Exposure Compensation Question

[smoke665]

When using a telephoto lens with a polarizing filter in aperture priority mode, shouldn't the TTL take care of exposure compensation, or is that only if you are set on full automatic?? Yesterday outside at an event in bright sun, I had to adjust the EV to as much as -1 to -3 which seemed opposite of what should have been. What am I missing?

 

[Overread]

The sensor in the camera uses the light that is reflected through the lens and off the mirror inside. As such if you fit a filter to the front of the lens that blocks part of the light then the camera has already taken into account that light loss when it reads the light and provides its meter reading.

However this doesn't mean that the sensor is infallible. It can still produce undesirable results.

eg
1) If you are spot metering then it will only meter for the small area selected for the spot metering (varies from camera to camera; some do it on the AF point others do it on the central AF point only). Thus if it were aimed at dark part of an otherwise bright scene it would overexpose the bright areas.

2) Centre weigthed and other modes are a bit more even; but can still result in meter readings that don't quit work perfectly when reviewed.

3) The sensor wants 50% grey; and thus if you were to expose for a scene of snow you'd get grey not white snow - an underexposure - because the camera isn't making whites white.


This is why we have EC and why reviewing photos on the LCD with the histogram showing is important.

 

[smoke665]

Centre weigthed and other modes are a bit more even

Pretty sure it's set on center weighted, but will check it out more this afternoon. Guess the point that's confusing me is that I'm having to set it to underexpose not over.

 

[Overread]

It suggests that the light was bright or near midday - often as not the highlights are easily overexposed in such lighting

 

[KmH]

The metering sensor is calibrated based on an assumption that most scenes average out to the reflectance of a uniform 50% gray surface.
If the scene is has more or less reflectivity then the meter is calibrated for the photographer has to dial in some exposure compensation.

Snow covered scenes reflect more and so require some + exposure compensation be dialed in.
A scene of a pile of coal would be just he opposite reflecting less that a uniform 50% gray surface and some - exposure compensation would be called for.

The photographer has the final arbiter of the average reflectivity of a scene and has to use experience and/or bracketing exposures to ensure at least 1 frame has a good exposure.

 

[Alexr25]

Are you using a circular polarizer or a linear one?
You should be using a circular polarizer since most cameras use a beam splitter to direct light to the metering sensor and beam splitters polarize light. Using a linear polariser on the lens could result in wrong meter readings due to cross polarization between the filter on the lens and the metering beam splitter.

 

[Overread]

Alexr raises a very good point! Often overlooked as with digital so mainstream most polarizers these days are circular polarizers

 

[smoke665]

It suggests that the light was bright or near midday - often as not the highlights are easily overexposed

Think you just answered my question. It was between the hours of 11-2 pm. Bright sun. New Pentax K3 II which is very close to operation of the previous K30, however several of the controls are in different places. Had the AE set to multi-segment, not center weighted as I thought. Image tone not set correctly for the scene, and some other menu options for highlights, and shadows needed to be corrected. Pain going through the learning curve again. Think I'm back on track again. Thanks

 

[smoke665]

Menus set correctly, no filter, exposure set to center weight, in full shade, still looks like best exposure is setting the EV to -1/3. Seemed to be constant across multiple lenses.

 

[Overread]

Sounds about right for bright conditions.