# Re-sizing images for the web



## Jon_Are (Aug 8, 2009)

I thought I had this figured out, now I'm ultra-frustrated.

It is my understanding that web images should be around 500-800 pixels in width and have a resolution of 72 ppi. 

Yes? No?

Here's my problem: I have images saved as JPEGs in Lightroom (which were processed from RAW images). If I open up an image to edit in CS4, here are the stats from the Image Size window:

Pixel Dimensions: 18.0M
Width: 2098 pixels
Height: 1499 pixels

Document Size:
Width: 8.742 inches
Height: 6.246 inches
Resolution: 240 pixels/inch

So, this is too large to post on the web and requires resizing.

When I change the resolution to 72 pixels/inch, this changes my width to 629 pixels and my height to 450 pixels.

So, I click OK and the image looks horribly jagged.

I'm at a loss as to why this is. Could it be that, as a JPEG, I've opened and re-opened it too many times (although I don't think this is the case)?

Does it have anything to do with 16 bit vs. 8 bit? I'm certain that I saved it as 8 bit, but now, when I re-open and click Image>Mode, 16 bit is checked.

Any thoughts?

Thanks!

Jon


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## UUilliam (Aug 8, 2009)

no need to lower resolution
resolution is for printing, just change the Width and height...


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## Josh66 (Aug 8, 2009)

I just do mine 800 on the long side, with whatever resolution is the default in Lightroom.


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## Andrew Sun (Aug 8, 2009)

UUilliam said:


> no need to lower resolution
> resolution is for printing, just change the Width and height...


Yeap this. If it's just for screen viewing, 300 or 72 dpi doesn't matter.


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## Opher (Aug 8, 2009)

I just take my horribly large photos and resize them with the gimp.
GIMP - The GNU Image Manipulation Program

It is free; I mostly just use it to resize individual images for the web.


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## Jon_Are (Aug 8, 2009)

Thanks, guys; that makes it easier.

Can you re-size in LR, or must I go into PS?

Jon


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## Jon_Are (Aug 8, 2009)

Also, what about checking (or unchecking) the following:

Scale Styles?
Constrain Proportions?
Resample Image?

And leave it on Bicubic??

Jon


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## musicaleCA (Aug 8, 2009)

Yes. It's part of the export dialogue.

Edit: Constrain proportions makes sure that whenever you change the height or width, the other is changed simultaneously to maintain the same aspect ratio. Let it resample the image using a bicubic algorithm; it'll do fine for downsizing.

Whatever people may think about resolution, for images that are going to be displayed on a screen, 72 PPI is standard.


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## Josh66 (Aug 9, 2009)

Very easy in Lightroom.

Go to the export menu, set it up how you want (tell it the dimensions, or just the dimesion of one side), then save that as a preset.


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## Jon_Are (Aug 9, 2009)

> Whatever people may think about resolution, for images that are going to be displayed on a screen, 72 PPI is standard.



This is what I thought. But why do my 72 ppi's look like crap - all jaggedy - but my 240 ppi's look OK?

Jon


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## Jon_Are (Aug 9, 2009)

Alright, so I've done a bit more research...

*THIS:*



> no need to lower resolution...resolution is for printing


...is incorrect.

*AND THIS:*



> If it's just for screen viewing, 300 or 72 dpi doesn't matter.


...is correct, but is not what I was asking about.

Here is what I found:

PPI refers to the number of pixels viewable on a screen. There is no correlation between the resolution of digital data (ppi) and the resolution of a printed image (dpi). A dot is a droplet of ink on paper and a pixel is a ray of light on your monitor.

So, my question remains...if 72 ppi is plenty for screen viewing, why, when I change the ppi resolution of my images, do they look horrible?

Thanks,

Jon


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## Perniciouspoof (Aug 9, 2009)

Jon_Are said:


> Alright, so I've done a bit more research...
> 
> *THIS:*
> 
> ...



People change the resolution of images for the web only because it makes it a smaller file that is faster to load, and because having larger images is unnecessary when web pages are going to be viewed primarily on computer moniters. The Quality aspect of ppi or dpi is shown primarily in printing.  

as to the question of why your photos degrade in quality when you change resolution-

When you change a photograph from 300 to 72ppi photoshop is throwing away extra pixel information to match the smaller print resolution. Which explains the degredation of quality. 

Instead of straight up converting from the actual file, try creating a new file that's set at 72 ppi and whatever measurements you want and place the photograph on that file. You should get better results.


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## Jon_Are (Aug 9, 2009)

> Instead of straight up converting from the actual file, try creating a new file that's set at 72 ppi and whatever measurements you want and place the photograph on that file. You should get better results.



Thanks, Perniciouis, but I really don't get what you mean (or how to accomplish it).

Do you mean to edit the RAW image, then, when I convert it to JPEG, select 72 ppi  (rather than 240, then later converting to 72) ?

Now that I give that some thought, it makes some sense. I'll try it and see.

Jon


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## Peano (Aug 9, 2009)

Jon_Are said:


> So, my question remains...if 72 ppi is plenty for screen viewing, why, when I change the ppi resolution of my images, do they look horrible?



You're probably resampling when you change the resolution. Resampling certainly can change image quality; but resolution (ppi) by itself has zero bearing on image quality or file size.

*72 ppi, 149K:*






*300 ppi, 149K:*





When you're sizing image for the web, these are the things that matter in the Image Size dialog box. Ignore the middle section.


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## Jon_Are (Aug 9, 2009)

Thanks, Peano.

Your explanation makes sense, except for one thing: 

If the middle, 'Document Size' box refers only to printing and not the screen image, why does it indicate 'Resolution: 72 pixels/inch (when pixels are relevant only to screen resolution)?

Is it just a poor choice of words from Adobe?

Thanks again for taking the time to answer with clear graphics as you did.

Jon


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## Perniciouspoof (Aug 9, 2009)

I would avoid saving the LightRoom files as Jpegs and opening them in photoshop for editing. Jpeg is a finished format and shouldn't be used for editing. 

Lightroom also has an 'Edit in Photoshop' option which works similar to the back and forth editing in Illustrator and photoshop. It'll place the file in photoshop with all the tweaks you made in light room without saving and editing a JPEG. Whenever you finish in photoshop just save it as a PSD or Tiff and it'll go back in Light Room with all your Photoshop edits and layers, then you can finish up in lightroom and change it to whatever format you want. (or if you were finished in photoshop you could just save it as a JPEG instead of a Tiff or PSD) 

Just some suggestions.


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## KmH (Aug 9, 2009)

Jon_Are said:


> PPI refers to the number of pixels viewable on a screen. There is no correlation between the resolution of digital data (ppi) and the resolution of a printed image (dpi). A dot is a droplet of ink on paper and a pixel is a ray of light on your monitor.
> 
> 
> Jon


This is not correct. Actually, it is an over simplification.

There is pre-press and there is continuous tone. 

Dpi is used for pre-press printing, which is not photographic image quality.

Ppi is use for web and continuous tone printing of photographic images.


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## Peano (Aug 9, 2009)

Jon_Are said:


> Thanks, Peano.
> 
> Your explanation makes sense, except for one thing:
> 
> If the middle, 'Document Size' box refers only to printing and not the screen image, why does it indicate 'Resolution: 72 pixels/inch (when pixels are relevant only to screen resolution)?



You're confusing two kinds of pixels: image pixels and screen pixels. The screen resolution of your monitor is a function of screen pixels -- the little dots of light on the panel that make up what you see on your screen.

Image pixels are the little bits of data that make up the digital image. No matter how you change screen resolution (screen pixels), you aren't changing image pixels. The best thing you can do at this point is to stop thinking that "ppi" has anything to do with the quality of the image on your monitor.

Maybe this will help make it clearer ...









> Is it just a poor choice of words from Adobe?



No, Adobe has it exactly right. The "document" referred to here is the print. The dimensions are the dimensions of the print (for example 4 inches x 6 inches). The resolution is the ratio of image pixels per printed inch.


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## musicaleCA (Aug 9, 2009)

And regardless, 72 PPI is still the standard for screen viewing. The reason being, not that it decreases file size, but that it more closely correlates to the actual dimensions that it will be presented on-screen with at that resolution. An image resized to 600x600 pixels is going to be the same size, yes, but if you say that the resolution is 72 PPI, it'll be displayed differently than if you said it should have a resolution of 300 PPI. Similarly it changes the defaults for printing when you import the image into, say, a word processor. At 72 PPI, the image is going to take on a large document size (our 600x600 would take-up 8.33x8.33 inches) initially until it's resized down. At 300 PPI, it'll be rendered much smaller on the page and be useful for printing (2x2 inches).

Edit: Um, yeah. Basically what Peano said.


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## Peano (Aug 9, 2009)

musicaleCA said:


> And regardless, 72 PPI is still the standard for screen viewing. The reason being, not that it decreases file size, but that it more closely correlates to the actual dimensions that it will be presented on-screen with at that resolution.



No, you're confusing image pixels with screen pixels. 



> Edit: Um, yeah. Basically what Peano said.



Right.


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## daveren (Aug 10, 2009)

hi Jon
This whole ppi thing is really confusing.
Like Peano, I pretty well ignore what it says in that box. 

But..... I think what it means is that Size, Height and Width would be the physical size of the image ON YOUR SCREEN - assuming the resolution of your screen works out to be 72 pixels / in.  I think the 72 ppi is an old standard though as most screens have more pixels now.  Generally I think the 72 ppi figure may be useful for web-page layout.  

I got used to just setting my "pixel dimensions" with the constraint on.  For web-posting in galleries, I use 1800 pix on the long side and 700 on the long side for Emails - maybe 300 on the long side for sticking in a forum post (that would give me roughly 4" on the screen, right?)

Careful! .....If you change only the figure in the Resolution box you will resize the image to fit the figures in the dimensions boxes which stay fixed.  So changing from 300ppi to 72ppi would greatly shrink your image size.  Its a similar situation when you change a Height or Width figure.  

Try changing the ppi figure to what you want then, before hitting OK, re-enter the original pixel deminsions into the upper box, hit OK and then the correct physical size shows up in the middle box without shrinking your image.  For example, opening a 1800 X 1200 pixel image with the Resolution set for 300 will give me 6" X 4" (a good print size).   When I change the Resolution to 72ppi, the image size indicated is 432 X 288 pix.  Without hitting OK, if I change the top image size boxes back to 1800 W and 1200 H, then the dimention figures change to 25" X 16.7" - thats a big photo on your screen.

For printing, you want to watch this box that is at least 200ppi or more for the physical size you want.

- sorry for the long wind.

Hope that helps, ..............  Dave


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## Peano (Aug 10, 2009)

daveren said:


> I think what it means is that Size, Height and Width would be the physical size of the image ON YOUR SCREEN - assuming the resolution of your screen works out to be 72 pixels / in.



No, one more time: *The resolution setting (ppi) in the Image Size dialog box doesn't affect the size of the image on your screen.* Look at the two images I posted earlier. 72 ppi and 300 ppi display at *exactly the same size*.



> Careful! .....If you change only the figure in the Resolution box you will resize the image to fit the figures in the dimensions boxes which stay fixed.  So changing from 300ppi to 72ppi would greatly shrink your image size.



That happens if you change the resolution *and also resample the image*. But when sizing for the web, there is no reason to change the resolution. Simply leave "Resample" ticked, change the pixel dimensions in the top box to the size you want for the web, and hit Enter. Pretend that the "Document Size" adjustments don't even exist.


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