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Is there a way to dissolve between one instance and the next to create a kind of "staggered mix" effect? Or even just to hold an image and then dissolve to the next, within the project timeline?

There are several ways to do this, I use this:
- have a layer of several frames
- grab handle at the right and pull until it's a multiple of the original frame count
- in the dialogue, I check interpolate and click stretch.

To be sure to have the original frames untouched somewhere, I extend their exposures first.

That's how I would proceed too.

Hmm. I think I need more information because it's not working for me. To be specific, I have two instance I want to dissolve between. Each instance is 100 frames. I want to make them dissolve between the two instances over 36 frames. So my timeline will be 200 frames in total and there will be a 36 frame dissolve between the first instance and the second.

In trying to follow your instructions, I got a very tiny dissolve over maybe 4 frames. How do I achieve what I want? Thanks.

It should help :

In your case you first need to isolate the 2 frames you want to dissolve from the 99 exposures. Just copy them into a new layer, which now should consist of only 2 frames, and proceed as I wrote to stretch this to the desired length of dissolve.

(I don't know your workflow habits, but I permanently copy stuff to temporary layers, do stuff with it, and paste it back in place.)

Thank you. The video really helped. I think the word "interpolate" confuses me. I guess it means dissolve, or transition.