Adventures, Photography, And more

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Adjusting auto focus (The problem with sheep part 2)

Many SLR cameras now come with a way to adjust the auto focus calibration for lenses. Most cameras will save the adjustment for each lens and when you reattach the lens it will automatically apply the adjustment. Those camera’s that just allow a global adjustment will require the photographer to make the adjustment for each lens every time they attach it. Annoying but better than nothing.

As I explained in my post “The problem with sheep” my telephoto lens was focusing behind my subjects.  To fix this I had to setup a quick little test environment and run some tests.

The first step is to print a focus chart.   Next setup your camera on a tripod at a 45 degree  angle to the focus chart.  If you are like me and do not have any of your high school geometry sets around you can fold a square piece of paper diagonally so it forms a triangle which will have two 45 degree angles and a 90 degree angle.  You can use this with your camera on your tripod to see if the lens is at 45 degrees.

Set your lens to the longest focal length, max aperture and the closest focus distance.  In my case this was 320mm, f5.6 and about 4 feet away from my focus chart.  This should give you the narrowest depth of field.  Now move your lens so it is out of focus and use the camera’s auto focus to focus on the line that says “this text should be perfectly in focus”.  I focused right on the “s” in focus.

After each test I unfocused the lens, dialed in a focus adjustment (see your camera’s manual on how to do this), refocused using auto focus and took a picture.  Below are 100% crops of the different test shots with different focus adjustments.

Focus with default focus calibration

Focus with default focus calibration

Focus with a -20 focus adjustment

Focus with a -20 focus adjustment

Focus with a -40 focus adjustment

Focus with a -40 focus adjustment

As you can see the -20 focus adjustment is the closest to the line being perfectly in focus.

Please follow the link to see the specific instructions I used for focus adjustments on the Pentax K10d.

Now I just have to find the sheep to validate my results in the field!


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The problem with sheep (aka, autofocus problems) part 1

On my way to work I often see sheep outside grazing on a farm. Each time I passed I wanted to take a picture so I started carrying to work my camera with the telephoto lens attached.

Now my telephoto lens is not anything special. It is a Pentax FA 80-320 F4.5-5.6 that has a reputation for being fairly sharp between 80-200 and very soft 200-320mm. I have been very disappointed trying to get my telephoto lens to be sharp at the long range since switching to digital.

Back to the sheep. I stop on my way to work and take some pictures of the sheep. When I put the pictures on the computer I am very disappointed that none of my sheep are sharp, but after careful inspection of the picture below I see that the grass & leaves behind the sheep is what is in focus.

Back Focus Example Photo

Classic Back Focus - The grass just behind the sheep's legs are in focus.

I know that I had the focus point on the head of sheep that is on the right. Then it occurred to me why so many of my pictures may be “soft”. My camera has a back focusing problem where the auto focus system focuses behind the subject with my telephoto lens attached. My other lens I regularly use does not appear to have this problem. Now that I know the problem I have to find a solution.

Check back later in the week for part 2 where I try to correct the back focus problem.