Hi! I started using the adjustment brush for the auto mask feature to speed up lightening backgrounds in product images to pure white without overexposing the subject since every once in a while, I expose the subject and background poorly. This question isn't about my photography technique, which manual camera settings I use, or what setup I have. It's just about the automask feature.
I know the adjustment brush is supposed to sample the tone at the cross hair and only mask out that tone and I guess similar ones. For masking in general, I came across this thread suggesting that to get the cleanest result, you're better off brushing over the edge and using the eraser to refine.
I was confused by it originally because I assumed the eraser used the same algorithm as the brush "in reverse" so I didn't understand why subtracting would do a better job of deciphering edges than adding, but after experimenting with images, I found it to be true (for me at least). Back to using auto mask on backgrounds, I was looking for quick tips and came across this video:
I didn't realize the brush is non-contiguous, so this was a great tip to find. I whipped out an image of a necklace on white to see if I could use this technique to speed up the process of selecting just the background and here's where the problem came in that I think relates back to the discussion about the brush versus eraser. Unfortunately, no, I can't post the image.
I set up the adjustment brush as demonstrated so it was large enough to cover my entire image and clicked once on the background (auto mask on). For some reason, it selected the entire image with the necklace at nearly 100% opacity instead of staying isolated to the tones that closely matched the point selected. The background is pretty evenly lit and even though much of the necklace is light in tone, it's a different darker color from the background so there's a lot of contrast between them. I didn't expose it so closely that the brush's auto mask should mistake nearly every tone in the necklace for being the same tonal range as the background.
I had a flashback to the earlier discussion about the eraser doing a cleaner job with auto masking, so I reset and combined the techniques. I selected the entire image with the brush (auto mask off), then switched to an eraser large enough to cover the entire image. I clicked once on the background in the same area as before (auto mask on) and lo and behold, it cleanly removed the background from the selection leaving only the necklace masked. The only overlap was where expected, a handful of highlights on the necklace that do run somewhat close to the background.
I know that's the opposite of what I'm doing since it's the background I plan to mask for adjustment and not the necklace, but the point is that I want to know what the difference is between the adjustment brush (add) and the eraser (subtract) because it almost seems like the brush inherently has a much higher "tolerance" or something so it includes a broader tonal range in its auto mask than the eraser does, making its auto mask results dirtier and less accurate.
I don't see any kind of setting for that and I don't know if that's true, but I feel like that would explain what I'm seeing and was just able to replicate with a different image. The eraser just works better. I don't know if that's the case for everyone, though. Anyone have any idea what could be going on?