GIMP Version: 2.8.4
Operating System: Windows
OS Version: XP - SP3
GIMP Experience: Beginner Level

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I open a gif animation (18 frames) and want to change the opacity of animation.
So I must change the opacity of each layer.
But there is no option to select multiple layers (or I can't see it).
So I changed the opacity for each layer separately - from 100 to 50.
File - Export - Gif... - the resulting animation has unchanged opacity !

Gif's always have full opacity, there's no transparency in gif's.

Cant' believe. Thanks.

The opacity of the layers within a gif image can be changed to make something appear somewhat transparent/ghostly, or make it seem as though the image is fading out/in, but each layer has to be flattened and the gif animation attributes possibly changed to 'replace'.
It is also true that animated gif image can not retain partial transparency. It's either all or none.

+1 about the strictly binary nature of opacity in GIF

By and large, GIF isn't an "editable" format, it's really the final output of the process; a bit like a PDF from a word processing software.

APNG is an extension to PNG format supported in (at least) Firefox and Chrome.

Googling [gimp apng animation] shows there is an extension available to provide open/save (actually open/export I guess after v8). I haven't tested it myself.

One thing to note is that when you're doing animated .gifs with transparency, make sure you semi-flatten the layers. if you don't, it will look like crap.

See this thread for a good explanation of semi-flattening..

viewtopic.php?f=23&t=214

Any time i use partial transparency i use APNG, or a flash program.With APNG files are bigger but you get a much cleaner result as you are allowed to use partial transparency and more than 255 colors so gradients look much more fluid.
Flash is vector math so it is constantly sharp at any resolution.There are FREE Flash Programs out there.
Giotto Effects List-

3D Rotate
Aircraft Landing
Alpha Split
Appear/Disappear
Beads on a wire
Blaster
Blinds
Blur
Boomerang
Burn In
Cartoon Runner
Circle
Circle and Back
Cross
Cylinder
Dice
Disco
Drop and Haze

Drop Shadow
Drop to Ice
Elastic Impact
Explode
Falling Leaf
Fish
Flash
Flatten
Helicopter Landing
Loop In
Merge
Mirror
Pool Jump
Pulse
Quiver
Radiate
Rainbow
Rocking Chair

Roll In
Rotate
Rubber Band
Shake Off
Shreds
Simple Effects
Spiral
Star Spawn
Starfall
Steam Roller
Stretch
Strobe
Swell
Typewriter
Wave
WaveX
Whirlpool
Wiggle
Zipper

Thanks a lot to everyone, especially to Rod.
Let's say - solved.