TC + Telephoto Zoom
 

Can someone explain to me why a telephoto zoom used with a teleconverter, especially when wide aperture and full zoom used, produces CA, fringing, ghosting, and other undesireable effects? I would like to understand the mechanics of the issue, so I can better guard against it in future purchases and future photographs. Thanks

This is largely down to the compexity of zoom lenses. On the big bucks pro jobs this is less of, or not a problem, when compared with lenses at the budget end. Some zooms can have as many as 20 lens elements. Zooms also come in a variety of zoom length flavours. A 3x zoom like 100-300 will produce better images than a 10x 28-300, as distortions within the design are easier to correct.

Now add a converter with between 5 and 7 elements and not only do you have a lot of glass, but the converter will magnify any defects from the zoom before adding its little bit. There is a thread running on the subject in the Lens Forum - TC Musings. It goes to show its not an easy question to answer.

Don

Hi Nancy welcome to WPF.

To put a different spin on what Don has said.

The first rule of thumb with optics is you do pretty well get what you pay for.
In the real world there are probably only a dozen or so expensive Zoom lens that give very good quality with a TC. Secondly there are various grades of TC aswell. So as a general point zooms & TC are best avoided.

Budget Zoom lens do tend to suffer from CA in bright contrasty lighting conditions anyway. Expensive lens use different types of glass to correct for this. Budget Zooms also tend to give a softer focus at the extreme focal lenght. Their optical performance normal don't start to peak until stopped down to f8-11. (This lens peaking is often referred to as the lens sweet spot). The TC will just amplify these lens defects. Plus the lost of light introduced by the TC means you have to reduce the f stop and bring the lens out of it's sweet spot range.

So to get good results you need the bests lens optics and the best quaility TC. The TC is normally designed to work with longer telephoto lens (200 - 300mm and above).

To achive better results many people use a prime telephoto lens and not a zoom.

Rob and Don, Thank you so much for your explanations. I have visited the cambridgeincolour.com website to try to better understand lens function, and with your explanations, I now have a better sense of the CR, blurring, and other TC/Zoom abberations. I have a good collection of Nikkor zoom lenses, mostly from my SLR film camera days, but none of them really works well with my Nikon D200. Yesterday, I ordered the Nikkor 300mm f/4 AF-S and I already own a Kenko 300 Pro 1.4 TC. I am hoping that the Kenko will not be the weak link in the chain. Again, many thanks. Nancy

Nancy, there is nothing wrong with the converter unless its a bad one (unlikely), the kenko 300 pro works perfectly with digital, I have the 1.4 and the 2x times to work with my Canon gear and get great results.