[nobody]

Putting a 1.4 TC on the 100 2.8 makes it a 140 ƒ4...

Pentax K-30
According to DoF Master
Online Depth of Field Calculator

100mm , ƒ2.8, @ 10 feet, total DoF .17 feet.
100mm, ƒ4 ---,@ 10 feet, total DoF .47 feet
140mm , ƒ4... , @ 10 feet, total DoF .24 feet

So wide open, more DoF wide open on the 140 ƒ4. From the same position you actually get more DoF wide open with the TC.
Diffraction tends to become an issue at ƒ8 and goes downhill from there.

100mm 15 feet DoF = total DoG 2.16 feet
Back up to 21 feet to keep the framing roughly the same

140mm, 21 feet, ƒ8 - total DoF = 4.28 feet.

You can actually double your DoF without changing your framing by using the TC and backing up, at the same ƒ-stop.
These kinds of things are usually a lot more complex than most people expect.

From HD DA 1,4x Pentax User Manual:
Chapter "Operational Precautions", point 5:
"The rear converter decreses DOF by one f/stop, eg if the aperture is set to f8, the DOF is equivalent to that of f5.6" In case of 2x TC it will probably decreased by 2 f/stops and that is alot imho especially in case of using Pentax M200 f/4. But it is only my opinion and I may of course be wrong.

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Last edited by nobody; 03-13-2018 at 02:32 PM.

[UncleVanya]

Here's more stuff that's interesting to read on the subject. I think the most important phrase in this article is that we overthink this. The other important point is that you shouldn't think about the lens as it was before the TC was added - think of it as the new focal length and aperture. A 600mm f/5.6 and a 300mm f/2.8 with a 2x TC added work exactly the same from an exposure, framing, and DOF perspective. That's what really matters.

The Online Photographer: Depth-of-Field Hell

[nobody]

So what is the precaution from Pentax TC manual for?

[THoog]

2x converters aren't typically that good optically compared to 1,4x. Some dedicated models exist that are quite good but most aren't.

Compared to the DA 1.4x and the F 1.7x, the 2x converters I have tried were roughly equivalent to smearing vaseline on the front of the lens and cropping 50% in post, only worse.

[UncleVanya]

Compared to the DA 1.4x and the F 1.7x, the 2x converters I have tried were roughly equivalent to smearing vaseline on the front of the lens and cropping 50% in post, only worse.

LOL - The dedicated 2x for some lenses is good I'm told. The A 2x-L wasn't too bad when I used it back in the day, but the majority were junk.

[MossyRocks]

Compared to the DA 1.4x and the F 1.7x, the 2x converters I have tried were roughly equivalent to smearing vaseline on the front of the lens and cropping 50% in post, only worse.

Sounds like I bought one of your old 2x converters. I really hate mine and get results like what you describe. It disappoints every time I use it, but thankfully I only paid $5 for it so it isn't like I was out any real money.

[THoog]

Sounds like I bought one of your old 2x converters. I really hate mine and get results like what you describe. It disappoints every time I use it, but thankfully I only paid $5 for it so it isn't like I was out any real money.

Maybe everyone goes through the "ooh, a cheap teleconverter would give me twice the reach!" phase. I had pretty much given up on teleconverters until I tried the DA 1.4x on a DA*300. A good TC on a great lens gives at least good results, but an average TC on an average lens gives... much less than average results.