Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
02-26-2011, 10:21 PM   #1
Senior Member

Join Date: Dec 2008
Location: N/E Victoria, Australia, on the mighty Murray River.
Photos: Albums
Posts: 141
Front/Back focus testing.

I have a K20D with Sigma 70-200 f2.8 EX DG HSM II & Sigma 17-70 f2.8-4.5 lenses. I have been trying to check the focus using the chart from "Pentax DSLR Blogspot" & am a bit stuck. With the 70-200 @ 200mm f2.8 & 2 metres from the chart it needs -7 on the focus adjustment but @ 100mm it needs around +3. When tested at 200mm f5.6 it needed +2 adjustment.???.
What is the correct settings I should use in regards to focal length, aperture & distance to chart. Have been using around a 45 degree angle. I know the 70-200 isn't that great with close up shots & wide open so should I move the chart back away further or keep using f5.6 or a similar aperture.

02-26-2011, 10:49 PM   #2
Inactive Account

Join Date: Aug 2008
Location: Michigan, USA
Photos: Gallery | Albums
Posts: 7,484
Every thing I've read about adjusting AF on a Zoom says go for the middle range of the focal length. At the risk of sounding like an ass, what happens with real subjects? That is something other than a piece of paper.

02-26-2011, 11:18 PM   #3
Veteran Member

Join Date: Jan 2008
Location: Southern California, USA
Photos: Gallery
Posts: 729
Either one is correct!

What I meant is, for a long zoom like 70-200, adjust the focal length that you use the most.

I have the same problem trying to adjust my 80-200. At the end, I adjusted it based one the 200mm because that's the focal length I use the most out of this lens.
I've posted this same question long time ago and that's the best answer for my case.
Or if you have a good memory, just remember what's the setting for 70mm and what for 200mm, then adjust the micro AF setting according to your need in the field.
02-26-2011, 11:56 PM   #4
Senior Member

Join Date: Dec 2008
Location: N/E Victoria, Australia, on the mighty Murray River.
Photos: Albums
Posts: 141
Original Poster
Yeah, I was thinking about just doing it at 100mm f5.6 and seeing how that works. I use all focal lengths on it for motorcross & other general images so thats why I was trying different focal lengths. If I keep the aperture a few stops from wide open it doesn't seem to bad with different focal lengths & depending on how close I am also makes a difference specially wide open.
I might check at 100mm & 200mm f5.6 & just set the adjustment in the middle.

02-28-2011, 12:15 PM   #5
Site Supporter
Site Supporter
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 16,966
I think there are a couple of things to consider here, before you go too far in testing.

first of all, is the target.

For a telephoto zoom, I would make my own target with something rather large, like a 4 inch square painted in a black and white checker board pattern (maybe 1 inch squares, and place a yardstick (meter stick?) at the side, angled at 45 degrees to the target. the target shoudl be at right angles to the stick.

FOcus at the preferred working distance is a good idea, and try min max and 1-2 middle focal lengths to make sure it is consistant over the range.

Use a tripod to take all shots.

The reason for this is so that you present a realistic target to the camera, not an infinately thin line, that will have background subjects. A sloping target is no good because you don't know what the camera is actually focused on, since the focus sensor is larger than a line across the page.
02-28-2011, 04:56 PM   #6
Senior Member

Join Date: Dec 2008
Location: N/E Victoria, Australia, on the mighty Murray River.
Photos: Albums
Posts: 141
Original Poster
I used an A4 peace of paper with the chart on it. The chart has a big half black, half white circle in the centre & measurements down each side, I camera on tripod mounted on a tripod & set to the right angle as well. I use the lens for different things from motorcross to cats. Seems to be more of a problem at wider apertures larger than f4.5 but not as bad from f4.5 & smaller. I have set the chart from just over a meter to a bit over 2 meters, it seems the lens just isn't up to the task of focusing properly at closer distances but at further objects say 5 metre's & more it seems not to bad depending on aperture. Not sure if having it calibrated with the camera will fix it but may have to look into it & find out.

  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
200mm, aperture, chart, f2.8, f5.6, focus, front/back focus, pentax help, sigma
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
Trying to understand front focus/back focus thing 41ants Troubleshooting and Beginner Help 16 08-31-2010 08:42 AM
Front/back focus or focus shift? fulcrumx29 Pentax SLR Lens Discussion 0 08-17-2009 04:56 PM
Front focus/ back focus on Manual lens. Possible? WangJianWei Troubleshooting and Beginner Help 6 04-20-2009 07:50 PM
How to check Back Focus/Front Focus grishazzz Troubleshooting and Beginner Help 14 03-18-2009 11:35 AM
I've done preliminary testing. Now what can I do about Back Focus? Pentakun Pentax SLR Lens Discussion 1 04-18-2008 09:20 AM

All times are GMT -7. The time now is 01:37 PM. | See also:, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]