Flattening Images in Photoshop

[rabanito]

After processing an image with lots of layers it happens that after flattening the image changes visibly.
It looks like I'm not the only one to have noticed the effect.
Has somebody an explanation for this and perhaps a cure?
Thanks

[Slobodan Blagojevic]

Changes how? Darker/lighter, more/less contrasty/saturated? Example? Was the visibility turned on for all layers before flattening?

[Mark D Segal]

What version of Photoshop?

I have never ever experienced this.

Slobodan's question about layer visibility may be the answer if you had any of them turned off.

Another question that comes to mind is whether the problem is systematic, or only with one photo?

[smahn]

After processing an image with lots of layers it happens that after flattening the image changes visibly.
It looks like I'm not the only one to have noticed the effect.
Has somebody an explanation for this and perhaps a cure?
Thanks

I have experienced this in the past and invariably it was because I was not viewing the image at 100% magnification.

[rabanito]

Changes how? Darker/lighter, more/less contrasty/saturated? Example? Was the visibility turned on for all layers before flattening?

I'll try to make an example and post it
I let PS discard the hidden layers so it should have no effect

[rabanito]

What version of Photoshop?

I have never ever experienced this.

Slobodan's question about layer visibility may be the answer if you had any of them turned off.

Another question that comes to mind is whether the problem is systematic, or only with one photo?

Ps CC

I noticed it only today. If it is systematic it was not noticeable before

[Mark D Segal]

OK, that's useful information because it may indicate that the problem has something to do with the treatment of this photo rather than the application as whole - something to watch for going forward.

[rabanito]

I'll try to make an example and post it
I let PS discard the hidden layers so it should have no effect

Here is an example

[rabanito]

I have experienced this in the past and invariably it was because I was not viewing the image at 100% magnification.

I tried that. It looks like this could be the reason.
What a fright! 

[Mark D Segal]

It shouldn't be necessary to view an image at 100% in order to see it's basic tonality and colours properly. Magnification of this kind is usually more useful in looking for sharpening and smoothing performance and artifacts. As for the example you posted, it's not large enough for fine comparison; but as it stands, I see precious little difference, if any. Could you remake this comparison using the full allowable image size for posting here?

[rabanito]

It shouldn't be necessary to view an image at 100% in order to see it's basic tonality and colours properly. Magnification of this kind is usually more useful in looking for sharpening and smoothing performance and artifacts. As for the example you posted, it's not large enough for fine comparison; but as it stands, I see precious little difference, if any. Could you remake this comparison using the full allowable image size for posting here?

Usually I look at the image at screen size or somewhat smaller, to take in the whole. Sometime I may even rotate it 180 to check the composition 

But I checked the following:
I resize the image to full size and only then flatten it. The tones don't seem to change. Then I resize it to working size and it looks like it doesn't change anymore.

[digitaldog]

It shouldn't be necessary to view an image at 100% in order to see it's basic tonality and colours properly.

No necessarily so. The pixels are subsampled for the zooming out of the image, that can absolutely affect color and tone. Hence, it is always recommended for precise inspection of images to view at 100% (1:1). Ditto for Lightroom but in Develop not the other modules.

[rabanito]

No necessarily so. The pixels are subsampled for the zooming out of the image, that can absolutely affect color and tone. Hence, it is always recommended for precise inspection of images to view at 100% (1:1). Ditto for Lightroom but in Develop not the other modules.

The problem is that you want to see the whole picture as you would do with a print or on a screen. In the end the tonal relationships are more important than the details
Most screens aren't big enough to display 5000x 3000 or bigger 1:1.
So you have to do most of the work in a smaller scale and when you're finished and satisfied, then flatten.
And then "it" happens 
It seems to me that flattening with the 100% view solves the problem, no changes that I noticed, even scaling back to 33% e.g.
If you flatten instead at 33% then I'm not sure of what I'm getting
Please correct me if I'm wrong

[digitaldog]

The problem is that you want to see the whole picture as you would do with a print or on a screen. In the end the tonal relationships are more important than the details
Most screens aren't big enough to display 5000x 3000 or bigger 1:1.
So you have to do most of the work in a smaller scale and when you're finished and satisfied, then flatten.
And then "it" happens 
It seems to me that flattening with the 100% view solves the problem, no changes that I noticed, even scaling back to 33% e.g.
If you flatten instead at 33% then I'm not sure of what I'm getting
Please correct me if I'm wrong

You're not wrong per se. But if your goal is to precisely view tone and color AND sharpness, you need to view the pixels 1:1 without interpolation. Here's one example. The speaker looks darker (on this end) in the zoomed out version than the one at 100%

[Mark D Segal]

No necessarily so. The pixels are subsampled for the zooming out of the image, that can absolutely affect color and tone. Hence, it is always recommended for precise inspection of images to view at 100% (1:1). Ditto for Lightroom but in Develop not the other modules.

Interesting - never having observed such effects looking at photos at various magnification ratios I wouldn't have suspected it, but kind of makes sense in principle.

[digitaldog]

https://helpx.adobe.com/photoshop/using/viewing-images.html

[nirpat89]

I tried that. It looks like this could be the reason.
What a fright! 

Try using the Navigator to set the magnification required to fit the image in the window.  It goes 100%, 66.67%, 50%, 33.33% and so on.  Anywhere in between there is a chance that the re-sampling will show some artifacts that may make the image look different. 

In any case, the difference is only in what you see on the display, not in the actual image.  Try comparing prints from flattened vs non-flattened and see if you get a difference.

[rabanito]

Thanks everybody for the interesting ideas!!!