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I baked a smoke / fire simulation and then created this animation. After that I rendered an image of one frame and that image looked ok. After that I wanted to render some other frames and each time the rendered image was unsharp. I am not aware that I changed something and I also did not Free Bake. how can this happen ? This is one of those unsharp images.

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I think you're referring to the motion blur. Your scene has motion blur enabled which is what you want for smooth animations. But if you just want to render a still image and you want it to render sharp (without motion blur) then you'd just disable motion blur for that one shot. The amount of blurriness depends on the strength of the motion at any given frame, so you should not be suprised that some frames render with less motion blur than others (depending on your animation). enter image description here Motion Blur is not enabled by default but is recommended to use for animations so that movements are perceived as smooth, but if you're rendering at very high fps (60+) you don't need it. You can also use it for stills in order to depict rapid movement. (Take for example a render of a racing car moving fast on the race track: if motion blur is disabled then it would appear as if the car was just parked on the street since everything is rendered sharp and thus no sense of motion is transmitted). As a final side note: As far as I can tell cycles does not support motion blur for the fire simulation per se, but in this case applies the motion blur from the rotation animation.

coCoKNIght
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  • where can I find that motion blur ? Or did I add that myself ? And how can one image be sharp and the other one unsharp ? –  Mar 30 '16 at 08:20
  • It may be a subframe setting interpolating the "crispness" Probably a side effect of this - @JanScherders – Rick Riggs Mar 30 '16 at 08:23
  • it's in the render settings on the properties pane. It's not enabled by default, so... :-) Well, the way motion blur works is the faster something moves the more motion blur is applied to it. Thus if you render a still image at a frame where there's not much movement, you won't see much motion blur. If you render your image at another frame where there's fast movevemts, then there'll be much motion blur applied to those fast moving objects. – coCoKNIght Mar 30 '16 at 08:25
  • @coCoKNIght You are right, this motion blur box is checked. I will rerun the render of the still image. I hope that solves the problem. But I still don't understand why my first render did not have this problem –  Mar 30 '16 at 08:35
  • @RickRiggs Can you explain ? I am new to animation ... as far as I know I did not create any subframes, but perhaps I don't understand your remark –  Mar 30 '16 at 08:40
  • @coCoKNIght Is "Motion Blur" best to have checked standard when you render animations ? –  Mar 30 '16 at 10:14
  • Yes unless you're rendering at very high fps, then you don't need it since the human eye will do it automatically then. – coCoKNIght Mar 30 '16 at 10:16
  • @JanScherders - I just saw that other post you were asking about, and thought you may have possibly introduced sub frames, if you re-timed your animation. – Rick Riggs Mar 30 '16 at 15:21
  • Try again based on your motion blur setting as that is quite likely the culprit, I was just trying to give you a little trouble shooting help based on some of the other factors that may have got you to this point. - @JanScherders – Rick Riggs Mar 30 '16 at 15:56
  • hi Rick, I have finished the render and now "motion blur" unchecked and yes, this was the cause of the unsharpness. No idea how this box got checked ... maybe when working on other stuff. Anyway, motion blur was responsible for the unsharpness –  Mar 30 '16 at 15:58
  • @coCoKNIght: you could include the important explanations and details from these comments into your answer. ;-) – Samoth Apr 05 '16 at 19:33
  • @Samoth: Please let me know what details you consider important to add to the answer. Btw. I was just rendering a cycles fire animation myself yesterday and it would appear that cycles doesn't support motion blur for the fire simulation itself. Can someone confirm that? – coCoKNIght Apr 06 '16 at 09:32
  • You could add a Screenshot of your mentioned Motion Blur Checkbox. And the explanation about the further questions Jan had. So that the complete information (without the need to read all the comments) is directly available in your question. – Samoth Apr 06 '16 at 09:36