|
To test whether the theory on the previous page holds up I shot a series of
photos of a lichen covered slate roof approximately 50 meters away using a Nikon
D2h and a Nikon 300mm f/2.8 lens and up to 3 teleconverters. They were taken with no in-camera sharpening
and then processed on the PC for optimum sharpness. However even the 1680mm shot
only required slightly more sharpening than the 600mm shot.
I have added some extra examples taken since which show the best performance
I have seen. Again none have been excessively sharpened compared with the
others.
| Photograph
(at 100%) |
|
Taken with 600mm f/5.6 lens stopped
down to f/10
Taken with same lens at f/5.6
from center of image
|
|
Taken with 840mm f/8 lens stopped
down to f/12
Taken with this lens at f/8
near center of image
Same image almost at the corner
corrected for chromatic aberration
|
|
Taken with 1200mm f/11 lens stopped down
to f/16
|
|
Taken with 1680mm f/16 lens stopped down to f/32
(slightly different lighting conditions)
This lens stopped down to f/22
the center of the image
|
The last shot clearly shows the most detail of the subject, but is not as
sharp as the earlier ones. Taking an estimate of the resolution in the last shot
as ~1.5 pixels gives an angular resolution of ~1 arcsecond which is about equal
to the theoretical prediction for a perfect lens of this diameter imaging green
light (Stopping down does not significantly worsen the resolution as the
aperture diaphragm is much closer to the imager than the first lens
element). Given that this must be less than a perfect lens the extra
resolution must be due to the applications of un-sharp masks used to
sharpen the images. On a practical note the images shown are crops from the
center of the frame. For the 1680mm shot there is a chromatic aberration of about 2 pixels at the edge of the frame. 600 and 840mm seem to show equal amounts,
about 2/3 of a pixel, at the edge. This can be removed
effectively with this setup by resizing the red channel only. With experience
of these setups in the field I feel quite happy to stack the tc20e II and tc14e
II and expect to get sharp images even without stopping down. Autofocus still
works, all be it very slowly, as the camera does not recognise that there is an
extra teleconverter. My third teleconverter is an old tc201 and images with this
added are always obviously of a lower quality than otherwise. Replacing this
with another modern teleconverter might produce good images stopped down.
|
