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07-05-2010, 08:06 AM   #1
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What happens when I use a teleconverter?

If I use a 2x teleconverter with my Tamron 70-200mm f2.8 what will I get?

I know that it will effectively double the focal length, I will lose approximately 2 stops of light and I will see a drop in quality.

What I don't know is...

Will I end up with a 140-400mm f5.6 or a 140-400mm f2.8, with 2 stops slower shutter speeds? Will my depth of field be based on f2.8 or f5.6?

How will it effect the macro function? Infinity focus will stay the same, but does that mean that minimum focusing distance stays the same but with double the magnification? (I have better alternatives for macro... but I may not have them with me when I'm carrying this beast around)

Thanks for your help -Dave-

07-05-2010, 09:42 AM   #2
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I have read many times (but never tested) that the minimum focus remains the same.
07-05-2010, 10:01 AM   #3
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It's f/5.6. You have changed the focal length of the lens without changing the physical dimension of the aperture - that means a larger f-number. Minimum focus should remain unchanged, or would at most change by a very small amount.
07-05-2010, 10:14 AM   #4
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QuoteOriginally posted by dmoon911 Quote
[snip]If I use a 2x teleconverter with my Tamron 70-200mm f2.8 what will I get?

I know that it will effectively double the focal length, I will lose approximately 2 stops of light and I will see a drop in quality.

[snip]


When I think of teleconverters and read stuff like "lose 2 stops of light and see a drop in image quality" I remember why I don't use teleconverters. This is just a personal preference so TC users please don't flame me on this one.

Tom G


Last edited by 8540tomg; 07-05-2010 at 11:33 AM. Reason: typo
07-05-2010, 10:21 AM   #5
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With regards to DoF:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/100919-tc-dept...-question.html
07-05-2010, 11:12 AM   #6
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QuoteQuote:
Will I end up with a 140-400mm f5.6 or a 140-400mm f2.8, with 2 stops slower shutter speeds? Will my depth of field be based on f2.8 or f5.6?
Mark put it correctly. You actually loose two stops of aperture.

when in doubt, remember that except with ND filters, if you loose stops it's because something has changed in the optical path.
07-05-2010, 11:24 AM   #7
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Thanks for all the answers. I think a light just went on! I did a bit for digging based on the answers I received here and found a pretty simple diagram on wiki that breaks it down.

Diagram
File:Equal illuminance.svg - Wikipedia, the free encyclopedia

Full Article
F-number - Wikipedia, the free encyclopedia

I guess it was the math I needed to understand.. Please correct me if I'm still wrong.

A 200mm f2.8 lens has a max opening of approx 71.5mm by adding a 2x teleconverter I increase the focal length to 400mm and the max opening remains at 71.5mm therefore my max aperture is 400/71.5=5.6 or 2 stops difference.

Plus the extra glass I guess I would lose a bit of light there too?
07-05-2010, 06:08 PM   #8
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Your math is spot on.

The extra glass will make you loose about 4% per surface (glass/glass or air/glass) unless there are coatings (those improve transmission).

The worst issue with that new glass is that it will create new distorsion effects and inetivably degrade the image quality.

07-05-2010, 08:43 PM   #9
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The change in quality caused by a teleconverter is much reduced with a high quality converter. However, a 2x converter will multiply any lens defects by 2 as well as everything else, even if the converter is perfect, and that is not possible.

I have two converters, the A 1.4X - S, designed for shorter focal lengths, and the F 1.7X AF converter which was designed to make manual lenses autofocus lenses when the SF-1 came out.

Here are the results of my shooting with an old lens and the 1.4X-S converter.

Unfortunately for me, my M 400/5.6 has a throat that does not physically allow use of the 1.4X-L converter, designed for lenses over 300mm. The -S does work fairly well on the lens, but I must work at it. When I am using the 1.4X, I am shooting a 560mm f/8 lens, and that is not an easy combination to work with. Even with a heavy tripod, the lens shakes in the slightest breeze.

The depth of field is somewhere between a piece of paper and a paperback book when the lens is wide open, so in addition to losing a stop of light because of the combination, I really should stop down to f/11 on the lens, giving me f/16 effectively. This combination is aimed at taking bird pictures, so the best times are - you guessed it - when the light is less than bright, at dawn and dusk. My K10D (as do most cameras) has its best resolution and colour at ISO 100. I find myself using the converter with the lens set to f/8 (f/11 in reality) and upping the ISO to as much as 400, just to get a shutter speed that will get a bird without being a complete blur.

The proper solution (other than for my bank account and my back hauling it around) is the recently discontinued 600mm f/4. At $10,000 new, give or take a bit, and about 10 Kg, I would have a lens that would really help with birds. Unfortunately, retirees don't usually have $10K to throw around, so I make do. That's what converters are for: they make do with less than optimal equipment.
07-08-2010, 08:34 AM   #10
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Thanks for all your answers...

I decided to get a 1.5x and reduce the light and IQ loss. The Kenko 1.5x af dg teleconverter arrived yesterday. It's supposed to be similar to the Tamron 1.4x? I'll give it a workout this weekend and see for myself. Thanks again.
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