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03-03-2011, 07:49 AM   #1
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Prime lens

I am interested in purchasing a telephoto prime lens for my Pentax K20 in the 400mm range. After researching what is available I find a 400mm is not an option. All that appears to be available is the Pentax DA* 200mm or 300mm.

If I was to purchase either one of these along with a 2.0 teleconverter this would be the only way of achieving my goal of having a lens in the 400mm range.

I would be interested in hearing other people experience regarding the following.

What make of teleconverter works best with either lens?
Will I be able to utilize the auto focus feature?
How many F stops will I forfeit on either of these lenses?


Any other helpful suggestions would be appreciated.

03-03-2011, 07:54 AM   #2
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If you can find it, and you don't mind manual focus, an A series 400mm (though not for the $1,000 asked by a current Ebay sellter) or a third party lens is a decent option.
03-03-2011, 07:59 AM   #3
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QuoteOriginally posted by Kootenay Image Quote
I am interested in purchasing a telephoto prime lens for my Pentax K20 in the 400mm range. After researching what is available I find a 400mm is not an option. All that appears to be available is the Pentax DA* 200mm or 300mm.

If I was to purchase either one of these along with a 2.0 teleconverter this would be the only way of achieving my goal of having a lens in the 400mm range.

I would be interested in hearing other people experience regarding the following.

What make of teleconverter works best with either lens?
Will I be able to utilize the auto focus feature?
How many F stops will I forfeit on either of these lenses?


Any other helpful suggestions would be appreciated.
There are some 400 mm lenses available on the used market, notibly the FA400F5.6 but you are correct, there is no current production pentax 400mm lens.

the 200F2.8 and 300F4 are both available, however note that there are no current production teleconverters from pentax.

ALso note that there are NO teleconverters that support fully SDM lenses, other than some older tamron adn kenko TCs that were designed for the FA power Zoom lenses and therefor ehave the required power contacts for SDM lenses used in the KAF2 mount.

One of the most common teleconverters that IS used with specifically the DA300MMF4 lens is the SMC-F 1.7X AF converter.

This is a special 1.xX teleconverter that has it's own internal focusing mechanism. You would manually focus close to correct focus with the lens, and then the camera would do the fine focus with the teleconverter.

This turns the 300F4 into a 500mmF6.7. There are many posts by users with this combination. I believe pentax still offers limited production quantities of this converter in the asian markets, and they are available on E-BAY.

I have had mine since 1991, and use it almost exclusively with the SMC (K) 300F4 on my K7. It is a great hand holdable 500MM lens.

Edit note,

I realized I did not really properly answer one point.

with respect to F stops lost,, any 1.4x TC costs 1 stop, a 1.7x TC costs 1.5 stops and a 2x TC costs 2 stops.

also note, any lens aperture plus teleconverter combination that puts the true aperture beyond F6.7 will NOT autofocus reliably.

Last edited by Lowell Goudge; 03-03-2011 at 08:05 AM.
03-03-2011, 10:58 AM   #4
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QuoteOriginally posted by Kootenay Image Quote
I am interested in purchasing a telephoto prime lens for my Pentax K20 in the 400mm range.
If 500 is "in the 400mm range", Sigma makes a 500mm f4.5. Not cheap.

03-03-2011, 03:10 PM   #5
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There is an A 400/5.6 for sale in the market place. This might do the trick for you.

Tom G

Last edited by 8540tomg; 03-06-2011 at 09:42 AM.
03-05-2011, 07:47 AM   #6
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Thanks for this, need some clarification re your comment about using the SMC-F 1.7X AF converter. You indicate it "will turn the 300F4 into a 500mmF6.7".

Below you say the "1.7x TC costs 1.5" which I interpret as turning the 300 F4 into 500mm F5.5, am I missing something?
03-05-2011, 09:59 AM   #7
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Use the TC factor as a multiplier. So 300/4 times 1.7x becomes 510/6.8, just as with a shorter lens and a 2x TC, 50/1.4 times 2x becomes 100/2.8, that's all.
03-05-2011, 10:38 AM   #8
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I tested a few TC on my 200mm recently and only the Pentax 1.7 worked without IQ loss, so I'm doing further testing with it. The AF, as others have mentioned, has a limited range, so you need to be somewhere near that range for it to work. In other words, you can't go from closest to infinity, but if you start in the middle, it can reach either end.
However, I found to my surprise, that if the focus is anywhere within the range when you AF, it is faster and more accurate than the 200 without the TC. It turns my f.2.8 into an f-4, if I remember correctly. Not only do you lose stops on the wide end, you lose some more stops throughout the normal progression. I can't remember which ones, and it really doesn't matter a lot, because you're only losing the number of stops, not effective stops.
Another thing I've noticed is a softness at infinity. My 200 has a slight softness at infinity to begin (either in AF or MF), and this seem to be multiplied by the TC.
Since the effective range is limited this way, I'm not actually gaining as much physical distance from the subject as the between 350mm and 400mm calculations would suggest.
Haven't made up my mind whether to buy one or not, but probably not.

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