Originally posted by TeeK Hey everyone,
My question is specifically...
While a TC can magnify a subject, the trade-off seems to be your resolution is halved (assuming a 2x, TC).
Umm, no. The resolution is determined by the sensor, not the lens. What a TC does is add extra glass to the lens. Since your SMC lens has been designed with great precision to produce an image with the glass used in its own construction, when you add a TC you
will get a degradation of the
image quality. Of course, some TCs are better than others and the degradation may not be noticeable.
The resolution of the picture remains at whatever you set the camera to, but when you add extra glass the image quality will change. Now, there is a bit of a terminology problem here. Another way to describe image quality is to talk about the resolving power of the lens (ie, how good is the lens at showing detail). But again, that is not the same as the sensor resolution.
Originally posted by TeeK Am i correct in assuming my new 10mp K200D, would effectively be out-putting @ 5mp (that can;t be right!) when a TC is introduced?
Nope, you are talking about the resolution of the image coming from the sensor, not the resolving power of the lens. So you cannot say that there is any connection between that and the use of a TC. Also, note that your resolution value set in the camera (on my K100D that is 6/4/1.5M) is only available for jpegs, and it determines the number of pixels that are in the final image. A TC will not affect this - it cannot! If you shoot RAW (as you should) the number of pixels will always be the same - it is the number of pixels in the sensor.
But the TC can reduce the amount of light, it can add flare from reflections caused by lenses with poor coatings, it can add distortions and colour aberations to the image, and there will be nothing that you can do to repair the image. It is important, therefore, that the quality of the TC is at least as good as the lens you will attach it to.
You're right that digital zooms are a rip off, but they rely on the fact that most people will view their digital images on 6x4 or relatively small images on screen and that modern cameras (even P & S cameras) have high resolution sensors, producing images with lots of pixels, and so digital magnification is possible to a point. In this case, you
are reducing the resolution because the magnification uses fewer of the original pixels (most likely by creating more using some form of interpolation). But if you intend to view your images printed out large, say A3, then a digital maginified image will be apparent as chunky pixels (interpolation methods are not perfect), but that is because you are magnifying it
twice.
So it is swings and roundabouts. If you have a good quality lens then you can afford to digitally magnify it and how much depends on the lens. If you are used to viewing images as 6x4 and people tell you that the lens produces sharp results at 10x7 then you know that you can digitally magnify by 1.7 and crop to get a good image (ie just get the 6x4 from the 10x7 image). In that case your image would be better than taking the photo using the lens and a x1.7 TC.
For example, recently I have been photographing damsel flies on my garden pond. I find that I get better images with my SMCP-A 70-210 at 210mm than with my Sigma 70-300 DL at 300mm when the A70-210 image is multiplied by x1.4 to get the same size as the Sigma images. The Pentax lens is simply a better lens (at least 1.4 times better
). If I added a x1.4 TC to my Pentax lens would it still be better quality than the Sigma? I don't know.
Richard