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I took four photos using two lenses, each with and without a Promaster 1.7x Teleconverter.
What we have is both a comparison of two lenses, as well as the effectiveness of the teleconverter, although my goal is simply to determine if it's worth using the teleconverter. Recently, it was posted online that an inexpensive (~$80) teleconverter was not worth using, as you could get just as good results by resizing/cropping the photo without using the teleconverter.
To compare, I cropped the same scene from each shot, resizing the normal photo up to match the teleconverter's view. Click on each photo to see the full resolution version; I've downsized the below photos to fit on the page side-by-side.
Without teleconverter.... .... and with.
Without teleconverter.... With.
18-70mm/3.5-5.6 (kit) lens
As you can see, the teleconverter has much additional detail. (Click on the photos to see the intended view, where it'll be easier to see the detail. The sizes have been reduced in this web page for ease of display.)
The color changes with the teleconverter. It's a bit warmer/less blue, but not necessarily more washed-out. Is this to be expected? Is it because the scene was slightly different, causing a different color cast? Now I have to re-test, setting a constant white balance.
Note that the kit lens produces shots that look better than the other. I should probably prefer this lens over the 35-70, unless the light is particularly low. Which it often is. Unfortunately, the 35-70 looks a lot worse with a more open aperture (at least at 70mm). My shots using less than f/7.1 looked really poor, so I went with the 7.1 above. There were places where the 35-70 has purple fringing where the kit lens does not. (Also, I wasn't very consistent with my settings, as you can see, but again, my main goal was to try to get the most detailed shots as I could. I wouldn't use these photos here as a test of the two lenses, although, in doing these tests, I have a better idea of how they compare.) I also had problems using the autofocus with the kit lens, with it not being able to focus on my intended target at all. So, I'm really in a quandary about which lens to use with the converter.
Aside from the resizing (standard bicubic), no other modifications have been made. SuperSteadyShot was turned on, but I also braced against a wall. Fine JPEG mode was used (although these crops have been resaved with a medium compression).
Even though there is light loss, leading to a slower shutter speed, I think it's evidence enough to me that you do get some additional detail using a teleconverter, as opposed to using "digital zoom" (or cropping and enlarging). I judge it as worth using, although you're probably better off getting a quality zoom lens. Still, this is definitely better than nothing, and not a waste.
Negatives? While it's not too heavy, it's one more thing to carry. It probably adds some distortion just due to the extra glass. It's a little less convenient than switching to a normal zoom lens.
Originally, I tried a Sony VCL-DEH17VA 1.7x Teleconverter. This teleconverter was designed for the DSC-V3 digital camera, but since I don't have a long zoom, I wanted to see if I could use it with my Sony A100.
It actually worked pretty well, at first glance. As long as you were happy with the subject in the middle of the photo it was fine, but there was distortion on the edges. This could be cropped out, and that might be OK for some situations, but it is not generally usable, I would think. It's a shame it had that flaw, as it otherwised performed well.
Even though there is some light loss, leading to a slower shutter speed, I think it's a lot less light loss than you'd expect from a conventional behind-the-lens 1.7 teleconverter. (This is probably due to the much larger front lens, which probably is able to gather more light.) My guesstimate is that the loss is like going from f/5.6 to 6.3, while a conventional converter would be equivalent to 9.5. This seems like a big advantage to this type of teleconverter.
With a Tamron 55-200 mm lens, I got different results.
The first photo was up-sized by 120% to match the size of the one from the teleconverter, which is a 100% crop. I do not see much additional detail in the teleconverter version. (Perhaps a very slight bit, but I'd say not enough to be worth the trouble.) I had poor results using the teleconverter at 200mm. Even resting the camera on a surface, I could never seem to get a clear photo. Using different f-stops did not seem to help. (Meanwhile, I'm pretty happy with the results of this lens with the aperture wide open.)
Also, it's a known "feature" of 1.7x teleconverters that autofocus will only work with f4.5 and lower. Sure enough, 160mm is the longest the lens will go and still be at f4.5, and that's the last point where I still have autofocus. Having to manual focus might be one reason with why I couldn't get a clear shot at 200mm.
I made a fresh attempt on another day, and was able to get better results manually focusing at 200mm with the teleconverter. It took several attempts, as many were a bit too blurry, probably due to the slow shutter speed and difficulty in hitting the right focus. I decided that I needed a smaller aperture as well, to ensure the best results, which only made it more difficult to steady.
The first photo below was resized 150% to approximate the view of the 100% crop on the right. As always, no other edits were made (no added sharpness, etc.).
For this lens, the teleconverter is much harder to use. The difficulty means that I probably won't often use it, or risk not getting good results. While it would give me the equivalent of a 340mm lens, it probably does not compete with an actual 340mm lens. Judging by the test at 160mm, my guess would be that you'd get only about half the improvement that you'd expect you should get from that magnification.
I had pretty good results with my airshow pics, to the point where it is hard to tell what photos were taken with the teleconverter and which were not. AF didn't work for me, so I had to focus manually, which meant that probably most of my photos were at least somewhat out of focus. Still, there were enough winners to make it worthwhile.
Recently, I took the opportunity to photograph some geese. I started with the teleconverter, knowing that they wouldn't let me get close.
Overall, it looks pretty good, but everywhere there is a highlight, there is a bit of "purple fringe". Also, things are not as sharp as they should be. I think that has partly to do with the shutter speed (1/250 sec.), partly the narrow depth-of-field, but probably partially the loss of resolution to be expected from an inexpensive teleconverter.
Without the teleconverter, the 55-200 produces photos with a distinctively different color, much sharper resolution, but, of course, not as much zoom.
There is not much "purple fringe" or CA distortion either.
So, is it worth the cost of distortion to have more zoom? The truth of the matter is that a lot of these distortions only are really noticeable when you zoom into the photo on the computer screen. Viewing photos normally, a slight loss of absolute resolution may not be noticeable, while you'll see more real detail in a zoomed-in photo. The difference is subtle, but I think there is slightly more detail in the feathers in the teleconverter version.
To better illustrate, I wanted to show zoomed crops from the above photos. The teleconverter version looks so poor at 100%, I've taken a 100% crop from a completely different photo. For the non-teleconverter version, I up-sized it by 150% to match in size.
I tend to think that the teleconverter has a slight advantage over the use of "digital zoom", but the difference is so subtle that I wonder if it makes a practical difference. Either way, with a reasonable size print, both methods should result in a good print. Meanwhile, I still wrestle with whether or not it is worth the effort to use the teleconverter.
Interestingly, with the bright sunlight, the autofocus did work with the teleconverter most of the time. (I won't say that it worked well or that it worked all of the time, but it was good enough.) On another day, without the teleconverter, the 55-200 couldn't autofocus on a bird in flight. So, the issue of when autofocus works isn't as straightforward as is usually stated (needing an aperture of f8, for example). I would think of it in terms of, the camera will autofocus given ideal conditions, and the less ideal the conditions, the more difficulty it will have. Apparently a bird against a blue sky is a very non-ideal situation.
Also, it's worth pointing out that the Super Steady Shot does not seem to work well with a generic teleconverter (as the correct focal length is not reported to the camera), and camera shake becomes more of a problem.
Last change: June 8th, 2008