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The Bug Geek
Insects. Doing Science. Other awesome, geeky stuff.
Menacing crab spider with prey, focus-stacked
Posted by on October 1, 2011
I was very impressed with this spider, but then I really should stop being surprised by the ability of crab spiders to ambush and subdue large and potentially dangerous prey; I’ve encountered many flower crab spiders that have taken down bees or wasps twice their size.
This spider seemed particularly menacing for some reason. Legs spread wide, motionless, chelicerae buried deep, completely unbothered by the invasive camera lens – its prey utterly helpless.
Crab spider (possibly a ground grab spider, Xysticus sp.) with Camponotus prey
This photo represents my first attempt at what’s called a “focus stack”. It’s a post-processing technique where two or more photographs are essentially overlain in order to obtain a greater depth of focus. In this case, I had one photo where the spider’s face was in focus, and another where the ant was in focus (well, more or less). I stacked the two to get both in focus in this final image. Some people have perfected the art (think Thomas Shahan, from whom I learned about this technique at BugShot) and can manually stack five, six, seven or more frames to get the perfect photo with piles of depth, even when the magnification is really high. Here’s another great example from one of the others students who attended BugShot.
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Man, that’s a really great first attempt at focus-stacking. I’ve not tried it myself yet – what program did you use?
Photoshop, I did it manually! The two shots were hand-held and note quite the same angle, and I was feeling too lazy to research software, so I did it myself. It took a bit of tinkering, and a careful eye would notice some issues, but it’s pretty good at this size…
Cool. Is this ‘File, Scripts, Load files into Stack’ in Photoshop? I may have to try it – although I don’t think I could hold my camera steady enough to get two shots that differed only in focal point.
CombineZP (http://en.wikipedia.org/wiki/CombineZP) is a great open source program for stacking. The images have to be exactly the same pixel size, so don’t do any cropping until after you have the stack. Also, you can get some distortion near the margins and if there is a databar on the images the program flips. Otherwise, though, it is pretty good.
Nope, this is “load two images as layers, make one semi-transparent, rotate one manually to compensate for slightly different hand-held vantage point, frig around with position until they look pretty good, manually erase unwanted components from both layers, merge layers to create one image…realize you’ve got a big blank spot that you missed, discover you can’t “undo” the merge, swear lots, repeat entire process (only this time a little more carefully), merge layers, soften a few iffy-looking bits with Blur and Clone tools.” Simple, no?
(I should have called this comment “There’s a Stack tool in Photoshop? lol <–newb)
Ha – that’s what I used to do, more or less, with SEMs. It always pays to keep a backup with the layers separate and only merge them for some finished product.
But heck, CombineZP is so easy that I’ve given up on the John Henry method. Haven’t tried the stack tool in Photoshop yet, so it is only there in theory.
I’ll have to check it out, thanks a lot for the tip, Dave!
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