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03-01-2012, 10:01 AM   #1
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Back-Front Focus Problems using FA*300 f4.5 and Tamron 1.4TC

Hi, i use a K7 with the FA* 300mm for wildlife, since i wanted more reach i used a Tamron 1.4 TC (which according to the popular opinion goes very well with this lens) but i never got the results i expected when pairing them together, the lens alone is tack sharp, bur adding the TC gives very unsharp images.

Trying to find out what the best combination of aperture was using the Tc i did a brickwall test with a white lens cloth as additional target (Autofocusing in Live Mode, on tripod, cable release, Iso 400 and sunny conditions, Target was about 8 meters away)

Without the TC focus was best around f 5,6 - 6,3, but with the TC even at f8 to f10 the sharpness was not quite there.

Im pretty new at this, but could the TC cause Back or Front focus? If so would the Focus tuning of the camera also affect the lens when used at its own? Or would i have to turn the Focus adjustment off when using the lens without the TC?

Here are 100% crops - non processed - jpgs 75 bit resolution (from RAW files)

First two are without TC (at f5,6 and f9,0)
Second two are with TC (at the same apertures)

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PENTAX K-7  Photo 
View Picture EXIF
PENTAX K-7  Photo 
View Picture EXIF
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03-01-2012, 10:07 AM   #2
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You should be able to confirm this using a test chart, though it could also be that the TC simply gives poor IQ

If it is FF, you should be able to use the focus adjustment setting to fix it:
Fixing Front and Back Focus - Introduction - PentaxForums.com
03-01-2012, 02:41 PM   #3
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QuoteOriginally posted by Adam Quote
You should be able to confirm this using a test chart, though it could also be that the TC simply gives poor IQ

If it is FF, you should be able to use the focus adjustment setting to fix it:
Fixing Front and Back Focus - Introduction - PentaxForums.com
You where right i checked for FF and BF using a focus chart i found online, no problem whatsoever, so i checked with a Promaster TC (1.7x) and the quality was much better. Seems that i will be converting the Tamron TC into an auto extension tube... (got a bad copy i guess...)
03-01-2012, 02:53 PM   #4
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QuoteOriginally posted by mgbirder Quote
You where right i checked for FF and BF using a focus chart i found online, no problem whatsoever, so i checked with a Promaster TC (1.7x) and the quality was much better. Seems that i will be converting the Tamron TC into an auto extension tube... (got a bad copy i guess...)
The fact that there is no FF/BF issues with the lens itself (or the lens with the 1.7x) doesn't preclude the possibility that the combo of the 300+1.4x needs a focus adjustment. Before you remove the glass and render a $200 TC into a $50 tube, I would confirm this by either using LV to focus or manually focusing to see what the image looks like when properly focused. Assuming we're talking about the same copy of the 1.4x that I sold you a little over a year ago, I know for certain that it worked very well (optically) when paired with my DA*300. I have a feeling the combo of the 1.4x and 300 might need some AF adjustment.

03-01-2012, 03:32 PM   #5
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QuoteOriginally posted by dgaies Quote
The fact that there is no FF/BF issues with the lens itself (or the lens with the 1.7x) doesn't preclude the possibility that the combo of the 300+1.4x needs a focus adjustment. Before you remove the glass and render a $200 TC into a $50 tube, I would confirm this by either using LV to focus or manually focusing to see what the image looks like when properly focused. Assuming we're talking about the same copy of the 1.4x that I sold you a little over a year ago, I know for certain that it worked very well (optically) when paired with my DA*300. I have a feeling the combo of the 1.4x and 300 might need some AF adjustment.
I think you are right, i used live view to focus on the chart and when focussing manually i can get a sharp picture. With AF in Liveview the image passes through a moment that the image becomes clear but stops at a point where contrast is minor... I tried all 20 different positions on the focus adjustment menu but none of them give me a clear picture ( i was using an LCDVF in Live Mode to monitor the results)...
03-01-2012, 03:54 PM   #6
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MF vs. AF results

Here are two images to show the differences. First one is AF in Liveview second is MF (100% crops)
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03-01-2012, 04:51 PM   #7
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QuoteOriginally posted by mgbirder Quote
I think you are right, i used live view to focus on the chart and when focussing manually i can get a sharp picture. With AF in Liveview the image passes through a moment that the image becomes clear but stops at a point where contrast is minor... I tried all 20 different positions on the focus adjustment menu but none of them give me a clear picture ( i was using an LCDVF in Live Mode to monitor the results)...
That makes more sense in that it's more of an AF issue than an optical issue. As for why the particular combo of the FA*300/4.5+1.4x isn't giving you good results, that I'm less sure about. I don't know if the loss of a stop giving you an effective aperture of close to f/6.3 is a contributing factor, although I wouldn't think so as I've used the a 1.4x TC on both the FA*300/4.5 and F*300/4.5 without any issues in terms of AF. It could be that the combo is just outside the range of what you can adjust for in terms of AF fine tuning (without going into the debug menu). Does it get progressively better as you go towards one end of the AF adjustment scale and you just can't go far enough to make it perfect?
03-01-2012, 05:07 PM   #8
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QuoteOriginally posted by dgaies Quote
That makes more sense in that it's more of an AF issue than an optical issue. As for why the particular combo of the FA*300/4.5+1.4x isn't giving you good results, that I'm less sure about. I don't know if the loss of a stop giving you an effective aperture of close to f/6.3 is a contributing factor, although I wouldn't think so as I've used the a 1.4x TC on both the FA*300/4.5 and F*300/4.5 without any issues in terms of AF. It could be that the combo is just outside the range of what you can adjust for in terms of AF fine tuning (without going into the debug menu). Does it get progressively better as you go towards one end of the AF adjustment scale and you just can't go far enough to make it perfect?
I did another series of shots but now without liveview (i though it could be a problem of liveview) using every finetune adjustment level and looking at the results at 200%, it seems that -9 is the sharpest setting, but i donīt have good light anymore to test the results on other targets.

03-01-2012, 05:11 PM   #9
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QuoteOriginally posted by mgbirder Quote
I did another series of shots but now without liveview (i though it could be a problem of liveview) using every finetune adjustment level and looking at the results at 200%, it seems that -9 is the sharpest setting, but i donīt have good light anymore to test the results on other targets.
FYI, AF fine tuning only affects focusing through the viewfinder (phase detection) and doesn't have any effect on focusing in LV (contrast detection). So regardless of the AF adjustment setting, all of the LV pictures should look the same.
03-01-2012, 05:16 PM   #10
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That makes a lot of sense! i was going crazy changing values and seeing the same image over again
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