Stacking Telephoto Converter Lenses

We all know that magnification increases if we stack two or more close-up lenses together. The same holds true if we stack two teleconverter lenses. In general, if one converter has power A and the other has power B, stacking them together would yield a teleconverter of power A×B. For example, the Sony VCL HGD1758 and Olympus TCON-14B have powers 1.7X and 1.45X, respectively. If we can find some way to stack them together, the resulting lens will have a power of 2.465X = 1.7×1.45.

But, what is "stacking lenses" anyway. Actually, it is very simple. We just connect a number of teleconverters together. Because the combo has a higher power, it extends the focal length of the camera lens. The following shows an extreme example: a Sonly VCL HGD1758, an Olympus TCON-14B and a Nikon TC-E15ED 1.5X teleconverters are connected together, forming a "new" teleconverter with a power of about 3.7X = 1.7×1.45×1.5. This combo is mounted on a Nikon Coolpix 5700, and extends the longest focal length of the 5700 from 280mm to 1035mm!

However, stacking teleconverter lenses does have some drawbacks.

1. Since the use of a converter lens reduces the image quality of the resulting image, the lenses to be stacked must be of very good quality. Stacking two average lenses will get you poor to very poor results. The reason is very simple. The converter mounted on the camera lens magnifies all sorts of defects coming from the front converter, and delivers a not-so-good image, coupled with its own defects of course, to the camera lens. Consequently, the defects of the first and second teleconverter lenses will be magnified twice and once, respectively. In this way, how could we expect an image of good quality?
2. Not every two randomly selected teleconverter lenses can be stacked. Let us don't get into too much technical details. The first converter projects an image to the second converter; the second converter takes this image and magnifies and projects the result to the camera lens; and the camera lens takes this image and projects it onto the sensor. If the projected image from the second converter is not large enough to cover the angle of view of the camera lens, vignetting occurs. The same applies to the image projected to the second converter by the first one. Therefore, making sure that the second converter can see completely the image projected by the first converter and the camera lens can see a complete image projected by the second converter is the key of avoiding vignetting. Unfortunately, no lens maker publishes technical data telling us the entrance and exit pupils that are important for us to avoid vignetting. As a result, we can only do experiments. In general, mount the converter lens with a higher power on the camera lens and stack the less powerful lens on the higher power converter. In this way, we are sure the second higher power converter will see through the image projected by the first lower power converter properly. However, the camera may have to be zoomed all the way in to see the final image.
3. But, the key question is whether two converter lenses can be stacked together. Certainly, one of them must have a front thread. If the higher power one has a front thread, we are lucky. If only the lower power one has a front thread, we may see some vignetting, although we must test to determine if vignetting is really a problem. Most people can live with some vignetting and crop the image. But, no one would like to see a significant vignetting that only shows an image circle!
4. All front mounting teleconverter lenses will increase the minimum focusing distance. As a result, stacking teleconverter lenses will usually increase the minimum focusing distance drastically. It may be worthwhile if you wish to shoot distant subjects. On the other hand, if the subjects are close to the shooting position, the use of the camera lens or only one teleconverter lens may be more promising. Click here for more details.
5. Good teleconverter lenses are usually heavy, and stacking two together would be even heavier. Furthermore, the image stabilization feature may not function well because the combined focal length could be too long for the OIS system to work properly. A tripod is definitely required. However, the FZ-10 has a tripod hole not on the line of view and mounting heavy converter lenses on a camera may tilt the camera downward and may also damage the lens barrel and tripod hole. As a result, a lens support may be needed.

Given the above reasons, unless you can live with a lower image quality, can find two good teleconverter lenses and mounting mechanism that will not cause vignetting, and do not mind the weight and size, you perhaps would not go into this stacking teleconverter lenses direction.

Teleconverter Lenses Used

To illustrate the above points, two very good teleconverter lenses are used: the Sony VCL HGD1758 and Olympus TCON-14B. Click here for more information about these two lenses. The Sony VCL HGD1758 does not have a front thread, and the TCON-14B has a 86mm front thread. This prevents us from putting the TCON-14B in front of the higher power Sony VCL HGD1758. I managed to find a 86-77mm step-down ring. Therefore, a 86-77mm step-down ring, a 77-72mm step-down ring and a 72-58mm step-down ring are used to mount the Sony VCL HGD1758 in front of the TCON-14B. Of course, the use of three step rings is not very promising because they push the Sony VCL HGD1758 away from the Olympus TCON-14B. However, this is the only choice I have. The left image below shows the huge combo, and the combined power is 2.465X = 1.7×1.45. Since the Minolta ACT-100 1.5X also has a 86mm front thread, it can be used with the Sony VCL HGD1758 as well. The right image below shows this combo with a combined power of 2.55X = 1.7×1.5. Since TCON-14B has a better center resolution than the Minolta ACT-100, I chose the TCON-14B for this test.

Sony VCL HGD1758 and Olympus TCON-14B	Sony VCL HGD1758 and Minolta ACT-100 1.5X

With this combo mounted on a FZ-10, since its power is 2.465X, the longest combined focal length of a FZ-10 is 1035.3mm = 2.465×420!

Lens Support

With this combo mounted on a FZ-10, it is huge and longer and heavier than the Panasonic DMW-LTZ10 teleconverter lens. The left image below shows this combo mounted on a FZ-10 with the help of a Chen adapter. Since this is a huge and heavy combo and since the tripod hole of the FZ-10 is not on the line of view of the lens, a lens support is needed so that the combo and the camera can be settled on a tripod in a balanced way. The right image below shows the combo mounted on a Bogen/Manfrotto 3420 Telephoto Lens Support. This is the only lens support of good quality I can find. But, this Bogen lens support has a minor problem. Since FZ-10's tripod hold is not on the line of view, when the combo is mounted on this Bogen lens support the camera and the tripod mounting bar are not aligned in parallel. In other words, the line of view of the lens and the tripod mounting bar make an angle. This is usually not a problem; one just needs some effort to get used to it.

Sample Shots

To see how good the resolution of this combo is, I framed a tree trunk at the center and took a shot. This is shown in the left image below. In order to see the sharpness near the edge area, another shot was taken by first focusing on the tree trunk and then recomposing so that the trunk is at the right edge. This is shown in the right image below. The images used ISO 50 and LOW for CONTRAST, SATURATION and SHARPNESS. Thus, comparing the images with the trunk at the center and at the right edge would give us a sense of sharpness difference. It is not difficult to see that the very center portion is sharp. Outside of this 1/9 area of the image, the result seems soft and image quality deteriorating quickly toward the border. The right edge portion may show some sharpness; however, the image quality is getting much worse near the right edge.

Center Portion (F5.6 and 1/125 sec)	Right Edge (F5.6 and 1/160 sec)
Click on the image to see a full size version

The center area of the left image above, as marked by the red frame, is cropped and shown in the left image below. Similarly, the right image below shows approximately the same area (of the tree trunk) of the right image above. Now, it is very clear that the center sharpness and edge sharpness of the combo have a huge difference, because the two 100% crops are basically the same area. Some chromatic aberration appears in the form of purple fringes. If the scene contains high contrast areas, chromatic aberration will be even more significant. Moreover, contrast is also lower!

Center Portion	Right Edge
Click on the image to see a 100% crop

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