I've created a single channel document (28MB) and uploaded it to Dropbox - the values that I'd like to enter in Levels on this are 8, 1.00 and 10. You should see a clear difference between the preview and the result.
You write about a problem with 16 bpc documents but provide a 8 bpc doc. Didn't you mean to supply a 16 bpc doc?
Anyway, even with your 8 bpc doc and working in 8-bit mode, the reason for the difference between preview and actual adjustment is that when zoomed to less than about 64% and with Cache Levels greater than 1, the preview image is calculated with very low precision. Photoshop does very low precision display calculation with Adjustment Layers and layer blending in general, too, unless the zoom is at least 64% or Cache Levels is 1.
Low precision display tends to be more disturbing in 16-bit mode than 8-bit mode because of the greater reduction in precision in the former mode.
To prevent this lowering of display precision, you can set Cache Levels to 1 in Preferences > Performance then relaunch Photoshop, but some graphics cards can have problems in 16-bit mode when CL is limited to 1. Also, reduced cache levels can result in noticeably jaggy interpolation of the display pixels when zoomed out and reduction in Photoshop's speed.
Ironically, Photoshop CS6 requires us to have a quite powerful modern computer for its newest features, while it still uses display shortcuts which I presume to have been deliberately made very imprecise so the program would work reasonably fast on antique equipment.
Thank you conroy, I suspected it may be something like that - the reason my Curve appeared to provide better results was that I was zoomed in at 100% when I ran it. I'll definitely remember in the future! Yes, the file provided was 8bit, sorry about that - I was getting the same result with that file because of the low Zoom level. Thanks for the clarification regarding cache, zoom, and previews - I'm assuming that this applies across the whole application, would that be correct?
1 person found this helpful
As far as I know, the low precision display affects previews of adjustments and the blending of layers in both 8-bit and 16-bit modes. Here is an 8 bpc example which compares a Subtract blend of two layers of noise with the merged layers at zoom levels of 100%, 64%, 63% and 33.33%. The left doc has the separate layers and the right has the single layer which results from merging the two layers. The transition from precise to imprecise display happens at just below 64% zoom when Cache Levels is greater than 1.
Thanks again - I had been trying to work out why most often I got accurate results with level adjustments on layers and it was only occasionally that the 'problem' occurred. Purely down to Zoom level when running the adjustment. The differences you illustrate above are very handy - between 64% and 63% they are immense, and very important to be aware of, especially when working on very large (in pixel dimensions) images.
It's strange that Photoshop provides 16 bpc document mode, supports 10 bpc display if you have Windows and a suitable graphics card and monitor, yet, much of the time, it's doing very low precision calculation of the image to be displayed.