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Old Jan 4, 2006, 1:00 PM   #1
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Having recently purchased a *istDL, I find that obtaining a precise focusing on portraits seems challenging. The depth of field using a 28mm-110mm lens seems very shallow at about the 80-110 range. For instance, when I shoot a portrait of my daughter,her hair, is in very sharp focus, but her nose and eye lashes appear soft.

I find that using a auto focus pocket digital camera seems much more forgiving resulting in very sharp pictures.



I would like to know if other Digital SLR users are having similar issues, or if I should have my camera exmined for any faults.

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Old Jan 4, 2006, 4:36 PM   #2
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For technical reasons (censor-size in relation to focal-length) pocket digicams always has a very large depth of field. For creative photography this is in fact a problem, and one of the reasons for choosing a DSLR is much more control of DoF.

To control the DoF you have to set the aperture manually. Thethe larger the F number - the larger DoF.

To get a visual clue of your DoF, youturn the ring around the release button opposite the Off-position. This will make the camera stop the aperture down to the chosen F number, and - depending on the light conditions - you should be able to judge the effect on the DoF in the viewfinder.

Of course there is allso the possibility that your AF misjudges the situation. Im one of those stubborn types, who thinks I should decide what part of the picture should be in focus - not the camera.So I normally have my camera set to only center-AF. This means that you have to half-press the shutter with the part of your motif that should be in focus in center, then recompose and press to take the shot. Sounds more complicated than it actually is.

Yours

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Old Jan 4, 2006, 5:31 PM   #3
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This all has to do with shooting technique if you post one of your pics and outline the problem I am sure that someone can explain what happened.
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Old Jan 4, 2006, 6:35 PM   #4
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also, when previewing the dof you will have to develope an eye for it. at smaller apertures the view finder is very dim..

but the first time i saw an ist my first comment was ''OH MY GAWD,, DOF!!! PREVIEW''



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Old Jan 14, 2006, 7:43 PM   #5
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Ikch (Lars) is right, set the camera on just the centre focus point and be careful what you aim at to focus, then recompose. Using the Av mode with the aperture set at f8 or higher will increase the DOF as well but be careful of low shutter speeds.

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Old Mar 2, 2006, 8:26 PM   #6
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I sent my *istDL back to Pentax for warranty service on the problem of focusing. They repaired it having recalibrated and adjusted the focusing mechanism. I can't say that I am entirely pleased with the result. There now appears to be a slight back focusing problem. I guess I will need to use smaller apertures and take advantage of the larger DoF to ensure that my images are in crisp focus. Thanks to all who gave feedback.
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Old Mar 2, 2006, 8:42 PM   #7
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A quick and dirty check that you can use to see if you've got an AF calibration problem is to setup some books on a table, spaced very closely.

Make sure to use a tripod so that camera movement is not an issue.

I'd make sure to use a sharp prime (preferrably a 50mm or more, so that you don't have to contend with a focus plain that's not flat).

Shoot at closer to wide open apetures so that you'll have a shallow depth of field., Then, take a photo of the book in the center and see if the books closer or further away from the camera are blurrier.

There are far more sophisticated tests around. But, I've found that this is an easy one to rule out AF calibratoin issues. If the book in front of your focus point is sharper, your camera is "front focusing". If the book behind the book your focusing on is sharper, your camera is "back focusing".

If the test shows one or the other, then I'd send it back to the factory for calibration, after checking with more than one lens (lenses can impact AF, too).

If not (with a good prime), it's more likely user error (if you lean after locking focus with a shallow depth of field, you'll get out of focus images).

Here is an example of what I'm talking about (select viewing size from under the image).

http://www.pbase.com/jcockfield/image/51015183

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Old Mar 2, 2006, 8:57 PM   #8
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As for depth of field, see this handy online calculator. But, keep in mind that if you use a shorter focal length lens, you'll need to be closer so that your subject occupies the same percentage of the frame:

http://www.dofmaster.com/dofjs.html

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Old Mar 2, 2006, 9:09 PM   #9
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When you get it right with an old 50mm MF lens the results are great.

Pentax *istDL, SMC Pentax-M 50mm f1:1.7, wide open, ISO 1600, Shutter speed 1/125.
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Old Mar 9, 2006, 9:02 PM   #10
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Jim, Thanks for the suggestion of the test. I set up the test using 2 lenses, with my *istDL mounted on a tripod and used the remote control in order to avoid camera shake. With my Sigma 28-105 lens wide open (F/4) there appears to be a slight back focusing issue., which disappears at smaller appertures. I switched to my Pentax 28-90 lens and did not see any evidence of back focusing. I am concluding that my Sigma lens may be at fault. Too bad since I use it most often. I guess I will have to contend with using apertures that are not "wide-open" in order to avoid any discrepancies in focusing.



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