If you've made it this far, then there's a chance that you're curious, or maybe interested, so I'll continue.
1.2 My equipment
- Canon T3i 18 MP camera with APS-C sensor 22.2 x 14.8 millimeters. I got it used in late 2013 after it was highly recommended for what and how I shoot. Its vari-angle display makes using live-view easy for framing and focusing so I rarely use the viewfinder.
- Canon 100mm f2.8 non-L version macro lens.
- Sigma 18-250mm f3.5-6.3 DC zoom lens for general purpose and some close-up work.
- Photoshop CS5 to do the stacking and any post processing as I already had it.
- An extreme lens: Canon 65mm f2.8 MP-E macro lens (1-5X life size) - I got mine used so it was more affordable. This isn't a normal macro lens but once you get used to its operation and capabilities, it really can open up whole new worlds. Section 4 is dedicated to it - Its table and usage are in sections 4.2 and 4.3.
- Cognisys Stackshot motorized focusing rail. This is the 'icing on the cake' as it does the very small steps required for the MP-E and eliminates ALL of the frustrations involved with a manual device.
Lens usage: I use the 100mm macro for 95% of my shooting, the MP-E for 4% and the Sigma for the other 1%.
2. DoF tables for focus stacking
In its simplest form, the depth-of-field for any shot Is a function of focal length, aperture, subject distance and sensor size. These tables are meant for a Canon APS-C sensor camera using the same focal length lenses. There's a description of how easy it is to make your own table for other lenses and/or cameras in section 3.
I took the time to create these tables and now all I have to do is:
- Focus on the closest required element.
- Measure the distance to the camera's focal plane - it can be approximate.
- Find that distance in the table for the step size.
- Repeat the 'move and shoot' until it's done
The stacking process works best with some overlap between images so the third line, marked "Step", is the one to use as it's two-thirds of the full depth-of-field.
2.1 Canon 100mm macro
Its closest focusing distance is 12 inches where it produces a life-size image. Most things aren't that small, so I opted for 2 inch increments up to 24 inches and then 3 inches up to 48 inches - most of my shooting is in the 12-24 inch range. I did many tests when I got this lens and for me, f11 gives the best results after stacking but do your own tests and if need be make a table for your own shooting style.
2.2 Sigma 18-250 zoom
Because of my studio's layout I have to be less than five feet from the subject. If it's too big to shoot with my 100mm macro, then I use this set at 35mm, 50mm or 80mm to get closer. A prime lens also works as it's the focal length, not the construction, that determines DoF.
Its closest focusing distance is 13.8 inches where it produces a one-third life size image. I used a 4 inch increment over a distance range of 12-32 inches. I don't use it often so didn't test it as thoroughly as the other lenses and I also use it at f11.
2.3 A butterfly example using the 100mm and table
Note: Butterfly's scales don't all have clearly defined edges so there will be areas that aren't 'tack sharp' - blame mother nature, not me.
- I turned the Blue Clipper butterfly so its wings were at about 45 degrees to the camera to ensure there was a significant depth to shoot. The width of the whole butterfly is about three and a half inches
- I setup the shot and focused on the closest element, the bottom of the right wing by the body - excuse the clamp that holds it in place.
- I measured the distance from there to the camera's focal plane using a yard stick marked in inches, and it was about 17 which isn't in the table. At that distance, the lens provides just over a one-third life size image.
- The table shows the step size at 16 inches is 3.52mm and 4.69mm at 18 inches, a difference of 1.17mm. I find focus isn't linear so I add one-third of the difference or 0.40mm, not one-half, to the smaller value to get 3.92 mm and use it - that's only about one-sixth of an inch.
The closest in the series showing the right wing's scales by the body.
The furthest in the series showing the antenna on the left.