Why does save to jpg or save to web change colours?

[Cassidy]

Can anyone give me any advice. I am finding more recently that often skin tones are dramatically changed when I save to web and/or jpeg. It seems to desaturate the reds out of skin tones, but will leave other true red items in tact. Any ideas please?

[Jerryb]

hi,
my thoughts.... my opinion...
1. you need to remember that jpeg format is a lossy format. by that I i mean the more compression the more loss in quality
a. when a file is saved in jpeg or resaved to jpeg, data bits are thrown away ...also the alogrythms that is used effect quality, and this results certain subtle shade being losst, real fine lines may appear to be dotted, edges are unsharpt or ghosting appears, artifacts may appear, etc....
example... saving in jpg ,let say a shade of color has value 111011111 well save to jpeg that color value changes to 111011100 , you noticed the last 2 digits have changed and that represent a very slight change in shade of color!!

b. the more compression the more likely that the change is more noticable..

2. for storing those precious pictures and doing editing... it recommended you never save in jpg... stay with png or tif or psd or some other NON-lossy format.... they will retain your quality....... use jpg for your email and websites...

3. if for posting websites, and if you need high quality..... the only thing i can say... use a large resolution size picture and at the same time try not to compress too much .... in your photshop "save for web" you have several settings you can work with..... experiment a little and see what you satified with...

4. did a little search to find some examples of the effects of different formats pictures are saved in. you may find this informative...
http://www.cafepress.com/cp/info/sel...es/help_images

this has some additional info but few samples...
http://www.imagemagick.org/Usage/formats/#jpg_color

well i hope that helps to understand why you have the distortion..

Originally Posted by Cassidy

Can anyone give me any advice. I am finding more recently that often skin tones are dramatically changed when I save to web and/or jpeg. It seems to desaturate the reds out of skin tones, but will leave other true red items in tact. Any ideas please?

[perrush]

are you working under sRGB or under adobe RGB. Save for web always convert to sRGB and that can mean a difference when you're working under adobr rgb in PS

[Swampy]

I work in RGB, but have found that this setting in the Save for Web dialog works well to preserve colors.

[Cassidy]

Many thanks for your time all of you. A combination of Swampy and Perrush's info seems to have fixed my prob.

[Swampy]

Glad to be of help, Cass.