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10-26-2010, 12:03 PM   #1
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Focal length input

I recently aquired a manual focus Tak-A zoom lens. When attaching the lens to the camera (K7), it requests input of the focal length. The lens I'm using is a 70-200. Do I input 70 or 200? Do I need to change that number if I change the focal length I use?

10-26-2010, 12:21 PM   #2
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QuoteOriginally posted by K(s)evin Quote
I recently aquired a manual focus Tak-A zoom lens. When attaching the lens to the camera (K7), it requests input of the focal length. The lens I'm using is a 70-200. Do I input 70 or 200? Do I need to change that number if I change the focal length I use?
I'm pretty sure the minimum is the best: with my copy of that lens, though, I usually want to be using more toward the longer end, and I also find it pretty soft at 200mm, so I'm in the habit of setting the FL to 150 or so or even just leaving it at 135mm (I use my 135 more, anyway, so that's where it's usually set. )
10-26-2010, 12:30 PM   #3
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Common wisdom is to input the lowest value, as it is better to under correct for shake than over correct.
You might consider doing a little bit of pixel peeping and see what entry works overall the best and report back.
It's entirely possible that putting in a median number in the 135mm range might work better for the long end and still be acceptable for the short end.
Play around a bit and see what you find.
10-26-2010, 01:26 PM   #4
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I try to optimize it by selecting the most used range (say, 135 - 200) and set it to 135. If I go below 135, I just switch off shake reduction.

The way I see it... for me to have an acceptable shutter speed w/shake reduction at 135, would be good enough for the long end without the shake reduction (say, 80).

Depending on the range of your lens, this optimal point will change depending on your light.

10-26-2010, 07:10 PM   #5
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This is debated endlessly, often with calculations whipped out.

But really, there is no optimal answer, just favorite solutions.

Mine is: I'm selling off most of my manual zooms, only keeping the very best.

Those I keep, I either set to one FL and tell the SR-bot, or I turn off SR.

If you're interested in sharp images, that's how to do it.

Last edited by RioRico; 10-28-2010 at 01:09 PM. Reason: spell.goof
10-26-2010, 10:24 PM   #6
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QuoteOriginally posted by K(s)evin Quote
I recently aquired a manual focus Tak-A zoom lens. When attaching the lens to the camera (K7), it requests input of the focal length. The lens I'm using is a 70-200. Do I input 70 or 200? Do I need to change that number if I change the focal length I use?
The reason the camera asks you about focal length is that it needs to know this in order to calculate how much to move the sensor to counteract camera shake - so it's all about the camera's shake reduction (SR) mechanism. (If you have SR switched off, it won't ask.)

It's certainly a safe option to set the FL to the lowest value, but you'll be losing a lot of SR towards the top end of the zoom range (where you most need it). So it's better to input a compromise value, in order to provide acceptable SR for all FLs (unless you want to input the actual FL every time you shoot).

I did some analysis of the situation in a very recent thread:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/119297-what-fo...al-zoom-2.html

What came out of this is a means of calculating the best FL to input for acceptable SR at both ends of the zoom range. As you will see, I didn't do the calculation specifically for a 70-200, but I have now, and the answer is 100mm. This will give you about 1 stop of SR at the zoom extremities, and full SR when you set the lens to 100mm.

By the way, the worst thing you can do is to input an FL which is greater than twice the lowest FL (70*2, or 140mm in your case), because then the camera will then actually magnify any shake you put in!
10-27-2010, 04:03 PM   #7
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Very good info! Thank you all! I will test and retest and detest and post anything significant I find.
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