Working with digital images

[Peppergal]

I've seen some issues re: resolution addressed here but I don't think I've come across this particular one.

I work mostly with images to be put on websites, mainly real estate websites.  I often receive small images from the realtors that are at 300 ppi, and I have been resizing them to be the same dimensions but at a lower resolution, usually 70 ppi, because I was under the impression that 70 ppi is a smaller file size than 300 ppi.

I had a discussion with a friend who says she leaves her pictures at 200 ppi because it causes less distortion when resizing them (however, I think she's using Front Page to resize them, rather than resize them in a graphics program.)

I made this page to show that whether it's at 300 ppi or 50 ppi, the resizing is better if using Paint Shop Pro (or any good graphics program, for that matter...) rather than using Front Page to set the image sizes.

http://karenricedesigns.com/resolution-experiment.htm

The original image is a 300 ppi image, that has been resized using PSP and also, using Front Page.

My question lies in this:  the file size for the 300 ppi is exactly the same as the file size for the 50 ppi images.  Therefore, what am I gaining by using 50 ppi (or 70, or 150, etc) if the file size is going to be no smaller than the 300 ppi?  I'm just wasting my time resizing the realtor submitted images from 300 ppi to 70 ppi...aren't I?

These pictures are not destined for print, unless a client wants to print out a listing for reference...it's certainly not going to be a work or art.  

I just don't understand...if the file size of a 300 ppi image is exactly the same as a 50 ppi image, why bother reducing the resolution at all?  If the file size is the same, the download time is the same, right?

[ShutterbugGail]

I typically save my images for web presentation at a resolution of 72 pixels per inch. Even if I'm working on an image that is 300 ppi, when I use the Save for Web menu in Photoshop, the image is saved at 72 px. I believe PaintShop Pro has a similar menu. You need to get a good balance between the way an image looks, the visual size and the file size so it loads as quickly as possible into a web page.

FrontPage is not really intended to be a substitute for an image editing program. Neither is it the best way to resize images, particularly if you go from a very large size to small size. And when you resize an image in FrontPage, you must also resample it using the resampling tool in the FP Pictures Toolbar. When images are significantly reduced in size, they often must be sharpened. You can not sharpen images in FrontPage.

I think you will find this article about resizing images for web presentation very informative:

http://www.digicamhelp.com/learn/image-editing/image-resizing-for-web.htm

[ShutterbugGail]

If you want to learn more about using the FrontPage Tool Bar more effectively, read the article I wrote about it for Tomas Brunt's OutFront:

http://www.outfront.net/tutorials_02/fp_techniques/pictures-toolbar.htm

[Peppergal]

Oh, I don't plan on using Front page at all for the images...my question was - does resolution matter with file size after all?  the 50 ppi and the 300 ppi are the same size, so I guess I am wasting my time when I resample the 300 ppi images I receive from the realtors to a 70 ppi image.

It's just that I thought a higher res meant a bigger file size...but evidently, that isn't so.

[Deb]

Hi Peppergal, maybe this will help.

An image that is 4x6inches at 300ppi will be 1200x1800. Similarly, one that is ~17x25.5inches at 70ppi will also be 1200x1800. So you see, both these images are the same "size" in terms of file size yet are vastly different in terms of dimensional size.  

Now, higher resolution will result in larger file size if the 300ppi image and the 70ppi image are of the same dimensional size.

[Peppergal]

That's what I thought, but if you look at the page I linked in my first post, look at the properties of the two pictures at the top.  The dimensions are exactly the same.... - one is 300 ppi and the other is 50 ppi...and the file size is the same.

So, is this a quirk in my image editing software?  Is it not really changing to to 50 ppi when I tell it to resize to the smaller res?

[Deb]

Yes, the file size is the same but your dimensional size is different. That's why you aren't seeing any change in file size when you change ppi.

Your "50full%20size.jpg" is 279x358pixels at 50ppi BUT its dimensions are 5.58x7.18inches in size.

Your "300full%20size.jpg" image, though 279x358pixels at 300ppi, is actually only 0.93x1.193inches in size.

This difference in dimensional size is the reason why you are not noticing a reduction of file size. The reduction in resolution from 300 to 50 must be accompanied by identical dimensional size for the reduction in file size to be achieved.

In other words, you aren't keeping all other variables contant when you are changing ppi. In the case of these two images, two variables are being altered. That is, both dimensional size and ppi are being altered which is negating the effect of the ppi reduction.

[Peppergal]

okay....I think I understand....

but then how do I achieve this -

picture display must be 200 x 300

picture display received is 200 x 300 at 300 res.  How do I get a 200 x 300 at 50 res, that is acutally a smaller file size?

I'm not computing here.  LOL.  Maybe it's just too late for me to be thinking along these lines.  

[ShutterbugGail]

picture display received is 200 x 300 at 300 res.  How do I get a 200 x 300 at 50 res, that is acutally a smaller file size?

I believe you have a Save for Web menu, or something similar, in PaintShop Pro? If so, use it. I will preserve the size of the image but compress the file size. This method typically saves to a resolution of 72. Are you locked into 50? 72 is the standard resolution for web images.

You can do this using a program such as Irfanview (free). Use the Resize/Resample Image dialog box which is accessed from the Image menu.

http://irfanview.com/

[Deb]

but then how do I achieve this -

picture display must be 200 x 300

picture display received is 200 x 300 at 300 res.  How do I get a 200 x 300 at 50 res, that is acutally a smaller file size?

Hi, let's see. In PSP9 go to IMAGE > RESIZE. Fill in the resolution you wish (50), then fill in the pixel dimensions you wish (200xwhatever; remember, your size needs to be proportional to your original or it will be distorted.) RESAMPLE (bicubic) and LOCK ASPECT RATIO need to be selected. Finally, hit OK.

That'll do it.  

[ShutterbugGail]

Fill in the resolution you wish (50), then fill in the pixel dimensions you wish (200xwhatever; remember, your size needs to be proportional to your original or it will be distorted.) RESAMPLE (bicubic) and LOCK ASPECT RATIO need to be selected. Finally, hit OK.

By chaning the resolultion to 50, then changing the pixel dimensions higher, isn't there a chance of a significant loss of image quality, even with resampling.

Just curious, Peppergal, why is chaning the dpi to 50 so important?

[Deb]

Hi there ShutterbugGail, Peppergal should be working off of her original 300ppi image (which, from her example, had been 279x358pixels in size). Resampling that image to 200x257pixels at 50ppi would be downsampling. The pixel dimensions are being reduced not increased here.

In any case, yep, reducing resolution to 50 will reduce image quality as compared to the original image.

BTW, since most monitors are at a resolution of 72 (and 96), how about using one of those for your image resolution.

[ShutterbugGail]

Sorry, I missed the dimensions and didn't realize she'll be downsampling. But I do think Peppergal should be using 72 or 96 dpi for the reason you pointed out.

[Deb]

Yep, I think using either 72ppi or 96ppi would be dandy.  

[Peppergal]

The 50 ppi I used was just an experiment.  I typically downsize to 72 dpi...however....

It did NOT change the file size.

As you can see on the page I posted previously:

http://www.karenricedesigns.com/resolution-experiment.htm

there really is NO significant loss of quality between the 300 ppi image on the left, and the resampled 50 ppi image on the right.

I took the 300 ppi image and then chose "Resize" and changed the resolution, but kept the pixel size the same...saved the file...and then discovered that the file size for the 50 ppi image is the same as the file size for the 300 ppi image.

The 50 ppi isn't important; it was just a number I picked just to "see" what would happen...the fact that the 50 ppi image and the 300 ppi image that are the same pixel dimensions are also the same file baffles me.

I know that if I run the JPEG Optimizer it will further compress the file size no matter what the resolution.  That's not my question...I'm just puzzled that simply  lowering the resolution didn't lower the file size some.  They're both set to display 279 x 358 pixels.   I've been changing the resolution on all my real estate photos and now after this experiemtn, I'm wondering if I was wasting my time.

Maybe I'm just being dense here and maybe I am on the edge of an "AHA, now I understand...." but I don't understand at this point.    

[Deb]

Maybe I'm just being dense here and maybe I am on the edge of an "AHA, now I understand...." but I don't understand at this point.    

(Note: I've reworded this reply. Hopefully it's less confusing. )

Nah, you're not being dense. It's commonly misunderstood, so don't fret it.

Think of it this way. An image it is made up of X number of pixels in length and Y number of pixels in width (say 1200x1800). In our example, the resolution you select merely determines how large the image will appear, that is, how spread out those pixels will be.

For instance, a 4x6inch image at 300ppi (4x300=1200; 6x300=1800; therefore 1200x1800 pixels) will appear as an 8x12inch image if the resolution is changed to 150ppi (8x150=1200; 12x150=1800; notice that it is still a 1200x1800 pixel image). The software merely spreads out the pixels to match the resolution so 4 inches worth of pixels (1200pixels) is spread out to 8 inches when resolution is reduced to 150ppi. Remember, we're dealing with "pixels per inch" (PPI). The file size will not change unless you also make the appropriate change to the dimensions. So, if you change the resolution to, say 150ppi, AND bring the dimensions back down to 4x6inch (4x150=600; 6x150=900; therefore 600x900 pixels), you will now see that your file size (overall file size in kilobytes) will be reduced (in this case by 1/2). To resize both the appearance of the image and the size of the file itself, you need to alter both the resolution and the dimensions.  

[Peppergal]

So, if you change the resolution to, say 150ppi, AND bring the dimensions back down to 4x6inch (4x150=600; 6x150=900; therefore 600x900 pixels), you will now see that your file size (overall file size in kilobytes) will be reduced

Okay - I think I get it now.

[Deb]

Hi Peppergal, that's super! If you do happen to run into a snag though, don't hesitate to holler back.