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03-27-2014, 05:44 AM   #1
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Pentax K-R

I'm having trouble getting crystal clear focusing on my K-R. Is this common? Also, I have a sigma 18-250 lens and the DOF doesn't seem to change even though I change it on the camera. Is this normal?

03-27-2014, 06:46 AM   #2
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You could have front or back focusing issues which can be corrected by calibrating the AF. From what I've read, the calibration adjustment is no. 21 under the custom settings menu.

As far as the DOF changing, like all superzooms, the Sigma 18-250 is a fairly slow lens, so you're probably not going to get really thin DOF no matter where your aperture is set.

Good luck!
Bob.
03-27-2014, 07:08 AM   #3
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Thanks, and I'll check the front and back focusing. What "all purpose" lens would you suggest that would compensate for several different distances instead of having several different lenses?
03-27-2014, 07:26 AM   #4
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To get a good recommendation, you need to know a few things.

What do you want to do with the lens - action, kids, landscape, walkaround, wildlife etc?

How long do you need it to be? superzoom like you have now, close zoom, longer zoom?

What's your budget?

There are some very good zooms out there.

03-27-2014, 08:50 AM   #5
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QuoteOriginally posted by rfrodman Quote
Thanks, and I'll check the front and back focusing. What "all purpose" lens would you suggest that would compensate for several different distances instead of having several different lenses?
A couple of things to consider:

1) With just about any zoom, (especially superzooms) you're never going to achieve razor sharpness at every focal length because most zooms have a range in which they're less sharp. That's not to say they'll be unusable in that range, just not razor sharp to the pixel-peeper.

2) Is the 18-250 your only lens? If so, I'd set the AF point optimized for somewhere in the middle (around 125mm) if you want a "happy medium" value. Or, you could optimize it for the focal length you most often shoot at, but it might make it less sharp in the other range.

3) If you have a fast 50mm lens, I'd open it up all the way and set the AF point with that. This is the method I prefer. I recently adjusted the AF points on both my K-x bodies using a manual focus 50mm f:1.4 lens. The advantage of this method is that with that fast of a lens, the DOF is extremely narrow, and you know what you have the focus "nailed". Set the AF mode to center point and use a tripod. Focus on a high contrast flat target (a printed page, for instance) 5 or so feet away. Use live view to carefully focus without moving the camera and take a photo. With the camera still on the tripod, zoom up all the way and check that the photo is in focus. If it is, turn the live view off and press the shutter release halfway without moving the camera at all or adjusting the focus. If the viewfinder focus indicator lights up, you should be ok. If not, it needs adjustment. Leave the camera on the tripod and don't move the focus ring at all while tweaking the adjustment until the focus indicator lights up. If it lights up at more than one numeric value, set it to the one in the middle of the "lit up" range.

That method seemed to work well for me.

Hope this helps.
03-28-2014, 12:27 AM   #6
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Bonjour from France and welcome to PF, rfrodman ... looks like you got some great advice and hope to see some images soon ... Salut, J
03-28-2014, 05:18 PM   #7
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Welcome and good luck with the adjustments.
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