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05-08-2013, 09:16 AM   #1
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Blurry pictures with Sigma 18-55mm

Hello

I have a problem with my Sigma lens (18-55mm F2.8-4.5) and i want to make sure im doing everything right. I have a K-r and a K20D and the pictures are coming out the same way. I set my AF focus point to the center so i can keep the entire picture in focus but my subject feet and head become a lil burry and then it sharpens up near the center. Im shooting at 18mm, 1/180, F4.5 and my iso is at 400. Plus im shooting on a tripod and using a remote. HELP!!


Last edited by johnlewis804; 05-08-2013 at 09:43 AM.
05-08-2013, 09:51 AM   #2
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What is the distance to the subject? Are the feet and head at the outer edges of the image?
05-08-2013, 10:15 AM   #3
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QuoteOriginally posted by jatrax Quote
What is the distance to the subject? Are the feet and head at the outer edges of the image?
Outer edges?? do you mean out of the frame in the view finder? no i try to keep everything in the frame and im only about 7 feet or less away

Last edited by johnlewis804; 05-08-2013 at 10:23 AM.
05-08-2013, 10:39 AM   #4
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John,

I think jatrax may be on the right track.

At 18mm it is possible that the curvature of field of your lens - and wide angle lenses are quite prone to curvature - may cause the edges of your picture to be out of focus. Indeed, your entire image may very well be distorted.

Portraits are usually shot with a short (90mm) telephoto lens to minimize any distortion.

However, standing back from your subject and using 55mm should give you what you want.

Mickey

05-08-2013, 11:01 AM   #5
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QuoteOriginally posted by mickeyobe Quote
At 18mm it is possible that the curvature of field of your lens - and wide angle lenses are quite prone to curvature - may cause the edges of your picture to be out of focus. Indeed, your entire image may very well be distorted.
Yes, and depending on the lens that can be slightly noticeable or really bad. I'm not familiar with that lens so I've no idea how bad the distortion is on it. But as Mickey notes, portraits are usually shot with longer lenses so that the person does not distort. That would be 75mm to 100mm (full frame equivalent) lenses which on your camera would be 50mm to 70mm approximately. Try the same shot at 50mm or so and see if it looks any different.
05-08-2013, 11:21 AM   #6
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The only reason im shooting at 18mm because im trying to get a full body shot in a vertical position. I'm limited on space since i dont have that much room to move back to get the entire body shot using anything higher. I don't believe i had this problem with the kit lens 18-55.
05-08-2013, 11:42 AM   #7
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QuoteOriginally posted by johnlewis804 Quote
I don't believe i had this problem with the kit lens 18-55.
The Pentax kit lens is quite good for what it is and may not display as much distortion as the Sigma. Or this could be another issue entirely. Try posting an example with the EXIF intact, maybe someone will see something else.
05-08-2013, 02:00 PM   #8
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Use manual focus [and/or focus setting the distance from the distances marked on the lens (I assume it has this)]. Then if there is curvature of the field decide what is more important--or use an average. Or maybe the lens is damaged. But don't just assume auto focus is correct.

05-10-2013, 07:10 PM   #9
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is there a setting in the k20d and k-r to help with lens curvature of field?
05-10-2013, 08:34 PM   #10
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For Pentax lenses there is on the k-r, no idea about the k20. But it is only Pentax lenses AFAIK. This can also be corrected in Lightroom if there is a profile for the lens.
QuoteOriginally posted by johnlewis804 Quote
I don't believe i had this problem with the kit lens 18-55.
Do you still have the kit lens? You might try and see if it works any better.
05-11-2013, 05:23 AM   #11
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Here is some EXIF data from my K-r, when i review the pictures it looks like the faster the shutter speed the more blurry the feet are...

Manufacturer: PENTAX
Camera model: PENTAX K-r
Exposure time, sec: 1/160
Aperture (F): 4.50
ISO speed rating: 100
Lens focal length, mm: 18.0
Orientation: Normal
Copyright: JOHN_LEWIS_DESIGNS
File change date and time: 2013:04:07 14:41:25
Flash: Flash did not fire, compulsory flash mode
Exposure bias (APEX): 0/10
Metering mode: multi-segment
Exposure mode: Manual exposure
White balance: Auto white balance
Focal length in 35 mm film, mm: 27
Scene capture type: Standard
Contrast: Normal
Saturation: Normal
Sharpness: Normal
Subject distance range: ExifA40AValue2
Artist: JOHN_LEWIS_DESIGNS
Sensing method: One-chip color area sensor
X Resolution: 300/1
Y Resolution: 300/1
Units: 2
SoftWare: K-r Ver 1.00
YCbCrPositioning: Co-sited
Exif version: 0221
ComponentsConfiguration: YCbCr
05-11-2013, 05:48 AM - 1 Like   #12
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You haven't included a sample photo to demonstrate your problem. Is it possible the feet are further away from the camera than other parts of the body?

Unless you are also increasing the ISO (doesn't look like it), then the camera has to compensate for the faster shutter speed by opening the aperture wider (smaller number). A wider aperture equals a narrower depth of field (DOF = the near/far range of things in acceptable focus). Conversely, if you go for a slower shutter speed and/or increase the ISO, your aperture will be smaller (bigger number) and increase the DOF.

Not to sing to the choir, but ISO, shutter speed and aperture are the tripod that defines how much light you collect to make a picture.

ISO is like a collection bucket, and you want to fill the bucket each time. The higher the ISO, the smaller the bucket and the faster it will fill. However the bucket has a set amount of 'digital sand' regardless what size you make the bucket. In a big bucket (low ISO), you don't notice the sand, but in a small bucket (high ISO) the sand starts to become noticeable.

Shutter speed determines how long you fill the bucket from the light hose. Too slow and it is hard to hold the hose on target, too fast and you don't fill the bucket.

Aperture is like the valve that controls how much light gets into the hose. As noted above, it also impacts focus.

Balancing these 3 items is the key to proper exposure.
05-11-2013, 06:04 AM   #13
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Ok ill boost the iso and slow the shutter speed down today at my shoot i will post pictures soon.
05-11-2013, 07:34 AM   #14
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QuoteOriginally posted by johnlewis804 Quote
Ok ill boost the iso and slow the shutter speed down today at my shoot i will post pictures soon.
The most important thing is to decrease the aperture a couple stops for greater DOF .... then either reduce the shutter speed OR increase the ISO to compensate. No need to do both. Personally, I would likely increase the ISO to at least 400 and keep the shutter at its current setting.
05-11-2013, 07:54 AM   #15
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well i already did a shoot and had my iso at 400 and shutter speed at 1/180 and her heels were still kinda blurry, So im going to keep the iso at 400 and change the speed to 1/90 or below and see if that helps
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