If you want to experiment into the macro photography world the best cost efficient option is to use extension tubes. These are exactly what their name says, an extension tube we place between the camera body and the lens. This has the effect of moving the lens further away from the sensor increasing the magnification of the lens. The larger the magnification the closest we’ll get to a true macro shot. What is a true macro and how does this work? We’ll see more about this a bit later.
But first a few words about extension tubes.
We have two types of extension tubes, passive and active. The active ones have electrical contacts that connect the lens with the camera body to allow for a full functioning of auto focus and diaphragm operation. And passive tubes are exactly that, just a tube with no contacts at all. Auto focus and aperture cannot be set with this ones. The aperture will be the maximum that the lens allows.
First lesson: be aware of the limitations of a passive extension tubes before you jump at the low cost and buy them. You’ll have to deal with extreme shallow DoF which will make shots very challenging.
I did exactly this and I wish I would have gone with the active ones. Here’s the one I got.
A photo is considered true macro when it represents the object with a 1×1 magnification ratio. In other words, the image captured in the sensor has the size of the original object. Hence the 1×1. Some manufacturers call their lenses Macro when they reach a 1×0.5 magnification ratio. These are actually close up lenses.
Back to extension tubes. How do they work? The extension tube moves the lens farther away from the sensor and allows to focus closer to the subject. By moving away from the sensor the image captured will be larger. See the diagram below which illustrates this.
To know how much magnification an extension tube has we have to do some math. Extension tubes increases lens magnification by an amount equal to the extension distance divided by the lens focal length. So, in my case if I add the each of the tubes to my 24-70 lens that has a 1×0.29 magnification ratio and I focus at 70 mm the resulting magnification will be:
7 mm / 70 mm = 0.1 + lens magnification 0.29 = 0.39
14 mm / 70 mm = 0.2 + lens magnification 0.29 = 0.49
28 mm /70 mm = 0.4 + lens magnification 0.29 = 0.69
49 mm / 70 mm = 0.7 + lens magnification 0.29 = 0.99
So, I have to use all three tubes stacked up with my 24-70 lens to get almost to a 1×1 true macro.
Here are some tests I ran using the extension tubes on my Canon 24-70 mm lens on a full frame EOS 6D body.
First the original shot without any extension tubes:
Now adding the 7mm extension:
The 14mm extension:
The 28 mm extension:
And finally all three extension tubes adding up to 49 mm:
Looking at these examples you can see how shallow is the DoF. I used a round subject on purpose to highlight this. I also took these photos with the camera on top of the table and I focused by physically moving the camera back and forth.
All I’m trying to say is using extension tubes handheld is almost impossible. And shooting anything that moves (flowers, etc) will be impossible to focus.
But if you know the limitations and are willing to compromise on what type of macro shots you can take, then extension tubes are really fun and economically accessible to try.