TC or ET for close up?

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Originally Posted by xpatUSA

Bill, the ratio you are seeking is (background minus focusing distance) over background distance. The blur size is directly proportional to that, all other things being equal.

For a camera perfectly focused on the background, blur is therefore zero (ignoring diffraction, aberrations, shaky hands, etc).

For a camera very close to a subject with a far background, the blur size starts to approach the aperture diameter and could even exceed it if we get into macro territory (m greater than 1).

As to Grahame's specific comparison, I think that the only way to quantify the bokeh difference would be by ray-tracing through all the lens elements at their actual distances and thicknesses, like here , and I certainly can't do that. My calc is only for a perfect thin lens.

HTH ...

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Thank you. I have not checked my old notes and I assume that the ratio you cited is correct. (i.e. regarding to it being the precise ratio of the mathematical relationship).

[Aside - I have used the Ratio of Camera to Subject Distance :: Subject to Background Distnce so often in practical examples to show how one can attain 'nice' Bokeh with 'slow' Lenses, I reckon that I have forgotten the actual Mathematics. I admit, I had to look up the formula for Exposure Compensation when using Extension Tubes: twenty years ago I knew all that stuff by rote. We're spoilt with our TTL Metering these days.]

However, my point remains the same (taking the ratio you cited) there is still very little difference in that ratio - and - (more importantly) all other things are not equal: the light path and elements through which the light travels are considerably different.

As you stated an accurate way would be to map the ray traces, I am disappointed that you've not taken up that challenge - there will be no Walker Texas Ranger Award for you this week (I'm joking of course).

I reckon that there could be more practical field work done and I am setting a task for myself.

Still thinking. Very Interesting.

WW

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Originally Posted by billtils

Grahame, thanks for posting the results of this much more rigorous experiment.

Overall it looks like it doesn't matter much in regard to image quality so it comes down to other factors, and the tubes lose out there.

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It may also be that using the TC with your 'prime' 300mm gives improved IQ than when used with a zoom.

One thing I did find with my 1.4 TC was that AF required a significant fine adjustment.

When I undertook the IQ comparison test I wish I had also taken a shot with the tube at minimum focus distance to assess it's magnification benefit. I suspect 20mm on a 300m lens is not going to really give much advantage.

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Originally Posted by Stagecoach

... One thing I did find with my 1.4 TC was that AF required a significant fine adjustment.
... I suspect 20mm on a 300m lens is not going to really give much advantage.

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Grahame, I calibrate all my lenses with the Reikan software and have the AFT adjustments loaded for each +/- the TC.

Yes, you are probably correct on the limited impact of the 20mm tube but went with it as the least complicated option for what was something stimulated more by idle curiosty than anything else, as it's the middle size of the set of three. However, if the weather holds over the next few days I may look at other combinations.

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Originally Posted by William W

Thank you. I have not checked my old notes and I assume that the ratio you cited is correct. (i.e. regarding to it being the precise ratio of the mathematical relationship).
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However, my point remains the same (taking the ratio you cited) there is still very little difference in that ratio

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Yes, agreed for the close-up shots in this thread.

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- and - (more importantly) all other things are not equal: the light path and elements through which the light travels are considerably different.

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Agreed also that all other things are not equal but, regrettably, we don't know how close they are either.

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As you stated, an accurate way would be to map the ray traces. I am disappointed that you've not taken up that challenge - there will be no Walker Texas Ranger Award for you this week (I'm joking of course).

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Grump!

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I reckon that there could be more practical field work done and I am setting a task for myself.

Still thinking. Very Interesting.

WW

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Later Bill,

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Originally Posted by billtils

. . . However, if the weather holds over the next few days I may look at other combinations.

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Good ho!