sowmya parthasarathy , Nov 20, 2009; 07:23 a.m.
Hi, I'll try to get this question as precise as I can. I have about 50 images, converted from tiffs to jpegs at 300 ppi resolution. I now need to upload these 50 jpeg images on the web and I'd like to reduce the resolution to 72 ppi. I usually use Genuine Fractals plugin in Photoshop to increase / decrease image size / sharpening, etc as its faster and does a clean job. Is it okie to directly convert my 300 ppi Jpeg images to 72 ppi in photoshop? As its gonna be tough for me to source out all the original tiff files. If this isn't okie, what is the best / quick way of reducing the image resolution (lossless compression). Cheers, SOwmya
Peter Y , Nov 20, 2009; 08:25 a.m.
Every time you save a jpeg, you have some loss.. Nature of the file format.
A better way, sorry that I don't know..
Matt Laur 
, Nov 20, 2009; 08:54 a.m.
Please be clear: are you actually looking to reduce the number of pixels in the image (say, from 3000 pixels wide to 600 pixels wide), or are you just looking to change the DPI meta data in the files? These are not the same thing.
Hal B , Nov 20, 2009; 09:41 a.m.
"Upload to Web" and "Lossless Compression" don't really go hand in hand. Just what are you talking about?
nikolai jerabek 
, Nov 20, 2009; 09:49 a.m.
Every time you save a jpeg, you have some loss.. Nature of the file format.
sorry but that is a myth !
A number of alterations to a JPEG image can be performed losslessly as long as the image size is a multiple 1 MCU block (Minimum Coded Unit) (usually 16 pixels in both directions, for 4:2:0 chroma subsampling ). Utilities that implement this include jpegtran , with user interfaceJpegcrop, and the JPG_TRANSFORM plugin to IrfanView .
further there are very simple algorithms just flipping the matrix order or spoken naturally the "orientation" of native blocks.
JDM von Weinberg 
, Nov 20, 2009; 02:48 p.m.
I'm sure you know better, but just in case, do not downsize the original files. Use "save as" to save lower-res copies either under a slightly different name or in a different place.
Peter Mounier , Nov 20, 2009; 03:00 p.m.
And to add a point, most monitors these days resolve at 96 pixels per inch, so for the web, you might consider downsizing to 96 ppi. rather than 72 ppi.
Peter
Robert Johnston , Nov 20, 2009; 03:59 p.m.
One reason I could not function without Lightroom. We can keep the original files as is, and do all conversions so easy, including importing to PS for things LR does not do. Using a plugin, can size files even as they are exported to my website. Then go do something else as it does. If I want tiff's, it converts and sizes them. If I want JPGs can set the pixel dimensions and resolution to just what I want and just archive the originals.
If you have to do it frequently think it would save lots of time.
David Thomasson , Nov 20, 2009; 04:11 p.m.
In this context, the ppi resolution is meaningless. For web images, it doesn't matter what resolution you set, because that applies only to print.
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For the web, you need to size the images in absolute pixels dimensions -- for example 800 px on the long side, or 1024 on the long side, something like that. When you save the tiffs as jpegs, the compression isn't lossless. But it also shouldn't harm image quality if you use the right amount of compression. In Photoshop, I would use quality level 7 to 10, depending on how much I wanted to limit file size. If you don't have Photoshop, you can use IrfanView (free) to resize the images and save as jpegs. IrfanView can also do them in a batch.
David Thomasson , Nov 20, 2009; 04:23 p.m.
Just to illustrate that resolution (ppi) has no meaning on the web, these were saved at the resolutions shown:
