects to relate the physical image size in inches to 72 DPI.

This has no relationship with the printer DPI.

Depending on teh software you use you can change this ratio. For example, I use PSP X2. If yo select Resize from the Image menu, you can see the image size in either inches, centimeters or millimeters, based upon 72 DPI resolution. You can also see the image size in pixels.

If you select advanced settings by checking the check box, you can then elect to re-size with or without resampling (the default is with resampling)

If you turn re-sampling off, you can change the pixel per inch setting and change nothing else, and this will be reflected in the overall dimensions of the image in inches (or what ever units you select)

I di not know, in other editors where this is buried, but you should have this capability in most image editors. The issue is that you MUSTturn off resampling to change the DPI setting without changing the number of pixels in your shot.

[Marc Sabatella]

In terms of DPI and this has come up many times, for some reason pentax elects to relate the physical image size in inches to 72 DPI.

Someone recently pointed me to the actual EXIF specification, and it's actually suggested/mandated right there that 72dpi be used.

Anyhow, here's the deal as simply as I can put it: the number stored in that field is *meaningless*. It has no relevance whatsoever; it's just there because there is place for a number that is relevant for a *scanner*, not a camera, but the camera has to put a number there, and the EXIF spec suggests/mandates it be 72. Could just as well be 7, 42, 666, or 2012 for all the difference it makes.

That's because the *actual* dpi of your image has nothing to do with the number in that field. it's the product (ok, literally, quotient) of two things and two things only: the number of *dots* in your image, and the size of your image in *inches*. That is, it is quite literally *dots per inch*. Your images don't *have* a resolution in dpi until you actually print them, any more than your car has has a speed in mph until you actually drive it. Your image might be 300dpi, 72dpi, 100dpi, or 20dpi depending on how big you print it, just as your car might go 72mph, 100mph, or 20mph depending on how fast you drive it (good luck on the 300 there, though :-).

So if you want a print of at least 300dpi, then find out how many dots (pixels) wide your image is, divide that by 300, and that is the size you can print at in order to get 300dpi. The old 6MP cameras made this easy: their images were 2000 dots tall and 3000 dots wide. 3000 / 300 = 10 . Meaning a 10 inch wide print would be exactly 300 dpi, because a 10 inch print would be 3000 dot / 10 inches = 300 dots per inch. Simple as that.

Your camera probably has more pixels/dots than that (check the specs; or the images themselves - the actual pixel dimensions are usually displayed by most photo viewers. Meaning you can print big than 10 inches wide and still have 300 dots per inch.

So if theyy are asking you to submit an image that will be 300dpi when printed, you need to know how big they plan to print it in inches. Multiply those numbers by 300, and you'll know how big your image needs to be in pixels. But that number in the EXIF field won't have anything to do with any of this.

[Lowell Goudge]

Marc makers some excellent points.

the only thing I would add, is that if you have an intended image print size, then make sure that that is how the image is scaled. But, if you are submitting a hard copy, and some publications ask for this, then you need to print your image at 300DPI for what ever size you want.

[tigershoot]

Just to add from a print perspective that 300DPI is regarded as the book/magazine publishing norm. However, as I learned in the Photoshop WOW book, for many photos you can easily get away with 225 and you won't see any degradation in the printed image. The authors reckoned that you must use 300DPI if your image has any hard edges (as else you may see some pixellation).

The 300DPI thing is way overused. It comes from the rule of thumb that the DPI should be twice the LPI (lines per inch of the halftone screen) used for any given paper. So, on quality coated paper, the norm is 150 LPI. On something like newsprint though the LPI may be way lower - perhaps just 85LPI so 170DPI would suffice. However, 300 is such a magic figure that if you submitted a 170DPI image you would probably get them complaining that image is too low res! I had Yellow Pages once insisting that the images must be 300DPI - they didn't know what they were talking about but 300DPI is the figure you will always here as the minimum for high res images.

Just to add from a print perspective that 300DPI is regarded as the book/magazine publishing norm. However, as I learned in the Photoshop WOW book, for many photos you can easily get away with 225 and you won't see any degradation in the printed image. The authors reckoned that you must use 300DPI if your image has any hard edges (as else you may see some pixellation).

The 300DPI thing is way overused. It comes from the rule of thumb that the DPI should be twice the LPI (lines per inch of the halftone screen) used for any given paper. So, on quality coated paper, the norm is 150 LPI. On something like newsprint though the LPI may be way lower - perhaps just 85LPI so 170DPI would suffice. However, 300 is such a magic figure that if you submitted a 170DPI image you would probably get them complaining that image is too low res! I had Yellow Pages once insisting that the images must be 300DPI - they didn't know what they were talking about but 300DPI is the figure you will always here as the minimum for high res images.

300 lpi is the new "norm" And I've got some suppliers going 500 lpi. My Heidelberg does ok at 200 lpi but it's only 2 color.

The rule of thumb was any detail over 2X lpi was lost. The press couldn't render detail any finer. Of course this was with a true dot. Many printers are now using stochastic screening.

[K-9]

Thanks everyone, especially Marc, who always seems to be the smartest tech guy on here (not that anyone else isn't either).

So, when I shoot a shot straight from the camera, and I have it as a PEF raw file, I would have to actually enlarge it when I save it as JPG/TIF to be able to give someone a file at 300dpi print ready resolution, or else they would see it as 72 and think it's not what they need? I would think that any file straight from my K20d RAW highest MP setting would be all they would need.

[Marc Sabatella]

Thanks for the nice comment, but read my response again. If you don't know the specific print size, there is absolutely no way to begin to answer any question about dpi. All you can know about an image is how many dots it contains. If you wat to know how many dpi it is, you need to know how big you're going to print it.

To re-use my car speed analogy, this would be like saying: I drove 600 miles from point A to point B; how fast is that on average in MPH? That's impossible to answer until you how many hours it took. And the answer is simple division.

OK, yes, there is always the chance that the person requesting the file has no clue what they are doing and will see the 72 in the EXIF and freak out regardless of what the *actual* dpi turns out to be (number of dots in your photo divided by size in inches they plan to print). Anyone who even *looked* at that dpi value before printing would be completely unqualified for the job they are performing, but if you still need to placata them, just go into the EXIF and change the number. Don't actually resize the file; that would be a mistake. I understand with Photoshop, there is probably a way to trick it into only changing the EXIF value by going to the resize window and specifying a new size in inches but then saying don't resample or something equally indirect, but frankly, that scares me. If all I am doing is changing a meaningless number in the EXIF, I'd rather just do that directly.

[jeffkrol]

Thanks everyone, especially Marc, who always seems to be the smartest tech guy on here (not that anyone else isn't either).

So, when I shoot a shot straight from the camera, and I have it as a PEF raw file, I would have to actually enlarge it when I save it as JPG/TIF to be able to give someone a file at 300dpi print ready resolution, or else they would see it as 72 and think it's not what they need? I would think that any file straight from my K20d RAW highest MP setting would be all they would need.

Yes it is but some only see the dpi number (I do some advertising for where I work and have seen this happen)... "dots" as in dpi can be made as big or small as you want. When an image is seen at 72dpi (and has not been re-sampled) it's dimensions will be reported as something like 42x28 INCHES (size depends on mp count, this is based on 6mp btw). Pixel count (which is all thats really important) will stay the same.

pixel count................ dpi........................ size in inches

3032x2016............... 72 ....................... 42.11x28

3032x2016............... 300 ...................... 10.1x6.72

most RAW editors can be told at what "dot size" to save your file (non destructive) to........ AND can be told at what jpg compression to use (usually destructive).
which are 2 different things... and resampling is different yet.
Dpi, misunderstandings and explanation, what is dpi
Deliverspecification for printed matter 300 dpi. What can go wrong.

Photos intended for printing must be delivered in 300 dpi (deliverspecification). It happens that in practice this is required too even when the printsize isn’t known yet. Actually ridiculous (see preceding paragraphs) but it happens regularly. This is no problem when the customer knows that it is possible to uncheck resampling in Photoshop (or Irfanview). If the customer doesn’t know this, things can go pretty wrong.

All About Digital Photos - The Myth of DPI
A Word of Warning
Some programs will resize a photo when the DPI is changed (see my example "The Horrible DPI Mistake"). Be very careful of this. To change the DPI without changing the pixel size of the photo you should click on the "maintain original size" (i.e. Corel Photopaint) or similar option that some programs offers, or click off "resample image" that other programs offer (i.e. Adobe Photoshop). See the page titled Changing the DPI of a Digital Photo.

[Wheatfield]

The only time the dpi tag matters is when you are sizing an image for a specific purpose.
As an example, the Noritsu lab that we have at the studio prints at 320 dpi.
So, when I'm sizing images for printing, I set the crop tool (Photoshop) to 320 dpi at whatever print size I am making and I get the right crop and the right number of pixels in one operation.
As was mentioned above, if you need to submit a 300dpi file for some reason or other, change that tag in your image size dialog, but make sure the program you are using won't try to resample the image. This way, the pixel count stays the same and the physical dimension tag chages.

The three tags: Image size, pixel count and dpi are interrelated in that if you change one, something else has to change to compensate. By forcing the resize to not allow the pixel count to change (do not allow resampling), if you change the dpi number, all that will change is the physical dimension that the file will print at the dpi you have input.
The converse also happens, change the print size withput resampling, the dpi changes as well.
However, this is all that is changing, the file itself remains untouched, dpi and print size are there for information only, they don't actually alter the file.
Did this make any sense?

[WerTicus]

dpi is a factor concerning the output device only.

its only important to you when your resizing or cropping an image to a specific measurement that you're doing so in the same dpi as your intended output.

monitors typically display 72dpi... that is fixed

printers can print at all sorts of dpi... typically printers do 300dpi.... mine does 360 and 720 if i want.

So if im in photoshop and i want a 10x8 i need to make sure i set the canvas to be 360dpi and 10x8 and not just 10x8 and 72dpi which would be 3x too small.

dpi is a factor concerning the output device only.

its only important to you when your resizing or cropping an image to a specific measurement that you're doing so in the same dpi as your intended output.

monitors typically display 72dpi... that is fixed

printers can print at all sorts of dpi... typically printers do 300dpi.... mine does 360 and 720 if i want.

So if im in photoshop and i want a 10x8 i need to make sure i set the canvas to be 360dpi and 10x8 and not just 10x8 and 72dpi which would be 3x too small.

You could still leave it at 300 and it'll print fine. All my stuff is 300 and I print to high end output that is 400dpi and it prints fine.

LCDs are generally 96 CRTs generally 72.

[K-9]

Thanks everyone, and I may even be more confused now than before, but I'm trying to follow.

Anyway, I have a publisher who is requesting what they call "high resolution (300dpi) files" be submitted for review. To me, this is easy when I scan my negatives and slides, because I can set the scanner to scan them at 300dpi and I'm done. My main issue is how do I supply them with the required 300dpi file from a shot taken with my K20d when it says 72dpi? From what I gather from Marc, I should just change the value myself (not sure how to do this) just for peace of mind of whoever may be looking at it, but all in all, even the 14.6MP, 72 dpi, full size image taken from my camera should be sufficient for them? Is this correct and I should just submit the 72 shots because they are a high resolution despite the EXIF data and the publisher will be aware of this?

[Wheatfield]

If you are using Photoshop, change the DPI number in the image size dialogue after unchecking the resize image tick box.
This will change the dpi tag without actually changing the image.
Other image editors should work in a similar fashion.
My experience is that if the publication says 300dpi, then that's what they want, not because it's what they need but because they don't know that it's a meaningless tag.

[Marc Sabatella]

Anyway, I have a publisher who is requesting what they call "high resolution (300dpi) files" be submitted for review.

You need to ask them what size they intend to print at, otherwise their request is meaningless. If the intended print size in inches is smaller than the size of images in pixels divided by 300, you're set. If not, then you can try upscaling the image (preferably using a "smart" program designed to do this well, like Genuine Fractals), but you'll also need to consider whether in the future you will need a camera with more pixels to satisfy these requirements. Or perhaps you've cropped your image heavily and that's why there aren't enough pixels left to yield 300dpi at the intended print size - in which case you may want to consider longer lenses.

Of course, as Wheatfield says, it's possible they are confused too and hav no idea that number of pixels and print size are what's important, not some meaningless number stuffed in the EXIF field. I'd start by assuming they are competent, and when you ask about print size, they'll tell you, and won't bat an eye if the images come in with an EXIF value that reads 72MPH. But if they respond with a blank stare when you ask them about print size, or object that the meaningless number on the EXIF says 72, then just change the number, hand them the file again with no other changes, smile, and take their money - no need to annoy them by trying to educate them on the matter.

As for how to change the number, that would depend on what program you might be using, but it *should* be as changing any other field in the EXIF, like copyright. i think because the topic is so poorly understood, software manufacturers often try to protect users from themselves and make it harder to change this field than it has any any right to be.