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10-28-2014, 02:32 AM   #1
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K-3 doesn't focus in low light conditions

Hello all !

I just bought a new Pentax K-3 - woohoo ! But I tested out the low light focus ability in a dimly lit room, and it could not achieve focus on a gray bedspread and posts. I was using a Sigma 50mm 1.4, and also a Pentax FA 50mm 1.4, and both hunted. Only once did the Sigma get a lock. All at f/1.4.

I had it on the center focus point, then tried it on using all the focus points.

Does that sound right ? Its supposed to achieve focus in complete darkness, and this was a very dimly light room with the adjacent rooms light on. I was expecting it to achieve focus in complete darkness.

(Also my flu card doesn't work and the K-3 doesn't see the card in slot 2. But I'll make another thread for this)

Is this camera DOA ? Or am I doing something wrong(By the way, its an upgrade from my K-5, and its faster, sharper, stabler, quicker OS and snappier so I'm real happy ! )

10-28-2014, 02:39 AM   #2
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QuoteOriginally posted by zoolander Quote
Its supposed to achieve focus in complete darkness
Seriously?
10-28-2014, 02:41 AM   #3
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Have you updated the firmware for both the K-3 and the Flucard?
10-28-2014, 02:42 AM   #4
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Two things:
1. The camera needs some contrast (lines, structure, pattern) to be able to focus. You can't expect it to focus on a featureless surface such as the clear blue sky, light or no light. Gray bedspread - that sounds a bit low contrast to me.
2. It's not supposed to achieve focus in complete darkness. It's spec'ed to work down to light level EV -3, which is equivalent to 1/2 second at f/1.4 and ISO 3200. Try some manual settings to work out which exposure renders the grey bedspread as midtone, and that's your light level.
3. These experiments are only interesting with the focus assist lamp disabled.


Regards,
--Anders.

10-28-2014, 02:52 AM   #5
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The K-3 can certainly do it's job in very low-light.

To test low-light AF, find a contrasty focus target, confirm focus in bright light, then dim the lights progressively until the AF stops working.

You may find this useful BTW, from the Nikon D4 manual:

10-28-2014, 03:12 AM   #6
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QuoteOriginally posted by Steve.Ledger Quote
Seriously?
I thought -3EV was complete darkness. Thats what people have been banging on about.

---------- Post added 10-28-14 at 08:14 PM ----------

QuoteOriginally posted by RollsUp Quote
Have you updated the firmware for both the K-3 and the Flucard?
The K-3 is the Prestige edition and comes with the latest firmware - I checked it already.

Adam cleared up the flucard for me on the other thread. The camera was bundled with an 8gb card which is not the original Pentax 16gb flucard, so I can only share photos and not have the tethering, which is a real bummer.

---------- Post added 10-28-14 at 08:18 PM ----------

QuoteOriginally posted by asp1880 Quote
Two things:
1. The camera needs some contrast (lines, structure, pattern) to be able to focus. You can't expect it to focus on a featureless surface such as the clear blue sky, light or no light. Gray bedspread - that sounds a bit low contrast to me.
2. It's not supposed to achieve focus in complete darkness. It's spec'ed to work down to light level EV -3, which is equivalent to 1/2 second at f/1.4 and ISO 3200. Try some manual settings to work out which exposure renders the grey bedspread as midtone, and that's your light level.
3. These experiments are only interesting with the focus assist lamp disabled.


Regards,
--Anders.
Thats exactly what I thought !

I was in AF.S, so I tried AF.C, and bingo ! AF points are lighting up and its hitting focus on the bedspread, and anything I like and even darker spots ! In total darkness its not metering the picture but it is achieving focus in AF.C.

Thanks

---------- Post added 10-28-14 at 08:20 PM ----------

QuoteOriginally posted by rawr Quote
The K-3 can certainly do it's job in very low-light.

To test low-light AF, find a contrasty focus target, confirm focus in bright light, then dim the lights progressively until the AF stops working.

You may find this useful BTW, from the Nikon D4 manual:
Thanks, I'll keep that all in mind.

Gotta love this forum !
10-28-2014, 07:34 AM   #7
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QuoteOriginally posted by zoolander Quote
I thought -3EV was complete darkness. Thats what people have been banging on about.
No, -3 EV is not complete darkness, but it is dim enough that you probably would not be able to make your way without stumbling over something.

QuoteOriginally posted by zoolander Quote
In total darkness its not metering the picture but it is achieving focus in AF.C.
It may have taken the photo in AF-C, but neither that nor the focus point lights in the viewfinder are evidence of acquired focus.
  • AF-C with default settings will allow shutter release before focus is acquired
  • The red focus point indicators merely show which point the camera is using
  • Focus acquisition is signaled by the green hexagon at the bottom of the viewfinder display along with a beep


Steve
10-28-2014, 08:46 AM   #8
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QuoteOriginally posted by zoolander Quote
I thought -3EV was complete darkness.

Gotta love this forum !
Exposure value - Wikipedia, the free encyclopedia

Exposure times - 15 sec for f1.4 and EV-3. You need tripod to make photos in such condition.


EV
TYPE OF LIGHTING SITUATION
-6
Night, away from city lights, subject under starlight only.
-5
Night, away from city lights, subject under crescent moon.
-4
Night, away from city lights, subject under half moon. Meteors (during showers, with time exposure).
-3
Night, away from city lights, subject under full moon

10-28-2014, 08:50 AM   #9
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QuoteOriginally posted by ogl Quote
Exposure value - Wikipedia, the free encyclopedia

Exposure times - 15 sec for f1.4 and EV-3. You need tripod to make photos in such condition.


EV
TYPE OF LIGHTING SITUATION
-6
Night, away from city lights, subject under starlight only.
-5
Night, away from city lights, subject under crescent moon.
-4
Night, away from city lights, subject under half moon. Meteors (during showers, with time exposure).
-3
Night, away from city lights, subject under full moon
That's cool, I'll remember that...
10-28-2014, 12:47 PM   #10
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Original Poster
QuoteOriginally posted by ogl Quote
Exposure value - Wikipedia, the free encyclopedia

Exposure times - 15 sec for f1.4 and EV-3. You need tripod to make photos in such condition.


EV
TYPE OF LIGHTING SITUATION
-6
Night, away from city lights, subject under starlight only.
-5
Night, away from city lights, subject under crescent moon.
-4
Night, away from city lights, subject under half moon. Meteors (during showers, with time exposure).
-3
Night, away from city lights, subject under full moon
I tried the AF in AF.C in complete darkness and it works, the picture taken was at ISO 51200 so its murky. But yeah, it does work.

It didn't work in AF.S mode, where I selected the single central point. I remember reading from reviewers through their personal experiences, that the center point and the one above andfbelow it were the best for low light auto focusing. So I'll test it out again tonight. I'm just a little surprised that it didn't work in AF.S mode.

So it works like a charm with the 50's and my 16-50 2.8 in AF.C mode - absolutely brilliant !
10-28-2014, 01:38 PM   #11
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QuoteOriginally posted by zoolander Quote
that the center point and the one above andfbelow it were the best for low light auto focusing.
The three sensors in the center column are optimized for f/2.8 maximum aperture and wider. It is a matter of AF sensitivity* and is not related to light levels. As for AF-C vs. AF-S, that is not the way my K-3 works. I can get shutter release, but AF will not lock focus if there is not enough light regardless of mode.


Steve

* Most of the sensors are tuned for f/5.6 maximum aperture. What this means is that the PDAF system has the same ability to detect an out-of-focus state at f/5.6 as at f/1.4. With the center column this changes to f/2.8 meaning that those sensors are much more appropriate for use with fast lenses. I some ways the f/5.6 sensors may be more likely to (inappropriately) lock focus than the center column f/2.8 sensors.
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