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03-20-2012, 02:35 AM   #1
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Zoom lens and converters

Hello all

I've looked in the forum and can't find an answer to this, so apologies if this has been covered. I have the Pentax 55-200 DA lens that came as a pair with my K-x and 18-55. I like it a lot, but want more reach, for wildlife and close up work. Would a converter work for this? Would I still have autofocus? I have my eye on the Sigma 70-300 APO as it has decent reviews and won't make my wife divorce me but would my money be better spent on a converter (ie its cheaper, see earlier comment about divorce!).

Thanks for any help/advice.

03-20-2012, 03:19 AM - 1 Like   #2
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Your DA 50-200 is a "slow" lens, with a maximum aperture of f/5.6 at 200mm. I used to have this lens, and it is definitely NOT recommended to use it with a converter: you will lose IQ, AF, light in the finder... The Sigma is a better choice in reach, but not necessarily in quality. If money is really tight why not try to crop your photos with your image management program on your computer? Your K-x has 12 mpix, and it is generally accepted that even 6 mpix are overkill for 10x15 cm prints (oops, 4x6"). So you can crop at least half of your pictures to make the subject appear bigger, "as if you had a 300mm lens".
Otherwise if you're going to spend money, spend it well. I hear a lot of good things about the Pentax DA 55-300, highly regarded as the best budget option. I'm considering getting one for myself...
Good luck!
03-20-2012, 03:35 AM   #3
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if i was in your shoes, i'd just sell the 50-200 and try to pick up a used 55-300
03-20-2012, 03:42 AM   #4
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QuoteOriginally posted by adpo Quote
if i was in your shoes, i'd just sell the 50-200 and try to pick up a used 55-300
Is that the pentax 55-300?

03-20-2012, 03:55 AM   #5
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QuoteOriginally posted by joshfishkins Quote
Is that the pentax 55-300?
More than likely
03-20-2012, 05:04 AM   #6
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QuoteOriginally posted by sterretje Quote
More than likely
Thanks. I was more interested in the macro function though, and that's A LOT more usually, although the SIGMA has it from 200-300 (I think).
03-20-2012, 05:31 AM   #7
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For "cheap" macro setup with any lens, I've heard good things about the Raynox adapters. It's probaby as good as it gets if you want <400 USD and with AE/AF! I wouldn't rely to much on the "Macro" is the Sigma lens title as they tend to use this word too often! Beware of marketing...
03-20-2012, 05:46 AM   #8
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I have Sigma 70-300 f4-5.6 APO Macro Super. I do not know what is the difference between "Super" and "non-super" versions of the lens though. I boought mine back in 2001 when I wanted more reach with my Pentax A3. The lens was with me ever since and it is still part of my "portable" lens collection. It has a macro at 300mm which goes to 1:2 magnification and it produces nice photos.

Now I do not want to make it a review, but if I would have to make a choice to take only three lenses with me, Sigma 70-300 would definitly be one of them (other 2 been Sigma 10-20 f4-5.6 and Tamron 28-70 f2.8). If I'm going out on a rainy day, then I put DA 18-55 WR (I guess I would use 18-135 WR if I had it).

Now for some comparison: If I am going to take photos of birds or wildlife, then I take Sigma 50-500 (bigma). For macro work I use Sigma 105 f2.8 macro, and for indoor events, better choice is Sigma 70-200 f2.8. Given price of all of these "beasts" 70-300 is a good deal. It is just a bit bigger than 105mm f2.8 macro and considerably smaller then 50-500 or 70-200 f2.8.

As for teleconverters, there are not many AF teleconverters out there. Sigma has 2 in current lineup (1.4x and 2x) but any of them costs as much (or more) then a decenr zoom reaching 300mm. If you look on the second hand market, with some luck, you could fine Kenko Pz AF 2X for under 100$, but there are just not too many out there. On the other hand you have to keep in mind that 2x teleconverters multiplies both the focal length and the apperture value, so your 55-200 f4-5.6 becomes 110-400 f8-11. That can work on bright sunshine for autofocus, but do not expect much in less than perfect light. Moreover the camera does not know that there is a teleconverter attached, so that the SR is going to work on the focal length reported by the lens (effectively on half power).

Now, If yor needs can get sattisfied by a prime, manual focus lens, you can find some decent deals on 400mm f6.3 (K or M42 mount) or even 500mm f8. If you have 30-70$ to play around that can be at least a temporary solution without having to cost a fortune.

03-20-2012, 07:44 AM   #9
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just to help you in your quest for more reach, and the use of TCs there are a few things to keep in mind.

the camera AF system will only be reliable and auto focus for lens plus TC combinations that are F6.7-F7 or faster. Considering that a 1.4 TC costs 1 stop, a 1.7x costs 1.5 stops and a 2x costs 2 stops on the base lens, you need to consider FAST lenses only when looking at TCs. generally if a lens is slower than F4 forget it

second, not all TCs (especially sigma) work on all lenses, so be careful;
03-20-2012, 09:19 AM   #10
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Behind-the-lens (BTL) TCs usually work best when a quality TC is matched to a high-end fast lens. Money is required all around. As mentioned, a BTL-TC on a long slow lens just doesn't work. If you want more reach for less money, your choices are basically:

* Buy longer lenses
* Crop images from an existing lens
* Use a front-of-the-lens TC which doesn't eat light

I haven't compared my behind-the-lens and front-of-the-lens TCs to see which impacts image quality most. But I'm not displeased with my Sony 1.5x front-loader. An Australian member here is selling in the marketplace a set similar to mine, Sony VCL HG strap-ons with 58mm rear thread and 0.7x and 1.7x optics. The tele adapter would turn a zoom at 200/5.6 into 340/5.6 (and it retains AF and auto-aperture), whereas an expensive 1.7x rear-loading TC would make it a 340/9.6, and a cheap rear TC would lose AF.

But for better image quality, look to a DA55-300 instead. Or boost your budget / rob a minimart to buy a Tamron 70-200/2.8. Or just learn to sneak up closer to birds.

PS: For macro, I'll second the Raynox recomendation.
03-20-2012, 09:44 AM   #11
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A teleconverter will work to turn an ordinary zoom into a longer zoom, but the result is almost always *worse* than simply cropping. So you're spending money to get worse image quality. Complete waste of $100.

On the other hand, a good quality close lens like the Raynox 150, when paired with an ordinary zoom lens, turns it into a *far* better macro lens than any quasi-macro zoom like the Sigma 70-300 could ever hope to be. An excellent use of your $50.
03-20-2012, 11:43 AM - 1 Like   #12
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what does eating light really mean?

just to be clear about RioRico's comment that in front of the lens macro converters (or close up lenses / diopters) do not eat light, a simple explanation of how close up lenses, tele converters and macro lenses work in general is needed.

Since the image size, is the ratio of subject to lens / lens to sensor, to get a bigger image you either need to reduce greatly the subject to lens distance or greatly increase the lens to sensor distance or a combination of both.

As the image gets larger, and is spread out over a larger area the image brightness goes down. That's life.

But depending on how you achieve magnification has a big impact on the image brightness, although it is not clear at the onset.

If you use extension tubes (to move the lens further away from the sensor plane, and allow closer focusing) the image naturally and intuitively gets dimmer because you have not changed the lens opening,

The same is true with a tele converter, because a tele converter increases the focal length, with out changing the aperture diameter, therefore the spreads the image out over a larger area (based upon its magnification ratio) and the light falls off in a similar manner to extension tubes for the same final image size.

Close up lenses, however are different. when you add a close up lens to the front of the lens, you are changing (reducing) the focal length of your lens, but you are not changing physically the diameter of the aperture or front element of the lens, so the effective F stop actually goes down,

For example your 50mmF2 with a +10 diopter close up lens becomes a 33mm F1.3 lens. extended on a 17mm extension tube.

Another way to consider this is because the focal length is reduced, to achieve the same magnification, you move closer to the subject and the lens takes in more of the total light reflected off the subject.

as a result close up lenses give brighter view finders, but regardless of the manufacture of a close up lens, they will regrade the image , especially off center) comparted to using extension tubes. FOr macro however, unless you are using the lens for true reproduction of flat objects, you may not care about off center image quality, and loss of sharpness at the edges might be creatively interesting. You can, after all crop off center as long as you consider the final cropping when taking the shot, if you want off center subjects sharp
03-20-2012, 12:00 PM   #13
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I agree, simple answer, sell the 50-200 and get a Pentax DA55-300. You wont be disappointed.
03-20-2012, 02:44 PM   #14
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QuoteOriginally posted by Lowell Goudge Quote
what does eating light really mean?
It means you are a black body.
03-20-2012, 06:07 PM   #15
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QuoteOriginally posted by Lowell Goudge Quote
just to be clear about RioRico's comment that in front of the lens macro converters (or close up lenses / diopters) do not eat light, a simple explanation of how close up lenses, tele converters and macro lenses work in general is needed.
I was actually referring to screw-on tele-converters, but the principle is the same: changing the effective focal length without changing the iris or objective diameters.

QuoteOriginally posted by civiletti Quote
QuoteOriginally posted by Lowell Goudge Quote
what does eating light really mean?
It means you are a black body.
Only at absolute zero, right? Brrr...
[/me dashes through desert snow]
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