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04-26-2012, 06:07 AM   #1
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Pentax 1.7 TC and FL

I searched and could not really find an answer here (I am sure it has been asked and answered before) so excuse my relative laziness - When using the Pentax 1.7 TC and the camera asks for Focal length what is the preferred method and what effect does that have ? Do you generally round up or down ? (eg: DA* 60-250 ==> 102-405). I did a google search and there seems to varying opinions. I tired that combo last night and used 400 for a FL when asked. Pictures seemed to come out fine, but then I shut the camera off and choose 300. Did not really see much difference in quality , just in the EXIF data.

04-26-2012, 09:39 AM   #2
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there is no hard and fast rule

some suggest the mean focal length, some work on weird and wonderful minimize the square of the errors, etc.

for me, I put the minimum focal length, unless I KNOW i am shooting at one specific focal length, i.e. maximum.

I do this because I rely on my technique to help, and it is better to undercompensate than over compensate. With good technique, this is the way to go, because let's be honest, perfect technique needs no compensation (i.e. a tripod) therefore good technique needs much less
04-26-2012, 07:40 PM   #3
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Thanks Lowell I kinda figured from some searches - do this , bring your slide rule , make sure you are facing east and ....
04-26-2012, 09:39 PM   #4
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There are a few options for using MF zooms, with or without a TC. If a TC is there, multiply the focal length by the TC factor. Also multiply the lens aperture: a 19-35/2.8 with a 1.7X TC becomes 32-60/4.8. Then choose one of the following to handle SR:

* Enter the mean focal length, or the least-means-square product. (These are maybe OK with short 2x zooms but generally suck with longer, greater-range zooms.)

* Enter the closest focal length every time you zoom. This is most accurate but quickly becomes a royal butt-pain.

* Switch SR off. Use the 1/(5FL) rule for SHARP images, or the 1/FL rule for acceptable images. Shoot in good light.

Buying an AF zoom to let the camera handle SR automagically won't work with most TCs because most won't report the adjusted focal length. Also consider that 1) zooms tend to be slower than primes, 2) TCs eat light, 3) the camera might not get a focus lock with optics slower than f/8, and 4) seeing to focus without assistance can be hard.

I am not a big fan of TCs. YMMV.

04-27-2012, 05:29 AM   #5
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Thanks Rico - I went out and tested a few shots the other day. I know TC's eat light (the 1.7 is more or less 2 stops). It's not something I would keep permanently on , however it is handy to potentially get a shot you may have otherwise missed. From my testing , the results seem acceptable. A few were suffering from camera shake (actually the camera was fine let's call it what it is , operator shake !). The scenario where I would find it handy is travel where I could bring let's say the DA*60-250 which in it's own right is a relatively big, heavy lens and the 1.7 TC as opposed to the Sigma 50-500 which generally is not a friendly 'travel lens'.

From my testing I just entered the FL once (tried both 300 and 400) when I initially turned the camera on , left SR on , bumped the ISO and I did not notice an appreciable difference in the photo with either of these settings. Of course it will never replace quality glass , and yes pictures without the TC were generally 'better', however it does offer an alternative that for me at least produces acceptable results. One test I did not make was to compare a crop to a TC shot. Might try that one.

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