Another dpi question regarding what we see on screen

[TMR Design]

First of all I am curious as to why each camera seems to output the image with different dpi indicated. my Canon A620 saves with 180dpi and my 30D daves as 72dpi. My understanding is that this number only comes into when printing.

The thing I find confusing is that although people say that number does not matter, it is confusing that the size of what we see on screen changes. When you have a 300dpi image it is absolutely huge on screen and opposed to the same image at 72dpi.

When resizing in PS for instance, you can change the image size and resolution but 800 x 600 pixels at 180dpi is a different size than 800 pixels at 72dpi. I would like things to be uniform and I want to make sure that everyone seeing my images sees the same thing.

So knowing I am not going to print what should I do for uniformity? Do I now size by inches and not pixels so that a 6" x 8" is just that and the pixel resolution does not come into play? When I change the 180dpi image to 72dpi the image size changes.

Can someone explain this without making reference to printing and only with regard to what we see on screen, what is uniform and compatible so anyone I send to or any upload to the Internet looks the same?

[Mcary]

Robert,

Do me a quick favor take any image and resize it to 800 on the long side next uncheck the resize box and change the PPI/DPI to 72 close the resize window and save the file as a JPEG. Next re-open the resize window again and change the PPI/DPI to 4000 close and save with a different file name.
Now post both images to the net and see if there's any difference.

A file thats 3504x2306 will appear huge when posted to the net no matter what DPI you use at the same time a file thats 800x600 will appear small no matter what DPI you use this is due to the actual phyical size dimentions of the file not the DPI/PPI
The net is basicially set-up to display images at 96PPI.

Mike

[TMR Design]

HI Mike,

OK, I did this test. The image at the higher resolution, when resized does appear huge. I did the same test but kept the image width at 800px with the high resolution and the iamges appeared the same. So by seeing this am I correct in assuming that the resolutoin that the camera spits out is unimportant to me? If I edit an image and make it 800 x 600 then a pixel is a pixel and that does not change? I think what confuses me is that when editing the images in PS they will appear on screen at different sizes when at different resolutions and if you have something resized that was at 180dpi and then change it to 72 the image is then too small and will distort if you were to attempt to bump up that number again.

[wei328]

When you resize an image in PS, the resolution you enter is "pixels/inch" NOT DPI, that why the image will be bigger or smaller according to number of pixels you specified.

[DavidW]

The whole "dpi" (which is really pixels per inch - ppi) thing is largely irrelevant for screen viewing and for photo editing applications. The zoom in Photoshop relates to pixels, not any notion of size - 100% is one image pixel per screen pixel. In other words, the "dpi" value is irrelevant to zoom ratios in Photoshop and most if not all other image editing applications. Browsers ignore the "dpi" value too - they display one image pixel per screen pixel unless you scale the image in the HTML (which is a bad idea - resizing the image using appropriate software will give much better results).

The "dpi" value is simply a number that translates the number of pixels along each side of the image to a measurement. This comes in when you move to a more print orientated application, such as InDesign. If I place a Photoshop image in InDesign, then the "dpi" value will determine the initial size of the placed image on the page, though, of course, I can override that size in InDesign.

The "dpi" value does determine the default print size in Photoshop, but as most people's printing workflows use Print with Preview, it often gets overridden even for printing.


The "dpi" value is simply an item of metadata which says "if you need a measurement for this image, place n many pixels per inch". When it comes to bitmap images, it only really has any link to reality when it comes to scanning - the appropriate "dpi" value may help you use the scan at the same size as the original..



David

[Mcary]

When you resize an image in PS, the resolution you enter is "pixels/inch" NOT DPI, that why the image will be bigger or smaller according to number of pixels you specified.

The image may appear bigger or smaller based on the number of PPI choosen but infact a file that is 800x600 pixels is still 800x600 pixels no matter if you've set the PPI to 10 or 10,000 and infact at 100% and fit to screen both will look exactly the same.

if you take a file that 800x600 @300 PPI and change the PPI to 150 and the resize block is checked you're changing the actual phyical size of the file from 800x600 to 400x300 that's why it appears smaller, because it is smaller.

[Mcary]

HI Mike,

OK, I did this test. The image at the higher resolution, when resized does appear huge. I did the same test but kept the image width at 800px with the high resolution and the iamges appeared the same. So by seeing this am I correct in assuming that the resolutoin that the camera spits out is unimportant to me? If I edit an image and make it 800 x 600 then a pixel is a pixel and that does not change? I think what confuses me is that when editing the images in PS they will appear on screen at different sizes when at different resolutions and if you have something resized that was at 180dpi and then change it to 72 the image is then too small and will distort if you were to attempt to bump up that number again.

Robert,

Resizing and just changing the PPI/DPI are two totally different things. Another little test open the resize window go down and uncheck the resize block now look at the numbers at the very top, while doing so change the number in the PPI box, did the top numbers change NO they didn't as you haven't changed the actual size of the file. Now recheck the resize box and do the same thing again now see what happens to the top numbers,they change because you're changing the actual dimentions of the image.

Mike

[Bodog]

This might help: http://www.scantips.com/no72dpi.html

[TMR Design]

This might help: http://www.scantips.com/no72dpi.html


Thanks man. This was a very useful link and really explains things clearly.
Unless I overlooked something the one thing which is not mentioned is what the recommended dpi is when you are printing images. I know many print shops require images at 300dpi as a standard. Is that what I should be doing in terms or prepping or planning for prints?

[tzalman]

If you are printing at home, 300 ppi for Canon and HP machines and 360 ppi for Epson. If you are sending them out, you can send them at any size and depend on the lab to resize and resharpen or send resized to 300 ppi which allows you to choose the resizing method and to do the final sharpening yourself.

[EOS_JD]

Resampling is not always a good thing. If you have to resample your image you are reducing the quality of your image. No matter whether you are upsizing (adding pixels that are not there) or downsizing (throwing away pixels).

On small prints you may not notice much diufference but on larger prints I prefer the native resolution down to around 180ppi. On my 20D that gives me an image size of around 19" x 13" (as big as my Epson R2400 printer will print).

All that matters in your digital image is the pixels. Number & quality!

[E-K]

Resampling is not always a good thing. If you have to resample your image you are reducing the quality of your image. No matter whether you are upsizing (adding pixels that are not there) or downsizing (throwing away pixels).

On small prints you may not notice much diufference but on larger prints I prefer the native resolution down to around 180ppi. On my 20D that gives me an image size of around 19" x 13" (as big as my Epson R2400 printer will print).

All that matters in your digital image is the pixels. Number & quality!

But don't printers like the Frontier resample to their native resolution if your image doesn't match theirs? If they do then it's a question of whether you want to have control over it or if you're fine with the printers algorithms.

e-k

[tzalman]

Resampling is not always a good thing. If you have to resample your image you are reducing the quality of your image. No matter whether you are upsizing (adding pixels that are not there) or downsizing (throwing away pixels).

On small prints you may not notice much diufference but on larger prints I prefer the native resolution down to around 180ppi. On my 20D that gives me an image size of around 19" x 13" (as big as my Epson R2400 printer will print).

All that matters in your digital image is the pixels. Number & quality!

My experience with my 1290 has been exactly the opposite. I print at 11x16 (A3 paper with a 1 cm. border). It may well be that the 2400 driver does a better job than the 1290, but if I send the 350D image to the printer at its native resolution (210 ppi for 11x16) the print lacks a good deal of fine detail. The detail is retained, however, if I up-res to 360 ppi and resharpen.

[EOS_JD]

It may well be that the R2400 does a better job. There's lots of info on this subject floating around the web.

How closely are you looking at your 11 x 16 print? Remember that the viewing distance of these prints is much more than say a small 6x4! At normal viewing distances you'll not see any differences. Don't pixel peep your large prints.

[EOS_JD]

But don't printers like the Frontier resample to their native resolution if your image doesn't match theirs? If they do then it's a question of whether you want to have control over it or if you're fine with the printers algorithms.

e-k

Generally I've found that the RIP pro companies use provides better resampling than you can do at home.

[E-K]

Generally I've found that the RIP pro companies use provides better resampling than you can do at home.

I agree their algorithms can be quite good, but I was responding to your comment here,

Resampling is not always a good thing. If you have to resample your image you are reducing the quality of your image. No matter whether you are upsizing (adding pixels that are not there) or downsizing (throwing away pixels).

The image always has to be resampled (unless it happens that the current PPI matches the printers DPI). It's just a question of whether you do it or the printer does it.

Personally, if sending the print to a pro printer I would prepare it however they ask for it :)

e-k

[RgB]

First of all I am curious as to why each camera seems to output the image with different dpi indicated. my Canon A620 saves with 180dpi and my 30D daves as 72dpi. My understanding is that this number only comes into when printing.

The thing I find confusing is that although people say that number does not matter, it is confusing that the size of what we see on screen changes. When you have a 300dpi image it is absolutely huge on screen and opposed to the same image at 72dpi.

When resizing in PS for instance, you can change the image size and resolution but 800 x 600 pixels at 180dpi is a different size than 800 pixels at 72dpi. I would like things to be uniform and I want to make sure that everyone seeing my images sees the same thing.

So knowing I am not going to print what should I do for uniformity? Do I now size by inches and not pixels so that a 6" x 8" is just that and the pixel resolution does not come into play? When I change the 180dpi image to 72dpi the image size changes.

Can someone explain this without making reference to printing and only with regard to what we see on screen, what is uniform and compatible so anyone I send to or any upload to the Internet looks the same?

Hi TMR

Try this out,

Resize an image to 800 px and 180 dpi. Save it.

Resize another of the same photo but this time 800 px and 72 dpi. Save it.

Now open each image and right click you mouse with the hand tool and try first Actual Pixels then Print Size

Notice how the 72 dpi doesn't change size when looking at print size.

The higher the dpi the smaller your print will be.

So if you save at 72 dpi it will print will be the size of your image at 100% crop or Actual Pixels.

Could someone with knowledge in this field correct me if i am wrong ;)

Thanks
Daniel

[Mcary]

Hi TMR

Try this out,

Resize an image to 800 px and 180 dpi. Save it.

Resize another of the same photo but this time 800 px and 72 dpi. Save it.

Now open each image and right click you mouse with the hand tool and try first Actual Pixels then Print Size

Notice something that doesn't happen to the 72 dpi.

The higher the dpi the smaller your print will be.

So if you save at 72 dpi it will print will be the size of your image at 100% crop or Actual Pixels.

Could someone with knowledge in this field correct me if i am wrong ;)

Thanks
Daniel

Daniel,

Sorry but this is pretty much totally wrong.

When you read in a printer manual or a commercial printing service that images files needs to be 300 or 360 DPI or any other number. They don't mean the DPI/PPI tag in the file needs to be set at that number they mean the total width and length of the file needs to = DPI x length in inches by DPI x Width in inches. IE 8x12 at 300DPI would be 2400x3600 and for 360 DPI it would be 2880x4320. The fact that file tags say 72, 300, 360, 10,000 or 5 PPI/DPI makes no difference to the printer all it see is 2400x3600 or 2880x4320.

Standard files size when printing at 300 DPI
4x6=1200x1800
5x7=1500x2100
8x10=2400x3000
8x12=2400x3600
9x12=2700x3600
11x14=3300x4200
16x20=4800x6000.

If the file is smaller then what’s required for that print size it will need to upsize, some printers require you to this others do it in house.

Mike

[EOS_JD]

Daniel,

Sorry but this is pretty much totally wrong.

When you read a in a printer manual or a comercial printing service that images files neeed to be 300 or 360 DPI or any other number. They don't mean the DPI/PPI tag in the file need to be set at that number they mean the total width and length of the file needs to = DPI x length in inches by DPI x Width in inches. IE 8x12 at 300DPI would be 2400x3600 and for 360 DPI it would be 2880x4320. The fact that file tags say 72, 300, 360, 10,000 or 5 PPI/DPI makes no difference to the printer all it see is 2400x3600 or 2880x4320.

Standard print size when printing at 300 DPI
4x6=1200x1800
5x7=1500x2100
8x10=2400x3000
8x12=2400x3600
9x12=2700x3600
11x14=3300x4200
16x20=4800x6000.

If the file is smaller then the required for that print size it will have to upsize some printer require you to this others do it in house.

Mike

When printing larger than a 10x8 my pixel per inch value does not need to be 300ppi. You can print a 16x20 at 150-180ppi without any noticable drop in quality at normal viewing distances. Remember these larger prints are looked at from a few feet, not a few inches like a 6x4.

8x12 @ 200ppi = 1600 x 2400
9x12 @ 200ppi = 1800 x 2400
11x14 @ 180ppi = 1980 x 2520
16x20 @ 180ppi = 2880 x 3600

This will allow large prints without resampling anything below 19 x 13 on a 20D 8mp image. A small amount of resampling perhaps for the 16x20 but you can print even this at 150ppi and it'll look great. Also think about the file sizes and the time it takes to process huge files.

[Mcary]

When printing larger than a 10x8 my pixel per inch value does not need to be 300ppi. You can print a 16x20 at 150-180ppi without any noticable drop in quality at normal viewing distances. Remember these larger prints are looked at from a few feet, not a few inches like a 6x4.

8x12 @ 200ppi = 1600 x 2400
9x12 @ 200ppi = 1800 x 2400
11x14 @ 180ppi = 1980 x 2520
16x20 @ 180ppi = 2880 x 3600

This will allow large prints without resampling anything below 19 x 13 on a 20D 8mp image. A small amount of resampling perhaps for the 16x20 but you can print even this at 150ppi and it'll look great. Also think about the file sizes and the time it takes to process huge files.

JD,

Per the last line in my previous post notice the "others do it in house" by that I'm refering to print services that do the upsizing/interpolation in house for those services, which I usually use. I just take the 20D file crop it to the desired ratio ie 16x20, or 30x40 and uplaod the file which might be something like 2920x2336 for a 20x16 print depending on how much of the original image was cropped. Heck I've sent 20D 3504x2336 files to be printed at 30x45 (78DPI) and gotten back lovely prints.

The main message I was trying to get across in the previous post is the total numbers IE what you posted for 16x20 2980x3600 are what matter not what the DPI tag of the file might happen to be.

Mike

[EOS_JD]

I agree their algorithms can be quite good, but I was responding to your comment here,



The image always has to be resampled (unless it happens that the current PPI matches the printers DPI). It's just a question of whether you do it or the printer does it.

Personally, if sending the print to a pro printer I would prepare it however they ask for it :)

e-k

I was talking more about printing from home. I rarely resample. I agree that if sending out, it's better to prepare yourself and you can be assured you get back what you want.

[EOS_JD]

They don't mean the DPI/PPI tag in the file need to be set at that number they mean the total width and length of the file needs to = DPI x length in inches by DPI x Width in inches. IE 8x12 at 300DPI would be 2400x3600 and for 360 DPI it would be 2880x4320. The fact that file tags say 72, 300, 360, 10,000 or 5 PPI/DPI makes no difference to the printer all it see is 2400x3600 or 2880x4320.


When printing ppi does make a difference. Does the printer see the resolution and the number of pixels to provide the appropriate print size. Or does it see the pixels and size to give the resolution? No matter all three are linked together.

If you want a 6x4 and the tag says 5ppi well you'll know what you'll get.

Resolution only matters not a jot when viewing on the web.

We're both saying close to the same thing though I think :-)
Jim

[RgB]

Daniel,

Sorry but this is pretty much totally wrong.

When you read in a printer manual or a commercial printing service that images files needs to be 300 or 360 DPI or any other number. They don't mean the DPI/PPI tag in the file needs to be set at that number they mean the total width and length of the file needs to = DPI x length in inches by DPI x Width in inches. IE 8x12 at 300DPI would be 2400x3600 and for 360 DPI it would be 2880x4320. The fact that file tags say 72, 300, 360, 10,000 or 5 PPI/DPI makes no difference to the printer all it see is 2400x3600 or 2880x4320.

Standard files size when printing at 300 DPI
4x6=1200x1800
5x7=1500x2100
8x10=2400x3000
8x12=2400x3600
9x12=2700x3600
11x14=3300x4200
16x20=4800x6000.

If the file is smaller then what’s required for that print size it will need to upsize, some printers require you to this others do it in house.

Mike

Mark thanks for correcting me I new i wasn't correct just wanted to show what happens at different setting when pressing for print size.

Since I haven't started printing any of my work yet i haven't looked into this field yet, I'm on a need to know basis;). As I am getting some new lenses soon I am going to look into this to understand printing more.

I knew that you should print @ 300 or 360 if requested as i have read in books in the past.

I just though it was cool and probably meant something how 71 dpi (or whatever it's called) doesn't change size when asking for print size.

[EOS_JD]

Daniel as a beginner get into the habbit of calling it ppi (not dpi). Images are made up of pixels not dots :-)

[RgB]

Daniel as a beginner get into the habbit of calling it ppi (not dpi). Images are made up of pixels not dots :-)

Okay will do.