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Old Jan 19, 2010, 11:30 AM   #11
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NothingRare wants to put a 2X teleconverter on a 300mm lens. The Tamron 70-300 Di LD has a 62mm filter diameter. If we presume that the filter diameter is an indication of the size of the objective lens, then a 2X TC would need an objective lens with a diameter of 124mm to make up for the 2 stop light loss resulting from the narrower angle of view.

Is that what we're talking about?
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Old Jan 19, 2010, 2:40 PM   #12
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Dave,

The front-mounted teleconverters you are talking about are for P&S cameras.

IQ is severely affected even on P&S cameras.

When it comes to SLR cameras it's much much worse.
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Old Jan 19, 2010, 9:14 PM   #13
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NothingRare wants to put a 2X teleconverter on a 300mm lens. The Tamron 70-300 Di LD has a 62mm filter diameter. If we presume that the filter diameter is an indication of the size of the objective lens, then a 2X TC would need an objective lens with a diameter of 124mm to make up for the 2 stop light loss resulting from the narrower angle of view.

Is that what we're talking about?
Pretty much, yes. If the max aperture is f/5.6, then the minimum objective dia would be around 54mm,(at 300mm) so a 2x TC would need to be at least 108mm for no light loss. Note that these are minimums, and don't account for losses internally.

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Old Jan 20, 2010, 5:55 AM   #14
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Originally Posted by peripatetic View Post
Dave,

The front-mounted teleconverters you are talking about are for P&S cameras.

IQ is severely affected even on P&S cameras.

When it comes to SLR cameras it's much much worse.
If I'm the Dave you are referring to, you are correct that the TC's I've used were originally intended for P&S cameras.

However, that does not mean they won't work on a DSLR, as evidenced by the results I showed with the Nikon TC-15ED (camera was a Pentax K100D DSLR and the primary lens a good 50mm prime.)

The physics determining image quality resulting from using a front mount TC is completely independent of the camera type.

You say "IQ is severely affected even on P&S cameras"... I urge you to look at CK Shene's test results carefully. Most front-mount TCs indeed degrade images too much but "most" doesn't mean ALL.

There are potential mechanical problems with using a front mount TC because they are heavy & must be supported and moved by the primary lens and camera. However, the Nikon TC-15ED certainly works ok on a Pentax screw-type AF system (the in-camera Pentax screw drive is very robust).

Really, I didn't make this stuff up, what I said is based on real, careful measurements.

Dave

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Old Jan 20, 2010, 6:44 AM   #15
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Pretty much, yes. If the max aperture is f/5.6, then the minimum objective dia would be around 54mm,(at 300mm) so a 2x TC would need to be at least 108mm for no light loss. Note that these are minimums, and don't account for losses internally.
It also doesn't account for a smaller exit pupil and any adapter required to mount the TC on the lens.
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Old Jan 20, 2010, 9:50 AM   #16
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Dave,

The front-mounted teleconverters you are talking about are for P&S cameras.

IQ is severely affected even on P&S cameras.

When it comes to SLR cameras it's much much worse.
This depends on the quality of lens used. Minolta made a 1.5x TC for the D5/D7/Axx cameras which was excellent, and Olympus had several with also excellent IQ. The minolta unit, with 49mm threads, would work very nicely on SLR lenses in that size.
The drawbacks were more due to the weight of the larger lenses, and the cost.
Less expensive TCs, with lower quality glass, do indeed lower IQ siginficantly.

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Old Jan 20, 2010, 11:46 AM   #17
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This depends on the quality of lens used. Minolta made a 1.5x TC for the D5/D7/Axx cameras which was excellent, and Olympus had several with also excellent IQ. The minolta unit, with 49mm threads, would work very nicely on SLR lenses in that size.
The drawbacks were more due to the weight of the larger lenses, and the cost.
Less expensive TCs, with lower quality glass, do indeed lower IQ siginficantly.

brian
I just ordered a Minolta ACT-100 1.5X TC so will know soon how it performs. Based on CK Shene's tests and its large front element, I'm hoping for good results - maybe a 450mm, f:6 lens based on my 55-300mm Pentax f:5.8

I have a couple of the Olympus TC's (a B-300 & TC-17); their optical quality isn't good enough for practical use with my DSLR lenses. The Olympus' front element is about 70mm so with a 300mm primary lens they yield about 500mm @ f:7.2. I expect the ACT-100 to be better in this regard, it has a front element about 80+mm so should yield about 450mm @ f:6
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Old Jan 20, 2010, 12:53 PM   #18
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I just ordered a Minolta ACT-100 1.5X TC so will know soon how it performs. Based on CK Shene's tests and its large front element, I'm hoping for good results - maybe a 450mm, f:6 lens based on my 55-300mm Pentax f:5.8

I have a couple of the Olympus TC's (a B-300 & TC-17); their optical quality isn't good enough for practical use with my DSLR lenses. The Olympus' front element is about 70mm so with a 300mm primary lens they yield about 500mm @ f:7.2. I expect the ACT-100 to be better in this regard, it has a front element about 80+mm so should yield about 450mm @ f:6
What are the sizes of their exit pupils? If they're not bigger than the objective lens of the camera lens, then there will be light loss because of the TC's smaller aperture (front or back).
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Old Jan 20, 2010, 1:42 PM   #19
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What are the sizes of their exit pupils? If they're not bigger than the objective lens of the camera lens, then there will be light loss because of the TC's smaller aperture (front or back).
I think there will be some loss as you suggest. The exit pupil of the TC is likely about 10% smaller than the entrance of the primary lens; we'll see. The bigger question is whether the optical quality is good enough.

So far I've no TC (or other lens) that can better the cropped 55-300mm - hmm, I haven't tested the Nikon TC-15ED on the 55-300, but there's way to much light loss for it to be useful (it is effectively about a 450mm f:8.5 at best).
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Old Jan 20, 2010, 4:46 PM   #20
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I guess I'll enter the fray.

1. An afocal (front mount) teleconverter can be designed for no f/stop loss with the only light losses being the transmission losses. I have confirmed this with the Schneider-Kreuznach 1.4x aux. lens designed for the Z series Kodak super-zooms and my Z612. That lens is an impressive five element chunk of glass. The Oly 1.7x lens for their super-zoom is even bigger and heavier. Bear in mind the S-K lens was designed to work with lens that are actually 70mm, F/4.8 or so. An afocal lens for a 300mm F/5.6 should sold with its own wheeled dolly.

2. Field resolution results. First I need to explain the two mode digital zoom on the Kodak Z612. Zoom ratios below 2x the camera first resamples (upsamples) the image and then crops such the resulting image has the same 6MP as an image that is not digitally zoomed. Beyond 2x the image is just a straight crop. The comparison results of the S-K 1.4x and a 1.4x digitally zoomed image were equal.

3. Cheap auxilary lenses such as the one the OP was asking about will not get anywhere close to the contrast/resolution of the S-K, Nikon, Oly lenses AND, based on posted sample do not deliver the magnification advertised. Those labeled 2x are really 1.2x and those labeled 3x are really 1.3x.

4. My Conclusion: Cheap aux lenses such as the the OP asked about are overpriced at $15. Some expensive aux lenses may have a place with some P&S cameras but I can't see the point with a DSLR. A used prime telephoto of the same resulting focal length would likely give better results.

A. C.
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