DPI Question?! - Confused!

[smc1377]

Originally Posted by ike

So should i only adjust the dpi if i need an amazing quality smaller image, or when printing at bigger sizes?

ike,

Oops, forgot to answer this part. It almost depends on how you print and how picky you are about your pixels. This advice is just for when you don't want photoshop to create any pixels at all so that you're only working with the true pixels from the camera.

So say you want to print a 4x6 image of your photo. Bring up that same Image Size window and uncheck the Resample Image box. But instead of changing ppi, you'll just want to change the actual dimensions (inches) to reflect the 4x6 that you're looking to print. You'll notice that the ppi will change to what the resulting ppi would be when it gets crammed down into the appropriate size.

Generally, the more pixels you give your printer (or more appropriately your print driver) to work with, the better it'll print. Your printer will basically take all these pixels and try to figure out how best to lay down the tiny little color dots to try to replicate the image you are sending it.

I have read though that home photo printers do have an "optimum" ppi that allows the printer to produce a superior image. It was from one of my "Shutterbug" magazine issues, but I can't recall which issue.

[ike]

Thanks so much SMC,

Everything makes sense now.

ike,

[EOS_JD]

Sorry to chime in very late on this coversation guys but it seems like there's an aweful lot of confusion in relation to resolution and it's the same everywhere you seem to go. Once you uinderstand this though, your image management and output to print will be much better.

Some of what I'm saying here has been noted above but I'll repeat just for clarification.

Images are made of pixels. So in relation to image resolution the term is ppi (pixels per inch). dpi is a term that relates generally to output. Your printed image is measured is dpi and your monitor output is measured in dpi too.

The resolution (ppi) of your image actually matters not a jot until you come to actually printing your digital image. The ONLY thing that really matters for your image is the number and quality of the pixels.

When printing, 300ppi is generally advised as a maximum resolution basically because the human eye cannot resolve any more detail than this. A link provided by an early poster seemed to suggest that ppi is converted to dpi which although true actually threw me a little because he's talking abpout something completely offline.

We are discussing photos (not newsprint). Most printers nowadays can print at very high resolutions. If a printer prints at 300ppi and you want a print at 300ppi, the printer will print one drop of ink for every pixel in the image. many though can now print at way higher than 300dpi and what happens is that the printers place these thousands of dots on paper and obviously the higher the resolution the more dots and therefore these dots overlap. My Epson printer can print at something like 5670 x 2400dpi. I can still send a 6x4 image at 72ppi to this and print at the highest setting and the image would still look crap..... because 72ppi is not a high enough resolution to print this small image. I hope I've not lost you. This shows that what matters is image resolution (ppi).

My 20D produces 3504x2336 pixel images. Providing I don't resample (add or delete pixels), the resolution won't matter until I come to print. You can make the ppi setting 10 or 1000 and it'll still be a 3504 x 2336 pixel image that will look the same on your PC screen.

When it comes to printing there's a very easy way to work out an unknown variable like print size or resolution. The equation I use is PS = R x P that is Print Size = Resolution x Pixels. Using this formula you can work out any unknown or required variable.

e.g.
1. You know the size of print you want (say 10x8) and you want to print it at 300ppi. You would need an image of 3000pixels x 2400 pixels in order to do this (do the equation for each axis) 10x300 and 8x300 = 3000x2400. If you have more pixels you can print at a higher resolution but this just means you have a larger image than you may need and most prefer to crop the image to the required pixel count. Quality from printing at higher than 300ppi is unlikely to be seen by the human eye so 300ppi seems to be the preferred quality for small images. You can actually print at around 240ppi without any noticable drop in quality in a small image like this and as noted below you can go a lot lower for large prints.

2. OK You know the number of pixels you have and you want to print a large 18" x 12" print. At what res will this print out at (without resampling) the image. Using the equation above R = P divided by Ps

Resolution= pixels/print size so:
3504/18 = 194.666
and again on the other axis
2336/12 = 194.66

Contrary to popular belief, a resolution of 194ppi will provide a very high quality print that will look fantastic from normal viewing distance.

Because viewing distance will be further than that of a 6x4, the ppi can be a lot lower for larger images. (look from a distance at a billboard then look close up and you'll see what I mean).

By printing these larger images at 300ppi all you do is increase the image size (greatly) and at the normal viewing distance you will not see a difference in the print! I print my 19x13s at just under 180ppi and they look amazing.

You can also add a third equation to work out how many pixels you need for a particular image. If you want a 12x8 print at 300ppi you need 12x300=3600 and 8x300=2400. So you need an image of 3600x2400 (just outside the normal range of an 8Mp camera.

Now with resampling switched on you can increase the pixel content of your image but all this does is allow the software to randomly add pixels that it thinks are suitable. In most cases this will look ok and you won't see much difference in your prints but when printing huge prints it can become noticeable.

I try not to resample too much because all you are doing is either adding information that is not there in the first place (adding pixels or upsampling) or deleting pixel information (downsampling). The only time I do this is if I require a very large print that takes my resolution below around 150ppi.

There's so much misinformation on this subject around the web it's amazing how anyone gets to grips with this stuff.

Does this make sense? Hope I can help those who are struggling to understand this. Your use of digital images will improve when you understand the basic concept.

Here's a pretty good link

http://www.steves-digicams.com/techc...uary_2005.html

Cheers
Jim

[smc1377]

Originally Posted by EOS_JD

If a printer prints at 300ppi and you want a print at 300ppi, the printer will print one drop of ink for every pixel in the image.

Jim, I'm afraid that this premise is false which then renders a lot of your explanation as false. A single pixel can be one of many millions of colors whereas a drop of ink can only be one of the ink colors available from your certain printer.

For example, you can have a single pixel that is PMS 344 which is an aqua color. You don't have any printer inks that are aqua in color so therefore your printer will have to mix multiple dots to try to achieve that aqua color. The colors are mixed on the substrate, not in the printer itself so it's not like it mixes the inks inside, then drops a single aqua color of ink onto the paper.

Therefore a single pixel DOES NOT equal a single ink drop unless your pixel happens to match the available ink colors in your printer.

Normally, a printer has to drop multiple sizes of multiple colors of ink drops to try to render a single pixel.

[RexK_Cozumel]

Originally Posted by EOS_JD

The resolution (ppi) of your image actually matters not a jot until you come to actually printing your digital image. The ONLY thing that really matters for your image is the number and quality of the pixels.

Now thats just silly. I would hate to view an image at say 20ppi on my computer screen about as much as I would hate to view it in print. And the whole it doesnt make a difference since you are viewing it from far away statement is silly as well... that is like saying there is no difference between Keeley Hazell and Roseanne Barr they look the same if you get far enough away.