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07-23-2013, 04:58 PM   #1
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Pre-A Zoom Settings

Probably a silly question, I think I know the answer, but when you mount a pre-a lens and it prompts you to enter the focal length, does that do anything other that write it to the EXIF? I think not, but if so....

what do you enter for a zoom? I've only had primes that meet this criteria...until a nice find today.

07-23-2013, 05:34 PM   #2
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It sets the focal length for the Shake Reduction system. It's usually okay if you are in the ballpark of the lens' focal length, but I have seen manual lenses that never seemed to be sharp (because I forgot to set the focal length and it was 'way off). You can choose a focal length somewhere in the middle of the zoom range, or if you generally work at one end of the range, pick a length in there.
07-23-2013, 05:41 PM   #3
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From the Pentax U.S. site: Using K-30 With Older Lenses | Pentax Support
In the he last paragraph the recommendation is to pick the midpoint of the zoom range. Seems to be a good compromise for using zoom lenses.

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07-23-2013, 05:43 PM   #4
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You must enter the closest focal length that you plan to use. If you zoom in or out significantly during the composition then you should re-input the new value. The easiest way is to turn the camera off and on again to be prompted.

As an alternative you can turn SR off from the menu and not wary about it. An incorrect focal length can actually add blur to the image.

07-23-2013, 05:44 PM   #5
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Cool, thanks guys. See I was about to write that off as unimportant. I'm not a real "zoomy" guy, but I picked up something old and perfect and cheap today and when I turned the camera on, I smiled and said "huh".

It's mostly intended for birds, squirrels and trips to the zoo, so I'll set it accordingly for what I'm planning.
07-23-2013, 05:48 PM   #6
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It's not just pre-A zooms. It's any MF zoom. The A lenses only transmit aperture info to the camera, there is no chip in them like the AF lenses have.
07-23-2013, 05:54 PM   #7
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QuoteOriginally posted by boriscleto Quote
It's not just pre-A zooms. It's any MF zoom. The A lenses only transmit aperture info to the camera, there is no chip in them like the AF lenses have.
Yeah, that stands to reason. I've never actually had an "A" lens, only manual primes and modern Pentax lenses.

You call all breath a sigh of relief knowing that somebody new is going to start posting a bunch of squirrel pictures soon. I know you were staying awake at night worrying about just that.
07-24-2013, 03:53 AM   #8
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QuoteOriginally posted by demp10 Quote
You must enter the closest focal length that you plan to use. If you zoom in or out significantly during the composition then you should re-input the new value. The easiest way is to turn the camera off and on again to be prompted.

As an alternative you can turn SR off from the menu and not wary about it. An incorrect focal length can actually add blur to the image.
It can make a lot of difference if you get it wrong - M75-150 zoom at 150mm, entered values 75, 150 and 300mm, three quick handheld shots at each value at the same focus etc.,1/50s, center crops with FastStone, K-7, no PP. :

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Last edited by kh1234567890; 07-24-2013 at 04:59 AM.
07-26-2013, 11:11 AM   #9
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I'm a bit surprised you haven't read any of my posts on this subject (which have probably bored the pants off an awful lot of readers), but here goes:

If you want to get full SR, you need to input the exact focal length that you're shooting at (or at least something close). If you'd prefer to accept a compromise, which permits you to set the zoom to any focal length, and still get useable SR, then you can use the formula (min + max)/2*min*max. If you do this, then you'll get the same amount of SR at both ends of the zoom range.

This means, for most telephoto zooms of the "vintage" era, you end up with setting a value of 100mm. With this, you'll get at least 1-stop's worth of SR (pretty much, and more for the 75-150).

However, if you find you're shooting mostly at either end of the zoom range, you're better off setting a value equal to the minimum focal length. That way, you'll get full SR at the min end, and still have a reasonable amount left at the max end.

The thing to avoid, when trying to match focal lengths to the value you've input, is a sudden change in zoom, such that the value you've input to the camera is 2 or more times the actual focal length. That will result in more blur than you would otherwise have had, had you turned SR off.
07-26-2013, 05:45 PM   #10
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Oops, I've given you the formula upside down: it should read 2*min*max/(min+max). Sorry about that!
07-27-2013, 12:39 AM   #11
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The point I was trying to make is that with telephoto zooms while you might sometimes get away with having the SR focal length set at a half of what it really is, you are unlikely to get away with having it set to twice the actual value. The SR setting is not so critical with wide zooms (e.g. the M28-50).
07-27-2013, 02:23 AM   #12
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QuoteOriginally posted by kh1234567890 Quote
The point I was trying to make is that with telephoto zooms while you might sometimes get away with having the SR focal length set at a half of what it really is, you are unlikely to get away with having it set to twice the actual value. The SR setting is not so critical with wide zooms (e.g. the M28-50).
Well, if you have set the value to half of the actual FL, you'll get half the blur you would get if you had SR turned off, so it's very much OK in that respect. The problem is that we're used to getting much more blur reduction than that, so the results may appear disappointing.

If it's the other way round, with the value set to twice the actual FL, you'll get, in theory, exactly the same blur as you would if you had SR turned off. But you're right, it is a situation you'd want to avoid, and you'd certainly want to avoid anything more than 2x.

Wide zooms are only less critical in that you tend to get less blur anyway at shorter FLs. But the amount of blur reduction remains the same (it's just that there's probably less in the first place).
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