I know similar questions have been asked, but this is a little different I suppose. All files for our client need to be sized to max:9600px. I'm debating if I process from ACR with default sharpening, then upsize OR turn off sharpening in ACR, upsize, and then use un-sharpen mask. I'm going to test some files come monday but was wondering if anyone has any ideas/experience. The files are from a 39mp hassie back, so they're reasonably sharp to begin with. Not sure how big of a difference it will make in the end. Thanks for reading!
Resize and then sharpen is the answer.
It is commonly said that sharpening should be the last step. This make sense because you do not want to stretch the effects of sharpening. Sure you could do more complicated things but the lest steps are applied the less change there will be of seeing artifacts in the final image.
Itai has given you the correct answer - but there is also another way of thinking - with the same result.
By upsizing an image you will invariably add some blur - no matter how sharp the original image is. (How visible the blur is, is another matter depending on the algorithm used and amount of magnification.) Thus if you were to sharpen first and then upscale, you would need to sharpen it again. If you upscale the image first, you need to sharpen it only once - the fewer steps get you to your desired result, the better as the less chances there are of any steps degrading the quality.
Having said that though: For certain artistic effects, it might be effective to sharpen the images first. In the end you need to try yourself, but upscaling first and then sharpening once should give you the better result.
Some interpolation algorithms used when resizing have sharpening builtin, like Lanczos, but for extreme resizing, on an engineering level, it is actually a well known technique to blur the image first and then resize in multiple steps. Ie, sharpening might actually be in vain. So I'd go with Itai's suggestion: resize and then fix it up.