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08-25-2018, 11:34 AM   #31
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Thank you for your knowledge!

10-07-2018, 02:56 PM   #32
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Great article. I never thought about where the meeting came from.. maybe some more experiments are needed. Question, how good is a low light long exposure with stop down? I'm assuming the metering accuracy gets worse with lower light or does a long exposure time help? I'm thinking of landscape photography here, should I meter in viewfinder or live view?
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10-07-2018, 10:11 PM - 2 Likes   #33
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QuoteOriginally posted by Martin KP Quote
Question, how good is a low light long exposure with stop down?
Metering and exposure time are not directly related, though I believe I get your drift. The explanation regarding low light is complicated, but I will give it my best shot.
  • The meter on your KP has upper and lower limits for sensitivity and linearity. The range (from the KP user manual) are stated as:
    EV -3 to 20 (ISO 100 at 50mm f/1.4)*
    Translation using the standard EV table**:
    • The low limit of meter sensitivity is the amount of light needed for a proper exposure at f/1.4 for 15 seconds at ISO 100
    • The high limit is f/11 for 1/8000s

    Normal open aperture readings outside that range will result in a blinking display indicating meter error.
  • Looking more closely at the low end sensitivity, stop-down metering results in less light to the meter for a given subject illuminance than if the lens is wide open. For example, at f/8, the lower limit for a stop-down metered exposure is met at 1/8s for ISO 100, 1/30s for ISO 400, 1/125s for ISO 1600 and so on.

    Back when stop-down metering was the norm on cameras such as the Pentax Spotmatic, the limits were published as a table in the user manual. On the Spotmatic, there is even an indicator window adjacent to the shutter dial for combinations of ISO(ASA) and shutter speeds known to be outside the meter range at f/1.4 and f/16.
  • If light to the meter is below its low limit of sensitivity, it will indicate settings that will result in overexposure of 1/2 stop or more (meter is no longer linear in its response).
  • The above points pertain to the RGB meter used with the optical viewfinder. Live view metering undoubtedly has its limits as well, though Pentax does not publish specifications for the main sensor used as a light meter. I would expect them to be similar to the regular meter sensor.
The take-away is to be very wary of using stop-down metering in low light situations. The camera will not provide the usual flashing warnings.

So, what to do? Conventional answers go something like this and none involve TTL light measurements:
  • Use a handheld meter, most will go to EV100 0 or EV100 3 lower sensitivity limit. None will provide readings below their limit of sensitivity and that is a good thing.
  • Use a table or calculator of common subjects/lighting and bracket +/- one stop from recommended settings. Sloppy as it might sound, I have found this method to work quite well.
  • Work out your own exposure guide for common low-light subjects


* Near as I can tell, the reference to use of available 50mm f/1.4 lenses allowing for a setting range of f/1.4 to f/16 or narrower. EV 20 at f/1.4 requires 1/219s.
** Wikipedia | Exposure Value, As Indicator of Camera Settings
10-20-2018, 01:55 PM   #34
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thanks, Steve

07-02-2019, 04:31 AM - 2 Likes   #35
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I found this article quite late, (1 year and 3 pages into comments)

The issue as discussed for a long time in the forums, dating back to the release of the K10D , as Steve says relates largely to the focusing screen and the way the meter takes light off the focusing screen.

This was characterized for several cameras with the following chart

What was found is that most cameras with stopped down metering were accurate between F4 and F5.6. There is a reason for that, it is the native aperture range of most consumer “Kit Lenses”.

Note in the chart, 18% grey exposure is about 128 greyscale on the histogram. Each F-Stop is about 40 greyscale.(at least between 30 and 225, then it becomes quite nonlinear.

Yes the metering improved greatly on the K10 with the focusing screen from the *istD but there were also other issues.

The use of teleconverter with feed through aperture pins also had metering issues, due to the reported aperture being different than the real native aperture. I demonstrated this with my sigma 70-200/2.8 with 1.4x and 2x teleconverter.

But that is not the end of exposure woes, my Tamron 28-75/2.8 for example has an error in the aperture movement that starts out under exposing by 1/2 stop wide open, to over exposing by 1/2 stop at F22. Exposure vs stops is a straight line between these two ends of the curve.

The bottom line is you should test your combinations to know how they work.

It takes about 10 minutes per lens / camera combo to do this.

Edit note, one issue with the K10D no one talks about, as it was changed in firmware, is that the initial stop down metering period was too short, and they metered before the lens was stopped down. The length of time for stop down metering was only about 1/2 the time of then*istD at the onset. But later software revert d it back to the same time. I know, because I reported the much shorter metering time to Pentax myself

Last edited by Lowell Goudge; 07-02-2019 at 04:36 AM.
10-03-2019, 03:59 PM   #36
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QuoteOriginally posted by Lowell Goudge Quote
Edit note, one issue with the K10D no one talks about, as it was changed in firmware, is that the initial stop down metering period was too short, and they metered before the lens was stopped down. The length of time for stop down metering was only about 1/2 the time of then*istD at the onset. But later software revert d it back to the same time. I know, because I reported the much shorter metering time to Pentax myself
Thanks for adding this note. I had no idea, despite being a K10D owner.

10-21-2019, 02:03 PM   #37
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Great post - I have only just read for the first time. This goes a long way to explaining why my stopped down Takumars (fixed aperture settings and already stopped down), exhibit quite a bit of error when metering with stop down.

LV is certainly a help though.

Thanks for posting!
11-01-2019, 11:04 PM   #38
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QuoteOriginally posted by stevebrot Quote
No, this is not another rant .....

Thanks for the tips. Very useful.

11-02-2019, 09:32 PM   #39
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This seems to jive fairly well with my experience owning the K50 and K3... I thought I was crazy for quite awhile in regard to the metering inconsistencies and the constant need to make EV adjustments. I have found the extreme (particularly low lighting) situation to be incredibly hard to get accurate metering from and have a general idea at this point of what EV adjustment I need to make when shooting in low light to prevent dark/blown out images. The odd thing with the K50 is that at some point I think something was damaged the camera completely lost the ability to adjust metering when stopped down, resulting in the need to manually correct .5 EV for every couple stops, or just shoot full manual.
11-24-2019, 10:41 PM   #40
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Excellent information from the OP and others.
My experiences using M mode green button metering with a K-30, K5, K5iis and K3 have been interesting to say the least.
The K-30 tended to underexpose by about one EV with great consistency. My K5 & K5iis are fairly accurate with manual zoom lenses and primes slower than f3.5. Fast primes, particular K series glass like the K200/2.5 and K125/2.5, overexpose by one EV. The K3, overexposes with all manual lenses regardless of maximum aperture or lens type. One EV with zooms and slow primes. Fast primes cause a nearly two EV overexposure, especially in bright light. I resort to chimping, dialing in compensation and shooting RAW plus jpeg. As long as I can keep the highlights from being blown out, I can make very good images from the K3 files since it preserves shadow detail extremely well.
11-24-2019, 11:52 PM   #41
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I've seen problems with an M35/2.8 that had sticky aperture blades. The green button didn't give it time to stop down but the 'preview' position on the on/off switch would do if held there for a moment (K-5).
12-10-2019, 11:34 PM   #42
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QuoteOriginally posted by stevebrot Quote
No, this is not another rant about the so-called "crippled mount". Instead, the intent is to provide a useful summary of why stop-down metering is often disappointing with modern dSLRs.

Stop-down metering

Stop-down TTL metering has a long history (see Pentax Spotmatic) and has long been considered the "Gold Standard" of TTL metering in that it measures the actual light at the taking aperture rather than an estimate based on a wide-open measurement. Even though all K-mount bodies support open-aperture metering, bodies having the so-called "crippled mount" (all Pentax dSLRs) limit this feature to lenses capable of automated aperture control by the body. In practice, this means lenses without an aperture ring or those with the "A" position on the aperture ring. Other K-mount lenses will still work using the aperture ring, but with in-camera metering limited to measurement with the lens stopped down.

K-mount auto-aperture actuation options include:
  • Maximum aperture only in modes other than M (default to Av)
  • Green button (instantaneous stop-down) in M mode to set shutter speed
  • EV scale stop-down using optical preview in M mode (menu option on some bodies) with shutter speed set using e-dial

Manual aperture (M42 and manual/preset aperture K-mount) options include:
  • Continuous metering in Av mode (all dial positions except M, X, and B)
  • Instantaneous metering using green button in M mode
  • EV scale using optical preview lever in M mode (menu option on some bodies) with shutter speed set using e-dial

Why it is a problem on many dSLRs
More properly, why is exposure with stop-down metering so often simply wrong?

My personal experience only goes back as far as the K10D and on that camera, stop-down metering simply sucked. Regardless of the method used, metered exposure was often 1 or more stops over or under-exposed. This was a puzzle since *ist D users at the time were happy with the feature on their cameras and stop-down metering was consistently good on Pentax film cameras. Much was made of the issue on this site and other places on the Web as users struggled to secure behavior similar to what they enjoyed shooting with K-mount film cameras when shooting with adapted M42 and non-A K-mount glass.

For example, the two graphs below show the meter variance (LV)* from expected** for two popular lenses using green button metering on the K10D equipped with the stock LF-80 focus screen. The X-axis is f-number.The intended takeaway is:
  • Variance existed even at maximum aperture where the meter should have been seeing exactly the same light
  • Variance may be both positive and negative even with the same lens
  • The degree of error varies by both aperture and lens
Those two graphs are a mild example. I have lenses that metered as much as 2.5 stops from expected, but which provided properly exposed frames if set to the expected shutter speed aperture combination.
Question #1: "What does the meter see different than the sensor?"
Question #2: "What is different between a lens wide open with the aperture ring in the "A" position and the ring in the wide open position?"

Answer #1: The viewfinder optical system, specifically the focus screen
Answer #2: When the ring is off the "A" position, the body detects that it cannot control the taking aperture and has NO reference as to what it might be.
PDAF autofocus cameras all share a common fault in that light is diverted from the viewfinder to drive the AF sensor. To offset the resulting dim viewfinder, focus screens have been designed to optimize brightness. Two side-effects happen as a result:***
  • The light intensity viewed by the meter sensor is not representative of or even proportional to that which will strike the image sensor. How much it is off depends on the aperture size (rear pupil diameter to be precise) presented to the focus screen, not light intensity.
  • Ability to show the out-of-focus state is also compromised...apparent depth of field is increased down making manual focus difficult. This point is another discussion and will be ignored for the present.
The variance from expected metering is most likely due to the focus screen brightening. This hypothesis has been strengthened by user experience substituting screens from *ist D series bodies as well as results from non-brightened aftermarket screens. In short, variance changed depending on screen, with none of the aftermarket screens providing a clear improvement.
Conclusion? The camera applies a set of correction factors based on the known maximum aperture of the lens mounted when information is available from the contacts on the mount. Without correction factors, the meter is basically reading spurious light from an unknown fall-back position as far as exposure calculation. As a side note, this problem extends to other brands as well. Backward compatibility for many F-mount lenses on Nikon dSLRs depends on stop-down metering and their users were reporting similar issues.

Summary points run something like this:
  • Both accuracy and linearity of stop-down metering are potentially compromised by the optical path present on modern dSLRs
  • The problem is not easily solved by simply dialing in a little exposure compensation
  • The degree of inaccuracy depends on the lens used and aperture set with some lenses being badly effected and others much less so

Is this still a concern? If so, what can we do?

The problem was widely known and had been acknowledged by Hoya/Pentax prior to the K20D release. There was great disappointment when stop-down metering was not improved on that model. When the K-7 was released in 2009, one of the announced improvements was better accuracy and linearity for stop-down metering. Happily, there was a significant improvement that joyously welcomed by many of us using vintage lenses. Since that has been almost a decade ago, one would think that incremental improvements should have resulted in near perfection.

Sadly, that is only partially true. Before sitting down to write this piece, I mounted up an XR Rikenon 50/2 to my K-3. In M mode at f/5.6, both green button and EV scale metering resulted in at least a 1.5 stop overexposure. Other users are encouraged to add their comments and experiences, but my present understanding of optical viewfinder metering runs like this:
  • The initial fix on the K-7 was limited to M-mode. Av metering in other modes continued according to the old rules from the K10D/K20D.
  • I believe that situation from the K-7 extends to the present. When I took delivery of my K-3, I checked both methods against the K10D and found that Av numbers were similar with both cameras. K-3 M-mode metering is better than I remember from the K-7, but still far from perfect.
  • Green button and EV scale metering in M-mode are fully equivalent. This applies to both K-mount and M42 lenses.
My personal recommendations run like this:
  • Use M-mode with either green button or EV scale for viewfinder metering with K-mount lenses
  • Similarly, use M-mode with green button for M42 lenses. I almost never use the Av option at present.
  • Seriously consider carrying a hand-held meter and gray card when shooting with non-A lenses. The hassle is often much less than chimping and figuring, and chimping some more.
  • I have been experimenting with center-weighted metering TTL in Live View using green button in M-mode. Once the exposure is determined, I change back to viewfinder and shoot at will until either the light or the subject changes. The results are very predictable and accurate. Paradoxically, I have been getting better results with M42 lenses using green button in live view than with Av mode. Go figure, they should be the same.
I anticipate some useful comments from our very competent members. Thanks in advance


* LV = Light Value, an expression of measured subject luminance equivalent to EV at ISO 100

** LV was about 7 on an evenly lit white wall using natural daylight. Expected values were metered with the lens aperture ring on the "A" position. A positive variance indicates an underexposure situation.

*** Both can be seen directly through the viewfinder when using the optical DOF feature. The screen does not darken appreciably until stopped down somewhere
around f/4.5. Similarly, there is little change in DOF as the lens is stopped down at wider apertures.
I always over exposed 1-2 stops with m42 lenses in stop down metering, without digging deep to the problem trying to fix it my lazy approach is to bracket the shots to get the prefect one, this is not scientific but I will get the shot at the right moment before it slips away.
12-20-2019, 07:54 PM   #43
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Imagine my surprise finding this thread on the boards here. I just today joined and submitted an issue with my K-X and using stop down in M with my M 2.8 lens. Adjusting to what the internal exposure meter would show, I would get almost almost a 3EV overexposure. I have not tried any of my old K lenses yet but will.

I was exited when I bought the camera several years ago as I had read using older "film" lenses would work. Well, I guess using external metering, then yes. Trying to use the internal meter...well.

I know the K-X is long in the tooth (old), and I have considered picking up a K5 ll as prices are pretty cheap now. Does anyone have info or experiences that would attest that a newer model camera would perform better using older glass?
12-20-2019, 08:50 PM   #44
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When I was regularly using my "Super Program" with any lens {it was kitted with a Pentax-A 50mm f/1.7 lens, which was also my main lens during most of the time} with Kodachrome 25 film, I had to bracket my exposures in order to get it right. My experience over the twelve years that I used the camera was that even at the "A" lens setting, the camera tended to over-expose slides, usually around + 2/3 EV. I did not compare against the "right" setting from a hand-held light-meter ..... when I got developed slides back from Kodak, my wife and I sat down with my projector and "graded" each slide, deciding on which was best.
12-20-2019, 09:38 PM   #45
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I found that camera of different made produces different reading, I mount a Pentax lens on a Fuji (both are APSC) via adapter which is simply an extension tube, and got about a stop difference forgot plus or minus, so there are different design to determine what is correct exposure, I do not have another mirrorless to compare.

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