Originally posted by stevebrot The stop-down metering on bodies prior to the K-7 is unreliable.
One footnote to the list of stop-down metering considerations is meter range. The K20D has meter low end sensitivity of EV(100) 0. This works well for open-aperture readings and is equivalent to the light available for a 4s exposure at f/2 at ISO 100. Yes, that is not very much light. What's more, it is a hard limit. There is no way to make the meter provide an accurate reading below that level and the camera display will blink as an indicator when the limit is exceeded. This is true
regardless of the ISO setting.
Where things get interesting is when you meter stopped down. The light to the meter (measured off the focus screen) may be a fraction of that coming from the subject, but the camera body has no way of "knowing" what that fraction is and cannot flag the user. Fortunately, the limits are easy enough to calculate and stop-down metered cameras such as the Pentax Spotmatic typically included a chart in the user manual showing the upper and lower limits for shutter speed at a given ASA(ISO) with the meter needle centered. Above or below those limits the meter would not be accurate.
What is not obvious is how quickly those limits might be approached in moderately dim conditions. Consider the following case:
- Dimly lit home interior...e.g. EV(100) 5
- Pentax-M 50/2 mounted to a K20D
- Shooting at ISO 800
At f/2 the amount of light to the meter is well above its minimum sensitivity and a green button exposure will be somewhere close to the appropriate 1/60s exposure at ISO 800. However, aperture ring to f/11 and the light to the meter will be at its low limit when stopped down. The green button may well set a time shorter than the indicated 1/2s shutter speed.
Translation?
If the light is dim and you are shooting with a lens lacking the A contacts, a sensitive hand-held meter or educated guess may be preferable to the camera's meter. For guidelines on "educated guess" see:
Ultimate Exposure Computer
The above link gives common exposure values (EV) for various subjects along with a chart for calculating exposure by EV and ISO.
Steve
Last edited by stevebrot; 12-17-2015 at 01:14 PM.