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10-23-2013, 03:18 PM   #16
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What are the cusps? I'm not familiar with that term.

10-23-2013, 03:37 PM   #17
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I found another with some crazy highlights with a 300mm (canon)and 1.4x wide open. There was a real good thread a long time ago about specular highlights and had some similar bokeh caused by aspherical lens elements.

10-23-2013, 04:16 PM   #18
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@G_Money & @borno, Thanks for the sample images! I think the Fox shots from borno are most representative of what I feel like i've been seeing in regard to the branches in the OOF background. It seems like there is more distortion on the branches than their would be without the TC. I suppose a direct with and without comparison would be needed to confirm that.
10-23-2013, 07:28 PM   #19
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You raise an interesting question I've used a variety of TCs with a variety of lenses but I've only thought about it in terms of reach, sharpness and mechanical characteristics but, since--unlike extension tubes--there's glass in these appliances, it stands to reason that they can have some impact on the image in terms of bokeh, color rendering, etc. I've never made any effort to suss out those subtleties, though and to be honest probably won't give it the methodical attention required to draw any conclusions.

10-23-2013, 09:04 PM   #20
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TC's

Consider that passing a beam of 'white' light through a prism (or lens, or any change in density) results in the full spectrum of frequencies being diffracted into the classic rainbow of colors. After magnification to achieve a given FL, optical formulas must re-combine the rainbow (frequencies) to a common point at one fixed registery distance on the sensor plane. Add extension tubes, TC's, etc, and the sensor is no longer at the original (designed) point of focal re-combination. (Who moved the cheese?)

Re-focusing the lens then 'fixes' the intended plane of focus, but the OOF part of the image begins to show dispersion effects more so than with the original configuration and relative to the amount of magnification applied. Donut highlights, coma, astigmatism, fringing, etc, would naturally not benefit from the changed registry distance even though the new plane of focus may remain fairly sharp.

I think it's amazing that quality TC's manage to optically re-register the focal plane and re-combine the rainbow as well as they do -- especially considering that, unlike manipulating the FL of one given lens with added optical elements, the designer doesn't know which of many possible lenses the TC will be tasked to pair with.

The tele converters designed for the Adaptall lenses are generally superior to most because they were limited to a specific family of lenses via the unique lens-to-adapter mount.

I've found that pairing TC's with lenses from the same designer/manufacturer and from the same optical design era usually gives the best relative results. That is, the optical formula of a modern, Pentax AF 1.7 TC may not be impressive when matched with an old SMC M 300/4 while an older M-class TC may perform fairly well considering their inherent IQ capability.

The extremely complex optical formulas used in the modern * and Limited lenses certainly inhibits the marketing of a "universal" TC. The "risk" of having a formula-matched TC used with just any ol' lens with a PK mount and then bad-mouthed for poor performance in these forums just isn't an attractive thing to take to market today. The R&D cost doesn't match the market at the ROI cost.
H2

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Last edited by pacerr; 10-23-2013 at 09:16 PM.
10-24-2013, 05:21 AM   #21
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QuoteOriginally posted by pacerr Quote
I've found that pairing TC's with lenses from the same designer/manufacturer and from the same optical design era usually gives the best relative results.
Me, too.
10-24-2013, 11:43 AM   #22
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QuoteOriginally posted by pacerr Quote
Consider that passing a beam of 'white' light through a prism (or lens, or any change in density) results in the full spectrum of frequencies being diffracted into the classic rainbow of colors. After magnification to achieve a given FL, optical formulas must re-combine the rainbow (frequencies) to a common point at one fixed registery distance on the sensor plane. Add extension tubes, TC's, etc, and the sensor is no longer at the original (designed) point of focal re-combination. (Who moved the cheese?)

Re-focusing the lens then 'fixes' the intended plane of focus, but the OOF part of the image begins to show dispersion effects more so than with the original configuration and relative to the amount of magnification applied. Donut highlights, coma, astigmatism, fringing, etc, would naturally not benefit from the changed registry distance even though the new plane of focus may remain fairly sharp.

I think it's amazing that quality TC's manage to optically re-register the focal plane and re-combine the rainbow as well as they do -- especially considering that, unlike manipulating the FL of one given lens with added optical elements, the designer doesn't know which of many possible lenses the TC will be tasked to pair with.

The tele converters designed for the Adaptall lenses are generally superior to most because they were limited to a specific family of lenses via the unique lens-to-adapter mount.

I've found that pairing TC's with lenses from the same designer/manufacturer and from the same optical design era usually gives the best relative results. That is, the optical formula of a modern, Pentax AF 1.7 TC may not be impressive when matched with an old SMC M 300/4 while an older M-class TC may perform fairly well considering their inherent IQ capability.

The extremely complex optical formulas used in the modern * and Limited lenses certainly inhibits the marketing of a "universal" TC. The "risk" of having a formula-matched TC used with just any ol' lens with a PK mount and then bad-mouthed for poor performance in these forums just isn't an attractive thing to take to market today. The R&D cost doesn't match the market at the ROI cost.
H2

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Hi pacerr,

Excuse me for going off on a tangent

I agree with just about everything that you have posted here, with the possible exception of the your implication that TCs should be matched to lens designer/mfg/era to get the best relative results. I find that statements like this in this era of too much information on the 'net are too often turned into "rules of thumb" which have a way of discouraging experimentation. Personally, I probably use TCs more often than the great majority of shooters, and purposely stay away from a "matching" mindset when choosing either to use one at all, or which one to use. Experimentation with a specific TC combined with a specific lens is really the only way to determine whether a combination will give acceptable results. I've found that great optics in the lens with a quality TC (or even stacked TCs if the lens is fast enough) is about as far as I'd go to predicting a reasonable "match". I also generally disagree with those who suggest that Sigma TCs are "matched" to their long tele lenses.

A few of the better combinations I've used that are pretty far from "matches" for era and design are the F 1.7x AFA with a Canon FD 300 f4 L (which predates the AFA by something like 15 years, and was obviously has no common design elements), and the Sigma EX 180 f3.5 APO DG Macro, which is a much newer lens (probably about 10 years post AFA's discontinuation). The common characteristics that these lenses share are that they both show excellent resolution and CA/PF control. With the mindset that best results come from "matched" designer/mfg/era, I might have dismissed even trying these combinations. . .

I'm not disputing your findings, nor am I saying that you're advocating anyone generalizing your observation into some kind of "rule of thumb". You've included appropriate qualifiers to your statement and I'm sure that they are justified by your experience with the lenses that you've tried. . . just saying that I have not found the same correlation.

Back to the OP though -- bokeh -- My observations have been that the TCs that I use -- Sigma EX 1.4x APO, Tamron F 1.4x PZ MC4 AF, Tamron Adaptall 2 mdl 140 F, 3 different Pentax F 1.7x AFAs + a Tamron "A" type 2x Macro Focusing TC for macro stuff with a fast 50 -- can all make bokeh a bit busy when the distance to background objects fall within a certain distance range. I'm no optical engineer, and have no idea what causes this. My reaction is that there's nothing I can do about this but deal with it in PP to get a pleasing final image. As an opportunistic birder, I usually have little choice when it comes to background distance since the birds choose where they're going to land. The only real alternative for me (for shooting long tele) is to spend multiple thousands of dollars per lens to get marginal IQ gains for a relatively small percentage of shots (regarding bokeh specifically). Bokeh is very low on the priority list for me when it comes to TC use. Just speculation since I don't own or use either, but I'd expect that the Pentax A Rear Converter Ls have the best chance of providing the best bokeh for the lenses they will mount to. I have an A RC 2xS, and don't use it since I get better performance using two 1.4x TCs stacked if I want 2x.

I use TCs to get more reach, and central area resolution and sharpness are really my only priorities. I prefer TCs over cropping because they give me better focusing and metering accuracy, and the magnified subject in the VF allows me greater opportunity to catch desirable poses and expressions.

I find this discussion enlightening, but in reality, it serves little practical purpose. As pacerr noted, there is little incentive for anyone (except Pentax for an SDM compatible TC) to spend their R&D capital on K mount TCs, and even if there were a number of K mount TCs in the works, I doubt that bokeh would be a high priority concern in the design of any general purpose TC.

Scott
10-24-2013, 12:25 PM   #23
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No doubt if one has a great enough selection of lenses and TC's to experiment with there's always the statistical possibility of finding serendipitous matches. but if I HAD to bet, my experience is the house odds favor pairings from the same design era.

Cheap older optics makes it a fun game to play and finding one of those outliers keeps the dream alive, eh? It's one of the least expensive forms of LBA too.

H2

10-24-2013, 02:37 PM   #24
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QuoteOriginally posted by pacerr Quote

Cheap older optics makes it a fun game to play and finding one of those outliers keeps the dream alive, eh? It's one of the least expensive forms of LBA too.
Indeed! I'd say that a relatively modest investment in TCs (less than $500) has saved me well over 10x that much in long glass. Using them also allows a lot more FL flexibility with virtually no increase in carry weight. Multiple fast Super teles add at least 7 lbs for every additional lens. . .

Scott
10-24-2013, 03:05 PM   #25
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QuoteOriginally posted by G_Money Quote

Here's a crop of an image with an A*600mm + 1.4X-L converter. I think it looks a little like the donut bokeh from a mirror lens. I'm guessing it's because of the way the tc extends its optics into the body of the host lens. I'm pretty sure that if the tc was removed, the oof highlights would look different.
That shot says to me the TC does impose a fingerprint on bokeh.

I noticed increased fringing with a Kenko PZ 1.5X TC used on an A*300mm f4. I never saw anything in the bokeh with either the DA*300 or A*300 that caused concern, other than the fringing on the older lens.
10-24-2013, 03:57 PM - 1 Like   #26
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TC bokeh

QuoteQuote:
. . . the TC does impose a fingerprint on bokeh.
How could it not have an effect?

I believe this image from the Wikipedia topic on Aberrations is useful.

Recombining colors to the common point of focus on the sensor after adjusting the optics for FL is a complex trick of optical magic. If we add additional registery distance between the lens and sensor with a TC or extension tube it's still possible to re-focus on the plane-of-focus at the sensor. But the OOF areas (bokeh) before and behind the new plane-of-focus are proportionally more difficult to restore to the original condition as seen with only the lens itself.

Just imagine extending the focal point by the physical length of the TC or tube and note the additional dispersion of the rays. Keep in mind that each point of 'light' passes through every point on the surface of the lens, not just the two points illustrated here. An entirely different optical formula would compensate for a longer prime lens . . . but how do you calculate the corrections in a TC that might be used with any lens?

It's been my experience that a superior lens and a well matched TC will often produce very nearly identical sharpness for the in-focus part of the image, but the OOF region will not be as sharp, bokeh will degrade, and a tendency toward more flare in the OOF regions will cause a reduction in contrast. The overall appearance is of a much less sharp image when using TC's. (Do TC images of a flat plane focus chart always seem sharper than real world subjects to you too? This is also one reason why hyper focal DoF is a subjective quality.)

This illustration doesn't show a point of light originating exactly centered on the lens axis that suffers no diffraction on axis or a point of light perpendicular to the lens at the outer rim (say, the top of the lens) that is diffracted more from the bottom to the plane of focus than from the top. Diagram a few ray paths to the OOF regions and it becomes easier to see how donut bokeh spots and blur occur.



H2

Last edited by pacerr; 10-24-2013 at 04:27 PM.
10-24-2013, 06:24 PM   #27
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I'm anxiously awaiting the Pentax SDM TC. Bokeh and aberrations are a concern certainly, but the main concern I have is auto-focus. Will my new Pentax TC compromise auto-focus on the DA*300?
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