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Thread: Fast Photoshop / GIMP editing for exposure "correction" mini tutorial

  1. #1

    Default Fast Photoshop / GIMP editing mini tutorial

    This writer (me-ow) uses GIMP on Ubuntu. But the same principles apply to the operation of either platform

    You will need "the Application"
    1a. Photoshop
    1b. GIMP (it's free and no need pay licensing, totally legal, safe, no malware. did I mention that it is free?)

    2. A photo which has either overexposed highlights or underexposed areas (or both!), for this practice

    Caveat: i am not a pro. If you are a pro and don't mind sharing, please edify us all!

    This technique may or may not be suited for pro work. Use at own risk

    1. Boot your computer up (duh). That includes turning on the power, monitor screen, peripherals, etc.

    2. Run the application

    3. Open the image which needs some adjustment

    4. Duplicate the background layer. In Gimp you can right click in the layer name and select duplicate. In Photoshop you may need to use Layers -> Duplicate.

    5.a Underexposed - set top layer blending mode to Screen.
    b. If the underexposure is general, adjust the opacity until "correct" exposure is achieved. Use your eyes and taste to judge.
    c. If the underexposure is regionally localized eg in the upper half of image, we must use "layer mask" - useful concept.

    d. Photoshop users click on the [o] button at the bottom of Layers dialogue (mouseover to see "Add Layer Mask") . PS user may get a prompt asking for "fill" of the added mask. Select empty/transparent if slightly underexposed. Select white/full opacity if severely underexposed.

    Gimp users can right click on layer itself and select "Add layer mask"

    e. Using layer mask: the idea is using various painting tools like gradient, paintbrush, etc, to "paint" in or "paint" out the top layer's blending with respect to bottom layer

    eg Selected transparent/black layer mask. This implies that the duplicated top layer is completely transparent having opacity zero and you will not see any of the top layer appearing. For white/filled layer mask, top layer is completely apparent, opacity 100% and top layer blends over below layers in the mode you had chosen.

    e-1 )) Straightfoward painting-in using gradient (eg, top part of image is underexposed and must recover this top part, without blowing out the lower part of image due to "screen" blend burning the already properly exposed lower half of image)

    Use gradient tool (hotkey G in PS, L in GIMP) drag the cursor across the layer mask to paint the part which needs recovery white. Observe if the desired part of top layer has appeared and has the blended effect on the lower layers.
    If the previously correctly exposed part is now burnt, there is a chance the gradient tool is not being dragged in the correct direction.

    e-2)) Dabbing using paintbrush - use paintbrush tool in transparent layer mask to selectively paint using white, in (eg faces) areas which need more brightness. User may wish to select 40-60% opacity of the tool itself, to achieve more gentle and slower paint-in.

    6) Over-exposed?
    Use layer blending mode of Multiply to recover. follow Steps 5(b) to (e-2) , substituting "underexposure" to "overexposure".
    Not all highlights can be recovered if too blown.


    1. It is possible to duplicate a layer with its mask on, and set the newly duplicated layer, to a different blending mode

    2. Setting a blending mode to "overlay" can increase color saturation. (GIMP)

    3. Save your work as a PSD file to stop the layers from being flattened, eg into a JPG . Export to smaller image formats as required

    *** End ***

    Thanks for reading, if there is anything inaccurate or plain wrong, please let me know and I will edit this post
    Last edited by Shizuma; 6th April 2013 at 12:40 PM.

  2. #2

    Default Re: Fast Photoshop / GIMP editing for exposure "correction" mini tutorial

    Just another quick tutorial. I am not a pro at Post Processing but I feel this is useful.

    Background: I saw a buddy on FB shooting LED continuous light at sunset. His model is beautiful, her outfit exquisite. The only issue was that due to the mixed lighting environment (warm sunset + cool LED), the model appeared sallow.

    It was not possible on a whole-image basis to manipulate the color balance to fit the model's skin tones as it would cause the beautiful sunset in the background to lose its original colors (which was not desirable).

    ***Selective color balancing ***


    1. Opened the jpg file (no high res available to me) in GIMP. You may use photoshop also, the steps are similar

    2. Duplicate the base layer. Assign the duplicated layer a meaningful name if required (but here we are only using 2 layers. it is not really necessary but a good practice nonetheless)

    3. Adjust the color balance on the top layer (dupe-layer) until the model's skin tones are correct.

    In the image the model's skin appeared yellowish and she hence appeared sallow (kind of like she is suffering from jaundice sickness - unpleasant color to look at).

    I used Color Balance and added more red (5 points) in the shadows, medium and highlight zones. I reduced yellow by 5 points in highlights.

    4. Add a layer mask to the top color-corrected layer. Set the mask to transparent. Use the brush, select white brush and paint the mask area where the model's skin is. (Layer mask can "paint in" and "paint out" the layer which is masked, affecting it's opacity and visibility).

    I used a soft brush to paint in the color corrected layers so that it will appear selectively over the model's yellow skin. She now has got the correct skin tones without adjustment to the background sunset colors.

    You may wish to experiment with the layering mode (normal, overlay, addition, etc) to find what is most suitable for your desired effect.

    ***End ***


    photo credit: Wilson Teh (thanks!)
    Used under fair-use --- post processing tutorial

    I hope this tutorial has been useful. If there is anything incorrect or better methods do let me know, I will amend accordingly!
    Last edited by Shizuma; 6th April 2013 at 11:40 AM.

  3. #3

    Default Re: Fast Photoshop / GIMP editing for exposure "correction" mini tutorial

    Just another quick tutorial. In today's tutorial, we will be examining how to make human subject's eyes brighter in less than 1 minute (if you use keyboard shortcuts! )

    (Shooting cats and brightening their eyes is an entirely different matter...serious )

    Background: I shot a human subject backstage (I volunteered as a makeup artist). She was adequately exposed but looked not that lively even though she had her stage makeup on and she was not tired, etc, physical conditions.
    She did not look that energetic because the catchlight was insufficiently bright in her eye? (uh...what's catchlight? no, not referring to the moderator. Catchlight refers to the reflection of light source, in subject's pupils. off topic: would teachers have more catchlights?)

    Anyway, we can make her look more lively by increasing the intensity of reflected light in the subject's pupils.
    Here's how.

    0. perform all your other post processing stuff like color balance, levels ,etc ,etc
    1. Make a duplicate of the background

    2. Set the blending mode to "dodge" or "screen" (more on the modes later)

    3. Select the duplicated top-most layer and add a layer mask to it. If you have the option of transparency/fill of layer mask, select "empty" or "black", such that none of the top layer is visible.

    4. Select paintbrush tool. Check color palette, ensure painting white. You may press D for default black/white palette and press X until you see white as foreground color. Mouse paintbrush over to eyes. Press [ or ] until brush is as large as iris. If the iris is not completely spherical, using a smaller brush to paint-in is sensible.

    5. Paint-in the mask, in the areas where the pupils are visible. Instantly brighter eyes. If the brightness is too bright for natural, adjust the layer opacity until satisfied.

    6. Some notes on the blending mode of top layer: screen may change the color a little. dodge brightens up (usually) without any color change.

    ***End ***

    *** Some stuff about layer mask. ***
    Layer mask allows user to "paint-in" , using any tool in Photoshop, the amount (in terms of opacity) and part of a layer.
    Conceptually think about white as "1" (binary TRUE) and black as "0" (bin FALSE), with other shades as values in between.
    The amount of "show" is determined by ( layer x mask-value) eg if masked area is white then show is all visible. if masked area is black then show is none-visible. if grey, partially visible.

    I hope this tutorial has been useful. If there is anything incorrect or better methods do let me know, I will amend accordingly!


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