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03-23-2010, 12:26 PM   #1
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DOF + Teleconverter

Here's my question: Let's say you have a 200mm f/2.8 lens and add a 2x's teleconverter. The lens is now equivalent to a 400mm f/5.6. Right? Is the DOF also roughly the same as a 400/5.6 or does it change to something else?

03-23-2010, 01:52 PM   #2
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So I gargle DOF TELECONVERTER and the first hit says:
A 200mm lens with a 2x tele-converter shot at f/5.6 will achieve identical depth of field as a true 400mm lens with no tele-converter also shot at f/5.6.
-harry
You should try gargling too. It's not hard. Really, it's not.
03-23-2010, 01:57 PM   #3
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Alex, thanks for asking that question as its had me wondering sometimes too. Sometimes we learn a lot more by starting a thread on a topic than just reading the first hit we get on a search.

...and welcome to the forum if you have not been welcomed yet.
03-23-2010, 02:13 PM   #4
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I apologize. I'm too snarky. I just sometimes see questions that look like they can be resolved with very little basic research, and ... ah well. I apologize. I'd better take a nap.

03-23-2010, 03:41 PM   #5
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Thanks for the answer. I guess I could have searched for it, but isn't this why we come to this forum? Well, anyway thanks!
03-23-2010, 04:53 PM   #6
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QuoteOriginally posted by RioRico Quote
So I gargle DOF TELECONVERTER and the first hit says:
A 200mm lens with a 2x tele-converter shot at f/5.6 will achieve identical depth of field as a true 400mm lens with no tele-converter also shot at f/5.6.
-harry
You should try gargling too. It's not hard. Really, it's not.
I agree with the advice. The problem can be, though, that you might get incorrect answers :-) And I think that's the case here. Or at least, the quote taken out of context above is misleading. Cehck out the link and the rest of the discussion, and you'll see "Harry" is actually talking about a 200/2.8 shot with the the aperture set to f/2.8 - thus making it "effectively" f/5.6. He's not saying that actually setting the aperture to f/5.6 on the 200mm lens with TC will result in the same DOF as a 400mm lens set to f/5.6.
05-15-2010, 10:57 PM   #7
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I'm back.
It took 7 weeks but I finally got around to reading every post on the interwebs about the impact of TCs on DOF. Yes, PF, dpreview, photo.net, muddybootsphotography, dslreport [yep], on and on. It seems no two "experts" agree. There are simulators that do not agree and circles of confusion that are, well, confusing. I now know FAR LESS about the topic than I did when I started.

All I wanted to do was calculate my DoF using the AF 1.7x TC and a couple of lenses but as it stands I can now create all kinds of numbers with no confidence in any of them. Good luck to anybody else who decides it can't be that difficult to sort out and can bring forth the light. I will kiss your ring. Tonight, I feel old and tired and stupid.
05-16-2010, 02:51 AM   #8
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QuoteOriginally posted by imtheguy Quote
Sometimes we learn a lot more by starting a thread on a topic than just reading the first hit we get on a search.
It may be true in some cases, but really, how many threads asking about "the noise of something moving inside the Pentax dSLRs", or "why is there is a flashing "F--" on the display" do we need?

05-16-2010, 03:42 AM   #9
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QuoteOriginally posted by alexcam Quote
Here's my question: Let's say you have a 200mm f/2.8 lens and add a 2x's teleconverter. The lens is now equivalent to a 400mm f/5.6. Right? Is the DOF also roughly the same as a 400/5.6 or does it change to something else?
I come back to the initial question:

A rear mounted tele converter (we are not talking about those add-on things, that screw into the front filter ring) does only one thing: it lengthens the focal length of the lens (onto which it is mounted) by a fixed factor. This fixed factor is usually something like 1.4x or 1.7x or 2x (and some modells give even a 3x factor).

So, the combination behaves like a lens with the resulting focal length in every single aspect: the aperture stops will be changed to a value corresponding to the engraved numbers multiplied with the tc factor and the DOF will be the same as that of a lens with the same fl initially.

In fact a tc is built into many tele lenses as the rear most group. This is the reason, why tele lenses can be much shorter in built, than their focal length. Manufacturers do not call this rear group a tele converter but a tele negative lens group - but that is basically synonymous.

Ben
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