Picture rezolution and size

[cocoalex]

I have this picture that has this size: 1807x1113 and this rezolution: 300.

I need to get this picture to a fix size: 640x480. This is easy. I go to Image Size and get 640 for my picture width. After that I adjust my canvas size so that the picture's canvas to be 640x480.

My question is refering to the resolution cause I need this picture to be on a site. The picture has a big rezolution > 300 dpi.
Should I change the picture rezolution to 72 dpi as it is recommended for web or this would be a very big mistake?

And can someday tell me if my method to change my picture size is a good one?

[Waxpants]

If you want to adjust an images' size and resolution all you need to do is go to image size (I usualll click on the images' for this) and change both the size and resolution to what you want.

Make sure you have resample checked as well.

A word of warning though.
shrinking an image by the amount that you want to shrink it will drastically reduce the images quality.

Think of this:
No matter what the image resolution is you will see it on screen at 72 dpi.
This means that even in Photoshop you are looking at your 300 dpi file at only 72 dpi.
All the pixels are still displayed however so the image on your screen will be about four times larger than it would print at.

Example:
If you resized the image to 640x480 and left the resolution alone, when you uploaded it to the web and look at it, it will appear to be 2663x1997 pixels (likewise when you look at it at 100% zoom in PS it will be huge).

So it is important to change the resolution as well.

As a final note, I don't think you have to worry about the canvas size as I think that it will automatically resize itself along with the image.

[cocoalex]

Thanks for your reply.

[Phil_The_Rodent]

Something I often find myself doing: there is a percent option in height and width. If you go 100% height and 100% width, you can change the resolution to whatever you want without changing the data of the image. 300 dpi to 72dpi with no resampling is exactly this... no data lost or fabricated. :)

[Demise]

why or why not would resampling be important? what is it for?

[Phil_The_Rodent]

Basically If you change a 300 dpi image to 72 dpi, you're going to find your pixel number reduced by a lot. If you blow this back up (resample) to the correct size again, you're going to get heavy blurring: because PS averages pixel information when blowing up an image. Resampling *is* important, however, if you don't want pixellation when you're blowing up an image. So basically, if you can avoid it, don't do it. It's the same reason you should never work on jpegs as working files, only psd or tiff or eps files...

[Demise]

I can understand that, ... so if I enlarge a pic resample it.... If shrinking don't?

[Phil_The_Rodent]

Yes you have to resample an image when it changes size, BUT when you're blowing up an image PS invents pixel data: which blurs it so is generally undesireable. Reducing an image size throws pixel information away, so you're not going to have the same problem (though I've noticed things do start to blur a titch when you reduce something drastically... I'll have to work on this a bit more).

Interestingly, thanks to your question, I tried reducing an image size with resampling clicked off. This was cool, it prevented losing or making up pixel information (in the same way that 100% would) but you can go backwards... increasing the width will decrease the dpi and vice versa. So there's probably the better and quicker way to change your dpi without changing your Pixel Dimensions...

[cocoalex]

Very interesting theories around here.
I have another question.

I set my picture to the desired size (640x480) and 72 resolution. I set for the same picture a resolution of 300.

When I tried to save it for web as jpeg I got the same size: 26 k.

Can somebody explain to me?

[Waxpants]

I think that this is because even though jpeg's can be saved at high resolution (I think). When you go to save for web it will automatically save your image at 72 dpi.
Now whether it trims out a bunch of pixel info or not I can't say

[Phil_The_Rodent]

Changing resolution does not change the size of the file: it's the number of pixels that do that. So a 1" x 1" file at 300 dpi (300px x 300px) would be a different size than at 1" x 1" file at 72 dpi (72 px x 72 px), but a 300 pixel x 300 pixel file at 300 dpi (!" x 1") would be the same size as a 300 pixel x 300 pixel file at 72 dpi (a little more than 4" x 4"). Is this what you mean?

Oh, and yeah, jpegs throw out pixel information during compression. Which is why it's only good as a final output step and not for savng a working file...

[TechZsue]

I tried going from 300 to 72 and I lost a bunch of layer style effects that were applied to the image.