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Old Jan 30, 2010, 7:36 PM   #31
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Besides this, in the first photo comparison, the image on the left is clearly out of focus...

Can't resolve detail with an image that's out of focus and compare it to an in focus shot.

dave
I think you are correct. However, that was the best approximation of focus I could achieve with either manual or auto-focus because the weight of the ACT-100 made focusing difficult.

These are very shallow depth of field shots - I suspect that a thru-focus series using the TC would show better results, however it is pretty clear that trying to use the heavy TC on front of the DA-55-300 is probably not worth the effort.

Maybe things would improve with a 300mm IF lens, but I don't have one to try.

I've also found that a 1.5X behind-the-lens Kenko TC does not improve the da 55-300mm lens fully zoomed; maybe other TC's would be better.
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Old Jan 30, 2010, 7:53 PM   #32
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I think you are correct. However, that was the best approximation of focus I could achieve with either manual or auto-focus because the weight of the ACT-100 made focusing difficult.

These are very shallow depth of field shots - I suspect that a thru-focus series using the TC would show better results, however it is pretty clear that trying to use the heavy TC on front of the DA-55-300 is probably not worth the effort.

Maybe things would improve with a 300mm IF lens, but I don't have one to try.

I've also found that a 1.5X behind-the-lens Kenko TC does not improve the da 55-300mm lens fully zoomed; maybe other TC's would be better.
No offense.

I know you are being sincere and making a real effort to be both objective and fair. But when I wanted to compare my Sigma TC to my Nikon TC, I mounted my camera on a tripod, took ten or twenty images with both, and filtered out the worst results in order to give a fair test - Mind you, I was only doing this test for my own benefit.

I no longer have these images; but all my results of comparing rear mounted teleconverters to front mounted magnifiers, left the front mounted stuff in the dust. Not that I'm "raving" about the results of ANY of the teleconverters - But at least the rear mounted ones produced useable images.

I'm a long lens nut job, and all of this is VERY important to my work.

Dave

PS. The Nikon beat the Sigma...

Dave
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Old Jan 31, 2010, 12:51 AM   #33
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A question.

It occurs to me that the front-mounted TC will have a variable distance to the front element of the lens they are mounted on depending on the design of the lens; literally how much air there is between lenses of a similar thread size could vary quite substantially - surely this is a recipe for potential disaster?

This of course will not be the case with a rear-mounted TC, or a TC specifically designed for specific P&S cameras.

I don't see any theoretical reason why, for a specific lens it would not be possible to design a front TC that works as well as a rear TC, but in the real world it can only work out that way if the TC is designed for use with a filter thread of Xmm and distance to front element of Ymm. Often Y will vary between lenses.

Is there something wrong with my hypothesis?
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Old Jan 31, 2010, 3:56 AM   #34
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A question.

It occurs to me that the front-mounted TC will have a variable distance to the front element of the lens they are mounted on depending on the design of the lens; literally how much air there is between lenses of a similar thread size could vary quite substantially - surely this is a recipe for potential disaster? ....
You are correct that the air gap can make a performance difference, however, the effect is usually small because the TC has an effective focal length of infinity.

In the case of the Panasonic FZ30, the camera lens does not move at all which also helps with the heavy adapter.

CK Shene's web pages have an excellent section on front mount converters.
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Old Jan 31, 2010, 4:32 AM   #35
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You are correct that the air gap can make a performance difference, however, the effect is usually small because the TC has an effective focal length of infinity.
Hmm.

http://www.cs.mtu.edu/~shene/DigiCam/index.html

Is the relevant link.

And from a bit of reading afocal lenses are very different from saying they have a focal length of infinity, rather they are focussed at infinity.

But I think you are correct in the implication that the distance to the front element will therefore affect only the magnification obtained, and not the lens' ability to focus as such. Also the difference will be small. I think it will work out that the greater the distance from the TC to the front element, the longer will be the effective focal length and therefore the greater the magnification.

The wikipedia article on zoom lenses is very interesting.

http://en.wikipedia.org/wiki/Zoom_lens

My hypothesis was wrong, and I have learned something today. :-)
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Last edited by peripatetic; Jan 31, 2010 at 4:46 AM.
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