Sunday, 13 July 2008

Epiphanies on Exposure

The Pope is in Sydney, leading the celebration of World Youth Day. My epiphany doesn't relate to that event but what Kurt Petersen took the time to find out and tell us.

Spot-Hi meters at ~215-220 on the histogram and is equivalent to Spot +2EV. Use this on something you want to be the brightest part of your photo.
Spot meters at ~115-120 on the histogram
Spot-Sh meters at ~15-20 on the histogram and is equivalent to Spot -3EV. Use this on something you want to be the darkest part of your photo.

I knew what Spot Metering does - point the center of your viewfinder it at something and the camera reports the Exposure Value you set on your camera (or the camera sets for you) to make that subject's tone equivalent to 18% Gray (or was it 12% Gray? - doesn't matter, it's about Gray).

By using Spot Metering in a Point and Shoot snapshot way, you get awful results - you may not be pointing at the correct area to make 18% Gray and you may be more concerned about not burning out the bright clouds or rendering some darker coloured ground object without noise rather than anything related a midtone.

So, if you realise that you don't want things to be Gray is an ephiphany. You actually want the opposite - to render bright highlights and dark ground objects in a pleasant way with detail,  colour and not much image noise.

That's where the ESP or "smart" biased averaging matrix comes in with these modern cameras. They detect all the tones in the metering areas and calculate a "middle" EV based an optimum chance of getting it right. But how do they do that? They don't know whether you want perfect cloud and sky tone at the expense of darkened ground, neither do they know you want the person's face or detail in grass in preference to the sky. The camera makers solve this by doing a lot of research on actual photos in the field, generating stats on scene types, metering patterns, capability of the recording sensor, people's choices in those photos and so on. So they come up with a "smart" calculation to recommend to the camera. That's the best they can do, but that is NOT YOU - You're the end of the line, the "moment of truth" - you and the scene and the camera are where it comes down to right at that moment you press the trigger.

So ESP or "smart" biased averaging can and will be wrong at times.

This is where Spot-Hi and Spot-Sh come in for the Olympus E-510. If you set the Spot-Hi to the Automatic Exposure Lock (AEL) button action, pressing the AEL button will activate Spot-Hi instead of ESP for that shot. You point it at a bright patch that you don't want to "burn" - for me, that's the clouds in the sky. You recompose and shoot. Sometimes, this will give a result similar to using the ESP pattern. Sometimes, it will darken the ground and give you a good sky tone. If you ensure that your ISO is as low as practically possible, i.e. ISO 100, then the amount of noise you generate by lifting the dark tones in post processing will be as low as possible.

What about Spot-Hi? Well, there is only one AEL button on the E-510, so I dedicate the Fn button to MyMode-1 and in this MyMode, Fn invokes Spot-Sh. It won't work like the AEL button, but if I point the camera at a dark ground object and don't re-frame, just shoot, then I am pretty sure that this dark ground object will not be inky, unrescueable black.

Below are some shots I've done using several approaches. They're at the Blackburn Lake area and are of fairly high Dynamic Range oppurtunities.

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