Thursday, June 25, 2015

Using PhotoShop Stack Mode to minimize high ISO graininess

Meet the Butterflyweed (Asclepias tuberosa), a species of milkweed native to eastern North America.  This seems to be a particularly good year for them.  I see prominent patches of their orange blooms off the side of the road as I drive around lately.

I took a series of focus-stack shots in the evening yesterday and ended up using 19 of them to make this picture.  Lately, I've been using the Auto ISO setting on my camera, so because of the time of day and the f/16 stop I was using, the Auto ISO ended up going all the way to 3200.

The first step in my process is to align all of the shots to each other.  Although I was using a tripod, a slight breeze was causing noticeable movement.  It wasn't so bad that the alignment process couldn't deal with it.

A bigger problem arose when I tried to do the Auto-blend step in PhotoShop to create the focus stack.  I had a very pesky photo bomber waltzing through my picture.  That's her, below, dancing from flower to flower.

Normally, like in the case of a fly, I'll just eliminate the shots that have the photo bomber.  In this case, however, the bomber was in quite a few of them.

I decided to invoke a technique that I have rarely used.  After I restacked the images, I used the PhotoShop command Layer --> Smart Objects --> Stack Mode --> Median, which analyzed the picture and removed anything that wasn't in all of the shots, i.e., the ant.

Try as I might, however, I couldn't find a way to Auto-blend the layers without bringing back the photo bomber.  As luck would have it, though, the f/16 aperture I had used already gave me a fairly decent depth of field.  While researching the other Stack Mode commands in the hope of finding something that would help me, I discovered that the Mean command was particularly good for removing graininess from high-ISO stacked images.  And it did it quite well.

If you look closely at the image with the photo bomber and compare it to the first image, you'll see the reduction in graininess that I'm talking about.  

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