Teleconverters or as Canon
calls them Tele-Extenders (TE) are added to a lens to increase the focal
length by inserting negative elements between the lens and camera. They
have been in use for decades and are rather simple optical devices. In
the case of the Canon 1.4 II there are 5 glass elements and no moving parts.
I have never heard of anyone having a defective one at least in terms of
optical performance. It is commonly believed that only a very good prime
lens would tolerate a TC, most of the older zoom lens would not give acceptable
results with a TC.
Whether to use a TC to increase image size or to "'rez up" the image in your photo editor was always a subject of debate. Back in film days (prior to 2003) I had performed a test with my Canon 300 f4L and Canon x1.4 Tele-Extender to determine if there was any benefit in using the TE. Several Fuji Pro via slides were shot and scanned into the computer and compared under magnification. At the time I determined there was an advantage in terms of visible detail in the image to using the Tele-Extender vs rezing up the image without the TE.
Tele-Extenders increase magnification
but reduce resolution by a certain amount. The image size gain of 140%
is partially offset by the resolution loss usually around 15% for a 1.4
TE according to my tests but also a loss of contrast which is clearly visible
but harder to quantify. Indeed Canons own modulation transfer data
(the modern measure of lens performance) shows a significant loss with
TEs on all the "L" lens. Tele-Extenders cause the lens system to exhibit
chromatic aberration, this can be seen as purple\green color fringing in
high contrast areas of the image and it gets progressively worse towards
the edges\corner of the frame. All of Canons long telephoto lens are well
corrected for chromatic aberration by themselves but adding a Tele-Extender
clearly causes purple\green color fringing.
When I acquired my first
digital camera in 2003 I again tested with and w\o a TE and I recall
I still found a benefit to using the TE over rezing up. Lately however
I had noticed in the field that my images using my 500mm f4L IS without
the TC seemed to have the same amount of detail as the TE ones. Newer
digital cameras have more resolution than the 6 megapixel 10D
and I have been using a Canon 50D of late with it's 15 megapixel sensor.
The detail in an image is limited by both the lens sharpness\contrast and
sensor resolution. In the 10D era the sensor had less resolution than the
lens could provide but with the 50D the sensor resolution and the lens
resolution are similar. With the 10D, loosing some resolution by adding
the TE did not reduce the image detail significantly but with the 50D's
higher sensor pixel count I began to suspect that the lens\TE combination
was now limiting the detail more than it had in the past.
With this idea in mind I
decided to test image detail for the straight 500mm and 300mm at optimum
apertures vs x1.4 and x2 at optimum aperture. The test was performed indoors
by shooting a target at a distance of 10 (300mm lens) and 15 feet (500mm
lens) using a tripod and flash, necessary to insure camera shake does not
impact results. Even slight focus errors can throw the test results way
off so I mounted my lens on the Arca ball clamp, focused on the target
with autofocus, one shot mode, then switched to manual focus placed tape
on the lens focus ring so it would not move and then shot several images
of the test target with different focus distances by sliding the lens foot
through the clamp 2 mm at a time about the point of focus. The idea was
to select the sharpest (best focused) image by looking at them under
high magnification on the computer screen and selecting the best (sharpest
All images were shot in RAW
and converted to TIF with no sharpening used. The best focused images were
selected and comparisons were made between the different apertures. The
best aperture for the straight 500mm was f5.6, for the 500mm x1.4 it was
f8 in line with what I know from use in the field. The 300mm is best at
f4 or f5.6 by itself and f5.6 with the x1.4 and f8 with the x2.
I then compared the images side by side on the computer screen with the non TE image rezed up 140% to match the 500 mm x1.4 image. The 300mm was compared with x1.4 and x2 TEs. Part of the target near the center and the corner was compared at a screen magnification of 300% where pixels are just becoming visible.
The above test procedure was performed with the 10D and the 50D. The image target used was a Canadian 10$ bill with a grid pattern for the focus point target. Crops from near the center and left corner approximately 60% off center were used for the comparisons.
To be sure my x1.4 TC was
not defective I repeated the same tests with a borrowed Tele-Extender (thanks
Lat) and tested again but did not see any difference.
The test target
Results and comparison images:
50 D image 500mm f4L IS
Disadvantages of using a
Advantages of using a
1 - TEs give best results
towards the center of the frame, color fringing gets progressively worse
Modulation transfer function curves from Canon Lens Work III
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Site and contents © 2009 Ken Newcombe