The only time I don't use the same aspect ratio is when I'm generating tall thumbnail images for the library. I have a library pose and camera setup which I adjusted by making the preview the same aspect ratio as the thumbnail I wanted. Now, when I want to render a tall thumbnail, I set the pose and the camera I've saved in the library, then make the render output to the thumbnail aspect ratio and render. I don't need to check the preview with this kind of preset, but in any other circumstance I can imagine, you would want to. In contrast, DAZ Studio has a render output box in their preview window, so you know what will be in scene and what will get cropped out of the render.

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