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06-13-2014, 07:23 PM - 1 Like   #1
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K-3 Stop-Down Metering Performance

A Little History

As with many other Pentax Forums users, I dabble in vintage, Pentax-compatible glass. In fact, LBA has graced me with quite a collection on K-mount and M42 lenses. It is common knowledge that in-camera meter support for M42 lenses and any K-mount lens lacking the "A" electric contacts is limited to stop-down metering. It is also fairly widely known on this site that stop-down metering with many older Pentax dSLR bodies is fairly unpredictable (neither accurate nor linear). That situation took a significant change for the better several years ago with release of the K-7. Stop-down metering in M-mode was much improved on that camera and most (all?) models released thereafter. After years suffering with the metering system on my K10D, it was with great anticipation that I ordered my K-3.

I was aware that the main metering system on the K-3 is unique and a significant departure from previous Pentax cameras and was concerned that the stop-down metering may not be very good. I had done a casual "ask around" of forum members regarding performance with legacy lenses and the general response was positive. Sure enough, my results using my older lenses on the K-3 was so very sweet. A touch of the green button and I had decent exposures. Finally, I could use my camera's meter with confidence.

That being said, my confidence was eroded a few weeks ago by a post in the troubleshooting section inquiring about overexposure with the K-3 and M and K series lenses. I made a couple of comments about my good experiences and the OP did a few tests. After a couple of comment exchanges, it occurred to me that I had never actually tested the accuracy or linearity of my non-A contact lenses on the K-3. I did a quick check with several of my lenses and there did seem to be evidence of a stop or more overexposure at narrower apertures.

At that point, I apologized to the OP for talking through my hat and resolved to actually do a few tests at the earliest possible convenience.

Test Conditions
  • Evenly illuminated subject
  • Center-weighted meter mode
  • Comparison with open-aperture metering and external Gossen Luna Lux meter
  • All results expressed as EV(100)
  • K-mount tested in M-mode
  • M42 tested in both Av and M-mode
  • Both viewfinder metering and live view metering were evaluated when possible
  • Measurements were taken at full stop intervals for the range f/2.8-f/16

Results

To save the reader time and trouble, I can say without reservation that the K-3 viewfinder metering system does not meter the same in stop-down mode as in open-aperture mode. The general tendency is to return a lower EV value (i.e. overexpose) for stop-down readings when compared to open-aperture readings or an external reference meter. This behavior is more pronounced at narrow apertures. The difference from an open-aperture reading was as much as 1.7 stops at some apertures. 

So...what to do? Skip to the summary or dig into the tables. A PDF of the original spreadsheet with an additional table is attached to this post.

Key:
OAV = Open-aperture metered using the viewfinder sensor
SDV = Stop-down metered using the viewfinder sensor
SDLV = Stop-down metered using the Live View sensor
EVAS = Estimated EV(100) at sensor, calculated
LL = Gossen Luna Lux metered value

All results in the tables are expressed as EV(100)

Table 1. K-mount, M mode
OAVSDVSDLVEVASLLΔ(SDV–SDLV)Δ(SDV–LL)Δ(SDLV-LL)
Aperture2.810.71111811000
410.710.310.7711-0.4-0.7-0.3
5.610.79.710.7611-1-1.3-0.3
810.79.710.7511-1.4-1.7-0.3
1110.79.310.7411-1.4-1.7-0.3
1610.7910.7311-1.7-2-0.3

Table 2. M42/Av mode
OAVSDVSDLVEVASLLΔ(SDV–SDLV)Δ(SDV–LL)Δ(SDLV-LL)
Aperture2.8N/A1211.79.312.30.3-0.3-0.6
4N/A11.311.78.312.3-0.4-1.0-0.6
5.6N/A10.711.37.312.3-0.6-1.6-1.0
8N/A1011.36.312.3-1.3-2.3-1
11N/A1011.35.312.3-1.3-2.3-1.0
16N/A1011.74.312.3-1.7-2.3-0.6

Table 3. M42/M mode
OAVSDVSDLVEVASLLΔ(SDV–SDLV)Δ(SDV–LL)Δ(SDLV-LL)
Aperture2.8N/A11.3N/A8.311.3N/A0N/A
4N/A10.7N/A7.311.3N/A-0.6N/A
5.6N/A10N/A6.311.3N/A-1.3N/A
8N/A9.3N/A5.311.3N/A-2N/A
11N/A9.3N/A4.311.3N/A-2N/A
16N/A9N/A3.311.3N/A-2.3N/A
Summary

I like bullet points, so here are a few. Note that all are in relation to stop-down metering and should not be taken as applying to open-aperture metering:
  • The viewfinder sensor in this application appears to be biased to higher sensitivity (yielding lower measured EV) as the light to the sensor decreases. This, despite the actual EV being well within the sensor's specified linear range.
  • Live view metering agrees well is reliably linear when available. This is true for both K-mount lenses in M mode and the M42 lenses in Av mode.
The first point is problematic in that there may be potential for overexposure regardless of the aperture used, say in the case where subject illumination is about EV(100) 6.

So is this behavior a basis for real-world concern? To tell you the truth, I don't think so. Yes, there is potential for significant overexposure in certain lighting, but in my short experience in general shooting, I have gotten really good results to date. At this point, my personal practice will be to remain aware of the potential and maybe use live view for much of my shooting using stop-down metering.

Caveat
This small test is based on only two lenses from my bag (FA 35/2 and ST 55/1.8) that I know to have accurate and linear aperture mechanisms. Behavior may be different with other lenses.


Steve

Attached Images
File Type: pdf stopdown.pdf (26.8 KB, 128 views)

Last edited by stevebrot; 06-13-2014 at 09:06 PM.
06-13-2014, 11:07 PM   #2
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Wow very thorough, thanks Steve.

I'm still on a K-5 but I think the K-5 is worse by my guestimates. I also have a FA 35 f2 and ST 55 f1.8, so if I get some time tomorrow I might do a test. I don't have a light meter though but probably sufficient to compare to LV which is always pretty good for an evenly lit scene (even on the K-5).
06-14-2014, 06:29 AM   #3
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Wow! Quite an effort! Thanks for posting the results. It looks like the K-3 still isn't perfect when it comes to older lenses like this.

Question : why didn't you run through stopped down metering with Live View? I find that metering in Live View on my K-30 produces good exposure values.
06-14-2014, 07:02 AM   #4
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Original Poster
QuoteOriginally posted by 6BQ5 Quote
Question : why didn't you run through stopped down metering with Live View?
I did (SDLV column), just not with the M42 in M-mode. Stop-down in live view is not supported for lenses lacking a conductive base. Go figure!


Steve

---------- Post added 06-14-14 at 07:04 AM ----------

QuoteOriginally posted by 6BQ5 Quote
Wow! Quite an effort!
Most of the effort was in doing the tables in BBCode...took forever.

Steve

06-14-2014, 07:06 AM   #5
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But you can still stop-down the lens and let the sensor metering system decide on an exposure value based on whatever light it happens to be receiving, no? I do this with my M42 lenses all the time.
06-14-2014, 07:17 AM   #6
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Original Poster
QuoteOriginally posted by 6BQ5 Quote
But you can still stop-down the lens and let the sensor metering system decide on an exposure value based on whatever light it happens to be receiving, no? I do this with my M42 lenses all the time.
In Av mode yes, but in M mode no. In M mode you need to use the green button to meter and that does nothing for M42 in live view.


Steve
06-15-2014, 10:30 AM   #7
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well done Steve, I'm not so technical, thank goodness that someone is so technical...
when I started the thread 'M lens on K3 - any problems?' I was sure there was something strange in the behavior of K3.
thank's for tables that are a good starting point, other users will be able to help us investigate further
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