Berlin Imagery wrote:
But if the Low layer is blurred then it seems like the fine detail on that layer should be gone and can't be separated and moved to the High layer.

You don't separate or move detail from the low frequency layer to the high frequency one.

When doing frequency separation first you clone the main layer to 2 new ones: name them low and high. Then you blur the low freq layer with the desired radius. At that time the original image is still in the high freq layer. You are in that layer when you do the Apply Image calculations in combination with the low freq layer. So you are not losing the detail.

JONATHAN RICHARD wrote:
Frequency Separation with Photoshop for retouching skin is mostly about Keeping Skin texture ( pores , creases etc. )  While controlling the blotchiness and color inconsistences (redness and minor highlight and shadow variances) seen in the skin.

Someone must have given you misleading information. FS has nothing to do with fixing blotchiness and color inconsistencies. It is just a method for separating higher frequency detail from the rest in order to work only on the detail, i.e. only on the high frequency. It allows you to use the clone tool which is more accurate than the healing brush and hence less destructive to the texture. Also it is definitely not only for skin.

You create 2 copies of the base layer.
On the low layer apply a Gaussian blur to the layer with just enough blur to get rid of what you want to keep in skin texture .( adjust to the point where the skin texture is just blurring ..This ideally would be where you stop applying blur)

The top layer is the layer you will applying a  filter on (Frequency Separation)
This filter will take the info on the top layer (texture)   and subtract out the info from the low layer, leaving the difference (this is where you are creating/keeping the skin texture)
Select (Image )
Select (Apply Image)
Select (Frequency Separation)  (Layer) select low layer
            (Blend) select subtract
(Scale) is set a 2
(Offset) is set to 128

Select (OK) ….resulting in basically a high pass filter layer that has automatically chosen the radius based on the lower layer because it subtracts that info out of the high layer….resulting in a perfect duplicate

The correct way to do this is in 16-bit mode and for Apply Image use these settings:

USE when retouching for  the skin blotchiness/color inconstancy

To use this Frequency Separation Filter keeping skin texture but controlling  the blotchiness and color inconsistences of the skin .......Use the Lasso Tool on the low layer (Add enough  feathering for a soft edge)

-Lasso the blotchy areas and add a Gaussian blur with enough radius to make the blotchiness in the lasso area disappear

-To see the  results with SKIN texture back in the high layer (minus the skin blotchiness  ) change the high layer Blend Mode to ( NORMAL…LINEAR LIGHT )  you will see the skin texture minus the blotchiness in the areas you worked on with-in  the Lasso areas .

When using FS you should never touch the low frequency layer. Never. If you touch it - you are filtering and destroying the image because you are destroying the correlation between the low and high layers created by the Apply Image step. That is not the purpose of FS. If you blur the low frequency the result is plastic dolls with unnatural ovsharpened details which is terrible. The only thing you should do after FS is cloning on the high frequency layer.

anchev wrote:
Someone must have given you misleading information. FS has nothing to do with fixing blotchiness and color inconsistencies. It is just a method for separating higher frequency detail from the rest in order to work only on the detail, i.e. only on the high frequency. It allows you to use the clone tool which is more accurate than the healing brush and hence less destructive to the texture. Also it is definitely not only for skin.

Keeping Skin texture ( pores , creases etc. ) to work on in the high layer  is  one  benefit  but not the only one for  me using FS  in  my  current work flow

Working   blotchiness and color inconsistencies  using Frequency Separation  and  select ( lasso ) Gaussian blur  on the  low layer  after an  initial blur to the low layer  is how I prefer to go  for controlling that skin issue ...

JONATHAN RICHARD wrote:
USE when retouching for  the skin blotchiness/color inconstancy

To use this Frequency Separation Filter keeping skin texture but controlling  the blotchiness and color inconsistences of the skin .......Use the Lasso Tool on the low layer (Add enough  feathering for a soft edge)

-Lasso the blotchy areas and add a Gaussian blur with enough radius to make the blotchiness in the lasso area disappear

-To see the  results with SKIN texture back in the high layer (minus the skin blotchiness  ) change the high layer Blend Mode to ( NORMAL…LINEAR LIGHT )  you will see the skin texture minus the blotchiness in the areas you worked on with-in  the Lasso areas .

anchev wrote:
When using FS you should never touch the low frequency layer. Never. If you touch it - you are filtering and destroying the image because you are destroying the correlation between the low and high layers created by the Apply Image step. That is not the purpose of FS. If you blur the low frequency the result is plastic dolls with unnatural ovsharpened details which is terrible. The only thing you should do after FS is cloning on the high frequency layer.

I appreciate you explaining your experience  in using FS but I find   that my use is what is  ideal and the plastic doll look will result is one over clones the skin texture  in the high layer  .

anchev wrote:
Someone must have given you misleading information. FS has nothing to do with fixing blotchiness and color inconsistencies. It is just a method for separating higher frequency detail from the rest in order to work only on the detail, i.e. only on the high frequency. It allows you to use the clone tool which is more accurate than the healing brush and hence less destructive to the texture. Also it is definitely not only for skin.

When using FS you should never touch the low frequency layer. Never. If you touch it - you are filtering and destroying the image because you are destroying the correlation between the low and high layers created by the Apply Image step. That is not the purpose of FS. If you blur the low frequency the result is plastic dolls with unnatural ovsharpened details which is terrible. The only thing you should do after FS is cloning on the high frequency layer.

Here we go again!  Anchev, you really should adopt a more responsible attitude, and stop coming out with ill-informed dogma. The only people who are going to respect your assumed authority are the poor learners you regularly misinform and confuse.

You say 'FS has nothing to do with fixing blotchiness and color inconsistencies." Not true - by separating the frequencies you isolate the colour and blotchiness from the fine detail. Are you suggesting that, having made the separation, the retoucher is then permitted only to deal with the high frequency detail, and that working on the low frequency layer is forbidden?

You also say that "When using FS you should never touch the low frequency layer." Again this is a dogmatic and ill-informed statement. You can do whatever you like with the low frequency layer. It depends on what you are setting out to achieve. Most people work on a copy of the low frequency layer, but that isn't necessary if you know what you're doing. Do you?

You state, "That is not the purpose of FS." Again you are presuming to lay down the law as an authority on the subject. The purpose of FS is whatever the retoucher chooses to use it for in any specific instance.

You warn against  "unnatural ovsharpened details which is terrible". Well I think we already know that!

Yes, there are inherent dangers in the overuse of FS. But it is a powerful technique with many applications that go far beyond the narrow, limited use you try to impose upon it when you state , "The only thing you should do after FS is cloning on the high frequency layer."

Berlin Imagery wrote:
In spatial frequency separation I learned to create the Low and High layers, blur the Low layer and then do Apply Image on the High layer against the already blurred Low layer. But if the Low layer is blurred then it seems like the fine detail on that layer should be gone and can't be separated and moved to the High layer.

So how does it work? Thanks sincerely.

What frequency separation does is very simple.. separates texture (high layer) from colour (low layer).

The key is to use the right radius, there is not a recipe that you can copy as every image is different in terms of size, light, focus, etc.

OP i would suggest to stay away from FS at all cost, it will give you more problems than benefits.

anchev wrote:
Then you blur the low freq layer with the desired radius.

Not correct, you don't blur to a desire radius, you blur to the correct radius otherwise the split won't be correct and you will end up blurring texture on some frequencies.

anchev wrote:
Someone must have given you misleading information. FS has nothing to do with fixing blotchiness and color inconsistencies. It is just a method for separating higher frequency detail from the rest in order to work on the detail, i.e. only on the high frequency. It allows you to use the clone tool which is more accurate than the healing brush and hence less destructive to the texture. Also it is definitely not only for skin.

Not correct!

You can work on the low to fix colour incosistencies, as long as you don't duplicate the low, blur the low again or tamper in any other way with the low, everything is going to be fine. Low layer contains colour information only if the split is done correctly. So yes you can work on the low for certain things.

anchev wrote:
When using FS you should never touch the low frequency layer. Never. If you touch it - you are filtering and destroying the image because you are destroying the correlation between the low and high layers created by the Apply Image step. That is not the purpose of FS. If you blur the low frequency the result is plastic dolls with unnatural ovsharpened details which is terrible. The only thing you should do after FS is cloning on the high frequency layer.

Not correct again, see above.. plastic dolls skin comes when you duplicate the low and blur it again or when you apply a filter to the low or tamper in any way with it but cloning on the low doesn't produce any bad results.