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11-24-2021, 04:51 PM   #61
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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.
Wow thanks for the info, that's a lot of research ! I rarely use the stop-down method unless I'm using my old Takumar lenses anyway. Most of my images come out pretty good using this method. Despite my attempts to try to learn the what the Green Button does for years, I'm still baffled...


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