If you use a teleconverter with a prime lens such as the 85mm f/1.8D, do you get the benefit of stopping down as well? For example, if you use a 2X teleconverter, you lose 2 stops so the lens is now 170mm f/3.5. Does that mean you get the sharpness you'd get if you stopped the lens down to f/3.5 without the teleconverter or does it still behave like it is at f/1.8?

1st off, only a 3rd party TC might fit. The IQ won't even be close. Loss of contrast/sharpness the least of your troubles.
Buy a nice AF 180 f2.8 ED (not the D-ED...cheaper) instead if you want that FL.

Both DoF and minimum focus distance stay the same. (I.e., DoF is determined by f-stop set on lens.) Generally, I avoid 2x converters.


Kent in SD

Why would only 3rd party TCs work? Can't TCs work on any lens?

I was using the example of the 2X converter just for simpler calculations thinking the same would apply if I got a 1.4X or 1.7X etc.

The DoF must change with the longer FL though doesn't it?

No, TCs don't just work on any lens. Meaning you will damage your lens if you try.

The depth of field aspect is complex.
If from the same focus distance you zoom from 85mm to 170mm with a constant aperture zoom the viewfinder image brightness and effective aperture (for exposure) are unchanged, but the magnification doubles.
Double magnification reduces depth of field by the equivalent of 4 f stops - to one quarter.
If instead a 2x is used on the 85mm the viewfinder is 2 stops darker and the effective aperture is 2 stops darker. You have effectively converted the 85mm from f2 (to keep the maths easy) to f4 - which gains you 2 stops dof. Overall you loose 2 stops dof (not 4) with a 2x but you also get a slower shutter speed.
Generally converters work best on lenses longer than 100mm. Also dedicated converters (there are none for this lens) work better on dedicated lenses.
A 2x discards 75% of the information coming out of the back of a lens. This discarded light increases lens flare quite a bit. If the taking lens and converter are good you can get decent results - but you cannot crop anything like as much as with no converter before the image breaks up.
AF slows down as well.
Despite all this good (as distinct from excellent) images are possible with a 2x if you lock the lens down on a tripod or pair something like the Nikon 20e on a 300mm f2.8.

Paul.K wrote:
Why would only 3rd party TCs work? Can't TCs work on any lens?

I was using the example of the 2X converter just for simpler calculations thinking the same would apply if I got a 1.4X or 1.7X etc.

The DoF must change with the longer FL though doesn't it?


Certain TC's have a flat front element and some protrude a bit. If you have a lens where the rear glass element is flush to where it mounts the TC that protrudes will hit the rear element.


(photo credited to "Steve's digicams")

Paul.K wrote:
If you use a teleconverter with a prime lens such as the 85mm f/1.8D, do you get the benefit of stopping down as well? For example, if you use a 2X teleconverter, you lose 2 stops so the lens is now 170mm f/3.5. Does that mean you get the sharpness you'd get if you stopped the lens down to f/3.5 without the teleconverter or does it still behave like it is at f/1.8?

To get the best out of a lens with teleconverter you need to stop down the lens to the "sweet stop" of the lens and additionally apply the loss of stops due to the extender factor.

Hmmm. Did some research. Minimum focus distance seems to stay the same with TC, but DoF actually decreases. Moose Peterson is saying a 2x will reduce DoF by half. Agree with the above--different brands of TC have different arrangement of elements. If you put a Nikon TC-20E on a lens not designed for it, the rear element of the lens can crash into the front element of the TC. This is why Nikon TC have an extra flange so they won't mount on lenses they weren't designed for. All in all, I'm not a fan of TC. I do own the Nikon TC-17E which I use when I have to on my 70-200mm f2.8 VR. There is some image degradation.


Kent in SD

SOOC, just shot these real quick for you.

85 1.8d with 2x converter, tripod remote release. 85 stopped down to 2.8




85 bare at 8

should have shot it at 5.6 to be "close to the same" but wasn't paying attention.



for comparison 135 series e at 5.6




dogs felt left out

85 at 4



at 2.8




As you can see a 2x third party converter really tanks image quality with this lens, imo for the focal length gain needed grab a cheap ai-s if budget is a concern, the 200 f/4 ais should be pretty cheap and I've seen the 200/4 af for 250ish locally lately.

Hmm, I didn't realize it was this complex. I was thinking since the 85mm is so sharp at f/3.5 I'd be able to get a really sharp 170mm lens essentially but I didn't realize about the difference TC designs and all the factors that are involved with it. It seems like a used 180mm f/2.8 is a much better option then as I don't want to stop down a ton to get good detail. Thanks for all the replies, lots of great information there to consider.

Thanks for the follow up as well, Kent.

Edit: WOW, thanks for that info, 90 5.0. I've never seen pictures that I knew a TC was used and that example you gave me (with the lens I was asking about too!) really is helpful to me. I'll have to check out some alternatives. Thanks again, everyone.