There is much confusion about dpi. Searching with
Google with the terms forum, 300 dpi results in a long string of
questions: how is it done, I don't get it. Some
people try to explain it, others meddle with this. The end is usually an
enormous confusion of tongues. For all these bewildered people I have written this article.
happened before you can read here.
per inch is ppi. Unfortunately this is usually called dpi.
DPI is a confusing conception. When it concerns the inking of prints, it means DOTS
PER INCH. But it is used too for PIXELS PER INCH. Though in that case one should speak of
PPI. But to make it even more confusing the term DPI is also used for
PIXELS PER INCH.
Alas. A better notation for PIXELS PER INCH would be:
px/inch. Or when we calculate it into centimetres: px/cm. More: a copy of an article that earlier was
is DOTS PER INCH? Read here.
1 inch = 2,54 cm, 1 cm = 0,39
dpi-value in the EXIF is meaningless. It is a fictitious number.
(A photo has no dpi, only
You can find the properties of your digital photos in a
photo editing program, but also in the Windows Explorer. Do a rightclick on your file
and next: Properties > Summary > Advanced. These properties are called the EXIF-data. Or short EXIF. In that row also a value
for dpi is given, f.e. the number 72, 180, 300 etc. This is a number
without any meaning, it is an invented number! A
box (field) has to be filled in, so every producer simply invents a number. Don't get confused, it
is nonsense. A photofile in a camera or (stored
at) a computer has no size in centimeters or inches. So no ppi (or
dpi). Such a file only has a subdivision, a resolution, in pixels. From
the moment you are going to print there
is the matter of size, given in inches or centimeters. Only then you can
speak of pixels per inch (ppi, regrettably ususally called dpi).
See fifth paragraph of: www.scantips.com/no72dpib.html#6.
resolution of a photo is expressed in Pixels (w x h)
The resolution of a print is expressed in Pixels per Inch
is a confusing conception
is just like DPI a confusing
term. It has several meanings. So watch out in which contexts DPI and RESOLUTION
Dpi (actually ppi) is only important in case of
printing. When you open a picture in Photoshop a dpi-number is mentioned.
Donít pay attention to it. It doesnít tell you anything, you can
change it in any number you want, it doesnít change anything
of the picture. But beware: you have to uncheck the resample-box! Resampling
means calculating a new amount of pixels. The moment resampling is off, you
can change the dpi-number, the according printing-measure is given. It
also works the other way around: change the printing-measure, the
dpi-number of the picture will be shown.
to calculate the dpi without software
course it is possible to calculate the dpi without Photoshop: pixels (height or width) divided by inches (heigth or width) is dpi.
image on internet or a monitorscreen is not 72 dpi
A computerscreen (monitorscreen) is nowadays usually
adjusted at 1024 x 768 pixels (this is called screenresolution). And
when a picture on your screen is shown, then on each
screenpixel is one picturepixel. Or
actually just the reverse: every pixel of the image takes up the space
of exactly one pixel of the screen. For example: a
photo sized 512 pixels wide on a 1024 pixels-wide screen, covers half of
that screen in width. It doesnít matter how large your monitorscreen is
measured in centimeters or inches. A photo on a screen has only a 'size' in pixels. Of course you can try to find out with a ruler
how many centimeters or inches that very picture on that very moment
measures on your screen. But it
is useless, your videosystem works with pixels only, not with
centimeters or inches. So forget this 72 dpi.
More information: www.scantips.com/no72dpi.html
gave birth to the 72 dpi-myth?
The dpi (px/inch) of the PRESENTATION of an image on the screen is identical to
the dpi of that screen. So on a screen of 72 dpi the presentation of that
image is 72 dpi too. But the image itself is not 72 dpi. Put it more strongly: such
an image hasnít any dpi, only pixels. And as computerscreens formerly
often had a resolution of 72 dpi, the misunderstanding arose that an image on a
screen or on a website is always 72 dpi. But on a screen of for instance 100 dpi
is an image on that moment 100 dpi too. On a screen of 96 dpi is
the image 96 dpi etc. etc. But only on the moment of presentation. The
image itself has no dpi, but only pixels.
Explanation: The image is reproduced 1 to 1 (pixel on pixel), thatís why it has the same dpi
the screen on which it is shown on that particular moment.
To determine the dpi(px/inch) of a screen:
Calculate the screenresolution (width x height in pixels). Determine the size of
your screen in one direction (width or height). Now divide the pixels (in width
or height) by inches (in width or height).
More: see previous and next paragraph.
and Save for the Web (72 dpi? No)
Why does Photoshop indicate 72 dpi at Save for
the Web? Simply because a box has to be filled in. But it is a
fictitious number. Other software for example mention 96 dpi or
The picture on the right side of this page was
1063 pixels wide. I wanted it to fit in the right column, so I resampled
it with Irfanview to 178 pixels wide. With Photoshop you can do this by
means of Save for the Web. (With this, other changes are possible too,
but are of no importance for this argumentation.)
When you have saved this picture and you open it again in Photoshop, the
imagesize-box will show you 72 dpi. When you uncheck resample, you will
be able to change the dpi according to your wish, the pixelsize remains
the same, and the printsize changes. The reverse is possible too: change
the printsize, the dpi changes. But this printsize and the dpi have no
significance if you want to put this picture on the web. Itís
only the pixelsize that matters.
That this picture is not 72 dpi, is visible when you look at Windows
Explorer or Windows Viewer. The Exif (properties box) mentions here 96
dpi. So Photoshop invents 72 dpi, and Windows thinks: letís make it 96
What printers have problems with, is that people get pictures from
internet to have them printed. In case of the picture on this page:
Suppose I want to print the picture of 178 pixels wide at the size of 10 cm wide (= 3,9 inches). Then the dpi is 45,2. This is of course far too
low. The difference with 300 is also too big for resampling (see below
the paragraphs Dpi and Printing and Numeric Example).
my dear printers: this picture wasnít and never has been 72 dpi as you
always maintain. This picture has far too few pixels for a good print at
the size of 10 cm, and far too low dpi at 10 cm (45), but never has been
When I want to print this picture of 178 x 248 pixels at the size of
6,28 by 8,25 cm (= 2,47 by 3,44 inch), then the dpi is exactly 72.)
and printing (300 dpi and printed matter)
printing is usually 300 dpi required. With Photoshop you can
calculate if your picture conforms to this requirement (uncheck the
resampe box!). When this is not the case (too little dpi), you put the
dpi at 300, check the resample box, and your picture is ready to be
pay attention: when your picture is too far under 300 dpi, you might try
to resample it to 300, but what you get then is a inferior print. Too
much difference is not allowed. So consult with your printer. It is also
better to resample a picture that is far higher than 300 dpi. This has
advantages for the printer, moreover you wonít be able to see the
difference with the naked eye when printed.
dpi? No! Pixels, thats's what it's all about.
4 MP camera produces (ratio 3:2) photos of about (rounded) 2300 x 1500 pixels.
made with a Nikon camera, you read: 2300 x 1500 pixels, 300 dpi.
made with a Canon camera, you read 72 dpi instead.
the newest Canon cameras itís usually 180 dpi.)
photos are to be sent to the printshop, and both
are to be printed in the same size.
The Nikon photo will be accepted because it is 300 dpi, which is according to
the demands of the printshop.
The Canon photo will be returned because of its resolution, which is too low,
and its too bad quality to print.
According to the prinshop, this photo doesnít meet the requirements because the
dpi is far too low to get a good print.
MADNESS! THIS IS NO EXCEPTION, THIS IS THE RULE.
is because one doesnít understand what dpi/ppi really is.
That photo by Canon has exactly the same quality as the one by Nikon (perhaps it
is even better, f.e. a sharper, better exposure, less noise) and can be
printed in the same size as that one by Nikon. Because the Canon photo has the
same amount of pixels. Only Canon chose at random the number 72 to be put in the
dpi-box, while Nikon chose number 300
in the same way. Kodak took 230. Another manufacturer chose 180, etc.
But such a number has no meaning whatsoever. You can rub or cross this number
out and replace it by another number (as it is a digital data, you can of
course not rub or cross it out really, you must do this by means of Photoshop).
And when you change in case of the Canon photo the number 72 into 300
in Photoshop (or Irfanview) (uncheck the resample-box!), you save this photo and send
it to the printer, he/she will say: 300 dpi, perfect photo.
This while the quality of the photo hasntí been changed at all!
Same exposure, same number of pixels, nothing has changed. Only the number of 72 has been
changed into 300.
I think the printer (or his preparer) could have done this quicker himself.
A 4 MP camera, ratio 3:2, produces a photo of about 2300 x 1500 px.
A 5 MP, ratio 3:2, produces a photo of 2600 x 1700 px.
Suppose the first photo comes from a Nikon camera, than you read: 2300 x 1500 px,
The second one is from a Canon camera, here the info mentions: 2600 x 1700 px, 72
Both must be printed at the same size.
The first photo will be accepted by the printer, the second one will be returned
because of its too low resolution.
is absurd, because it is just the reverse: the second photo has a higher
The resolution of a (not yet printed) photo
is the pixelsize ( see third paragraph on this page).
NEXT PARAGRAPH: WHAT CAN GO WRONG!
What Print Shops Really Want: www.rideau-info.com/photos/printshop.html
Watson applies to people from the printing-shops, and says STOP
DOING THIS, ask fot the number of pixels you need, and not for the dpi.
Changing the DPI: www.rideau-info.com/photos/changedpi.html
Ken Watson tells us here: If a printer still asks for a photo with so much dpi
instead for a photo with so many pixels, dontí try to explain the way it works,
give them what they ask for, change the dpi of the photo in whatever number they
want, 300 or it doesnít matter what, as long as you donít resample.
meaningless, and a change of that number doesnít alter your
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 isnít 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 doesnít know this, things can go pretty wrong.
1. Pictures made with my 4 MP Kodak Camera, are loaded in Photoshop as follows:
2304 x 1536 px, 230 dpi, and (of course because of that 230 dpi) 10,017 x 6,678
2. I change this to 300 dpi and I
donít know that I must uncheck resampling, so I get:
3005 x 2003 px, 300 dpi, printingsize stays 10,017 x 6,678 inch.
3. Suppose the printingsize must be 8 inches wide (of course the 300 dpi must
not be changed), then you get:
2400 x 1600 px, 300 dpi, 8 x 5,333 inch.
This means that at first there has been a resampling upwards at 3005 x 2003
pixels. After this resampling downwards to 2400 x 1600 pixels. But resampling was
quite unnecessary. Quite a pity to to spoil your beautiful photos in this way!
So: When the printsize is not yet known and the printer requires that
you must deliver your photos in 300 dpi, uncheck the resample-box in Photoshop (or
Irfanview)! In that case nothing can go wrong.
Read also: Ken Watson - The Horrible DPI
Mistake, on his page The
Myth of DPI
Wrong: Deliver photos at 300 Dpi. (This is what you read at the sites of the
greater part of the printshops, above you can read that because of this you will
get bad prints.)
Printers should say: Photos must be delivered with enough pixels, so that they
can be printed at minimally 300 dpi (actually ppi, px/inch).
Prints, pay attention!
When you have really
beautiful photoís which you want to have perfectly printed, then
itís worth the trouble to join hands with a printer who specializes in
that kind of printing (called art print). Then itís possible to
deviate from the strict requirement of 300 dpi. (This seems
contradictory to the preceding paragraph, but this is caused by the
finer grid which is used for art prints. Thatís why it makes sense to
work with a higher dpi.) Of course this procedure is more expensive.
Inquire and consult to avoid disappointment.
a 4 MP camera.
When Iím taking pictures, ratio 4:3, the picture will be 2304 x 1728
pixels, and W x H in pixels is then 3.981.312 = (rounded off upwards) 4
In ratio 3:2 they will be 2304 x 1536 = 3.538.944 = (rounded off) 3,5 MP.
mostly take pictures in ratio 3:2. Suppose I want a print of size 15 x 10 inch, then the dpi will be: 2304/15 = 153,6 (or when I calculate it with the
height: 1536/10 = 153,6).
This is far below 300 dpi. I could resample, but the difference between
153 and 300 is too much, the picture will be of a very bad quality.
When I start from 300 dpi, I can make the following calculation:
2304/300= 7,68 en 1536/300= 5,12. So a
picture of (rounded off) 8 x 5 inch is the maximun printingsize for the
photos made with my camera, starting with a dpi of 300.
In case of calculating this for your own photos: when a little below 300 dpi for a
certain printing-size, you can resample with Photoshop. When far above
300 dpi, you have to resample downwards.
∑ divide pixels
inches, is the outcome > or exactly 300, you get a good print
∑ divide pixels
by inches, is the outcome < 300, you have to add pixels (resamplen)
∑ divide pixels
by 300, and
youíll get the maximum printsize in inches
∑ divide pixels
by centimeters, is the outcome > or exactly 118, you get a good
∑ divide pixels
by centimeters, is the outcome < 118, you have to add pixels
divide pixels by 118, and
youíll get the maximum printsize in centimeters
But once more: the dpi must not be too low, for in
that case resampling has no use any more.
Youíll get a very bad picture (where is the limit, I donít know
exactly, so consult your printer).
now for something else:
What you read above is the official story, the thing you have to
reckon with as printers start from these. But as a matter of fact it is
no more than a directive.
At the beginning the
norm was 240,
but a broader margin was wanted, after rounding off upwards the norm was
at 300 dpi.
Yet there are more factors which determine a good print, for example the
quality of your cameralens. You can read more about this at
Ken Rockwellís site:
2. Ken Watson thinks nowadays 200 dpi is
sufficient for printing. On his site www.rideau-info.com/photos
find among others the following articles:
What Print Shops Really Want, and Changing the DPI.
Dots per inch (inkdots)
DPI as DOTS PER INCH (as distinct from
PIXELS PER INCH), are the small inkdots (per inch) that come on the paper during
the printing-procedure. The DPI (dots/inch) must be larger than the
(pixels/inch), otherwise the result will be a bad print. An example: A print
has 300 pixels per inch and 1200 dots per inch. In this case each pixel
has the average of 4 (ink)dots. Back to
Printing at home and
(dots per inch)
At home you are usually printing with an inkjetprinter. When printing a
colour print it puts on each pixel several dots (inkdrops) (see previous
paragraph). Some printers have a higher dpi than other ones, and
you tend to think: the higher the better. This neednít always
be true. Other technical aspects of a printer are no less determining
for the quality of the print. For example the way colours are mixed, the
space between and the size of the inkdrops, and much more. Instead of
relying on the dpi better read the reviews at the websites which are
known as reliable.
Dpi (pixels per inch)
When printing at home there is in general no need to adjust the dpi
(pixels per inch) (as described
in the paragraph Dpi and
Printed Matter). Although it is sensible to make an estimation at
what size your picture approximately (maximal) can be printed. A
homeprinter usually produces still good prints at 200 dpi (pixels per
inch). You can make a proof in order to see if the result is
satisfactory. If not, make a smaller print, or raise the dpi (ppi) with,
for example, Photoshop or Irfanview. See for explanation elsewhere on
(ppi) = 200 pixels per inch = about 80 pixels per cm. So a print of 15
by 10 cm has to be 1200 by 800 pixels minimally. If not, resample as
described elsewhere on this page.
A useful trick to be able to know quickly how large a picture roughly
can be printed in centimetres is: divide pixels by 100.
Explanation: 100 pixels per cm = 254 pixels per inch = 254 dpi (ppi).
An example: you want to print a photo sized
4 by 5 inch the same size. Than you have to scan it at 300 dpi. If you wish to print
8 by 10, youíll have to scan at 600 dpi, the
print will be 300 dpi. The question is, can your device manage it, it
makes great demands on the memory. But essentially this is the way it
has to be done. More info: www.scantips.com
and megapixels (MP)
The MP of a compactcamera is usually
based on the production of photoís of 4:3.
(Also see paragraph Numeric Example as mentioned above.) My camera is specifically called a 4 MP
camera. A photo of 4:3 (adjusted at best
quality or highest resolution) has 2304 x 1728 pixels = (rounded off)
4.000.000 pixels = 4 MP.
(The camera can also be adjusted at a lower resolution, in that case
you get a picture with fewer pixels in both width and height.)
When I set the camera at 3:2, in the highest resolution, the
picture will have 2304 x 1536 pixels = (rounded off) 3.500.000 = 3,5 MP.
picture is of the same quality, the same resolution as the picture of
4:3! The number of pixels changes, caused by ratio and not by quality!
Width remains the same (2304 pixels), but height changes from 1728 into
1536, so in all there are fewer pixels.
So if you know the MP, you can calculate the number of pixels.
= MP. So
12 x square is MP.
When for instance you own a compactcamera
of 5 MP, then the procedure is as follows: 12 x square = 5.000.000.
x and you will
see that the picture will be (approximately) 2582 x 1936 pixels.
(Explanation: divide 5.000.000 by 12, extract the root, multiply this number
with respectively 4 and 3, and youíve got W
x H in pixels.)
So by means of the MP of a camera you are able to calculate the printsize of your pictures. First calculate the number of pixels with the
x square. Then divide by 118, and you have the maximum
printsize in centimeters (for inches: divide by 300). (See scheme in the
paragraph Numeric Example.)
With a reflexcamera the MP-indication usually gives the number of pixels
of a photo ratio 3:2. So f.e. a 6 MP camera, adjusted at the highest
resolution, produces photoís of approximately
3000 x 2000 pixels. (Same calculation as with compactcamera, but now
x square = 6.000.000)
and disadvantages of a camera with a high MP
camera with a high MP better than a camera with a low MP?
quality of the picture has to do with other factors, not with the number
The advantage of a camera with a
The higher the MP, the more pixels a photo has, the larger this
photo can be printed. This especially is important when you frequently
make crops. The number of pixels
decreases, through which the
maximum printingsize gets smaller (compared with the original
(But for very large prints the MP can even be lower, as these photos are
viewed at a very great distance.)
The disadvantage of a camera with
a high MP:
The files are getting bigger (in KB or MB). So your memory card will
earlier be full, storage at the computer requires much room, sending by
mail often is not possible in the original resolution.
(It is often possible to set the camera in a lower resolution, a lower
MP. Donít mix this up with the JPEG-compression, this certainly has to
do with the quality of the picture!)
You can work it out for yourself now with the information as given in
the previous paragraph: a 6 MP camera will be amply sufficient for most
people. And as with printing at the customary size a 4 MP or 5 MP camera
meets too with the demands.
The Megapixel Myth - www.kenrockwell.com/tech/mpmyth.htm
Just like many other people I got
confused when I wanted to have pictures printed and I was told that I
had to deliver them at 300 dpi. The pictures I had made with my
Kodak-camera were 230 dpi, at least this was mentioned in
the Exif-data. Asking questions about the meaning of that number
in photostores resulted in rambling stories. Making inquiries at Kodak
Nederland gave the following result: one didnít believe that a
dpi-number was mentioned in the Exif, because a stored picture has no
dpi, I was told. When they found out that
really 230 dpi was mentioned in the Exif of the Kodak-pictures,
they found this odd. I searched on internet for the meaning of dpi. With
the information I gathered I couldnít explain the dpi of my pictures. And that may be right, for on an internet-forum I read the only
plausible explanation: it is a fictitious number, some number has to be
filled in. I think that this ďboxĒ is inserted in view of the
printing of photos. But as long as printing is out of the question, that
fictitious number is senseless and confusing. I take it ill of
camera-manufacturers that they omit to supply information about this.
The same applies to books about photography. Hardly ever youĒll find
this explanation there. Thatís why I have put this information on
internet. Back to first paragraph.
in the education for the graphical sector
various sides I learn that in the schooling for the graphical sector the
misunderstandings about dpi, as described above, are still taught (in
The Netherlands, but also in most other countries). It turns out to be
very difficult to convince the graduates form these educational
institutions (who find themselves in the printing trade) that they are
mistaken. They certainly find it hard to accept this from an outsider.
Illustrative is the reaction on a forum by a former pupil of a graphical
school: OK, so I understand that at the graphical school everything
has been explained incorrectly. This seems rather strange to me.
This canít be wrong, Iíve got this from school. It was drillled into
This attitude Iím afraid will only change when someone inside the
trade group realizes that something is amiss on this point in the
printing trade and starts to act to bring about a change in the
schooling. Maybe this will happen only when the sector realizes that it
profits by it: less frustration in the contacts with clients, less
mistakes, less loss of time. We will see.
See also: www.emdpi.com. The author
of this site states the sort of mistakes he came across in a
textbook on graphics. A ďthumbs downĒ indicates that the given
information is wrong.
Pictures of Groningen Holland