Originally posted by iDon All those lenses work completely fine with my film bodys.
Do you have an explanation or even a solution for this issue?
Originally posted by jatrax No solution really but that is typical.
jatrax is essentially correct. It is typical for stop-down metering across brands (e.g. Nikon, Canon, etc). Conventional wisdom is that the cause may be traced as follows:
- PDAF systems use a half-silvered main mirror which results in less light to the viewfinder
- The stock focus screen and open-aperture metering system are optimized* to work together to provide a bright viewfinder image and compensate for the half-silvered mirror
- Stop-down metering creates an additional challenge since the screen brightness varies according to both the light and the size of the aperture pupil. In simplest language, the brightness of the screen is not a consistent representation of the amount of light to the sensor.
The result is that stop-down metering may vary as much as 2 EV through the aperture range depending on camera model and lens. The problem was more acute on the older Pentax dSLR cameras (K10D, K20D particularly) and is much less evident on current generation Pentax cameras.** On current cameras, the meter will apply different processing in M-mode when the A contacts are not detected. Stop-down metering should be essentially linear. Av mode will still work for manual aperture lenses, but results will be less consistent.
Now...having said that, here are the notes:
- Don't expect to get the same EV from stop-down metering as for open-aperture. Dispel the notion that one is "right" and the other less so. Neither is "right". Both are approximations for acceptable exposure.
- Do expect that stop-down metering will be essentially linear for the range of lens aperture in adequate light
- Potential still exists for significantly inaccurate metered EV in dim light at narrower apertures. The meter still has a lower range for linearity and it is based on the amount of light reaching the sensor. If the ambient light would normally meter at EV 4 (f/2.0 and 1/4s @ ISO 100), the amount of light to the meter sensor would only be EV 0 at f/8. This is at the raw edge for meter linearity for SPD meters. The RGB sensor on the K-3 is supposed to be good for an additional -3 EV, though I would not push it. This is true regardless of the ISO setting.
Steve
* The focus screens are designed for optimum brightness. This has been described to me as "scavenging" off-axis light to increase the apparent brightness of the frame. How well this works varies by the aperture size. The open aperture metering system compensates for the false-brightness by factoring in the lens maximum aperture when calculating the appropriate EV.
** The "fix" was first implemented on the K-7 and migrated to the rest of the line-up in the years since that camera debuted.