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05-07-2012, 08:22 PM   #1
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Do teleconverters count toward diffraction equation?

If we are aiming for f4 or 5.6 and use a lens that is f4 with a 1.4 teleconverter do we get the diffraction of an f4 or f5.6 lens? How about extension tubes in macro?
thanks
barondla

05-07-2012, 08:28 PM   #2
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When using TC's, I think the degradation of IQ will become noticeable much faster than the diffraction. With that said, I believe that it's the effective aperture that counts.

Adam
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05-08-2012, 12:23 AM   #3
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QuoteOriginally posted by barondla Quote
If we are aiming for f4 or 5.6 and use a lens that is f4 with a 1.4 teleconverter do we get the diffraction of an f4 or f5.6 lens? How about extension tubes in macro?
thanks
barondla
It is the effective aperture which counts, i.e., "image width" divided by aperture diameter a. "Image width" is a term used in German optics texts (Bildweite b) and denotes the distance between the principle plane and focal plane (where the image is formed). "Subject width" (Gegenstandsweite g) would be the other distance between principle plane and subject. I.e., magnification M=b/g.

Effective aperture is b/a. Use the thin lens equation to compute b from g and f, and of course, a TC multiplies f by a factor.

Therefore both, a TC and focusing, change the image width, read, change the effective aperture.

Therefore, at 1:1 macro photography with a 2x TC using a lens set to f/4, the effective aperture (as far as light gathering, field of view, depth of field and diffraction blur are concerned) is f/16!

This is why Adam is not correct. Each lens has a sweet spot which is determined as the point where blur from optical aberrations and diffraction are equal. With a lens where this point is at f/5.6, you can't expect better with a 2x TC than f/11 performance which is soft due to diffraction. Of course assuming a TC which is optically excellent. An average TC adds aberration blur shifting the sweet spot further down, maybe by another spot.

This is why an excellent 2x TC works typically well with f/2.8 lenses having their sweet spot below f/4. And why an average 1.4x TC works not so well with f/5.6 sweet spot lenses. It's all due to diffraction.

Last edited by falconeye; 05-08-2012 at 12:38 AM.
05-08-2012, 02:29 AM   #4
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Adam: ". . . . I think the degradation of IQ will become noticeable much faster than the diffraction. With that said, I believe that it's the effective aperture that counts." [my stress]

Falconeye: "It is the effective aperture which counts . . . . . . . .This is why Adam is not correct
." [my stress]

Falconeye,
is there something here that needs explaining? Why do you echo Adam - practically quote him - and then say he's wrong?

Adam,
please enlighten this tyro (me). I presume that "IQ" in this context does not mean what it usually signifies i.e. "Intelligence Quotient". What does it mean? Does it signify what Falconeye calls "aberration blur"?

05-08-2012, 03:48 AM   #5
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QuoteOriginally posted by barondla Quote
How about extension tubes in macro

I may also be incorrect on this one, but... It is to my understanding that either extension tubes or bellows do not effect the otpical ability of the lens
05-08-2012, 04:24 AM - 1 Like   #6
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Diffraction is the bending of a wave around objects or the spreading after passing through a gap. The gap here is caused by the aperture blades. I have not seen any teleconverter so far with aperture blades. The diffraction is therefore caused by the narrow gap presented by the aperture blades in the lens. Diffraction processes are most noticeable when the obstruction or gap ( aperture ) is about the same size as the wavelength of the impinging wave. Diffraction occurs at all apertures but is not noticeable enough till the aperture becomes very small, i.e. closer to the wavelength of light. If you consider that a teleconverter is similar in operation to a magnifying glass, what it does is cause the dffraction effects to become more noticeable, as also any aberrations already caused by the lens itself, however a teleconverter itself does not cause diffraction.
05-08-2012, 05:55 AM   #7
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QuoteOriginally posted by selar Quote
Diffraction is the bending of a wave around objects or the spreading after passing through a gap. The gap here is caused by the aperture blades. I have not seen any teleconverter so far with aperture blades....<snip>
Excellent, and correct, answer. Thanks to selar for answering the OP's actual question without this forum's usual self-important dross of opinion and ad hominems.

QuoteOriginally posted by Kendrick Pereira Quote
I presume that "IQ" in this context does not mean what it usually signifies i.e. "Intelligence Quotient". What does it mean?
IQ = "image quality" (the subjective kind)
05-08-2012, 06:09 AM   #8
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Just taking an educated guess here, but I would expect that extension tubes would also increase the effect of diffraction, simply due to the diffraction "spreading" more as you increase the distance from the aperture blade on the way to the sensor. So again the effective aperture becomes the point of diffraction, rather than the actual aperture.

Again not certain on this, but following my understanding of how diffraction occurs, this would be the case.

05-08-2012, 06:58 AM   #9
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QuoteOriginally posted by jezza323 Quote
Just taking an educated guess here, but I would expect that extension tubes would also increase the effect of diffraction

Yes that would indeed be scientifically correct; and I stand corrected on that one. But for enhancing macro; minus an actual marco label on a lens, this would be the best result optically; much better than a TC or filters
05-08-2012, 02:46 PM   #10
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QuoteOriginally posted by Kendrick Pereira Quote
Adam: ". . . . I think the degradation of IQ will become noticeable much faster than the diffraction. With that said, I believe that it's the effective aperture that counts." [my stress]

Falconeye: "It is the effective aperture which counts . . . . . . . .This is why Adam is not correct
." [my stress]

Falconeye,
is there something here that needs explaining?
Yes, you highlighted the wrong parts.

Adam thinks that image quality is degraded by a TC such that diffraction is a non issue. And I disagree and I explained why.

QuoteOriginally posted by Medium FormatPro Quote
It is to my understanding that either extension tubes or bellows do not effect the otpical ability of the lens
Diffraction depends on the effective f-stop number. And both a tele converter as well as close focussing (such as enabled by extension tubes) increase the effective f-stop.

Note that the effective f-stop is the distance between the lens principal plane and focal plane in mm, divided by the aperture diameter in mm.

Both a TC and close focusing change that distance.

QuoteOriginally posted by selar Quote
Diffraction is the bending of a wave around objects or the spreading after passing through a gap. The gap here is caused by the aperture blades.
This notion isn't very helpful.

Diffraction is a result of the Heisenberg principle that you cannot both determine directional and positional information.

The more you constrain a photon's position (e.g., by making a lens smaller), the more the photon will choose an arbitrary direction leading to hitting an arbitrary pixel.

Whatever constrains a photon's spatial extent contributes to diffraction. Don't think in terms of passing a gap, that's just a historic experiment taught in university lectures.

BTW, that's basic knowledge all the people who report about light field cameras severely lack ... (read: there's a reason why the Lytro's resolution sucks. It has to.)
05-08-2012, 03:06 PM   #11
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I've been complaining (read; "whining" ) about IQ degradation in a few threads, specifically as it relates to my favorite lens, and most often used, the DA*300/4, when coupled with at least a couple of TC's: the Pentax-F 1.7 x AF Adapter and the Tamron 1.4X Pz-AF MC4.

I can almost feel that some users of this lens/TC(s) will jump in an offer samples of images demonstrating perfectly fine results but ... that should go to another thread.

The sweet spot for this lens, at least in my case, is F4.5-F5.0, when used without a TC.
Perhaps some people will say that their DA*300/4 sweet spot is F5.6 but that is not the point of the argument in this thread.

Is there a way to figure out what would be the combo, lens+TC, theoritical "sweet spot" where you'd get as little loss of IQ possible?

Thanks for any answer(s).

JP
05-08-2012, 03:45 PM   #12
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QuoteOriginally posted by falconeye Quote
Diffraction depends on the effective f-stop number. And both a tele converter as well as close focussing (such as enabled by extension tubes) increase the effective f-stop.

Note that the effective f-stop is the distance between the lens principal plane and focal plane in mm, divided by the aperture diameter in mm.

Both a TC and close focusing change that distance.
This bears repeating, because so many people miss this point. Some macro lenses actually have a scale to indicate the effective stop as you focus closer - unfortunately, that is not present in all macro lenses, so there is no reminder of this aspect.

Here is another helpful article: Macro Camera Lenses and a few selected quotes:

QuoteQuote:
A rule of thumb is that at 1:1 the effective f-stop becomes about 2 stops greater than the value set using your camera. An aperture of f/2.8 therefore becomes more like f/5.6, and f/8 more like f/16, etc.
QuoteQuote:
Finally, it's important to note that Nikon cameras automatically correct for the effective f-stop. In other words, the f-stop that is reported in your Nikon camera's viewfinder/LCD will increase progressively as your focusing distance decreases — even if you never specifically changed the f-stop setting using standard methods.
05-08-2012, 05:23 PM   #13
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QuoteOriginally posted by jpzk Quote
Is there a way to figure out what would be the combo, lens+TC, theoritical "sweet spot" where you'd get as little loss of IQ possible?
Not without actually measuring the combo.

But a perfect TC would maintain the sweet stop, i.e., if at f/5.0, the 1.7x TC would shift it to f/8.5. That's already close to diffraction territory for modern high MP APSC cameras.

A real-world TC may make it f/11 which is not as sharp one may wish but can still be sharpened up at least.

As a general note, I've found images from TC combos to sharpen well.

Photozone has tested one of the best tele lenses ever made (the Nikkor 200/2G) with 1.4x, 1.7x and 2.0x TCs.

Nikkor AF-S 200mm f/2 G ED VR (FX) - Review / Test Report - Analysis

In the center, the sweet spots shifts as follows (effective f-stops):

200: f/2.8!
280: f/5.6 (rather than f/3.9)
340: f/8.0 (rather than f/4.8)
400: f/8.0 (rather than f/5.6, with degrading max. resolution)

So, it seems the TC is eating about ~2 stops on its own for reaching the sweet spot. With even worse results in the corners. However, this isn't a shift as the 200/2G is so good the optical performance is almost only determined by the TCs.

I would have to run a numerical analysis to come up with a model which predicts the sweet spot of any combo. To simplify, it is SQRT(N_TC^2 + N_eff^2) with N_TC=5.6 or 8 for 1.4x or larger. And N_eff = sweet spot F-stop times TC-factor.

Something like SQRT(8^2+(1.7*5)^2)= f/12 for the DA*300/4 x 1.7TC combo. No idea if this sounds plausible but it certainly is close to the diffraction boundary for K-5 pixels. Feasible but needs lot of sharpening.


P.S.
Just noticing we're in the Q forum here: The Q's pixels are so small that the DA* at f/4 (f/4.5 maybe) is the only way to use this lens with the Q. Forget about stopping down or using a TC.
05-08-2012, 05:54 PM   #14
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All the scientific mumbo jumbo in the world does not change the fact that a physical gap (aperture, diffraction slit) causes diffraction not an effective one derived from calculation.

Once again, the real aperture of the real lens causes diffraction, and the TC increases it. That should be enough for most photographers, discussions of the Heisenberg Uncertainty princple do not contribute to the understanding of your equipment and a resultant improvement in photographic technique.

However, a flippant aside, the Heisenberg Uncertainty principle may be referred to and attributed to as a cause of bad photographic results when explaining to a client why you stuffed up their wedding shots. Not saying it will work, but worth a try. LOL.
05-09-2012, 06:38 AM   #15
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Adam: ". . . . I think the degradation of IQ will become noticeable much faster than the diffraction. With that said, I believe that it's the effective aperture that counts." [my stress]

Falconeye: "It is the effective aperture which counts . . . . . . . .This is why Adam is not correct
." [my stress]


"Yes, you highlighted the wrong parts.

Adam thinks that image quality is degraded by a TC such that diffraction is a non issue. And I disagree and I explained why"

"Don't think in terms of passing a gap, that's just a historic experiment taught in university lectures"

Since you represent your knowledge of the subject as more advanced than what is taught in universities it is not surprising that the likes of myself would have no idea what the technical jargon &c you use signifies. However something I have no problem understanding is that Adam does not say "that diffraction is a non issue". Not at all. But he does consider degradation of image quality to be significant and says so. That comment made, he addresses
barondla’s enquiry as to whether it is the aperture of the prime lens or the effective aperture of the optical system which is relevant to the diffraction issue. Would he have bothered with this if his point was that bothering is pointless? He has read the question and answers to the point and, incidentally, you say exactly the same even echoing his very words.

So what are you disagreeing with? Do you mean that you didn't really read what you were commenting on?

If what you mean is that other contributory factors to degradation of image quality are insignificant - i.e. other than diffraction - you have not said so.

Perhaps you would like to enlighten me as to which words I should have highlighted and how this would have clarified the issue?
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