Viewpoints

Exposure strategy for digital cameras

27/02/2020
Exposure strategy and execution is still super important even in an all digital workflow
20200223-63GFX-50Smiltun-copy

One of the many reasons why I finally switched over to medium format digital from 5x4 film was the huge dynamic range potential of the digital medium and the flexibility to make large adjustments to raw files in post processing, especially shadow detail.

However this flexibility can make you lazy and imprecise. For the longest time I let my exposure discipline lapse with digital cameras. It had been honed over years of shooting transparency film where a whole scene had to be squeezed into four stops of light and an exposure error of less than a third of a stop was totally noticeable when the transparency was placed on a lightbox. My approach was so precise and consistent that I never made second safety 'B' images that could be held back and processed to compensate if there was an error. I trusted my craft.

Digital raw files with badly clipped highlights can not usually recover detail. Conversely 'clipped' shadow detail in raw files can be recovered to a very large extent. At first sight, digital looks similar to negative film with highlight flexibility replaced by shadow flexibility.
‘Loch Torridon’

What I find in the digital world however is that deliberately underexposing as a strategy to protect highlights leaves you with files that can be recovered (usually without significant shadow noise) but often at a price to colour accuracy and quality. The image can look washed out and you are forced to start making even more radical adjustments to compensate. Over time I have refined my approach.

My current exposure strategy for digital files is closer to transparency than negative film. With negative film the approach is to meter for the shadows and let the highlights manage themselves. My digital exposure approach (assuming the GFX sensor) can be best summed up as meter for a mid point, let the shadows handle themselves and manage the highlights if necessary using either graduated filters and/or a 'hdr safety copy' exposure (a second image taken with the automatic exposure bracket technology of the camera at step -3 (and possibly a third at step -6).

The latter hdr approach allows me to blend in the lost highlights if needed using either the lightroom hdr tool or layers and luminosity masks in photoshop. This should preserve the colour quality of my midtones. The goal is not to use the HDR safety image unless strictly necessary i.e. unless the scene has an extremely wide dynamic range, grads are ineffective or I have accidentally blown my highlights and they are not recoverable. Given we are talking the marginal cost of making an extra exposure it is a sensible safety step.

If the dynamic range is even more expansive and I am desperate for lots of shadow detail, then a +3,0,-3,-6 bracket strategy might be required. However I have rarely encountered this need. I often visualise an image with plenty of blacks.

The more I understand digital files, the more I learn if and when to clip highlights in one or more channels in a print. Also the longer I work with digital files, the lighter I become with the shadow recovery tool. Recovery of every shadow pixel is not always necessarily the right visualisation.

So in short, I have found that if I want to make the colours of the mid tones really sing regardless of what I do in post production, I should expose them precisely. One of the first series of articles I wrote covered how to use a handheld spot meter to measure exposure. I now utilise the spot meter option on the GFX extensively and a large proportion of my images are metered using spot metering. This means picking a particular tone in the scene that I want to lock to a mid tone (not a white or black tone obviously) and then adjust (typically up to +- one stop) for its colour. Unless the scene has an insanely wide dynamic range below that midpoint and I have visualised an image with lots of shadow detail, I will typically trust that I can recover all the shadow detail I need in post production.
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Having locked the image to a specific midpoint, there is usually very little global exposure correction to do to the image. So when I have corrected the image globally for white balance, saturation, clarity and sharpness, I will focus on making local contrast, shadow/highlight and black point/white point adjustments with the aim of balancing the image tonally. This is best done by rotating the image 180 degrees as the level of abstraction this creates is a huge advantage to spotting the right adjustments.

I will also work on local/selective colour luminosity and saturation, depending on the image. The infamous Fuji greens often need some selective desaturation. Lightrooms's luminosity masks, brush tools and graduation tools are central to all these selective adjustments.

This discipline in exposure combined with my renewed ability to consciously control image shape with my view camera seems to have filtered through to improving the precision of other aspects of my photography including setup and visualisation.

The above refinements to the realisation stage of my photographic process was the direct result of my trip to Torridon with Guy Aubertin and David Tolcher this January. Once again having knowledgeable friends to bounce off ideas of technique has proven invaluable. The trip also spurred me on to fully resurrect my view camera practices using my GFX cambo. Apart from the 35mm Pentax 645 medium format lens, all my other lenses are now large format lenses and adding the base tilt option has given me full control of image shape again (as I used to enjoy when I used my Linhof Technikardan.)

All the above learning holds true for my current GFX sensor and workflow - no doubt as technology evolves, my approach will evolve with it.