DPI Question?! - Confused!

[ike]

Hey guys,

Just been messing with a photo, checking out the dpi of it.

Now, the confusing thing is...

I edited the dpi from 72 to 200 without changing any other settings,

This gave me a file that is 13mb and 7911x5933

and the original which is 3.14mb and 2848x2136

So, now my main question is, which is the better picture? looking at them on screen i cant tell any difference, so what is the difference, except for the dimensions and dpi?

Which would be the best image to get printed on canvas/poster 40x30 inches.

And what is the approximate maximum size print for each?

Also, (sorry for all the questions...) but would a 2848x2136 size image @ 200dpi be better than the other two?

ike,

[Nicole]

You can read more about the dpi / ppi thing here. This article explains it much better than I could . Though it sounds like your program must be using some sort of resizing to increase the size of the picture, so make sure you check carefully for any unwanted signs of this (like straight edges becoming jagged, etc).

[ike]

I'm using Photoshop CS3, i think it might have done a good job.

There doesnt seem to be any artifacts like jagged edges, fuzzy picture.

Guess its just one of those things, will print both of the pics off later on to check print quality at 6x4, as the only printer i have access to at the moment.

ike,

[Nicole]

Yep, that's pretty much what I'd do. Actually, there was a discussion about the resizing in Photoshop in another group recently... using the RAW converter to essentially upsize to more megapixels... the discussion was here if you want to see an example / read about it.

Let me know how it turns out I'd be curious to know.

[ike]

I understand it a bit more now after looking at the flickr discussion.

If i have this right, it seems photoshop clones the pixels to make them bigger, resulting in a bigger image, with the same quality (or almost the same)

kinda like this:

Original...

RGB
RGB
RGB

Enlarged...

RRGGBB
RRGGBB
RRGGBB

So its still the same image, just bigger. But it seems to work better with RAW instead of JPG.

ike,

[Nicole]

Yep, that's the basics of it (which is why you can get jagged edges and stuff since if it was originally one pixel it may become 3 or 4, and smooth lines may no longer be smooth).

[ike]

Yup, thanks for the help.

I think its going to be something that works great with some images, and really badly with others, luckily the image i was messing with isnt important to anyone apart from me,

ike,

[smc1377]

When changing ppi, your resolution shouldn't have changed.

The ppi is just a reference number telling your computer to shove 200 pixels into an inch instead of the 72 pixels from before. The resolution of 2848x2136 should have stayed exactly the same no matter if you tell it to have only 1 ppi or 10000 ppi.

If you ended up with a different resolution, you resampled the image.

I haven't worked with CS3 yet, but if the Resize Image window is anything like CS2, then make sure you have the bottom box unchecked where it reads "Resample Image". Once you uncheck this box, no matter what you change the ppi to, the resolution (at the top of that box) will never change. Only the resulting image size in inches will change.



By resampling an image, Photoshop is CREATING pixels (or combining them) that didn't come from the camera. It might look just fine, but just realize that these pixels aren't the same as when they came from the camera.

[ike]

SMC - Thats what i forgot to do. Uncheck that little box.

Now i understand when converting from 72dpi to 200dpi the overall image size is reduced, cramming all the pixels into the remaining space, unless resampled which results in a bigger image with 200dpi,

Now, i have one more question.

Lets say i adjusted the dpi to 200, which means i have a smaller image (Height X Width) would printing this out at the same size as the original image look better or worse? Because in theory, it should look worse, because the image is having to be stretched but the extra dpi would make it look the same, if not slightly better.

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

ike,

[smc1377]

Originally Posted by ike

Now, i have one more question.

Lets say i adjusted the dpi to 200, which means i have a smaller image (Height X Width) would printing this out at the same size as the original image look better or worse? Because in theory, it should look worse, because the image is having to be stretched but the extra dpi would make it look the same, if not slightly better.

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

ike,

Actually in theory, it shouldn't look any worse or better than it did before you changed the ppi cause print resolution is dependent on pixel resolution. So as long as your image contains the same amount of pixels, your print will be exactly the same.

Because, when you stretch the image back out to its original size, you are also changing the ppi back to original size. For example...

Original image is 1000x1000 @ 72 ppi = 13.9" x 13.9"

After changing ppi to 200, your image is 1000x1000 @ 200 ppi = 5" x 5"

So when you go to stretch that 200 ppi image back out to its original size, you are basically changing the ppi back out to 72 ppi. By stretching the image, you are basically just reverting your ppi back to 72, therefore your resulting print should be exactly as it would have been if you just left the image alone.