Originally posted by matroxication Any tips will be really helpful to understand where am going wrong with this.
You are not doing anything wrong. This problem has been with Pentax owners since at least the K10D and is not limited to Pentax. The reasons are complex and poorly understood by the user community. What we do know is that the focusing screen brightness (what is read for both open-aperture and stop-down metering) does not decrease linearly as the lens is stopped down. This is not a problem for open-aperture reading when the lens controls the aperture, but it is for stop-down metering. Here are the talking points:
- While stop-down metering used to be the gold standard, the implementation on Pentax dSLRs often results in inconsistent metering
- The inconsistency varies by physical aperture and by lens mounted
- Both over and underexposure may be demonstrated for a particular lens depending on the stopped-down aperture and may be as much as 2-3 stops over or underexposed
- Incremental improvements have been made starting with the K-7 model, but only in M mode. Believe it or not, metering on the K10D and K20D was MUCH worse.
- The root cause is the stock focus screen and brightening treatments applied to compensate for the half-silvered mirror required by the PDAF system. The brightness of the screen does not vary linearly as the lens aperture is stopped down.
So much for the problem, here is how to get good results:
- The most consistent meter on the camera the one used in live view (the image sensor itself). It does not suffer from this problem and will meter consistently and accurately both in Av mode with manual aperture lenses or using the green button in M mode with K-mount lenses.
- The second most consistent metering is to use M mode with either the green button or the optical DOF preview*. The camera is able to detect the case when a non-controllable lens may be mounted (no-A contact detected). In that case the meter output is treated differently than for lenses that the camera can control. The results are much better than on past cameras. This is true for both K-mount and M42 lenses. Yes the green button will still calculate an exposure even if its action does not actually stop down the lens. The meter does not care. This generally works very well except for wide aperture lenses.
- Least dependable is Av mode with manual aperture lenses
- For tricky lighting, a hand-held meter is a good tool to have in the bag
In actual usage, the flow goes something like this...
Meter once and shoot freely until either the light or subject changes.
Long-time users of stop-down or non-metered cameras are very familiar with this concept and it works incredibly well. Rare is the need to meter and massage exposure in real time. There is one more thing that all users of stop-down metering should be aware of and that historic users likely remember well. That is the so-called meter coupling range. The meter sensitivity in the camera's specification sheet is with the aperture wide open. While the K-1's meter may be linear to -3 EV(100), that level of screen brightness comes up much more quickly as the lens is stopped down. In a dim room at f/11, the meter may well be at or below its linear range and will report more light than is actually there with resulting severe underexposure. This happens regardless of the set ISO. The solution is to meter wide open and manually calculate the shutter speed for the desired aperture -or- use one of the more sensitive hand-held meters. In true available darkness, neither technique will work.
More than you ever wanted to know, right?
Steve
* The optical DOF preview displays an EV display in the viewfinder that allows for easy exposure compensation and old school center-needle operation. Very cool.