How can I use Photoshop to blend 2 photos together based on brightness?

Question

Basically, I have 2 exposures of the same seen: 1 dark, 1 normal.

I'd like all to replace all parts of the normal photo that are above a certain brightness with the same parts from the darker.

Right now I manually feather-select these areas with the marquee tool and just delete them (the darker image is under the normal image).

Does this sound doable? How can it be done?

Answer 1

Yes it's very doable. I do that pretty often to create fake HDR from the same photo at two different exposures. There are many ways to achieve this, it depends on how proficient you are with the selection tools and layer masks. I'm confident you can do it.

First, put your two photos in the same file, on different layers and create a layer mask in the top one. Without the layer mask, the top layer would completely hide the bottom layer. The bottom layer will be made visible "through" the mask by painting in the mask itself. To simplify a bit: anything that is black/grayscale in the mask acts as a "hole" through which you can see the bottom layer(s). If you are not familiar with masks, start with a black or white brush and experiment to get a feeling for it, it's fun. Now if your darkest photo is in the top layer, paint in its layer mask to bring back the corresponding regions of the lighter, bottom layer. Congratulations, you now have a composite of both layers.

Now there are two points to keep in mind, in my opinion:

1) How do you select the dark and bright regions to create your mask? I would recommend you go to "Select -> Color Range" and either: a) use the "Sampled Colors" mode and Fuzziness slider then just click (and shift+click) in the dark or bright regions of your photo to create/expand a selection, or b) use the "Highlights" or "Shadows" mode to directly select bright/dark regions (to some extent). Once you click OK, select the layer mask and use "Edit -> Fill" to fill the current selection with black (invert the selection before if needed using "Select -> Inverse").

2) How do you handle the transitions between the darker regions of your top layer and the brighter regions of your bottom layer. This is the tricky part. My way to do it is to apply a "Filter -> Blur -> Gaussian Blur" on the mask itself. Since black is fully "see-through" in the mask and white is opaque, a Gaussian blur will create a smooth grayscale transition in between these 2 areas, thus blending both layers somehow. Make sure to enable the filter's "Preview Mode" so that you don't overdo it and create unsightly halos. The layer mask is basically a grayscale picture, so most of the paint tools or filters you are familiar with will work there. For example, I use selection tools to Gaussian blur some part of the mask more than others (say, trees vs. sharp transitions).

Answer 2

What you can use is layer mask. Basically, you create a copy of the layer, use Threshold tool (or curves or whatever you want) to make the copy white when you want top level to be visible and black when you want iot to be transparent, and then use that as mask for the top layer.

Answer 3

David, you have opened a glorious can of worms. The technique you are describing is old school, can be done and sometimes is the best way to do it.

However, PS has some pretty cool features under automate. Go to FILE -> AUTOMATE -> MERGE TO HDR. You'll select the two images and let photoshop run. This is one way to do it.

You can also use software like Photomatix Pro of PTGui to blend your images. PTGui is geared more around creating panos, but I think you can use it to the same effect.

Back in PS, come to think of it, I wouldn't use the marque and delete technique because it is destructive. Rather, I would stack my layers, then use a LAYER MASK to paint out the sections or note. That you you can fine tune your selections (still using marque if you want, though I prefer brush as you have more control) without ditching portions of your image.

Good luck.