Understanding PPI

[bray]

Trying to get a better understanding of digital specs. Am I correct in thinking that the image size when a picture is taken by a digital camera deterimes a preset PPI? In other words can some cameras increase the PPI and still maintain the same image size?

I do not favor increasing PPI in the software as this from what I have read may distort the final print even without resampling

All the specs I am reading refer to various image sizes and megapixels

Thanks
Bill

PS looking at the Pentax digital, I think the ist 2.

[David Asch]

Really, the camera has nothing to do with PPI. The dimensions of a 6mp image will always be the same when you download it. The software/printer determines the print resolution.

[bray]

I thought printing was DPI and PPI was the image itself...

I have looked at a lot of my digital images and it seems to me that the PPI is 72...I am therefore wondering if thats the deafult for all cameras.

I understand that the DPI can be increased at the time of printing but it seems to me that the original image and PPI is more important in getting sharper pictures..

Lost and confused...back to google :confused:

Thanks for the reply
Bill

[Chuck S.]

Bill, set aside DPI for the moment (that's about printer dots and is not directly related to image PPI). As David mentioned, the image coming from your camera is an array of pixels; for a 6 megapixel image, that may be 3000 pixels long by 2000 pixels wide (or horizontal vs. vertical if you prefer). Once you've opened the image in Elements, it retains those pixel dimensions unless you do something to make them change.

Also once you've opened the image, the program selects an implicit ppi for your image (often supplied by the camera) and then calculates a set of linear dimensions (in inches or cm) that correspond to the implicit ppi. At that point, your image pixel dimensions are unchanged; the only thing the ppi and inch dimensions are good for at that point is to show you what the print size would be at the selected ppi.

Now if I'm ready to print, I know that 72 ppi is inadequate, so I need to change the ppi, generally with resampling unchecked. When I do that, say increase to 300 ppi, the linear dimensions will be reduced proportionally - and the pixel dimensions will remain unchanged. I can play around with the linear dimensions (again with resample unchecked) and watch the ppi go up or down as I do. All the while...yes, the pixel dimensions (the originals from your camera) remain unchanged, so nothing has happened to your image.

When you start getting into resampling, the ppi really does matter; that's a subject for a follow-up if what I've said here makes sense.

Chuck

[bray]

Chuck based on what you have said am I to understand that before printing say a 4 x 6, I could adjust the dimensions without resampling and then let the program select the appropriate PPI? This would be to get a specific print size.

After which I can still improve print quality by upping the DPI from the printer properties?

In playing around with the sizes as you noted, I also see in the print dialog (PE4) where I can select various paper sizes. Would this selection take care of the PPI issues?

Hope my assupmtions are on track.

Thanks again to both of you for the help
Bill
PS for some reason my spell checker in FF is acting up again so please forgive any errors. Will have to reinstall the extension:)

[Chuck S.]

Bill, if your camera takes an image that's in the proportion of 3:2, then yes, you can select 4 inches by 6 inches and the ppi will float up...way up. However, once you get much above 300 (or some say 360), then all the extra resolution doesn't help the print quality. What I do is this:

1. Open image. In my case, they're 3072 by 2048 pixels.
2. Go to Image>Resize>Image size and change linear dimensions to 6 inches by 4 inches with resample unchecked. PPI goes up to 512.
3. While still on the same screen, put a check mark in Resample.
4. Change resolution to 300 ppi. Note now that the pixel dimensions have changed to 1800 by 1200.

This is downsampling - throwing away excess pixels. It's generally better than upsampling, which adds extra (calculated) pixels to an image that doesn't have enough. If I downsample for printing purposes, I'll save the result with another name like image123_4x6 because I don't want to permanently throw away those pixels - I might want to print an 8x10 or larger later, and I'll need every original pixel I can muster for that.

Chuck

[TonyW]

After which I can still improve print quality by upping the DPI from the printer properties?

One thing to remember is that the quoted printer dpi is way higher than the native ppi of the printer. Printers need to use a lot of dots to make one pixel - the printer has only a few colours to work with so has to use a lot of dots of different colour to generate the pixel colour. So although your printer might say it has 2400 dpi it probably is only about 300 ppi which is why, as Chuck says, you won't get any better by going bigger.

Tony

[Triumphant]

According to the QImage site, the Epson printer driver asks for 720 ppi and Canon and other drivers ask for 600 ppi. If sent something else, the driver interpolates to get these values. Supposedly, this is not the best strategy if your program has a better set of algorithms than the driver.

(Makes sense to me. My son-in-law uses QImage and swears by it.)

That would suggest the image resizing should result in one of these values.

I have not experimented to prove the above, although now my printer seems to be working as it should, I think I will!

[TonyW]

You're probably right - Epsons usually quote dpi as multiples of 360 (1440, 2880, 5760) whereas Canon use multiples of 300 (1200, 4800). So theoretically I guess you should stick to that multiple to minimize interpolation. I guess I'll have to try the experiment to see if I can tell the difference :)

Tony

[Chuck S.]

Tony and Triumphant:

I would be really surprised to learn that you can tell the difference between a photo sent to the printer at 300 ppi and 600 ppi. If I can do 300 without upsampling, that's what I'll do; I don't trust Adobe to give me the calculated pixels necessary to get to 600 ppi. At 8 by 10 that would be a very large file indeed....!

But....whatever works is fine!

:)

Chuck

p.s I used QImage for a while, but its browser got confused on occasion and lost touch with my photos; had to keep reloading it. Finally just got fed up and abandoned it; printing out of Elements has done well by me.

[bray]

Chuck, thanks for your explanation and patience.

Things are certainly clearer to me.

Regards
Bill

[Chuck S.]

Bill, my pleasure. The Photoshop image sizing dialog is far from intuitive; I scratched my head for a long time before I settled into my own way of understanding what was going on. Once I decided that the most important measure was the pixel dimensions, the rest started to make sense. But it still brings me up short every once in a while...

Chuck