While I working on a new articles from the shots we have done in 2011, I’d like to re-post the full article form my guest post for LearnMyShot.com (great educational source for beginners). The material is really interesting, and I want to have it on my blog as well.
This is how we use focus stacking (focus bracketing) technique in our product photography.
We use this technique once a while, mostly for a critical product shots, where closed aperture or tilt-shift lens does not provide enough DOF and/or details. For this tutorial, I have photographed a silver bracelet, positioned at about 45 degree from a camera focusing plane and then used Photoshop CS5 to compose a final all-in-focus image.
First, the lighting setup. even though it is not related to the focus stacking technique, I think it will be cool if I explain it here.
Because the bright bracelet has black cast on some of its pieces, I wanted to make sure it will be correctly shown on the final image. I also wanted to use soft, even lighting (Tiffany-like, take a look at their silver pieces), yet to show the rough texture of the bracelet surface.
The lighting setup:
To get desired effect I’ve used two foam boards to hide the bracelet from direct light hit from a strobes: First board, white matte reflector is on the left (it is white on the other side) and the second one (to the right) is 50% gray.
First light, WL X-1600 (1) through 20 degree honeycomb grid was creating a spot on the right side reflector and on the table right in front of the bracelet.
Positioned at given angle, some parts of the bracelet was reflecting that light spot from the board, while the rest were reflecting gray board. Also, light from that spot on a grey board was reflected by the opposite white reflector, producing even and smooth lighting on our object.
Second light, AB-800 (2) through strip box was highlight the background, adding a soft reflected lights from that side.
None of the light sources were hitting bracelet directly.
Now, the focus stacking technique:
The idea is similar to a HDR photography, but instead of exposure bracketing, we do a focus shift: for each shot we move camera to cover another piece of the object, merging images in one during the post-production.
The correct way to change a focusing point is to change a distance between the camera and our subject by moving the camera: lens focusing point should not be changed (later I’ll tell you why). So, some sort of macro focusing rails (I use Manfrotto 454 Micrometric Positioning Sliding Plate) helps a lot; without it it will be problematic to move the camera in that direction. Obviously, we shoot completely manual: manual exposure, manual focus.
Why not to simply re-focus the lens? Re-focusing lens will work, but there is some drawbacks of this method, below are my thoughts:
When we re-focus the lens it will slightly change a perspective and bokeh will be changed as well, while moving the camera does not change anything except the focusing point. Think about this: when lens focus gets adjusted, some of the internal elements of the lens being moved relatively to each other and aperture blades, right? This inevitably (depending from a particular lens design) change bokeh and perspective, as the distance between the lens front element and a focusing point will be changed (meaning we’ll change a focusing distance).
Instead, by moving the whole thing (camera and lens) we guaranteed moving only focusing plane through our subject, while the focusing distance remains unchanged.
Some of you may think that refocusing the lens does not change the size of the image, but this is not true.
Re-focusing the lens will change the image size as well. Try it yourself on some macro object and you’ll see how image gets enlarged when you will re-focus from the closest to farthest point on the subject. If we re-focus lens we move internal elements in it: it works the similar way as we’ll move the whole lens, meaning object size will change.
So, yes, when we move the camera, image size gets changed. But the focus stacking software handles this very well. When I need to have my subject as close as possible, I often start shooting focusing from a closest (to a camera) part of the subject, moving to a farthest. Inevitably object gets enlarged so at the end of the sequence the front (closest, out of focus) part of it does not fit frame anymore. Photoshop ( I believe any other stacking program) does handle this very well, stacking the whole thing correctly: It simply use only focused part of the image, stitching only areas that are in a focus. So, the whole sequence of “usable” areas will have exactly the same proportions, as the distance between lens and focusing plane (focusing distance) will be the same, right?
This means the size of the focused part of the object will be always the same, and software won’t have any trouble stitching it together.
I’ve used F11 aperture for the shoot: this is right in the middle of “sweet spot” for the lens I’ve used, Canon 180mm F3.5L macro. Here is why:
As we all know, DOF gets increased when we closing aperture (larger number on F stop value). However, every lens loses contrast and amount of visible details (linear resolution) when aperture gets closed: diffraction starts playing a big role when light travels through a pinhole lens…
Each lens has it’s own “usable” range (you have to “master” your lens to found it), but even most expensive ones does not do a good job at it’s maximum F-stop number. You can see examples of how image gets changed when we go from F8 to F16, than F22 and f32 on my another article, where I’ve compared Hasselbald H4D-50 and Canon 1Ds MkIII.
I did total of 12 shots for the bracelet: by doing a little bit more then needed, overlapping focused area on each shot, we got that extra amount details on the final, merged image. Photoshop does great job of pulling all the usable information from each shot and combining it on one final super-image:-)
Those extra shots does not cost me much time, but the end result… that amount of details you can’t get even with 50+ megapixel medium format camera image done in one shoot.
100% crop of the left end of the bracelet:
100% crop of the right end:
100% crop of the center:
The rest is on the video:
Lighting, light modifiers and accessories:
Exposure specification: shutter speed 1/250 sec, F18, ISO 100
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