Resizing in photoshop.

[chris_arnet]

So I was messing around with a few images today in 1st period, and i accidentally entered 500ppi without changing the image size. i thought it would just have some sort of error message but it accepted. I didnt have the original image to compare it to, but i thought i noticed a slight increase in resolution. Did it actually somehow add pixels mathmatically to my image, or was i just imagining things? I thought i had to have special software to do that. If it really did, is there any harm to say, resizing to 1000ppi (just a random high number) at standard size so i can blow up to 20x30 with a little more success? I have a 20D, so I can do it anyway, but i would like it to have a bit more ppi so it has a bit more resolution. oh, and yes, i know it doesnt actually increase resolution like more MP would, but ive heard it does to some extent. Im just trying to gain as much knowledge as possible.


thanks in advance,
chris

 

[Big Mike]

You could change the PPI to 1,000,000 (without re-sampling) and it wouldn't change anything.
On it's own, PPI is only a measure of how large an image will appear on a computer display.
The important thing to know/change is the actual size of the image, in pixels.

It's also a handy reference for printing. Typically, you want 300 pixels for every inch of print. So if you set the PPI to 300, the size (in inches) will reflect the size of print you can make at high quality.

 

[chris_arnet]

allright, so its not actually mathmatically adding pixels to my images?

 

[Big Mike]

NO, not if you don't re-sample.
If you do re-sample, then you may be adding or removing pixels.

 

[chris_arnet]

allright. Is there any downside to adding pixels?

 

[Samurai Photographer]

Yes. You're asking the application to redraw pixels, which inevitably leads to image degradation. However, if you begin with a high resolution image, you can increase the image size in increments of 10 percent and wind up with good results. In Photoshop, choose Image|Image Size to open the Image Size dialog box. In the Pixel Dimensions section, choose Percent instead of the default Pixels from the drop down menu.Change the value to 110. Make sure you have the Resample checkbox selected and then click OK. You can resize by 10 percent several times to greatly increase the size of the image with good results. I've created an action that resizes by 10 percent. I run the action several times until the image is increased to the desired size. Here's a link to a video tutorial on creating actions: http://www.accessdigitalphotography.com/photoshop-tips-and-tricks/creating-actions-in-photoshop-cs3/
Cheers,
Doug

 

[Big Mike]

I had always heard that increasing in increments like that, was the best method...but from what I'm hearing, it doesn't make that much of a difference after all.

When I increase the size of an image, I just enter the size (in pixels) that I want it to be (make sure that re-sample is checked).
Also, I use 'bi-cubic smoother' when going bigger and 'bi-cubic sharper' when going smaller.
Lastly, I like to use Unsharp Mask after resizing.

 

[Mesoam]

I can honestly say I had no idea you could do this with such little negative effect on the image. I just repeated the above function 5x and there is barely any quality loss, perhaps on a large re-print. This is fantastic