Is this FF or BF?

gebco · 04-02-2010, 09:22 PM

I'm beginning to think my K-x has some focussing issues. I used to think that my kit lens was soft wide open, but I was trying some manual lenses today and the pics also came out soft, so I began playing with the focus. I found that if I use the focus indicator light, most times the pic is soft, but if I adjust a little beyond the object, just to the point where the focus indicator goes out, the pic is sharper. Is this FF or BF? I think I read somewhere how to access the menu to adjust focus, as opposed to sending it in. Any thoughts on this? The pics below are handheld, ISO 3200, f/4.5, 160th, 24mm, using the kit 18-55. TIA
Pic 2: autofocus
Pic 3: manual focus using indicator light
Pic 4: manual focus, adjusting just to point where focus indicator light goes out

Attached Images

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Last edited by gebco; 04-02-2010 at 09:23 PM. Reason: forgot to label pics

JeffJS · 04-02-2010, 09:39 PM

If you're adjusting beyond your subject to get good focus, then it's front focusing. This really isn't the best way to test it however. The recommended way is to use a focus chart set at 45 degrees to the lens center in good light. Google Nikon Focus chart to get more info..

gebco · 04-02-2010, 10:43 PM

I'm not sure if you can tell from looking at the resized photo. It looks to me like it is focussing on the text toward the bottom of the chart.
(EDIT: it was shot using a 50mm manual at f/2.0, 1/60th, ISO 200, focussed based on the focus indicator light)

JeffJS · 04-02-2010, 11:00 PM

Yep, Looks front focused to me. Can't really tell but somewhere between 10mm and 20mm on the chart. I would suggest doing this in sunlight however. You want to make sure you are as exactly close to 45 degrees as possible, as well as being dead on sure you're focused on that center line. Any deviation from that can give a false result (the amount of error). Indoor room lights have a tendency to fool Pentax's AF systems in some situations.

I don't know what to tell you about how to fix it, I don't think the Kx gives an adjustment option like the K20 and K7 do. Does it do this with ALL lenses? I would at the very least make sure I had the latest firmware. Others with actual Kx experience may be able to assist you further.

gebco · 04-02-2010, 11:09 PM

I will try the test in sunlight (the next time we get some).
This test was done in the kitchen under halogen and fluorescent lighting. It seems to do it with all lenses. I chose the 50mm for the test as it has the largest aperture of my lenses.

Thanks for your help. I'll post an updated pic.

Bulldogface · 04-03-2010, 04:17 AM

Hi, K-x will let you adjust focusing, but you need to enter debug mode...just google k-x debug mode, and you should be covered

. btw, regarding focusing and related sharpnes, here is thread about it: https://www.pentaxforums.com/forums/pentax-dslr-discussion/91982-k-x-sharpness-problem.html

noahpurdy · 04-03-2010, 09:38 AM

I had some back-focusing issues with my K-x and had to go into debug mode to take care of it. Everything is tack-sharp now!

Marc Sabatella · 04-03-2010, 09:45 AM

It also looks like you're not coming in at a steep enough angle to really get the full value from this test. But in any case, I wouldn't assume that any "debug mode" focus adjustment will affect the indicator at all. I've seen discussion of this both ways, but no controlled tests with incontrovertible findings. My assumption is the indicator is going to remain a little iffy - generally front focusing in yellow-tinged lighting, also just not as precise as you want, coming on before optimal focus is achieved - regardless of the af adjustment setting. I'd assume the af adjustment setting will affect the operation of af more so than the appearance of the af indicator - but this is something I'd like to see a good controlled test to prove or disprove.

04-02-2010, 10:43 PM	#3
gebco Veteran Member Join Date: Mar 2010 Location: Winnipeg Photos: Gallery \| Albums Posts: 329 Original Poster	did the test I'm not sure if you can tell from looking at the resized photo. It looks to me like it is focussing on the text toward the bottom of the chart. (EDIT: it was shot using a 50mm manual at f/2.0, 1/60th, ISO 200, focussed based on the focus indicator light) Attached Images Last edited by gebco; 04-02-2010 at 11:00 PM. Reason: additional info

04-02-2010, 11:09 PM	#5
gebco Veteran Member Join Date: Mar 2010 Location: Winnipeg Photos: Gallery \| Albums Posts: 329 Original Poster	thank you I will try the test in sunlight (the next time we get some). This test was done in the kitchen under halogen and fluorescent lighting. It seems to do it with all lenses. I chose the 50mm for the test as it has the largest aperture of my lenses. Thanks for your help. I'll post an updated pic. Last edited by gebco; 04-02-2010 at 11:10 PM. Reason: further info

04-02-2010, 09:39 PM	#2
JeffJS Inactive Account Join Date: Aug 2008 Location: Michigan, USA Photos: Gallery \| Albums Posts: 7,484	If you're adjusting beyond your subject to get good focus, then it's front focusing. This really isn't the best way to test it however. The recommended way is to use a focus chart set at 45 degrees to the lens center in good light. Google Nikon Focus chart to get more info..

04-02-2010, 11:00 PM	#4
JeffJS Inactive Account Join Date: Aug 2008 Location: Michigan, USA Photos: Gallery \| Albums Posts: 7,484	Yep, Looks front focused to me. Can't really tell but somewhere between 10mm and 20mm on the chart. I would suggest doing this in sunlight however. You want to make sure you are as exactly close to 45 degrees as possible, as well as being dead on sure you're focused on that center line. Any deviation from that can give a false result (the amount of error). Indoor room lights have a tendency to fool Pentax's AF systems in some situations. I don't know what to tell you about how to fix it, I don't think the Kx gives an adjustment option like the K20 and K7 do. Does it do this with ALL lenses? I would at the very least make sure I had the latest firmware. Others with actual Kx experience may be able to assist you further.

04-03-2010, 04:17 AM	#6
Bulldogface Forum Member Join Date: Jan 2010 Location: Akershus Photos: Gallery Posts: 69	Hi, K-x will let you adjust focusing, but you need to enter debug mode...just google k-x debug mode, and you should be covered . btw, regarding focusing and related sharpnes, here is thread about it: https://www.pentaxforums.com/forums/pentax-dslr-discussion/91982-k-x-sharpness-problem.html

04-03-2010, 09:38 AM	#7
noahpurdy Veteran Member Join Date: Nov 2009 Location: Pittsburgh, PA Posts: 479	I had some back-focusing issues with my K-x and had to go into debug mode to take care of it. Everything is tack-sharp now!

04-03-2010, 09:45 AM	#8
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	It also looks like you're not coming in at a steep enough angle to really get the full value from this test. But in any case, I wouldn't assume that any "debug mode" focus adjustment will affect the indicator at all. I've seen discussion of this both ways, but no controlled tests with incontrovertible findings. My assumption is the indicator is going to remain a little iffy - generally front focusing in yellow-tinged lighting, also just not as precise as you want, coming on before optimal focus is achieved - regardless of the af adjustment setting. I'd assume the af adjustment setting will affect the operation of af more so than the appearance of the af indicator - but this is something I'd like to see a good controlled test to prove or disprove.