After Effects is loaded with assistants and keying is at the heart of motion graphics and animation. A favorite assistant is available under the animation menu, 'keyframe assistant'. There are several options here but one that practically creates an animation for you with only a few steps is the 'sequence layers' option. This assistant does pretty much what is sounds like, it creates keys and transition for the layers you select. Whether a slide show or part of a presentation with a few timed sequences, this a tool you will want to keep nearby.
To get started, choose the images you want to sequence, ideally they are the same size but this is not a requirement. To create your composition, simply drag one of these images from your project panel onto the composition symbol at the bottom of the project panel. This is a quick and easy way to create a comp based on the resource properties. If this was a high definition video, the comp properties would match the video.
In this case we will simply create a composition with my default frame rate of 29.7 and the size of the image used to create the comp. Now drag the remainder of the images into the composition and they will all occupy the complete time window. It should default to the time of the previous composition since a single image does not have a length of time.
When you click on the first image, first layer then hold the shift key while you click on the last, this will choose all the layers at once. You can take this same action by entering 'Ctrl A'. With all layers highlightighted, drag their timeline setting back to the number of frames you want each to appear in this presentation.
Now its time for the assistant. Highlight all your layers, then under the 'Animation' menu, choose 'Animation->Keyframe Assistant->Sequence Layers'. This will open the pop out menu for sequencing with three options: overlap, duration, and transition. The sequence layer assistant is going to sequence your layers based on these three simple options.
If you think about a typical presentation slideshow, the images fade into fade into one another, they 'dissolve' as one releases the stage to the next. Typically they overlap just a bit with a brief moment of both sharing the stage but with decreased opacity so they really are sharing. These three options are just that.
When you click on the overlap checkbox the duration and transition options appear. You choose your duration which will typically be a product of a bit of experimentation. In my simple slideshow, showing some of my artists works, I've set each image to appear for two seconds with an overlap of four frames. When you choose overlap and set your duration you have three options for your transition: none, cross dissolve front layer, cross dissolve front and back layers.
Choosing the dissolve transition options simply tell After Effects to add an opacity fade over the duration window you specify. You can choose none but we usually like to see a fading transition letting our images or video 'share' and hand off the moment as the next takes center stage.
What is a bit funny about describing this assistant is, it really does take longer to describe it than to apply it. Try it out! Choose all your layers, choose the sequence layer assistant, check to overlap, then set duration, and type of transition. An extra benefit about it being so easy is that it is also easy to experiment, trying different durations to get just the overall time and effect you are seeking.
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