When to use a teleconverter?

When to use teleconverter and when not to? The easy answer is "it depends." Some people claim that a teleconverter destroys lens resolution and that cropping an image instead is often the better option. Some say that adding a good quality teleconverter is often better than cropping. Which is true? Is it testable, or do we have to trust purely on opinions, often opinions based on gut feeling?

Teleconverter is not a good idea when there's not enough light. Adding a TC will cause slower shutter speeds and higher ISO, and in dimming light that will mean camera shake and noise. Also when the lens itself is soft and slow a teleconverter will do no good, remeber the old adage "garbage in, garbage out."
And if adding a TC will hamper autofocus performance, a TC is usually not a good idea.
And as personal opinions (for what they are worth) I don't like teleconverters on zoom lenses and I'd never use one on a lens slower that f/2.8. But YMMV, as I said that's my personal opinion and choice, and yours may be different.

But when the lens is sharp to begin with, it is easy to prove that Nikon's new teleconverters are wickedly sharp. I shot three samples, one with a naked lens, one with a TC-20E III, and one with a ridiculous concoction... TC-34E aka stacked teleconverters.

In the samples below we can see that the 3.4x TC-34E delivers almost 3x the resolution of the naked lens. No cropping and enlarging will ever be able to catch up.

No teleconverter




Nikon TC-20E III




Stacked Nikon TC-20E III + TC-17E II aka TC-34E





Here is the gear




Here is the center part of the ISO 12233 chart. Find where the 9 black and 8 white lines are no longer distinguishable. That's the extinction resolution.




Here are the links to the original full-size images:

https://www.flickr.com/photos/34628704@N08/32993170764/sizes/o/

https://www.flickr.com/photos/34628704@N08/33707238081/sizes/o/

https://www.flickr.com/photos/34628704@N08/32993169904/sizes/o/

For those who just want a quick glance without looking at the full-res full-size images, here are the center crops from No-TC image at 200 mm and TC34 image at 680 mm. It is easy to see the TC34 image resolves all the fine lines all the way up to the "20" mark, whereas the cropped No-TC image stops at somewhere between 6~7. At that point the 9 black lines start merging into 8 black lines, which is false detail, aka aliasing. That point is where the resolving power ends.

Here it is obvious that using stacked teleconverters beats cropping by a huge margin, delivering at least 3x the resolution.


TC34 + 200/2VR2 @680 mm





200/2VR2 @200 mm, no TC

Good exercise and good detail, particularly since I don't have the kit to test that out on my own.

Interesting observations. Do you use that set-up for practical use ?
Its an expensive way to get to 600mm, but I guess a 600f4 aint cheap either.

Ironwood said:

Interesting observations. Do you use that set-up for practical use ?
Its an expensive way to get to 600mm, but I guess a 600f4 aint cheap either.

Click to expand...

In real life I don't use stacked teleconverters, except when in a desperately focal length limited situation. This setup still autofocuses but the focus is not fast enough to track movement (680mm f/6.7). With my other two tele lenses this setup does not AF (300/2.8 & 400/2.8).

I think the main observation is that Nikon has managed to push the III-gen teleconverters (14TC & 20TC) that extra mile. The hit on image quality is very low, if the taking lens has enough resolution. Especially the TC-20E III seems to deliver 2 times the resolution of the main lens, not 2 times minus 30% "blur factor" like the older TC-20E II did, which made its use always dubious.