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01-11-2012, 05:15 AM   #1
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Pentax K-x night/low light exposure problem

First of all, I have used a Minolta 5d for some 6 years, including night shots - always to a great result, so I know how to do it!
Last night I took my Pentax K-x out for some night shots. I used kit Pentax 18-55 lens. It was pretty dark, only remote city light and the moonlight through clouds. However I was able to see my step well enough, so it's not a pitch-black!
I set to shoot in Av mode with f.4 at 28 mm. Multi-segment metering. No exposure compensation.ISO 1600. The suggested shutter speed come up to 0.3", with indication blinking (according to the manual complaining about low light). The shot came out almost competely black, to my surprise! I took a few more shots, messing with exposure compensation and metering modes. All the photos came grossly underexposed, even those with compensation +3.The suggested shutter speen never went lower than a couple of secs. Tried manual exposure - not much better. The best shot came up at 30s, ISO 1600, f/4 - still somewhat on the under side.
Shooting at a road daylight the result is always excellent!
Two questions:
1. Am I right in thinking that at no expo compensation the camera should try to produce a shot close to medium-gray, no matter how dark it is?
2. Does anyone experienced something similar or is my camera f**ked?

01-11-2012, 07:14 AM   #2
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How dark is dark, and could you post some examples? If the shutter speed number is flashing, that means the camera isn't getting enough light to take a proper metering level. You have to remember our eyes adapt and it could be darker than you realize (while our eyes can perceive, the camera can only calculate). Your camera isn't necessarily broken, we'll need to see some examples before we can really help.
01-11-2012, 07:33 AM   #3
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I found the K-x tends to need quite a bit more exposure for lower light shooting. Not sure exaclty what is going on with your case maybe post a few shots for us to see?
I tended to add over a stop sometimes up to 2 stops more than the meter said for lower light shooting.
I've not used the 5d but I've known a few that have had it and it was apparently pretty good for longer exposures metering wise.

Post some samples and we can see what is going on
01-11-2012, 07:46 AM   #4
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30 sec exposure at 1600 ISO tells me it was dark indeed. I used to do a bit of night Photography using a Bronica ETRS.
My starting point was 8 seconds which was sometimes enough in a very well lit street using 400 ISO.
Then I would normaly just double my exposures, so then about 16 sec and 30 etc. This was done on the bulb setting of course and since exposure
accuracy was hardly critical, i would genraly just count in my head if i didnt have a watch with a second finger.
I would expect these light levels would poss be beyond the scope of built in camera metering.

01-11-2012, 09:14 AM   #5
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QuoteOriginally posted by nmilyaev Quote
Last night I took my Pentax K-x out for some night shots. I used kit Pentax 18-55 lens. It was pretty dark, only remote city light and the moonlight through clouds. However I was able to see my step well enough, so it's not a pitch-black!
I set to shoot in Av mode with f.4 at 28 mm. Multi-segment metering. No exposure compensation.ISO 1600. The suggested shutter speed come up to 0.3", with indication blinking (according to the manual complaining about low light). The shot came out almost competely black, to my surprise!
...
The best shot came up at 30s, ISO 1600, f/4 - still somewhat on the under side.
Shooting at a road daylight the result is always excellent!
Two questions:
1. Am I right in thinking that at no expo compensation the camera should try to produce a shot close to medium-gray, no matter how dark it is?
2. Does anyone experienced something similar or is my camera f**ked?
When the info display blinks in the viewfinder, as you've already said - it's because the light conditions are out of the K-x's metering range.

Once the metering limit has been reached
in this case the low limit -
the K-x merely sets the lowest exposure from the low limit -
as all bets are off at this stage.

Exposure compensation will not help as the K-x does not set any other exposure other than the lowest it can meter - so that behavior is quite "normal".


I shoot often in available light/dark conditions
sometimes it is below the metering and AF limits of the K-x.
Please see Posts #131 & #132 in Kx in Use ( 1 2 3 ... Last Page)
- where I experimented to see where those limits were.

eg: from Post #132
the metering low limit of the K-x using 18-55mm f/3.5-5.6 zoom

@ 18mm
ISO100, f/3.5, 13sec
ISO5000, f/3.5, 1/4
ISO1600, f/3.5, 0.8sec;
@ 28mm
ISO5000, f/4, 1/8

and I can confirm -
@ 28mm, ISO1600, f/4, 0.3sec


This is easy to do - simply put the lens cap on - set focal length ISO and aperture - and blinking display on the LCD will show the lowest limit reached, and therefore the lowest exposure the K-x will set - that is why we will have to manually override that "suggestion" if the conditions are far below this - as in your case.

K-x manual specs actually say:

the K-x is spec'd to EV=1 (at ISO200, f/1.4) = EV=0 (@ ISO100, f/1.4) -

for the 18-55 f/3.5 zoom:
ISO5000, f/3.5, 1/4sec (tested limit from above)
calculating for f/1.4 and ISO100 that's the same equivalent light level:
= ISO100, f/1.4, 2 secs
- this is EV (Exposure Value) = 0.3 (@ ISO100) better than published spec
= ISO1600, f/1.4, 1/8sec
= ISO1600, f/4, 1sec.

Using a moonlit exposure table
shooting in full moonlight - the suggested exposure is ISO100, f/8, 8mins -
translated to ISO1600 (4stops faster), f/8, 30 secs = f/4, 8secs
7 days after the full moon (is it called "quarter moon"? looks like half bright/dark to me)
ISO100, f/8, 1.5hours! = ISO1600, f/8, ~5.5mins = f/4, 1.5mins!

Another exposure table for confirmation
4th line from bottom in table:
Subject illuminated by full moon, high in clear sky = (ISO1600-3200) EV0
on second table -
EV0 = f/4 @ 15secs

So I would say you were trying to meter quite a way below the K-x metering limit

Last edited by UnknownVT; 01-11-2012 at 11:21 AM.
01-11-2012, 01:41 PM   #6
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Thank you everyone for your replies.
Here are the sample photos (sorry for the delay - I was at work):
https://picasaweb.google.com/108648874181776963060/PentaxLowLight?authkey=Gv1sRgCITG-oL-4ZTgVQ
Sorry for the image quality - I know I should have used a tripod, but I was so let down that I could not be bothered - decided to test on as many settings as possible instead.
I have also uploaded a few Monlta pics for comparison - on one of these you can just about make out the outlines of the skies there and the stars, but mind you that was quite a bit darker than on the pentax shots. Still we are talking ISO difference of 5 stops!
On the hindside, analysing these Minolta shot they looks comparable to the Pentax ones, so perhaps the sensor sensitivity is similar (my initial fear was that the Pentax is broken somehow and drops sensitivity in low light), so perhaps the sensor is ok to that end...
Still, the metering is rather disappointing - I am pretty sure that Minolta was much better at getting exposure right in low-light situations and was bravely going down below 1s mark in shutter speed in Av mode.
As to the cap-on test I'm pretty sure Minolta went down to about 4s...
Any idea why Pentax imposes these weird restrictions?
01-11-2012, 02:47 PM   #7
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I tend to do a LOT of night shooting myself. Luckily the K-x handles high ISO really well, so you can crank that up well past 1600 and have acceptable results, especially if you're comfortable shooting RAW and mucking about stripping the grain in post process.

I've shot aurora with my K-x using the kit lens, so despite the metering being unable to cope, the actual shots once you go manual can be really nice indeed.

Here are some of my better 'kit' shots in low light, just to show the camera isn't that bad once you stop relying on it's internal metering and just wing it with a bit of practice.

ISO12800, f/3.5, 1/25 second, obvious effects from ripping the grain out in this one. FWIW, this one also was metered by the K-x.


ISO 200, 30 seconds, f/3.5



ISO 1600, f/3.5, 30 seconds - this is about as dark as you're going to get. No moon, with true 'dark sky' conditions up here in rural Maine.

01-11-2012, 03:09 PM   #8
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QuoteOriginally posted by nmilyaev Quote
Still, the metering is rather disappointing - I am pretty sure that Minolta was much better at getting exposure right in low-light situations and was bravely going down below 1s mark in shutter speed in Av mode.
As to the cap-on test I'm pretty sure Minolta went down to about 4s...
Any idea why Pentax imposes these weird restrictions?
Pentax does not impose weird restrictions.

The K-x is spec'd the low limit of metering at EV0 at ISO100 = LV0
this is about the same as almost any other dSLR on the market.

Was your 4 secs for the Minolta at ISO1600?
this makes a difference -
the lens max aperture also affects the lower limit
as a more modest aperture lets in less light for the TTL metering

The 18-55 zoom at 28mm has a max aperture of only f/4 -
compare this to the f/1.4 for how most manufacturers spec their metering -
this is some 3 stops difference in the amount of light reaching the meter -
so the 0.3sec could have been lower at 2 secs -
which is in the similar ballpark to the 4s of the Minolta.

01-12-2012, 02:17 AM   #9
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Thanks Sandy. Good shots and nice illustration! It's kinda curious how 30s already created moving stars (they are slightly blurred).
westmill, thanks for the tips.
Vincent, I did similar tests cap on - my results match these of yours at #132. 18mm ISO 100 F/4: 20s.
And your math does add up. I still believe Pentax' low light metering is somewhat funny and underexposing.
What I found annoying in more tests last night is that when using spot metering pointed at a dark spot and being static in that position the exposure readings were jumping up and down between 4" and 10" as if the camera struggled to make up its mind.
In multi-segment too, once I half-press the shutter release for the first time after idle the reading jumped to lets say 15", then rolled down to 10".
Any explanation for that mystery?
BTW, higlights on or off didnt make difference to the exposure other than allowing ISO 100 to me. Thanks for the tables
Thanks all once again! My camera seems fine - may be a bit funny! Hurray! ;-)
01-12-2012, 09:34 AM   #10
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useful discussion

The above discussions have been useful in clarifying the limits of modern DSLR light meters. Regretably, even today, not nearly as good as they were 30 years ago.

Thirty years ago I routinely used a Miranda Range Finder with a CdS meter and it took well exposed images, up to 16 secs at 200 iso of street scenes at night.

Not a chance that modern DSLRS can do this or, at least at the level of the consumer cameras produced by Pentax.

The best is to use manual settings and teach yourself to estimate how many seconds your chosen iso rating will do for various night conditions. It is not an issue once you get over the disappointment that modern technology is not as good as old technology, in some cases anyway!
01-12-2012, 12:36 PM   #11
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QuoteOriginally posted by nmilyaev Quote
Vincent, I did similar tests cap on - my results match these of yours at #132. 18mm ISO 100 F/4: 20s.
I still believe Pentax' low light metering is somewhat funny and underexposing.
What I found annoying in more tests last night is that when using spot metering pointed at a dark spot and being static in that position the exposure readings were jumping up and down between 4" and 10" as if the camera struggled to make up its mind.
In multi-segment too, once I half-press the shutter release for the first time after idle the reading jumped to lets say 15", then rolled down to 10".
Any explanation for that mystery?
BTW, higlights on or off didnt make difference to the exposure other than allowing ISO 100 to me. Thanks for the tables
Thanks all once again! My camera seems fine - may be a bit funny! Hurray! ;-)
I did find a few anomalies in the K-x metering - as noted on Post #132 -
@ 26mm there seemed to be out of ordinary meter readings (with cap on)

When nearing the low light limit I can just about understand meter variences if using matrix where a slight change in scene/light could make a difference -
but I am at a loss to be able to explain that on Spot metering -
as long as you are sure the scene/light did not vary at all....

Olde meters had needle indicators - which had to be damped to prevent wild swings -
perhaps the meter in the K-x, although digital, is somewhat "under-damped" -
which in a way, may be a good thing as it may be "overly-sensitive" to changes -
personally I prefer over-sensitive to under-sensitive -
(in a meter that is....
not necessarily in a person )

....just a little irony -
how does one check if there is light variance?
with a light meter -
and what is the most accurate meter that most of us here own?
spot meter in our dSLR!

Personally I keep Highlight correction On - and sacrifice ISO100 as I mostly shoot low available light - and much prefer to protect blowing of highlights - since it is very feasible to lift details out of the shadows whereas it is almost impossible to recover blown out highlights.

So much so that that my normal setting is -1/3 stop exposure compensation (on Natural Color and the weakest Shadow Correction)

Last edited by UnknownVT; 01-12-2012 at 12:46 PM.
01-13-2012, 08:17 AM   #12
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QuoteOriginally posted by UnknownVT Quote
I did find a few anomalies in the K-x metering - as noted on Post #132 -
@ 26mm there seemed to be out of ordinary meter readings (with cap on)

When nearing the low light limit I can just about understand meter variences if using matrix where a slight change in scene/light could make a difference -
but I am at a loss to be able to explain that on Spot metering -
as long as you are sure the scene/light did not vary at all....
Nah, held it firmly pointing to the same spot within the room and there was no-one to fiddle with the light switches...

QuoteOriginally posted by UnknownVT Quote
Olde meters had needle indicators - which had to be damped to prevent wild swings -
perhaps the meter in the K-x, although digital, is somewhat "under-damped" -
which in a way, may be a good thing as it may be "overly-sensitive" to changes -
personally I prefer over-sensitive to under-sensitive -
(in a meter that is....
not necessarily in a person )

....just a little irony -
how does one check if there is light variance?
with a light meter -
and what is the most accurate meter that most of us here own?
spot meter in our dSLR!
Over-sensitive or undecided? Just like in person, pardon the pun, too ;-)
I feel there is as you hinted a bit of hesitation and variance in the light meter - perhaps something for Pentax to look into for the next firmware update? (I do hope that is FW rather than hardware problem).

QuoteOriginally posted by UnknownVT Quote
Personally I keep Highlight correction On - and sacrifice ISO100 as I mostly shoot low available light - and much prefer to protect blowing of highlights - since it is very feasible to lift details out of the shadows whereas it is almost impossible to recover blown out highlights.

So much so that that my normal setting is -1/3 stop exposure compensation (on Natural Color and the weakest Shadow Correction)
Yeah, you are right - keeping the highlight protection on at all times is worth it. Especially, as I read somewhere the difference in IQ between ISO 100 and 200 is hardly any if any at all (isn't ISO 100 non-native and is only simulated?). The only advantage of ISO 100 is shooting in very-very bright light when the shutter speed runs over the top limit (1/6000), which I hardy can think of a condition for - perhaps a snow-strewn Spanish beach? :-)
01-13-2012, 11:31 AM   #13
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QuoteOriginally posted by nmilyaev Quote
I feel there is as you hinted a bit of hesitation and variance in the light meter - perhaps something for Pentax to look into for the next firmware update? (I do hope that is FW rather than hardware problem).
The anomaly I found seems to me more like a miscommunication/coupling between the lens and the body -
as it occurs only at one spot - at about 26mm on the 18-55 (Mk. I) zoom
and does not seem to have a problem with the 50-200 (the only other lens I have)

QuoteOriginally posted by nmilyaev Quote
Especially, as I read somewhere the difference in IQ between ISO 100 and 200 is hardly any if any at all (isn't ISO 100 non-native and is only simulated?).
Although ISO100 is not a native sensitivity - some here will tell you that the noise floor at ISO100 is better - so when doing extended processing this can make a difference.

Found the thread: ISO 100 and 12800 on Pentax KX? ( 1 2 3)

But since I shoot humble JPGs it's kind of practically moot for me - except as you mentioned for the very bright
(that's lighting conditions,
and not the person....)

QuoteOriginally posted by nmilyaev Quote
...
Over-sensitive or undecided? Just like in person, pardon the pun, too ;-)
...
perhaps a snow-strewn Spanish beach? :-)
I'm undecided
about the beach....

but not insensitive
to good punny things

Last edited by UnknownVT; 01-13-2012 at 11:43 AM.
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