Saturday, March 15, 2014

Chasing that ‘perfect’ exposure

As we all know, our cameras are amazing pieces of technology; however, in one respect they don’t always tell us the truth, ie when we use their in-built metering. This potential weakness becomes important if we are trying to get a single capture and ensure the maximum quality of the RAW file.

In other posts I have spoken of the value of the adopting an Expose To The Right (ETTR) workflow. That is adjusting the camera metered exposure so that the highlights we wish to capture, with detail in them, ie not the blown out speculars in the scene, are as far to the right of the histogram as possible without clipping.

But the question is: how far should we adjust the exposure from the 0Ev, 18% grey level that our camera will meter at?

If you are shooting with a modern DLSR you are pretty safe to adjust the metered highlight by, say, 2Ev. But how much further can you go? Well that depends on your camera.

In the case of my 5DMkIII the situation is captured below. That is I am safe to meter on the highlight area that is of the most interest for my image and then adjust by 3 stops, ie if I’m using an ETTR strategy.

How do I know my 5DMkIII is safe up to 3Ev? I tend to use an external spot meter when I am doing landscapes, and have a Sekonic 758DR. This exposure meter has the advantage of calibration software, which allowed me to create the data plot above, which tells me my 5DMkIII has a dynamic range capture potential of about 7.2Ev.

I have set the clipping points such that the Sekonic warns me about clipping over an inner dynamic range of 6.5Ev.

The other advantage of using the 758DR is that it has a 1 degree spot. Whereas the camera’s spot size is lens dependent and only becomes close to 1 deg for long telephoto lenses, ie about 340mm on my 5DMkIII. Check out the data here:

So how do I use this calibration information in my capture workflow? Here is one (zone-based) strategy I use:

  • Evaluate the scene and decide on a composition; 
  • Use my 758 in spot metering mode to evaluate the ‘hottest’ areas, ie the brightest areas with detail I wish to capture and that I wish to reposition towards the ETTR clipping edge, ie away from the 18% 0Ev point that the camera has metered;
  • I then use the 758 to evaluate the dynamic range of the scene, ie once I have metered from the highlight I will check out the shadow areas and note what the meter is telling me is the delta-Ev between the two. If this is more than about 7Ev, I will consider bracketing;
  • I then use the 758 to (automatically) tell me the exposure at that ETTR edge (or work it out in my head). For example, if the 758 says I need to shoot at 1/120s to capture the highlight I’m interested in, and I know I wish adopt an ETTR shift at, say, 3Ev, I will shoot the scene at 1/15s.

Bottom line: if you are a regular user of ETTR-based metering, then knowing your sensor’s clipping limits is important. If you don’t have calibration software, then take a few images of the same scene, adjusting the exposure, until you see (non-specular) highlight clipping. Either way, you will know how much to adjust your exposure by; which, for a modern DSLR, will likely be between 2 and 3 Ev.

No comments:

Post a Comment