[BigMackCam]

Apologies in advance for the dumb question

If I use a good quality 1.4x tele-converter (like Pentax's own), I appreciate I'll lose around one stop of light. However, I'm not clear on how it will affect general lens performance.

Regarding CAs (fringing, etc.), will it *magnify* them at any given aperture, or will the lens perform as if it was stopped down by one stop (ie. CAs reduced)? My assumption is that, regardless of the light loss, the converter will increase the effect of CAs...

Regarding resolution, will the resolution be reduced in terms of what reaches the sensor? My assumption is that it will...

Hence, before I start using a tele-converter, I really need to be working with a lens that has very acceptable CA and resolution performance, right?

Thanks in advance

[Adam]

You'll get the best results with premium lenses, that's for sure. But modern TC's like the DA 1.4x, or telephoto-optimized TC's like the A 1.4x-L do deliver a clear benefit in terms of resolution.

[Craig66]

A TC should not affect your image quality in any way. Other than the aperture 1 stop loss. The auto focus might be affected if you use a lens slower than F4, like a F5.6, F5.8 etc. But your image quality should stay the same as if only using the lens without a TC.


I use mine with my DA*200F2.8 and find it has no affect on the image quality. I've also used it with a Sigma 24-70F2.8 and 70mm F2.8 with good results.


As with any photo, the better quality the lens used, the better the image quality will be. But the TC shouldn't make it any worse.


Pentax-DA 1.4x AW AF Rear Converter Review - Introduction | PentaxForums.com Reviews

[DHA]

Hummmm...nothwithstanding the light loss (an f stop or more), it's just not true that the image isn't degraded to some extent. I've used many TCs over the years, including the Pentax 1.7AF, good Kenkos, Sigma EX DGs, etc., and image quality is always affected...sometimes not by much, but the degradation is there. Anyone who tells you otherwise is parading fiction as fact.

[MJSfoto1956]

my recommendation is to stop it down one additional stop. So an f/4 lens with a 1.4x TC becomes and effective f/5.8, but you should avoid shooting it wide open, instead stopping it down to at least f/8.

M

[K-Three]

Anytime you add glass and especially the air/glass surface transitions to an optical system, you add problems; chromatic aberration, coma, flatness, distortions, and others. This true whether the added glass is on the front of the lens, or between the lens and the body
In a complete, well designed lens, the issues are minimized within the constraints of the design, especially cost.
When you add a converter, it usually has not been optimized for that particular lens.
Converters should be used on the longest lens you can get, when you need a bit more reach; they should be avoided on zooms, as zooms usually have much more glass in them already.
A good well designed converter ($$$) will minimize the loss of quality, but I can't agree that it won't have any effect on you image quality. It may make issues that were not as apparent greater, and may appear to create issues that you have never seen before.
Like all the solutions, it's a compromise, you have to decide if it's one you are comfortable with.

[gmiyamoto]

I agree w/K-Three. A teleconverter will increase the flaws that already exist in a lens. A lens that suffers from chromatic aberration will show even more chromatic aberration. That's why it's best to use a teleconverter on a good lens. However, I think that it's still OK to use a TC w/a zoom if you need to get the shot & are prepared to correct any problems in post processing.

[UncleVanya]

I find the Pentax hd TC works very well on my DA* 50-135 and 60-250 zooms.

[clackers]

Yep, I have the 1.4 HD as well and it's fine on the 50-135 and Tamron 70-200.

[Stone G.]

image quality is always affected...sometimes not by much, but the degradation is there. Anyone who tells you otherwise is parading fiction as fact.

Generally speaking, this could/should be so, because any abberations would be magnified by some factor, and which lens is totally free from any abberations?


However, speaking about 'Image Quality Degradation' as the inevitable result requires (to me at least) some additional qualification. To many, image quality is equal to little noise (which is a film/sensor property) and then sharpness, which is our perception of resolution and contrast combined, plus minimal abberations (CA, coma, geometric distortions etc.). And with any good TC and any contemporary sensor, resolution will be increased with a proper, matching TC. Some practical examples where I have found a TC beneficial:


Tamron Adaptall-2 SP 35-80mm CF macro: Sharpness from corner to corner is definitely increased. Yes, you might say, because the FoV gets smaller - but that together with the increases resolution might actually be what I want for macro. So, I should compare a croped non-TC with the full with-TC image.


Tamron Adaptall-2 SP 350/f5.6 mirror lens: The field is flattened; catch-in-focus becomes possible and vignetting is being reduced. Again you may want to compare cropped and uncropped images and to me, the increased resolution makes images of for example the Moon with TC the clear winner.


Pentax HD 1.4X AF TC with the DA* 200/2.8 and the DFA 100/2.8 WR lenses: I have pixel-peeped and pixel-peeped and cannot see any degradation in normal situations (typically with lenses stopped down a bit). I see what I went for: Increased resolution (and no decrease in contrast). Of course, If I shoot over long distances in turbulent air, external degrading effects may be magnified, but that's no different from using any long(er) lens over large distances on a poor day.

[normhead]

Hummmm...nothwithstanding the light loss (an f stop or more), it's just not true that the image isn't degraded to some extent. I've used many TCs over the years, including the Pentax 1.7AF, good Kenkos, Sigma EX DGs, etc., and image quality is always affected...sometimes not by much, but the degradation is there. Anyone who tells you otherwise is parading fiction as fact.

The actual degradation caused by the Pentax 1.4 TC has been measured at 3%, and I've seen not seen work that suggests anyone can see a 3% difference. You are losing 3% resolution, but you are enlarging your subject 40% so the gain in subject resolution is 37%. And in fact, when I ran tests on my lenses with both primes and primes plus TC, the only real factor in subject resolution was the length of the lens. The longer the lens, the more subject resolution. Whether or not a TC was used was irrelevant. I've only tested with the Pentax 1.4 and Pentax 1.7, but I would't pay much attention to the above post. You use a TC because you want the subject to be bigger, that's where you increase your resolution. The overall resolution of the frame is pretty much irrelevant.

IN my collection, either the HD DA 1.4 or F 1.7 AF TC used with a Sigma 70 macro, Tamron 90 macro, DA 60-250, there is no noticeable degradation. With the F 70-210 the purple fringing and CA the lens is prone to is amplified when it is present. However it is no more prone to occurring with the TC, and in many images there is no visible purple fringing or CA.











One of these was taken with the 1.4 TC and 60-250 and one with the 1.7 an A-400. Can you pick out the ones taken with the TCs? It's almost certain, other factors than whether or not you were using a TC will determine the usefulness of the final image.

[BigMackCam]

Thanks for the responses, all - greatly appreciated

@normhead - thanks for that last post with the photos. The 60-250 is a nice starting point, of course, but results like that with the TC in the mix are impressive. Great shots!

[normhead]

Thanks. many of the guys I shoot with use a 70-200 ƒ2.8 with a 2x Tc to get to the equivalent of the 400 ƒ5.6 I have with my A-400. A fast lens with a TC seems to be the most popular way to shoot for many wildlife photographers. The thing is, when you take the TC off you have a 2.8 lens for larger wildlife, and for smaller animals and birds, you have 400 ƒ5.6. But those guys are shooting Nikon and Canon, I don't know of a quality 2x TC for Pentax.

[BigMackCam]

Thanks. many of the guys I shoot with use a 70-200 ƒ2.8 with a 2x Tc to get to the equivalent of the 400 ƒ5.6 I have with my A-400. A fast lens with a TC seems to be the most popular way to shoot for many wildlife photographers. The thing is, when you take the TC off you have a 2.8 lens for larger wildlife, and for smaller animals and birds, you have 400 ƒ5.6. But those guys are shooting Nikon and Canon, I don't know of a quality 2x TC for Pentax.

I think my biggest problem is going to be the speed of my existing glass. Right now, the only sharp (by my standards!) long lens I own is the HD 55-300, and I'm still getting used to it (though very happy so far). I'm going to need very good light to use it effectively with the 1.4x TC, and here in the UK we won't see much of that until next summer. The more I think about it, I believe I need to exhaust the possibilities of the 55-300 with some cropping before I spring for better / faster glass and / or TC...

[clackers]

I think my biggest problem is going to be the speed of my existing glass. Right now, the only sharp (by my standards!) long lens I own is the HD 55-300, and I'm still getting used to it (though very happy so far). I'm going to need very good light to use it effectively with the 1.4x TC

I've never been happy with that combination, BMC.

Hope you have better luck.