Here, in Astaroth's Photography Blog, we could watch a qualitative comparative about the use of several different extension tubes. I have a pack of three of different sizes: 12mm, 20mm and 36 mm. You can use them separately or add several of them to the others. You could obtain in this way some configurations: 32mm (12mm + 20mm), or others as the most interesting, 68mm (12mm + 20mm + 36mm).
When I made the first comparative, I wrote about the problem of the lack of light and the vibrations. Both problems are solved with a additional source of light, as a ring flash.
Now I want to make a quantitative comparative, calculating the amplification factor. How We can do it? You have to shoot with the different combinations to a rule. We want to compare the different configurations in the maximum amplification, i.e., we want put the focus in the nearest distance as be possible to the camera.
We are going to test the next configurations:
- Only the lens (extensions tubes = 0mm).
- Lens + separate extension tubes (extensions tubes = 12mm, 20mm and 36mm).
- Lens + larger combined of extension tubes (extension = 68mm).
With above pictures, we can build the next table:
|It's a image. Please, click for enlarge.|
For this test, I've used always the same lens (Tamron 90mm Macro f/2.8 SP Di VC USD) and the same camera (Nikon D7000). In the website of Nikon you can find information about the size of the sensor of the Nikon D7000 (23.6mm x 15.6mm). With this data, and the size of the real object (you can measure it in each above picture) you can calculate the amplification factor.
For example, in the first case (without extension tubes), the horizontal size of the real object is approximately of 23mm (from 3.3cm to 5.6cm). The size of the sensor is 23.6 mm, so we are watching an object of 23mm projected in a surface of 23.6mm. 23.6/23 ~ 1.03 ~ 1, so the ratio in this case is 1:1. We've tested that the maximum ratio of amplification of the Tamron 90mm Macro f/2.8 SP Di VC USD is 1:1.
A more interesting question for me is: what is the maximum amplification that I could get with my gear? It's possible with the lens focused in the nearest plane and using all my extension tubes combined (68mm). In this case we are watching a 12mm object projected in a 23.6mm surface. 23.6/12 ~ 1.97 ~ 2: We are watching the object twice larger in the sensor (2:1).
It's also interesting measure distances in the focused plane. If the focus is in the nearest plane, we can compare the previous pictures of the rule with others pictures taken in similar conditions.
For example, in the next image we can view two images mixed. Both of them were taken in the same conditions (with the same gear and focusing as near as possible). The sharp parts are approximately in the same plane, at the same distance of the camera. The blurred parts are nearer or farther, so the rule is not useful for these parts. In the focused area, the rule marks the distance, so we can take a measure with it.
The diameter of the bigger eyes of this jumping spider is about 0.5mm.