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 FZ-30, and extends the longest focal length of the FZ-30 from 420mm to 1553mm!

However, stacking teleconverter lenses does have some drawbacks.

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 images of good quality?
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 (actually the entrance pupil) 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.
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 resulting images. But, no one would like to see a significant vignetting that only shows an image circle!
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.
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. If the lens being stacked are heavy, the combo may tilt forward and add too much weight on the lens barrel, even though the combo is on a tripod. Additionally, the added weight may also be too much for the tripod hole to handle. In general, a lens support as shown above may be needed to "balance" the weight.

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-30, since its power is 2.465X, the longest combined focal length of a FZ-30 is 1035.3mm = 2.465×420!

Lens Support

With this combo mounted on a FZ-30, 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-30. Since this is a huge and heavy combo, 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.

Light Fall-Off and Vignetting

This Sony VCL HGD1758 and Olympus TCON-14B does not cause vignetting at the 420mm maximum focal length; however, light fall-off at the corners is obvious if it is used to shoot uniformly colored scene as shown below. In general shooting, this light fall-off is not very obvious.

Sample Shots

To see how good the resolution of this combo is, I framed a tree trunk at the center and took a shot. Then, use the same focusing area but move the tree trunk to about 3/4 of the frame and take a second shot. Finally, repeat the same process but move the tree trunk to the right edge. Thus, in all images the tree trunk should be very sharp even though its position may be different due to different composition. The reason that the lens is always focused at the center and recompose is simple: we want to ensure that the tree trunk portion is always sharply focused, and, as a result, we would be able to know the image quality reduction when the sharply focused subject is moved to the edge of the image. In the images below, each row has two images, the left one showing the composition and the second being the 100% crop from the yellow frame. Click on the left image to obtain the original full size unprocessed image. Note that The images used ISO 80 and LOW for CONTRAST, SATURATION and SHARPNESS.




Click on the image to see a full size version

Not much chromatic aberration in the form purple fringes is shown in the above images, because I tried to avoid high contrast areas to minimize its impact. However, if the scene contrast is high, purple fringes will occur along high contrast areas. In fact, a touch of purple/blue can be seen in the above images, which may be due to the subject being in shade (i.e., white balance problem) and chromatic aberration. Note that contrast is also lower for off-center subjects.

My Words

The above images show the sharpness difference at the center and edge. In summary, if you wish to have some fun, stacking two or more teleconverters would definitely be interesting. However, the results may not be as good as you expected. In general, the sharpness is only good at the center small area, and contrast and resolution power are usually lower than expected. If your work can sustain a significant crop and/or you only use the images in small size, stacking teleconverter lenses may still have some good values. On the other hand, if you really need lens power larger than 2.0X, consider the use of one teleconverter (e.g., Raynox 2020PRO 2.2X) first because it may provide you with better results than stacking two not-so-good teleconverters. Note that the loss of image quality may not be obvious with small size images.

The following has some random testing shots. Most of them were cropped and slightly leveled.