Image changes when flattening layers in Photoshop

When I process an image in photoshop I use many layers. Working non destructive means I use adjustment layers, and merge layers into a new layer to run filters on them. The advantage is obvious. When I have overdone an effect, it is easy to just turn off that layer. or to reduce the effect with the opacity slider. But when I prepare for print or web, I always take a copy of the psd file which I resize and flatten to save disk space. Some times when I flatten the image, the tonality seems to change. I’ve done a lot of searching on this, finding nothing. Then, by coincident, when I tried to figure out how to flatten an image without changing it, I flattened while viewing the image at 100% zoom. To my surprise there was no change at all. I zoomed out to “fit on screen”, and the image seemed to change when flattening.

So, the image doesn’t really change when flattening, only the screen rendering may change. It seems that any odd number zoom factor may cause this effect. It also seems to be more visible when you got a lot of small high contrast specs in the image. I was happy for a while, but then I realized that the Photoshop view could not necessarily be trusted, even when using a calibrated screen.

I’ve read this regarding evaluation of output sharpening, but now it seems to me that it also applies to tone adjustment, and possibly all kinds of adjustments you can do in photoshop. Thinking of it, I’ve also seen color changea. Don’t edit, or evaluate results unless you have zoomed to 100% or any 2 division of that (50% or 25%).

Just something to be aware of when photoshopping using many layers.


5 Responses to “Image changes when flattening layers in Photoshop”

  1. December 18, 2008 at 8:56

    Thank you! I hadn’t noticed a difference between an unflattened and flattened image until today … probably because I don’t normally work in more than a few layers (today, it’s 16). I was confused when my images looked fine until I zoomed out to 8.33%, at which point a faint cross-hatching appeared. This totally explains it.

  2. December 18, 2008 at 10:17

    glad it was helpful. I took me a while to figure out, and the solution was so simple.

  3. March 30, 2009 at 23:04

    Thank you so much for this, just encountered this problem which I never have til now, and no amount of googling seemed to give me the answer and I was getting worried!! Thanks a million

  4. 4 Kjell
    March 30, 2009 at 23:11

    You welcome Susy. Glad I could help you out

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