Macro and Depth of Field

Tourmaline in quartz schist, The tourmaline is about ½ inch long.

Getting really close to a subject is not only more interesting it can be fun and frustrating.  I have used extension tubes attached between the camera body and the lens, and close-up lenses attached to the front of a zoom lens.  The results weren’t too bad but I noticed the depth of field (DOF) was noticeably shallow. Extension tubes worked best on something where the depth of the object wasn’t that deep.  I thought that maybe if I had a macro lens I could get different results!  After all there are plenty of images out there to prove it even though some were focus stacked.

After considerable research I decided to purchase a used macro lens from mpb.com.  The price of the used lens was half of a new lens supposedly in excellent condition so I opted for the Tamron 90 mm macro.  If this didn’t work out the loss wouldn’t hurt my pocket too much. It turned out that this was the best decision I made.  The lens performs beyond my expectations!

Of course I immediately started shooting everything from bolts and nuts to flowers, insects, and my rock collection.  I did notice that the depth of field changed as the distance from the camera to the subject increased or decreased.  When the distance was at its smallest the DOF was the shallowest. The other thing I noticed was the DOF would change when the aperture changed. By how much did it change I wondered, so I did a little experiment.

In my light cube, I placed a ruler vertically on a slant at an angle of about 20 degrees where the far end of the ruler laid towards the back of the light box,  then marked off the focus point with a piece of tape.  I then took four images at aperture settings of f/4.5, f8, f18, and f36 with the lens at about 14 inches from the mark.  I didn’t use a tripod but the camera was hand held with my arms braced at the edge of the platform.

 

                f/4.5 1/8" DOF                                                                          f/36 2" DOF

After processing the images I filled a table with my observations where the endpoints were in focus just before it went soft.

I then plotted the four points in Excel and used a two degree polynomial to connect the points.  Excel gave me the formula y = 0.0014x² + 0.0009x + 0.1155 which comes pretty close to my actual data.

 The conclusion is the smaller the aperture opening the DOF is wider.  However, this comes with some caveats.  Depending on lighting conditions, if the light is lower than full sunlight, the camera may not be able to reach the smaller apertures if your subject requires it.  Of course with the smaller aperture the shutter speed decreases.  If the shutter speed is too slow, use a tripod and/or increase the ISO.  Too large of an ISO increases sensor noise.

The bottom line is the macro lens is the less painful way to go for macro photography.  The challenge begins when you go from stationary objects to moving objects!

Carpenter Bee