DPI setting

T

tomtom

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Edit My Images
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Hello everybody
you will be able to tell by this question my serious lack of digital nouse.

I recently bought a canon 20D, which i mainly use for recording my work (sculpture)
I like to save my images at 300dpi as this, i am informed is the best size for printing postcards, images in books etc.
I also have an old nikon coolpix which does this, but cannot find how to do this on the canon.
It just appears to save all images at 72DPI

Any ideas ?
 
Hmmm! I thought all cams recorded at 72 dpi, even when using the best image quality settings ... I dont know about RAW though as I dont use it.

What prog do you use to edit your pics?
I use Photoshop to edit my pics, I click Image>Image Size, and there at the bottom, where it says resolution, just change the 72 to 300.
Voila!

Hth, Im sure someone with more know how will be along to help shortly!
 
Hi
thanks for responding,
i use photoshop and am preety sure you can decrease the size of a file...ie 300 to 72 but not the other way around ?
 
you can increase the file size, but it can look a bit pants. What settings are you using on the cam for image capture?

*moved to correct forum
 
Lets take a 4MP image at 72 DPI....It'll typically be 2272x1704 pixels with an image size of 80.15cm x 60.11cm - pretty huge for screen viewing but no good for commercial printing as for this it needs to be 300 DPI. So, in Photoshop (or whatever image editing program you use), take the tick out of the "resample image" box and where it says 72 pixels/inch, change this to 300 pixels/inch. You will see that the pixel dimensions stay the same - 2272x1704 (4MP) but the image size has now changed to 19.24cm x 14.43cm. You now have an image of approx 19x14cm at 300DPI. What you have done in effect is crammed 300 pixels into a square inch instead of the original 72 out of the camera.

Hope this helps.
 
Images from my 300d opened in elements via the adobe raw plugin are 240dpi, size 32.51cm x 21.67cm
If I then resize the image smaller the dpi increases proportionately and vice versa upwards.
 
My best advice for DPI is to ignore it. It is only a piece of information stored in the file to tell applications interested in taking notice how to handle it. The DPI setting does not in any way effect the pixel size or quality of the image, it is simply a ratio that lets you translate the pixel size into physical units like inches or centimetres.

If you want to go online and order a 6x4" then pretty much everywhere you go will do so at 300dpi, and only at that resolution, which means a file of 1800x1200 pixels. And if you supply them with a 1800x1200 pixel file at 72dpi, you will still get a 300dpi 6x4" print.

It is only when you are printing something yourself and have a choice of dpi options that it is something you need to worry about, any other time it will be ignored.

Michael.
 
mij said:
The DPI setting does not in any way effect the pixel size or quality of the image
Click to expand...

Sorry but that's just wrong! The DPI directly relates to the printable size and therefore quality of an image. If you have an image that is 800x600 pixels at 72 DPI and you print it at say A2 size, you will see every individual pixel with the naked eye. Trust me, I do this for a living!
 
Grendel said:
Sorry but that's just wrong! The DPI directly relates to the printable size and therefore quality of an image. If you have an image that is 800x600 pixels at 72 DPI and you print it at say A2 size, you will see every individual pixel with the naked eye. Trust me, I do this for a living!
Click to expand...


thanks for mentioning this, as i was of the impression 300dpi was just for when you are scanning photos to your PC, and i recently sent some files to photobox which i'm sure where 72dpi as i think photoshop does this by default, so i guess next time i will have to make sure i change the dpi to 300 manually as you have mentioned, can you set it in camera to produce in 300dpi then ?
 
Any good printers should change the DPI to 300 anyway before printing the file I would of thought.

I only shoot RAW and they come in natively as 300dpi with adobe cs2.
 
thailand tel said:
thanks for mentioning this, as i was of the impression 300dpi was just for when you are scanning photos to your PC, and i recently sent some files to photobox which i'm sure where 72dpi as i think photoshop does this by default, so i guess next time i will have to make sure i change the dpi to 300 manually as you have mentioned, can you set it in camera to produce in 300dpi then ?
Click to expand...

I don't believe you can set a camera to produce a 300dpi image, although I'd happily stand corrected if someone knows of one that can :shrug:
The whole dpi and file size issue can be very confusing - I posted this earlier in this thread:

Lets take a 4MP image at 72 DPI....It'll typically be 2272x1704 pixels with an image size of 80.15cm x 60.11cm - pretty huge for screen viewing but no good for commercial printing as for this it needs to be 300 DPI. So, in Photoshop (or whatever image editing program you use), take the tick out of the "resample image" box and where it says 72 pixels/inch, change this to 300 pixels/inch. You will see that the pixel dimensions stay the same - 2272x1704 (4MP) but the image size has now changed to 19.24cm x 14.43cm. You now have an image of approx 19x14cm at 300DPI. What you have done in effect is crammed 300 pixels into a square inch instead of the original 72 out of the camera.

Just to add further - the image doesn't HAVE to be 300dpi to be printable commerically if the image is scaled. eg: in the above example, the printer could take the 80x60cm 72dpi image and print in at approx 25% reduction in size and achieve the same quality as if you scaled the image yourself in photoshop and actually made it 300dpi and the printer printed it at 100% size.

Ultimately it about getting 300 pixels into a square inch so if you do it in photoshop that's ideal because you are supplying an image for printing at the correct size and quality for printing.

I hope that helps :shrug:
 
DPI is all about printing blobs on a sheet of paper , it doesn't relate in anyway to what the camera has captured.
The camera records a finite number of pixels at whatever resolution/quality it has been designed for / set to.
When the user decides on the size of print is required then the post processing scales the camera image to match the printer resolution to give the desired finshed size.

Its all about fiddling numbers , the only bits that are real is the number of pixels on the sensor & the texture of the potato used for printing :D
 
mij said:
My best advice for DPI is to ignore it. It is only a piece of information stored in the file to tell applications interested in taking notice how to handle it. The DPI setting does not in any way effect the pixel size or quality of the image, it is simply a ratio that lets you translate the pixel size into physical units like inches or centimetres.

If you want to go online and order a 6x4" then pretty much everywhere you go will do so at 300dpi, and only at that resolution, which means a file of 1800x1200 pixels. And if you supply them with a 1800x1200 pixel file at 72dpi, you will still get a 300dpi 6x4" print.

It is only when you are printing something yourself and have a choice of dpi options that it is something you need to worry about, any other time it will be ignored.

Michael.
Click to expand...

which is pretty much how I understood it to be.

Grendel said:
Sorry but that's just wrong! The DPI directly relates to the printable size and therefore quality of an image. If you have an image that is 800x600 pixels at 72 DPI and you print it at say A2 size, you will see every individual pixel with the naked eye. Trust me, I do this for a living!
Click to expand...

Grendel - trying to understand more than doubting you.....

DPI = dots per inch. If a picture has 1800 dots (pixels) in width and you don't throw any away or create new ones by changing the number of dots in an editor, How does the quality change if the file info happens to say 'print at 72 dots preference set' or 'print at 300 dots preference set'.... and you then ignore it as mij posted. mij did state the same pixel dimensions for both.

Not sure I see the connection to your example of a small file printed too large.
 
For printing a 6"x4" at Photobox, as long as you upload an image more than 1800x1200, it will get resized/cropped to fit that dimension (aspect ratio 3:2) to fit in 6"x4". It doesn't matter if you have the DPI set to 70 or 300.

Of course it is better if you cropped/resized yourself so that you know exactly what you're getting.
 
I think the quality changes, Robert , because they will be printing at the same size of 6X4. If you have a finite number of dots in an image.. then if you squeeze those dots in to each inch at 300 dots in each inch...... then the quality will be good but you will run out of dots after only a few inches so the image will be small. If you use the same number of dots and onlly put 72 in each inch...you will be able to carry on into more inches as you will have more dots left over.

So, if you send in a photo at 72dpi for printing and ask for 6 x 4 they print at 300dpi so that will be ok
Alternatively, if you want the image to be bigger than 6X4 and only send it in as 72dpi they wont be able to change it into 300 dpi for printing as by the time they have put 300 of the dots into each inch...they will have run out of dots at 6X4 so wont be able to print bigger at that good quality. The only way to print bigger than 6X4 would be to stretch out the dots and have less in each inch....which would indeed show al lthe dots and result in poor quality.


HTH either that or it has set everyone into more confusion!
 
But a 72 or 300 'tag' in the file won't change the number of pixels in the digital image. So how can the quality change if you just ignore that number? The file will have (say) 1800 pixels across whatever the DPI print number is.
 
RobertP said:
But a 72 or 300 'tag' in the file won't change the number of pixels in the digital image. So how can the quality change if you just ignore that number? The file will have (say) 1800 pixels across whatever the DPI print number is.
Click to expand...

Because by changing the dpi from 72 to 300 you are compressing more of the pixels (aprox 4 times more) into the same area therefore improving the PRINTING resolution. The amount of pixels in the image doesn't change but the PRINTABLE size of the image reduces
 
You can increase the number of pixels in a square inch to improve print quality but if you go the other way, ie try to print a 72dpi A5 image at A3 you are actually putting 4 times less pixels into a square inch and therefore reducing the PRINTING quality
 
RobertP said:
But a 72 or 300 'tag' in the file won't change the number of pixels in the digital image. So how can the quality change if you just ignore that number? The file will have (say) 1800 pixels across whatever the DPI print number is.
Click to expand...

The quality can change because you are deciding to print at different sizes and therefore stretching out that finite number of pixels over a larger area...therefore the quality is poorer. Why are we tallking about ignoring the numbers? the numbers are important when you are printing. the tag of 72 or 300 isnt important when looking at your file size but it is when you are going to print and put those dots into each inch.

RobertP said:
The file will have (say) 1800 pixels across whatever the DPI print number is.
Click to expand...

you are right...the file will have 1800 across but the size of the printout and quality of the printout will not be the same whatver the dpi print number is.
 
any image has a given number of pixels (which I think is what you quite correctly are saying Robert), it's about how many of those pixels you put into a square inch of printable size that determins the final printing quality. If you look at a picture in a newspaper up close you can see the individual dots that are used in the printing process - this is because newspapers print at a very low resolution, typically 150dpi. Now look at a high quality print in a hardback book or similar. You will only see the individual dots if you use a magnifying glass this is because it will have been printed at a much higher resolution, typically 300 dpi. More dots in a square inch.
 
1000 x 1000 pixel image. I want a print 5 inches square.

If the dpi is pre set to 300 but I still print it at 5 inches square, surely the print program will arrange 200 pixels per inch and them resample them to suit the printers nozzles or whatever. So the dpi in the file is ignored.

Where is the flaw in my logic ?
 
Ok so the confusion is if a printer really does make 72 or 300 dots spaced apart and that is the best it can do?
 
I think it will print 200 an inch because of your filesize and the size of the print you are requesting.... and the dots may be noticeable. the printer wont manage to squash 300 dpi in there..if it did then the print would have to be less than 5X5
 
No flaw at all Robert. The maths is:

1000x1000 pixel image at 300dpi would actually print at 3.33x3.33 inches. If you wanted to print it at 5 inches x 5 inches the dpi would become 200dpi, therfore slightly less quality
 
Janice said:
I think it will print 200 an inch because of your filesize and the size of the print you are requesting.... and the dots may be noticeable. the printer wont manage to squash 300 dpi in there..if it did then the print would have to be less than 5X5
Click to expand...

Or what Janice said! (y)
 
RobertP said:
Ok so the confusion is if a printer really does make 72 or 300 dots spaced apart and that is the best it can do?
Click to expand...

Ah now you're getting into printer resolution, that's another subject altogether! :bang:
 
DPI only relates to printers & scanners. They have a true , physical capability. All the rest is smoke & mirrors.

After all if a 1800x1200 camera truly was 72dpi it would need a ******* big sensor :D
 
CT said:
LOL.

PPI and DPI are two different things - the latter refers only to printer settings.
Click to expand...


Good point CT (y) ppi = pixels per inch, dpi = dots per inch

A lot of confusion comes in because people tend to use dpi as a generic term when refering to image resolution :bang:
 
The PRESET dpi isnt actually IN the image. It is just called that as a statement of the size and how the dots will be arranged in each inch. The output size and amount of dots are interlinked. SO if you decide to change your dpi then your output size will have to change.

You really just have a 2400 X 1800 pixel image. end of story. then if you are asked about the resoultion and you say it is 300dpi.(for print) that shows it can be printed out at say 8X10 and be good. if you said that your 2400X1800 image was at 72dpi and was to be printed out. people would know that either you were going to get a smaller image than 8X10 (using 300dpi to print with) OR that if you did indeed print at 72dpi and get the 8X10 size ...it was going to be very grainy and dotty.

Hope I'm not going round in circles, Robert, Im trying to help...truly I am! (y)
 
Grendel said:
Sorry but that's just wrong! The DPI directly relates to the printable size and therefore quality of an image. If you have an image that is 800x600 pixels at 72 DPI and you print it at say A2 size, you will see every individual pixel with the naked eye. Trust me, I do this for a living!
Click to expand...

Precisely! The DPI relates to the printable size and does not affect the pixel size of image quality of the file.

An 1800x1200 pixel file is an 1800x1200 pixel file regardless of the DPI setting in the file. And when printed out by a printer that only offers 300dpi output it will give you a 6x4" print regardless of the DPI setting in the file.

The DPI setting is an instructional field in a text file that can be used or ignored, in the majority of cases it will be ignored.

I use non-destructive cropping which means setting a size of 3x2" in Photoshop and it calculates the DPI setting relative to the pixel size of the image. And when I send off an image like that, with a DPI value of over 900, to a lab that only prints at 300dpi I always get back 6x4" prints. No one one has ever sent me a really high quality 3x2" prints.

If you change the DPI setting in Photoshop the image will remain the same unless you resample it. The only difference between an 1800x1200 image at 72dpi and an 1800x1200 image at 300dpi are the bytes which record the DPI setting. The image itself remains identical.

My point is not that the concept of DPI has no meaning, but that in the majority of cases the DPI field within the image file is ignored, and when it does matter you will be told it does and what you need to use.

So unless you are printing yourself the DPI setting is something you do not need to worry about as the printer (human or software) will handle this based on their own particular requirements.

If anyone can name a non-specialist lab that offers user-customizable dpi settings then I would be interested, but I have never seen any that offer anything but 300dpi or 254dpi at larger sizes. And they will crop or resample as appropriate to get the appropriate pixel count before printing. Give them a true 72dpi 6x4" file at 432x288 and they will upsample it to an 1800x1200 file before printing, they will not print it at 72dpi.

Try sending them an 1800x1200 file set to 1dpi and see what you get back, it will not be a roll of wallpaper.

Going back to tomtom's original question, which seems to have been lost, it does not matter what DPI setting he uses in his software because when he sends his shots off to be printed they will be 99% likely to be printed at 300dpi and the only factor that will determine the quality of his prints will be the pixel size of the files.

One thing to remember, is that Photoshop was meant to be a printers tool, not a photographers one, which is why DPI and CYMK etc. are all prominent features. And this is why Adobe are producing LightRoom in response to the need of digital photography.

Michael.
 
Are we all happy now cos my head hurts ;)
 
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