Extension Tubes
I have always been attracted by Extension Tubes and Bellows as they seem to make my whole gear look a lot more sophisticated. It also gives you a feeling that you are able to extract the best out of even simple lenses and extend their usage beyond what they are designed for. I asked my son to buy an extension tube set for my Pentax camera. After the same was delivered, he wrote to me " Father I have received your lenses but I do not find any glass inside the tubes". I chuckled to myself. Yes that is what makes the extension tubes so interesting. They seem to contain no glass and yet they increase the capabilities of your existing lenses.
Construction
Here is how a set of Extension Tubes look like. The commonplace Extension Tubes are generally sold in sets of three. They typically are of three different lengths, one with a length of 36 mm, one with a length of 20mm and the last with a length of 12mm. The reason is, it gives the Photographer a choice of combining them in multiple combinations to achieve various lengths. We will understand the reason for having extension tubes of various lengths later.
The Extension Tubes are simple metallic tubes with the inner surfaces blackened to avoid reflections. The extension tube fits between the camera and the lens. To achieve this, one end of the Extension Tube will be designed like the back of the lens and can be screwed into the body just the same way you would screw in a lens. The other side of the Extension Tube will be shaped like the bayonet of the body in which you will screw the lens. Because of this you can attach a lens to this side the same way you would attach a lens to the body.
When would I use an Extension Tube
Extension Tubes are a cheap, and some times, the only alternative for Macro Photographers. Let me explain why. Look at the diagram below (Skip this if you find this too basic):
I have shown four different scenarios here.
First, you have an "Object" (a tree here) at a far away distance from the camera, as in the case of Landscapes. It is so far away that you can assume that to be at infinity. The lens casts an "Image" of the tree (inverted) on the sensor. If this is a 50mm lens, the distance between the lens and the sensor would be 50mm.
The ratio between the Size of the Image and the Size of the object is called the magnification factor.
Magnification Factor = Image Size / Object Size
Another formula that determines magnification is
Magnification Factor = Distance to the Image / Distance to the Object
Second, now I am taking a walk and am getting closer to the tree. I look through my SLR's view finder and I am disappointed. The tree now looks bigger but at the same time it is looking fuzzier. This is because the lens is now forming a sharper image behind the sensor. On the sensor it would look like a set of bigger circles and hence will not be sharp.
How do I fix this? I cannot move the sensor. Hence, I move the lens forward by rotating the focusing ring. In some lenses this may be visible. Many of today's lenses may not move the front element of the lens but may move the inner elements to increase the distance between the sensor and the lens. Now the tree looks larger and sharper and I am happy.
I am encouraged by the results and I want to get a larger image of a part of the tree. I walk still closer to the tree and as I do that I keep rotating the lens so that the tree is in focus. Now what, I cannot rotate the lens beyond a point, due to mechanical limitations of the lens design. I have reached the "Closest Focusing Distance" of the lens. Using this lens, I cannot get an image larger than what I see on the view finder. Hence this is the "Maximum Magnification Factor" for this lens.
For me to get any closer, I need the lens to move forward. This is what an Extension Tube does. I slip in an Extension Tube between the camera and the lens, and the lens is now farther away from the Sensor than it was. I can now walk closer to the tree and photograph the Lizard perched on the tree branch. This is means that the Magnification Factor of the lens is increased without adding any glass to the lens ! That is the beauty of Extension Tube.
Of course, all solutions have their limitations. I now find that I can no longer take a picture of the tree at the distance. I can only make my lens to focus on nearby Objects. If I have to take the picture of a Landscape, I have to remove the Extension Tube and re-attach my lens to the body.
There are other limitations as well that are not as apparent.
a) The F stops marked on the lens are no more the same F stops. If you are manually metering and setting the F stop on the lens, you would find the image underexposed. Of course, this is no more a problem with SLRs that have the ability to meter the image through the lens.
b) The resolution of the lens would drop as the lens is designed to deliver the maximum resolution within the range of focusing distances for which it is designed. This could be an issue only if you are using low resolution lens on a high megapixel sensor.
A Bellows can do the same thing, with one difference. Extension Tubes have fixed length. A Bellow can increase or decrease in length. They can also do a host of other things which we will not discuss in this article.
How much of Extension Tubes to use
Ok, I understand the principle. I used the 36 mm Extension Tube for the first time with my Canon 100-400 lens at Waynad to take some close-ups. At that time, I did not seriously think of which Extension Tube to use and how much bigger my image became after using the extension tube. I did not think much as I was able to see that I could get closer to the subject for the Garden Lizard I was photographing. I was also amused with the ability to transform my 400mm Tele lens to a 400 mm Macro lens that allowed me to take close ups with a good working distance to the Lizard.
A couple of weeks back, I was taking pictures of a much smaller subject, a Hummingbird Hawk Moth flying around a Lantana plant. Taking pictures of Hummingbird Hawk Moth is a challenge as the Moth never settles down anywhere. Their flight paths are not predictable and they move very fast. Not ideal conditions for Macro Photography. I decided to use my Canon 100-400 that has a fast focusing speed and Image Stabilization and take pictures of the Hawk-Moth hand-held. I slipped in my 36mm Kenko Extension Tube to reduce the minimal focusing distance to less than 1.85 meters. I took most of the pictures at 400mm that seemed to provide a good working distance. The combo did work well. It took some time to intuitively determine the distance to which I can get closer. After some fifty clicks, I had a couple of images that looked good on the LCD. Later when I analyzed the pictures on my computer, I realized that I could have taken the pictures a bit closer. That is when this question bubbled up.
How much bigger your subjects can get by using an Extension Tube?
I looked around for answers in the web. There were many explanations. Here is one I liked.
The Maths behind Extension Tubes
If you over-simplify the construction of a lens, the lens is made of a "Tube" and a "Glass Lens". Let us focus on a very distant Object, say the moon, and turn the lens to focus the moon properly. The lens will typically be at its shortest length. This is typically equal to the focal length of the lens. At a minimum, the tube must of this length to make any image.
Now turn around and try to focus on some thing very close by, say a flower or a butterfly. You now has to turn the focusing ring to make the lens longer. Every lens has a physical limitation on the amount of length to which it can extend. At the lenses Closest Focusing Distance, the lens will be the longest. I am going to refer to the additional length by which the lens has extended out as "Internal Extension Tube" (marked as yellow ring in the image). As you can see, the length of the Internal Extension Tube determines the "Maximum Magnification Factor" of the lens. In over simplified terms, this can be expressed as:
Magnification = Length of the Tube / Focal Length of the Lens
Thus a 50mm lens that can extend outward by an additional 50mm can achieve magnification factor of 1 (or 1:1 as popularly expressed). At 1:1 magnification the Image will be equivalent to the Object in size. a Safety pin photographed at 1:1 magnification distance will occupy the entire sensor size !
You can either achieve this internally by virtue of the construction of the lens (say by buying a 1:1 Macro Lens) or by adding an Extension Tube to the Lens to make the 50mm lens to extend to an additional 50mm.
By this definition, any lens can achieve a magnification level of 1 or more by adding a tube of suitable length. A 100mm lens would need an extension tube (internal + external) of 100mm to achieve 1:1 magnification. A 400mm lens would require a 400mm Extension Tube (Internal + External) to achieve 1:1 magnification ! Bird Photographers with 800mm lenses would need an Extension Tube of 800mm to achieve 1:1 magnification.
Of course no one in their senses would use the 800mm lens (or even the 400mm lens) for a 1:1 Macro Photography. There is my mistake. I thought that a small 36mm Extension Tube can do wonders for me. It did of course help me get a bit closer, but not lot. So how close did it get me?
Here is a table with simple calculation that you can prepare for your lenses:
| Lens | Focal Length (F) | Maximum Magnification (M) | Internal Extension Tube (I = F * M) | Extension Tube (E) | Total Extension Tube (T = I + E) | Final Magnification (T / F) |
|---|
| Canon 100-400 | 400 | 0.21 | 83 | 36 | 119 | 0.30 |
|---|
| Canon 100-400 | 200 | 0.12 | 23 | 36 | 59 | 0.30 |
|---|
| Canon 100-400 | 100 | 0.07 | 7 | 36 | 43 | 0.43 |
|---|
| Canon 50 | 50 | 0.16 | 8 | 36 | 44 | 0.88 |
|---|
| Canon 18-55 | 55 | 0.37 | 20 | 36 | 56 | 1.02 |
|---|
You can construct a table for the lenses you own. The only information to be determined to construct the table is the Maximum Magnification Factor of the lens. This is generally available from the Lens Specifications on the web.
Verification
Having constructed the theoretical table, I wanted to verify the truth. I used a school graph paper to measure the length and the width captured on the sensor and confirmed that this table is correct (I had to make some allowances for the inaccuracies of focusing and angles of the lens).
Canon 100-400 without Extension Tube | 400mm |  | |
Canon 100-400 with 36mm Extension Tube | 400mm |  |  |
Canon 50 without Extension Tube | 50mm |  | |
Canon 50 with 36mm Extension Tube | 50mm |  |  |
Canon 18-55 without Extension Tube | 55mm |  | |
Canon 18-55 with 36mm Extension Tube | 55mm |  |  |
I continued to experiment with the Canon 18-55 lens to determine how close I can get with a 36mm Extension Tube. At 28mm I achieved the maximum magnification possible with this lens with the 36mm Extension Tube. I could achieve a Magnification of 1.66 and the front element of the lens was touching the juice bottle. At shorter focal lengths I need to place my subject behind the front lens which is not practical.
Conclusion
The conclusions are:
a) The Extension Tubes are more effective with normal lenses like the 50mm than with long focal lengths by virtue of the laws of optics.
b) Magnification can be increased by stacking all the tubes. By stacking all the 3 Extension Tubes, I would have a decent extension of 88mm. This would help me achieve close to 1:2 (Factor 0.5) magnification even with my 400mm lens. Need to find out how practical this is.
c) The 1:1 Macro lens would still out-perform the 100-400 with an extension tube due to the fact that the lens loses resolution as it moves farther from the focal plan than what the lens was designed for.
d) Extension Tubes are great accessories to have in the kit as it achieves magnification without introducing chromatic aberrations.
Links
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Krithik Kumar C, May 1, 2010
Well written article!
Chandrashekar S, May 3, 2010
Looks very interesting, you seem to have done good study of this and also have put it neatly as it can be, well done.
Vimal Parmar, May 3, 2010
Thanx a ton... I was actually looking for this info...!!
Sudhakar G, Feb 27, 2010
Superb info on extension tubes... TFS.
Pradeep Kumbhashi, May 25, 2010
Its one of the amazing article i have read about extension tubes. Very crisp explanation. Thank You
Gopinath Subbarao, April 16, 2011
Murali, Detailed & excellent research and simple & understandable explanation. Thanks for the great work