Should You Use A Tele-converter or Not?

Nikkor 500mm f/5.6 PF and Sigma TC-1401 tele-converter

I never did much questioning about using a tele-converter when I needed more optical reach. Since I tend to live by “trust but verify”, it seemed prudent to actually compare using a tele-converter to simple image cropping.

Besides magnifying the image, a 1.4X tele-converter will also slow the lens by one stop. This one-stop light loss also introduces extra image diffraction, which complicates matters a bit.

If you choose to crop an image instead of using a tele-converter, you don’t get the chance to alter the depth of focus for the same subject distance. For example, 500mm f/5.6 at 10 meters depth of focus is 0.13m. On the other hand, 700mm f/8.0 at 10 meters depth of focus is 0.09m. Most people would rather have the smaller depth of focus that the tele-converter would provide.

I have the Sigma TC-1401 1.4X tele-converter. It works just fine on my Nikkor lenses, even though Sigma and Nikon both tell me not to do it.  There are of course lenses that aren’t compatible with any tele-converter, and I’m not discussing those lenses.

I tested a few lenses to see how much the tele-converter messes up the resolution, and found that the tele-converter was well worth the slight drop in resolution. I particularly like this tele-converter mounted on my Sigma 70-200 f/2.8 Sport lens. My mirrorless Nikons have no trouble with the loss of one f-stop of light, either.

Before I got mirrorless Nikons, I never used this combination of the 500mm f/5.6 PF Nikkor and the Sigma TC-1401. The f/8 aperture resulted in unreliable focus with my DSLRs. Even my Nikon D850 and D500 cameras would often fail to focus in deep shade. My mirrorless cameras focus just fine with the tele-converter on this lens, in just about any light.

I use the free MTFMapper program to test lens resolution and other lens optical characteristics. This program gives me very comprehensive information on how a lens performs optically.

When I tested my 500mm lens with the 1.4X tele-converter on my Nikon Z9, I got a surprise. The following was what I found.

500mm f/5.6 PF Nikkor MTF50 resolution

The resolution plots shown above give me the overall lens resolution throughout the frame, in both the meridional and sagittal directions. The peak lens MTF50 resolution at f/5.6 was found to be 61.5 lp/mm, which is the same as 2940 lines per picture height on my Nikon Z9 camera.

MTF Contrast plot at 500mm f/5.6

The most common plot of lens performance, as shown above, is an MTF contrast plot. I have included measurements at 50 lp/mm, which most manufacturers omit.

Test chart edge measurement details at 500mm f/5.6

Overall, this lens is really good, even wide-open. The resolution is consistently high across the whole field of view, at least in the sagittal (wheel spoke) direction.

I mounted the 1.4X tele-converter on the lens and then repeated the resolution testing. This is the equivalent of a 700mm f/8 lens.

Nikkor 500mm with TC-1401: 700mm f/8 MTF50 resolution

The EXIF data when using the Sigma 1.4X teleconverter doesn’t notice that the teleconverter is attached, so it still indicates 500mm f/5.6.  In reality, this is 700mm f/8.

The peak lens MTF50 resolution at f/8 was found to be 43.9 lp/mm, which is the same as 2098 lines per picture height on my Nikon Z9 camera.

MTF Contrast plot at 700mm f/8

Test chart edge measurement details at 700mm f/8

The lens resolution at 500mm is 2940 lines/ph, compared to 2098 at 700mm, or 61.5 lp/mm versus 43.9 lp/mm.

So, how do we know if using the tele-converter is “worth it”?

Cropping changes the “picture height” portion of the equation, because there are effectively fewer millimeters of image height. The 1.4X tele-converter retains 71.43% of the image width and height, or 1/1.4.

Cropped resolution = (original_resolution) * (1/TC_magnification)

TC_magnification = 1.4X

Cropped resolution = 2940 * (1/1.4) = 2100 lines/ph

Lens MTF50 resolution with tele-converter = 2098 lines/ph

This resolution is the combination of the tele-converter effects and also the 1-stop-dimmer diffraction effects.

Here, it’s better to NOT use a teleconverter! You don’t gain any resolution at the effective 700mm.

If the cropped resolution is equal to or greater than the tele-converter mounted onto the lens, then you shouldn’t use a tele-converter. Especially when you consider that you’re losing at least a stop of light, it just isn’t worth it.

If the reduced depth of focus is important to you, however, you might opt for using the tele-converter anyway.

If you notice the autofocus getting too slow, then you may opt for cropping the 500mm setup instead.

I tried these same tests on my Nikon Z8 camera, and the 500mm with the 1.4X tele-converter performed better than on my Z9. In that case, using the tele-converter is worth it.

Not every lens responds the same way to a tele-converter. Take nothing for granted.