Photographing bright and dim objects at once?

[Rei]

To the bytephoto members, if I may borrow your expertise:

I'm having some trouble photographing clouds drifting over the moon in the evening with my DSC-H2. If I do a short exposure, I can see all of the craters and mares on the moon, but the clouds don't come out. If I do a long exposure, I can see the clouds fine, but the moon is a bright white ball that bleeds into the surrounding clouds.

Is there a way to get both the dim clouds and the bright moon to come out right?

[famjad21]

Here is a way using photoshop (not elements).

Use a tripod. Take the long exposure first then the short exposure ensuring that the camera hasn't moved.

Open copies of both images and move the cloud image onto the lunar image as a layer.
Add a mask to the second layer and paint black onto the mask where the moon was and the properly exposed lunar image will come through. Combine the layers and save as another name.

If this is confusing read up on layers and masks in photoshop. They are a very powerful tools.

Frank

[Rei]

I've tried something similar (GIMP not Photoshop, but same difference), but you get too stark boundaries that way.

If there's no way to do it with the optics, I may try a more sophisticated merge technique -- something along the lines of adjusting the brightness levels logarithmically on each image until they're close matches for each other. One will be overexposed on the bright parts and the other underexposed on the dim parts. I could then threshold alpha values by brightness on the overexposed one and darkness on the underexposed one, and merge them.

I think that would work, but I think it'd be a whole lot better if there was a way to do it with the optics.

[famjad21]

The trick with PS or any other editor that uses masks is to blur the mask at the edges so you don't get the stark transition.

Frank

[Rei]

Yeah, I did that. The problem is that it still looks odd because you're effectively dimming the brightest part of the image, but not the "second brightest part". Even if you blend in the seam, it doesn't look right. I think, to get a good blend in software, you'd have to do it the way I mentioned above. Haven't tried it yet, but it seems like it'd be the only realistic solution if there's no way to do it in hardware.

[famjad21]

Good luck and let me know how you make out.