The truth about teleconverters

[Tom3]

what is it?

Some claim they get good results, some seem to describe them as an abomination. Obviously the truth is somewhere in the middle, but... where?

I was looking at getting a 1.4 TC but the lens I'd use with it is not top notch to start with: it's a sigma 18-200 and it's 6.3 @ 200; would that even make sense? it would be to use in bright settings though.

 

[tsaraleksi]

Teleconverters, by their nature, cut image quality down, because they only use a portion of the image coming through the lens. This means that on really nice lenses, you can use them, because the IQ is high enough that you won't notice the drop. On lenses where the IQ is a bit more dodgy, stay away.

At the same time, a 1.4x TC will eat up a stop of light, and on the 18-200, that will mean that it won't autofocus on your camera.

 

[Big Mike]

It comes down to your expectation of quality...and everyone is different.

I've used a 2X TC on a cheap 100-300mm lens. The quality wasn't good...but it was fun to shoot with a '600mm' lens.

I've shot with a 2X TC, stacked on top of a 1.4 TC and you would think the quality would be terrible, but it wasn't that bad.

As mentioned, they do steal some light, which will make it hard or impossible to auto focus on some cameras when using slow lenses.

I wouldn't spend a lot of money or count on getting great results, with a TC & a slow lens...but if you can get it cheap, it might be worth playing with.

 

[davebmck]

At the same time, a 1.4x TC will eat up a stop of light, and on the 18-200, that will mean that it won't autofocus on your camera.

While it is correct that the 1.4 TC will lose you one stop of light, it does not mean your 18-200 will not autofocus. It may not autofocus at the maximum focal length in low light situations, but it will focus in most other situations.

 

[Big Mike]

While it is correct that the 1.4 TC will lose you one stop of light, it does not mean your 18-200 will not autofocus. It may not autofocus at the maximum focal length in low light situations, but it will focus in most other situations.

That's probably true...buy why would you bother to use a TC if you weren't going to shoot at the long end of the zoom?

 

[tsaraleksi]

While it is correct that the 1.4 TC will lose you one stop of light, it does not mean your 18-200 will not autofocus. It may not autofocus at the maximum focal length in low light situations, but it will focus in most other situations.

At the lowest part of the range, you will retain AF, but past 31mm you will no longer be able to AF on anything but a 1 series camera, as in, it will simply not function. The only reason that the Sigma can AF at its long end is that it misreports itself to the camera.

 

[Tom3]

ok, it all makes sense.
It seems like I can find some used ones for $50, so it might be worth buying just to play

thanks for all the info!

 

[icassell]

That's probably true...buy why would you bother to use a TC if you weren't going to shoot at the long end of the zoom?

I will frequently use my 1.4X EX sigma TC with my 100-300mm f4 EX. This is a combination of an excellent TC and an excellent lens, and I frequently get excellent results. I may use it at the shorter end of the zoom because, frankly, It is a pain in the butt to keep putting it on and taking it off -- even though I have a 50-150mm f2.8.

 

[dxqcanada]

There are some high quality TC's made by some lens makers ... usually they are made for specific focal lengths of lenses they make.
Usually these have LD glass and/or APO.

I have used low prices ones ... middle, and high end TC's.
It makes a difference in image quality.

 

[davebmck]

At the lowest part of the range, you will retain AF, but past 31mm you will no longer be able to AF on anything but a 1 series camera, as in, it will simply not function. The only reason that the Sigma can AF at its long end is that it misreports itself to the camera.

I have to disagree with that. My camera and others in its class are designed to autofocus at a minimum aperture of f5.6. However, that does not mean it will not autofocus with a lens with a smaller minimum aperture. I have had a lens with a F6.3 minimum aperture on my camera and although the camera's autofocus would hunt in low light situations, it worked just fine in better light situations. It's all a matter of how much light is available, not the absolute value of the minimum aperture.[FONT=&quot] [/FONT]

 

[gsgary]

what is it?

Some claim they get good results, some seem to describe them as an abomination. Obviously the truth is somewhere in the middle, but... where?

I was looking at getting a 1.4 TC but the lens I'd use with it is not top notch to start with: it's a sigma 18-200 and it's 6.3 @ 200; would that even make sense? it would be to use in bright settings though.

The Kenko 2x is great on my 300mmF2.8L

But wouldn't use it on a zoom lens, i have only used the Canon 1.4x on L zooms and it works very well

 

[Tom3]

that looks very good to me!

I think I should pick a used one to play with. I'm concerned that my 18-200 is 6.3 @ 200 though..

does anyone have a pic of a TC with a zoom lens?

 

[tsaraleksi]

I have to disagree with that. My camera and others in its class are designed to autofocus at a minimum aperture of f5.6. However, that does not mean it will not autofocus with a lens with a smaller minimum aperture. I have had a lens with a F6.3 minimum aperture on my camera and although the camera's autofocus would hunt in low light situations, it worked just fine in better light situations. It's all a matter of how much light is available, not the absolute value of the minimum aperture.

The Sigma 18-200 and the 50-500 are designed specifically to misreport their maximum aperture as 5.6 so that cameras that are made to not AF at anything small that f/5.6 will still work. The reason the camera is designed to turn off the AF at that aperture is the reason you've experienced, the focus will hunt. I'm of the opinion that you may as well leave the camera to work as well as it will work, but I dunno, I'm not a camera designer.

With some TCs, it's possible to tape down certain pins so that the lens will not tell the camera not to AF, which is a way around the problem.

 

[gsgary]

that looks very good to me!

I think I should pick a used one to play with. I'm concerned that my 18-200 is 6.3 @ 200 though..

does anyone have a pic of a TC with a zoom lens?

I don't know what camera you are using but unless you have a pro camera it won't AF

 

[gsgary]

The Sigma 18-200 and the 50-500 are designed specifically to misreport their maximum aperture as 5.6 so that cameras that are made to not AF at anything small that f/5.6 will still work. The reason the camera is designed to turn off the AF at that aperture is the reason you've experienced, the focus will hunt. I'm of the opinion that you may as well leave the camera to work as well as it will work, but I dunno, I'm not a camera designer.

With some TCs, it's possible to tape down certain pins so that the lens will not tell the camera not to AF, which is a way around the problem.

But be very careful i have heard of camera going wrong because of the pressure on the AF system