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03-02-2014, 11:27 AM   #1
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Odd behaviour with Teleconverter

So today I decided to go out and shoot some icicles. Loaded my K5 with my DA* and went out shooting. Problem is you have to be 3' away to get focus. So I figured I'd just stick my Vivitar 2x Macro teleconverter and be done with it. All went fine, camera still showed f/2.8 but I know that in reality I was two stop slower, fine, metering worked and pictures came up pretty nice. So I figured that I would get incredible shots if I were to use my best lens, so back in the house and the Sigma 35 is loaded on the teleconverter. Set everything up, drop the camera into manual, lower the aperture to 1... euh WTF... locked at 5.6?!?! Took everything appart and mounted everything back together, clearly I mounted it wrong. Still, no faster than 5.6. Oh well, it's still in the lens sweet spot. Hit green button to get exposure reading and get a starting point (I'm in manual) fire away and bam, a white blob of nothingness. I increased the shutter speed by 5 stops before I got something in a decent exposure. Re-tried the green button, same thing, white. So I tried in P mode, same thing, wayyyyy over exposed.

Got back inside to analyse the situation and I noticed that the bokeh was much too blurry for being shot at f/5.6. Did some math, drank a few coffee, thought about it for a while... Lens + tele would report 5.6 but it was actually at f/1.4. Makes sense now.

But why did the metering get it wrong? Isn't it TTL? So as I always do, I checked with every single lens I could put my hand on and they all work fine... all but my precious new little baby.

Anyone else ever had similar problem?

03-02-2014, 03:25 PM   #2
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Can't speak about the 35mm but I've run into problems with Sigma Lens and TC's. Sigma 170-500mm and Pentax 1.4 and 2.o TC's. Never did meter accurately.
If I had to guess, I'd say an incompatibility of the chip in the lens. Might only work with Sigma TC .
03-02-2014, 03:55 PM   #3
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Depending on the age of the lens and TC many TCs are just feeding straight through.

The camera is not perfect in metering and the body adjusts for the known maximum aperture. With fast lenses, this leads the toe apTC over exposing by about 2 -3 stops depending on body
03-03-2014, 06:44 AM   #4
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I recently tried my Sigma 70/2.8 on an A-type TC (i.e., with the 6 A contacts but without the data contact). Nominal f-stop as reported by the camera was not correct -- went outside the actual aperture range of the lens. Some test shots showed that the results were not predictable; changing the nominal aperture by a certain amount might produce the same amount of actual change, or more, less, or no change. Short answer, not a workable combination.

03-03-2014, 07:02 AM   #5
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QuoteOriginally posted by baro-nite Quote
Short answer, not a workable combination
Pretty much what I had figured. Still usable but it takes at least two shots to get one right, usually three... What bugs me is that my TC works flawlessly with ALL my other lenses... bummer
03-03-2014, 11:53 AM   #6
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if you have a teleconverter on a lens, the error is actually predictable if you are shooting in A mode on the lens.

take a shot of a uniformly lit subject, then review the greyscale, you will likely need to set exposure compensation down by between -1 and 3 stops,. but once set, it will be accurate for all shots. Using stop down metering is a little trickier as the exposure changes but not in a consistent manner.

when the K10D came out this was a big topic see the chart below. 120 greyscale is the neutral setting.

03-03-2014, 12:29 PM   #7
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Lowell, this is a different issue -- at least in the case I tested, the Sigma lens does not behave predictably. This lens does not have an aperture ring, and changing the camera's setting by one stop can result in anything from no change to a stop and a half, depending on where you are on the scale. It's just not usable.
03-03-2014, 12:44 PM   #8
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QuoteOriginally posted by baro-nite Quote
changing the camera's setting by one stop can result in anything from no change to a stop and a half,
Yeah that.

Being the first (and only) Sigma lens from their new "programable" line up that I own, I'll blame the lens firmware for that

03-03-2014, 12:52 PM   #9
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QuoteOriginally posted by fgaudet Quote
Being the first (and only) Sigma lens from their new "programable" line up that I own, I'll blame the lens firmware for that
Mine's an older lens; I think the problem is that you need all 7 contacts on the TC. The one I tried is plain A-type (no data pin), and some TCs with the data pin have one contact that doesn't continue through.
03-03-2014, 03:37 PM   #10
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QuoteOriginally posted by baro-nite Quote
Lowell, this is a different issue -- at least in the case I tested, the Sigma lens does not behave predictably. This lens does not have an aperture ring, and changing the camera's setting by one stop can result in anything from no change to a stop and a half, depending on where you are on the scale. It's just not usable.
Take your lens and shoot a uniformly lit surface, such as a block wall at each aperture and plot it.

That's how the curve was developed
03-03-2014, 05:26 PM   #11
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Well, thanks every one for the inputs, I'm glad (in a way) to see that I'm not the only one having some issues with TC.
But I will approach the issue in a different way than suggestd. I just won't use the 35mm with the TC and stick with my tried and true (and very decent) A50mm + TC for macro. I rarely use the TC as a TC... Mine does extend to allow 1:1 so it's more like a 2x macro tube.
Also, there's a DFA100mm that'll make it's way here in the near future. Soon I won't really need the TC at all. Except maybe as a TC ... and if I do, my longest lenses work #1 with it, so I'm all good.

Thanks again
03-03-2014, 06:33 PM   #12
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QuoteOriginally posted by Lowell Goudge Quote
Take your lens and shoot a uniformly lit surface, such as a block wall at each aperture and plot it.
I understand that; when I say "not usable", I mean I am not willing to keep a record of what nominal aperture gives what result. It would be one thing if the entire scale were shifted x number of stops, but it isn't -- it's practically random. In my case the lens turns out to be not particularly good on a TC anyway.
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