Photographing Insects: Extension Tubes
How extension tubes turn a normal lens into a macro lens
Extension tubes let you get much closer to your insects, resulting in more detailed shots. Here I report my first experiences using them on my Canon R7 with the RF-S 18–150mm lens. I also give some precise analytics.
An extended version of this article has appeared in my free book My Journey into Insect Photography that you can download here.
I planned to buy a true macro lens. A macro lens should have a magnification of at least 1, while my current lens gets to 0.47 at 50mm (which is still 0.75 on my APS-C camera). But then I heard about extension tubes as a cheap alternative. So I decided to try that route.
What are extension tubes?
Extension tubes are rings that you place between the lens and the camera body. They move the lens further away from the sensor. This changes the focal length of the lens. You can get closer to your subjects and the magnification increases. A downside is that you lose some light, so better use a flash. Extension tubes do not have any glass. They are just hollow. So they do not affect image quality.
There are extension tubes with different lengths. I bought the MK-RF-AF1 tubes from Meike. You get two tubes for only 45 euro (from Amazon). One has a length of 13 mm and the other 18 mm. You can stack them to get an extension of 31 mm.
Note that these are tubes with electronic contacts. So the camera can still communicate with the lens. This is crucial for auto focus and for changing the aperture. You also have cheaper ones without electronic contacts. Avoid these, unless you use a fully manual lens!
There is one big downside to using extension tubes. With normal macro lenses can focus up to infinity. When you use extension tubes you loose that possibility. Your maximum focus distance is severely reduced. So you can only use them if you want to do macro and nothing else. It also makes it a bit harder to find the insect om the viewfinder, as you cannot focus while you are still far away.
First experiences
The extension tubes are pretty tiny. Hard to imagine they have such a big impact. I put both on my camera and put the 18–150mm lens on them. That all works very simple. Then I started to approach some insects. You immediately realize that you can get much closer. Actually, I used to shoot insects at 60mm but that did not work anymore. The insects almost had to get into my lens and the flash light no longer reached them. So I had to move up to 70mm or more.
The details become considerably better. I could now photograph small ants and little spiders. I started to look for tiny insects that up to now I had skipped because they were too small. With the extension tubes you can make the insects at least twice as large (in width and height; four times if you look at the area). That made a big difference.
Of course this also had a downside. Focusing becomes considerably harder. The field of view is smaller and a slight movement of the camera destroys focus. Auto focus did help here and still worked without a problem. And with some practice manual focus also worked. This was definitely more of a challenge, but also way more fun. And the resulting images were clearly more impressive (in my personal, unbiased opinion :).
But I had to understand this all better. So I did a number of controlled experiments with the different tubes, focal lengths, and so on. The results were pretty amazing. Let us first look at an example (details follow at the end).
An example
The following image shows the effect of using the extension tubes on my RF-S 18–150mm lens. The top four images show the effect when the lens is at 70 mm and the bottom images when the lens is at 100 mm. From left to right you see the largest magnification possible without extension, with the 13mm extension, the 18mm extension, and both, so 31mm. The head of the man, including helmet and neck, is only 14 mm high.
The top-right image shows an area of about 27x18 mm, so that is a magnification factor of close to 1.4 (on APS-C)!
Minimal focus distance
The minimal focus distance at which such magnifications are achieved is rather small. Officially the distance is measure from the sensor. But what matters most is the distance from the front of the lens, as this is the point closest to the insect. Because the 18–150mm lens is rather long and extends when the focal length increases, the difference is significant. And remember, the extension tubes make the lens longer.
For the top row with a 70 mm lens, the minimal focus distance from the lens is 13 cm, 7 cm, 5 cm, and 3 cm, going from left to right. 3 cm is still doable but only for certain insects. Also, you have to be careful to avoid that the lens casts a shadow. For the bottom row with a 100 mm lens, these numbers are 23 cm, 13 cm, 11 cm, and 7 cm, which is more comfortable. For example, 70 mm with the 18mm extension gives almost the same image as 100 mm with the 31mm extension. But in the first case you have 5 cm distance and in the second case 7 cm. That is a noticeable difference.
Going below 70 mm can give a slightly better magnification, but the distance from the lens becomes impossibly small (smaller than 2 cm). So that does not make any sense. Going above 100 mm can help to achieve slightly larger minimal focus distance, and a smaller magnification. For butterflies I use 150 mm, otherwise the magnification is too large.
Magnification
So how much does the magnification factor increase when using extension tubes? Theoretically you can calculate this as follows. Let L be the length of the extension tube, F the focal length of the lens, and M the maximum magnification factor of the lens itself at that focal length. Then the new magnification factor becomes M + L/F.
An example will help. If the magnification of the lens is 0.35, the extension is 31 mm and the focal length is 70 mm, the new magnification become 0.35 + 31/70, which is 0.79. Note that this is for a full-frame camera. For an APS-C camera we must multiple the result with 1.6, so it becomes 1.26. Our measured magnification was actually better: 1.38, which is really nice for macro. For 100 mm, the maximum magnification become 1.09. With a focus distance of 7 cm from the lens this works great.
Details
For those of you who want to know all the details, here follow two tables. In the first table, for each focal length, I indicate the measured magnification factor (taking the APS-C sensor into account) achieved by the extension tubes.
The second table shows some additional detail when using both tubes (I always use them stacked), so 31mm. In the left part of the table you see the effect at the minimal focus distance. The numbers indicate the width in cm of the smallest area you can photograph at that distance, the distance from the lens where this is achieved, and the corresponding magnification factor. At the right you see the effect for the maximal distance at which you can still focus. Again, the width of the area, distance from lens, and magnification factor are given.
As you can see, you can cover quite a large range of magnification factors (from 0.35 to 1.38) and a large range of focus distances (from 3 to 68 cm). So that gives considerable flexibility.
Conclusions
Using extension tubes is great when you are photographing insects. I had no problems at all to use them. Of course focusing become harder because the depth of field gets smaller. But that would also be the case with macro lenses. Auto focus still worked without problems and the image quality seems great. Remember though that your focus range is severely restricted when using extension tubes.
From now on, when I am shooting insects, I will always use both of the extension tubes. It gives great magnification and, with the zoom, enough flexibility, as the last image shows.
Next: Composition
Previous: Some Theory
Mark Overmars is a dedicated insect photographer that loves to share and regularly publishes about his work and about photography in general. You can visit his website at www.insectphotography.org. Download his free insect photography book at www.insectphotography.org/book.
