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06-09-2008, 06:55 PM   #1
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Teleconverter vs Zoom & Crop?

Hey everyone,

I have some Q's regarding teleconverters. Hopefully someone can enlighten me?

Firstly, i'm really luv'n my new 18-250mm.
Now I don't have to keep changing out between my 18-55mm kit lens, and the 50-200mm, lens i grabbed when i purchased my K100D last summer.

In this humble amatures humble opinion, this has to be the altimate “walk'n around” lens.

So now that i don't have to stress fumbling between two lenses, i;m wondering if a teleconverter would be a good choise if i wanted to be greedy and grab a bit more reach (for macro and wildlife/birding.)?

My question is specifically...
While a TC can magnify a subject, the trade-off seems to be your resolution is halved (assuming a 2x, TC).
So are the results much better than “digital zoom” like a point and shoot, does in-camera.
Or in my case, shooting RAW and cropping merclessly untill the subjuct is compsed within the shot?

Am i correct in assuming my new 10mp K200D, would effectively be out-putting @ 5mp (that can;t be right!) when a TC is introduced?

So who's using a TC around here, do you like it?
Would I?

Thanks in advance if anyone has any insite.
Pretty new to the SLR thing,

Teek

06-09-2008, 07:06 PM   #2
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Well, I'm no optics expert, but I would assume that a theoretically perfect TC would be equal to a lens of the converted length... i.e. a 50mm on a 2X TC would produce the same image as a 100mm lens. Of course the problem is that TCs are not perfect, and alot of them seem to stand for Total Crap. I'd figure a high quality/price unit would be superior to a center crop, but with a bad one, you probably be better off cropping. I'm sure you'll get a better answer, but that's my .02.
06-09-2008, 07:29 PM   #3
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i think it really depends on the prime lens.

I would not consider a TC on an ultra zoom like you have.

I use one, but on a very fast 70-200 F2.8

See the results below

https://www.pentaxforums.com/forums/187646-post19.html
06-09-2008, 07:30 PM   #4
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QuoteOriginally posted by heliphoto Quote
Well, I'm no optics expert, but I would assume that a theoretically perfect TC would be equal to a lens of the converted length..
I was under the impression that since a TC is merely relaying what is diplayed on the actuall lens and passing <i>that</i> image to the camera. there would be a loss there.

QuoteOriginally posted by heliphoto Quote
... i.e. a 50mm on a 2X TC would produce the same image as a 100mm lens.
...but @ half the resolution no? In other words, you could'nt get the same result by taking the shot @ 50mm and enlarging half the pixels u want too keep by 100% ?

That <i>can't</i> be right, im on the wrong track here...


QuoteOriginally posted by heliphoto Quote
Of course the problem is that TCs are not perfect, and alot of them seem to stand for Total Crap. I'd figure a high quality/price unit would be superior to a center crop, but with a bad one, you probably be better off cropping. I'm sure you'll get a better answer, but that's my .02.
Never hurts to throw it out there, much appreciated.
Moneys not really a problame-o. Its weather or not there is even a quality TC available for my lens/body combo.
Thanks again for the .02 man,

Teek

06-09-2008, 07:49 PM   #5
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QuoteOriginally posted by Lowell Goudge Quote
i think it really depends on the prime lens.

I would not consider a TC on an ultra zoom like you have.

I use one, but on a very fast 70-200 F2.8
Bummer. You mean my lens won't let enough light in to be any good for much?

QuoteOriginally posted by Lowell Goudge Quote
Nice. Sharp too. Is that @ 200mm?

QuoteOriginally posted by Lowell Goudge Quote
I just wish the shake reduction could be set to accomodate the TC
Grrr. I did'nt even think of that.
06-09-2008, 09:50 PM   #6
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QuoteOriginally posted by Lowell Goudge Quote
i think it really depends on the prime lens.

I would not consider a TC on an ultra zoom like you have.

I use one, but on a very fast 70-200 F2.8

See the results below

https://www.pentaxforums.com/forums/187646-post19.html
I think you are 100% correct.
Expanding slightly, getting into focal lengths past 250mm benefit greatly from using a tripod. Shake reduction does not mean shake elimination, so some sort of support should be used, especially with the apertures he would be working at
I'm wondering if, when Pentax puts a TC back into the line-up, how they will deal with it in the firmware, though I'm not terribly surprised that they aren't dealing with it now.
06-10-2008, 02:36 AM   #7
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QuoteOriginally posted by TeeK Quote
While a TC can magnify a subject, the trade-off seems to be your resolution is halved (assuming a 2x, TC).
So are the results much better than “digital zoom” like a point and shoot, does in-camera.
Or in my case, shooting RAW and cropping merclessly untill the subjuct is compsed within the shot?
There have been numerous comparisons between upsized images vs. images taken with teleconverters. When done seriously (same subject, stable tripod), these comparisons are usually in favor of the teleconverter (see this comparison for example). Of course, the old rule applies: garbage in, garbage out. If you start with a low quality consumer zoom or with a crappy teleconverter, your results will surely be disappointing.

Cheers!

Abbazz
06-10-2008, 04:43 AM   #8
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QuoteOriginally posted by TeeK Quote
Bummer. You mean my lens won't let enough light in to be any good for much?
that is my own view, others will say differently, but for autofocus to work well the total needs to be kept to F6.7 or faster.
QuoteQuote:

Nice. Sharp too. Is that @ 200mm?
Thanks, this was using the 2x TC zoomed all the way in so effectively 400mm. this is a very small falcon, only about 11-12 inches high. No crop and no PP just resize

QuoteQuote:
Grrr. I did'nt even think of that.
yes shake reduction uises the passed through focal length not the real one, because there is no way to override it. The shot is a little light also, due to the exposure error, which is an issue with the K10D where it must know the real wide open apature because the metering is not linear as a function of apature. Pentax correct this in software, but the TC does not modify the apature value, and with Fast lenses, introduces an exposure error. I have actually plotted this issue in fact for all my lenses, just to know how to set them.

06-10-2008, 02:54 PM   #9
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QuoteOriginally posted by TeeK Quote
Hey everyone,
My question is specifically...
While a TC can magnify a subject, the trade-off seems to be your resolution is halved (assuming a 2x, TC).
Umm, no. The resolution is determined by the sensor, not the lens. What a TC does is add extra glass to the lens. Since your SMC lens has been designed with great precision to produce an image with the glass used in its own construction, when you add a TC you will get a degradation of the image quality. Of course, some TCs are better than others and the degradation may not be noticeable.

The resolution of the picture remains at whatever you set the camera to, but when you add extra glass the image quality will change. Now, there is a bit of a terminology problem here. Another way to describe image quality is to talk about the resolving power of the lens (ie, how good is the lens at showing detail). But again, that is not the same as the sensor resolution.

QuoteOriginally posted by TeeK Quote
Am i correct in assuming my new 10mp K200D, would effectively be out-putting @ 5mp (that can;t be right!) when a TC is introduced?
Nope, you are talking about the resolution of the image coming from the sensor, not the resolving power of the lens. So you cannot say that there is any connection between that and the use of a TC. Also, note that your resolution value set in the camera (on my K100D that is 6/4/1.5M) is only available for jpegs, and it determines the number of pixels that are in the final image. A TC will not affect this - it cannot! If you shoot RAW (as you should) the number of pixels will always be the same - it is the number of pixels in the sensor.

But the TC can reduce the amount of light, it can add flare from reflections caused by lenses with poor coatings, it can add distortions and colour aberations to the image, and there will be nothing that you can do to repair the image. It is important, therefore, that the quality of the TC is at least as good as the lens you will attach it to.

You're right that digital zooms are a rip off, but they rely on the fact that most people will view their digital images on 6x4 or relatively small images on screen and that modern cameras (even P & S cameras) have high resolution sensors, producing images with lots of pixels, and so digital magnification is possible to a point. In this case, you are reducing the resolution because the magnification uses fewer of the original pixels (most likely by creating more using some form of interpolation). But if you intend to view your images printed out large, say A3, then a digital maginified image will be apparent as chunky pixels (interpolation methods are not perfect), but that is because you are magnifying it twice.

So it is swings and roundabouts. If you have a good quality lens then you can afford to digitally magnify it and how much depends on the lens. If you are used to viewing images as 6x4 and people tell you that the lens produces sharp results at 10x7 then you know that you can digitally magnify by 1.7 and crop to get a good image (ie just get the 6x4 from the 10x7 image). In that case your image would be better than taking the photo using the lens and a x1.7 TC.

For example, recently I have been photographing damsel flies on my garden pond. I find that I get better images with my SMCP-A 70-210 at 210mm than with my Sigma 70-300 DL at 300mm when the A70-210 image is multiplied by x1.4 to get the same size as the Sigma images. The Pentax lens is simply a better lens (at least 1.4 times better ). If I added a x1.4 TC to my Pentax lens would it still be better quality than the Sigma? I don't know.

Richard
06-10-2008, 04:29 PM   #10
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I want to add some general remarks about TCs and lenses:
– TCs are generally computed to work with tele lenses, mostly prime ones
- some TCs are dedicated modells, optimized for use with a limited number of certain lenses, sometimes even zoom lenses (like the Sigma APO tcs)
- wide angle lenses (or the wide end of a zoom) usually don't go well with TCs
- zoom lenses don't mix well with TCs, unless there is a dedicated TC available for that lens.
- TCs vary greatly in image quality. You really get what you pay for.

- a 1.4x TC will reduce the maximum aperture of the lens 1 stop - so a f/4.0 lens becomes a f/5.6
- a 2x TC will reduce the max. aperture of the lens 2 stops, so a f/4.0 lens will become a f/8.0 lens
- to obtain decent image quality with most TCs you need to stept down the aperture at least 1 stop, sometimes 2 stops.. Example: a f/4.0 lens + 2x TC = f/8.0, stopped down 2 stops, will effectively be used as an f/16 lens. In this are the image quality already breaks down, due to diffraction.

- the resolving power of a lens is determined by its open diameter. A TC does not change that. So the lens +TC will in theory have the same resolving power (nothing added or lost) as the lens on its own. But ofcourse the TC takes its toll (see above)

- TCs available for Pentax usually do not communicate their presence to the camera. So the shake reduction will only work with the focal length of the lens attached. The EXIF data will be ignorant of the TC as well and show only the data for the lens.

So, I think, that was the short list of things one needs to be aware off, when working with TCs. In the "good old days", photogs only used TCs for their longest focal length lenses, to give just that occasionally needed extra bit of reach. I think, this is still wise to do.

Ben
06-10-2008, 04:42 PM   #11
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QuoteOriginally posted by richard64 Quote
For example, recently I have been photographing damsel flies on my garden pond. I find that I get better images with my SMCP-A 70-210 at 210mm than with my Sigma 70-300 DL at 300mm when the A70-210 image is multiplied by x1.4 to get the same size as the Sigma images. The Pentax lens is simply a better lens (at least 1.4 times better ). If I added a x1.4 TC to my Pentax lens would it still be better quality than the Sigma? I don't know.
My understanding (not having yet used one) is that a good 1.4x will not appreciably degrade the image. Though of course you lose light. Also that Sigma is soft-ish after about 200mm, methinks, as is my Sigma 70-300 APO DG. This combination of facts would certainly explain your result.

Damselflies -- cool!

Last edited by rparmar; 06-10-2008 at 07:29 PM.
06-10-2008, 05:26 PM   #12
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You say you like the super zoom so you don't have to change lenses.

You will have to take off the lens to use a TC. Forget about using the TC with your super zoom.

Your basic choices are:

) get a 200mm and a TC and always use tem together to get more reach than your super zoom. Or just buy a longer lens without the need for a TC.

) get a 300 or 400 and a TC. Use the lens both with anxd without the TC to get two lens lengths.
06-11-2008, 10:01 AM   #13
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QuoteOriginally posted by rparmar Quote
My understanding (not having yet used one) is that a good 1.4x will not appreciably degrade the image. Though of course you lose light.
Indeed, but the quality is the important point. A cheap TC will degrade the image, and you may find that digital magnification is better. That's the point I was trying to make.

QuoteOriginally posted by rparmar Quote
Also that Sigma is soft-ish after about 200mm, methinks, as is my Sigma 70-300 APO DG. This combination of facts would certainly explain your result.
I guess so. It is not as heavy, or as large, as the A70-210, and it is AF, so it does have its good points

QuoteOriginally posted by rparmar Quote
Damselflies -- cool!
I'll post some when I have the time. :-)

Richard
06-11-2008, 03:30 PM   #14
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OK.
Any tips for GOOD 2x AF teleconverters?
I mean not just brand, but which ones specificaly are the good ones?
Thanx
06-11-2008, 06:45 PM   #15
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QuoteOriginally posted by axl Quote
OK.
Any tips for GOOD 2x AF teleconverters?
I mean not just brand, but which ones specificaly are the good ones?
Thanx
See my post above, if it is to go with a fast Sigma, take the APO 2x EF DG.

It also works with a select list of Pentax long lenses
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