Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
12-15-2011, 02:29 PM   #1
Pentaxian
ducdao's Avatar

Join Date: Dec 2009
Location: Montreal/Vermont
Posts: 2,160
K-r and 50/1.2 Front focus

Anyone tried the combination above? I never experienced FF with the K-r until I put the K50/1.2 on it and shot wide open. The shots was done indoor under tungsten lightning. I already manually set WB to tungsten and it didn't help.

I'd tried the K50/1.4 and Bower 85/1.4 and didn't have the FF issue.

Any thoughts?

12-15-2011, 02:41 PM   #2
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 42,391
Might just be the shallow DOF that's tricking you, combined with the natural inaccuracy of the AF system.

Nevertheless I would test the lens using a chart, trying to achieve focus confirmation coming from both the tele end and the close end of the focusing range. If they're off by the same amount, I would say it's not FF but just the slack in the AF system.
Fixing Front and Back Focus - Introduction- PentaxForums.com

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

12-15-2011, 02:49 PM   #3
Pentaxian
ducdao's Avatar

Join Date: Dec 2009
Location: Montreal/Vermont
Posts: 2,160
Original Poster
QuoteOriginally posted by Adam Quote
Might just be the shallow DOF that's tricking you, combined with the natural inaccuracy of the AF system.

Nevertheless I would test the lens using a chart, trying to achieve focus confirmation coming from both the tele end and the close end of the focusing range. If they're off by the same amount, I would say it's not FF but just the slack in the AF system.
Fixing Front and Back Focus - Introduction- PentaxForums.com
Thanks for the tip, Adam. However, I'm certain that it's not the shallow DOF that caused the issue. As you can see, I have several other Pentax DSLRs as well, and I don't have any FF with neither of them.

Going back to the K-r, my only solution to overcome the FF with the 50/1.2 is to slowly over focus by turning the focus ring for bout 1mm whenever I reach the focus and/or focus confirmation is on in my viewfinder, that works all the time but it really annoys me.
12-15-2011, 05:25 PM   #4
Veteran Member




Join Date: Oct 2007
Location: France
Photos: Albums
Posts: 804
There is no AF on the 50 1.2 so you can't blame the AF system ! The issue (I have also with my 50 1.2 on the K5) is that the focus point confirmation is not accurate enough for proper focusing with 1.2 lens; there is also a slight difference between the focus plane on the focusing screen and the actual plane on the sensor; some guys on the forum have fixed that by changing the trim; the best way is to use live view for focus
Regards

12-15-2011, 05:33 PM   #5
Veteran Member
Philoslothical's Avatar

Join Date: May 2011
Posts: 1,723
As a general observation, I've found the focus confirmation when using manual lenses to be less than accurate overall. Certainly less accurate than using an AF lens. I use it as a rough guide, but not more than that, especially with faster lenses.
12-15-2011, 05:34 PM   #6
Veteran Member
Smeggypants's Avatar

Join Date: Sep 2010
Posts: 1,536
I always use live view to focus my 50/1.2 - I also magnify the live view often too. The standard focus screen isn't accurate enough to render such shallow DOFs even if it does need shimming.

focus confirmation for my 50/1.2 is way way back focussed, more than -10. The same on my old K10 and K20 and on my current K-5s
12-16-2011, 04:30 AM   #7
Senior Member




Join Date: Jan 2011
Location: Paradise, NL
Photos: Albums
Posts: 106
QuoteOriginally posted by Philoslothical Quote
As a general observation, I've found the focus confirmation when using manual lenses to be less than accurate overall. Certainly less accurate than using an AF lens. I use it as a rough guide, but not more than that, especially with faster lenses.
I find the same....I get the red focus confirmation square with MF lenses but I have found it to be very inaccurate. In fact, I get different results if I approach the focus from the infinity side rather than from the close side. With the red focus square illuminated...I can actually turn the focus ring slightly in each direction without the red square turning off.

I generally ignore the focus confirmation square for MF lenses and use my eyes only. The fastest MF lens I have is the SMC-A 50 1.7....I find that really hard to focus using my eyes at 1.7...I can only imagine what 1.2 must be like. I've debated if I should get a focusing screen...just trying to justify the cost.

AB
12-16-2011, 05:29 AM   #8
Pentaxian
ducdao's Avatar

Join Date: Dec 2009
Location: Montreal/Vermont
Posts: 2,160
Original Poster
QuoteOriginally posted by ablundon Quote
I find the same....I get the red focus confirmation square with MF lenses but I have found it to be very inaccurate. In fact, I get different results if I approach the focus from the infinity side rather than from the close side. With the red focus square illuminated...I can actually turn the focus ring slightly in each direction without the red square turning off.

I generally ignore the focus confirmation square for MF lenses and use my eyes only. The fastest MF lens I have is the SMC-A 50 1.7....I find that really hard to focus using my eyes at 1.7...I can only imagine what 1.2 must be like. I've debated if I should get a focusing screen...just trying to justify the cost.

AB
Just to be clear, the red square is not the focus confirmation, it's the indicator of the focus point. The focus confirmation is the green hexagon.

12-16-2011, 05:39 AM   #9
Senior Member




Join Date: Jan 2011
Location: Paradise, NL
Photos: Albums
Posts: 106
QuoteOriginally posted by ducdao Quote
Just to be clear, the red square is not the focus confirmation, it's the indicator of the focus point. The focus confirmation is the green hexagon.
Don't they light up at the same time when "focus" is achieved? Can't remember off the top of my head. If not then my same comment applies with the green hexagon.
12-16-2011, 06:35 AM   #10
Pentaxian
hoanpham's Avatar

Join Date: Nov 2009
Location: Strand
Photos: Gallery | Albums
Posts: 1,365
it's the AF sensor that is inaccurate. My K20 is better than my K5. AF sensor when achieve correct focus, it triggers the green hexagon to lit. 1mm focus throw might be too much for f1.2.
The 50/1.2 is much better with a *calibrated* focus screen. Even that, I still need to fire multiple frames to be sure the few cm in-focus area.
12-16-2011, 06:54 AM   #11
Pentaxian
ducdao's Avatar

Join Date: Dec 2009
Location: Montreal/Vermont
Posts: 2,160
Original Poster
QuoteOriginally posted by ablundon Quote
Don't they light up at the same time when "focus" is achieved? Can't remember off the top of my head. If not then my same comment applies with the green hexagon.
No, they don't always light up at the same time. The red square always lights up first to indicate where the focus point is then the green hexagon comes after once you've reached the focus.
12-16-2011, 06:59 AM   #12
Pentaxian
ducdao's Avatar

Join Date: Dec 2009
Location: Montreal/Vermont
Posts: 2,160
Original Poster
QuoteOriginally posted by hoanpham Quote
it's the AF sensor that is inaccurate. My K20 is better than my K5. AF sensor when achieve correct focus, it triggers the green hexagon to lit. 1mm focus throw might be too much for f1.2.
The 50/1.2 is much better with a *calibrated* focus screen. Even that, I still need to fire multiple frames to be sure the few cm in-focus area.
I've noticed the same. My K10D and K20D both perform much better with regard to the focus confirmation on the 1.2 aperture. Another funny thing is my K100d is also better than the K-r when it comes to focus confirmation.

My K-r focus screen is probably is not calibrated as you said. How does one calibrate the focus screen? Can you direct me to the relevant threads if any?
12-18-2011, 11:51 AM - 1 Like   #13
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
There is no way to calibrate it. It,s very design guarantees that it shows too much DOF when shooting at f/1.2 - far more than the picture will actually contain. This is trivially easy to demonstrate by shooting some text on an angle, comparing how many lines appear in focus in the viewfinder versus how many appear in focus in the actual image. Becaus the viewfidner shows too mich in focus, there is guaranteed to always be something that appears in focus in the viewfinder but will turn out not to be. This doesn't happen when shooting with slower lenses - it starts to show around f/2.8 but doesn't get really bad until below f/2. Aside from installing a split prism focus screen, there is no solution except to learn to live with it - to anticipate how much of what appears in focus really will turn out to be.
Reply

Bookmarks
  • 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!
camera, ff, k-r, kr, pentax k-r, tungsten
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
K-5 Manual Lens Front Focus & Focus Beep woes fixed kneonx Pentax K-5 14 04-12-2014 03:47 PM
How to Fix Front Focus and Back Focus Adam Pentax DSLR and Camera Articles 11 07-17-2012 10:01 AM
Front focus far away, back focus up close hjb981 Pentax K-5 14 05-05-2011 01:28 AM
Trying to understand front focus/back focus thing 41ants Troubleshooting and Beginner Help 16 08-31-2010 08:42 AM
front focus, back focus or other? imfinetoday Pentax SLR Lens Discussion 23 07-08-2010 11:49 AM



All times are GMT -7. The time now is 01:06 AM. | See also: NikonForums.com, CanonForums.com 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?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top