Sunday, January 15, 2023

Rules of Thumb and capture Workflow for Wide Angle Landscape Photography

In previous posts I've discussed various models and rules of thumb for wide angle landscape photography. In this post I'll layout the workflow I personally use.

First, let's remind ourselves of a few rules of thumb that cover both focusing and exposure:

  • The hyperfocal distance,(H) in meters, may be approximated by dividing the lens focal length (in mm) by 10. This assumes the aperture (N) is set to f/10 and you are content that the circle of confusion (C) is the focal length taken as microns. To find the hyperfocal at another N or C simply use the following expression to adjust the F/10 value:
  • If the lens throw between the nearest point of focus (x), as measured from the front principal, and infinity is T, then this should be divided by H/(2x) to derive the number of focus brackets;
  • A reasonable estimate for the position of the front prinicpal of a wide angle lens, eg when doing landscape photography, is to assume it is at the non-parallax point of the lens, ie the entrance pupil, which may be esimated in the field in realtime, if not known in advance.
  • For high dynamic range scenes, ie where you are not confident you can capture both the sky's highlights and the foreground's shadow details in a single exposure, set an ETTR exposure for the sky and adjust the exposure for the foreground by +4Ev, ie four stops. Together with noise reduction software, such as Topaz PhotoAI, a +4Ev approach will cover most/all situations. This allows us to take a fixed exposure when focus bracketing for the foreground, and an ETTR exposure for the sky, ie at infinity.
All the above rules of thumb are designed to be 'calculable' in your head. The accuracy of the rules are appropiate for wide angle lens use, but not macro lens use. For example, by assuming the front principlal is located at the entrance pupil, the most we will be out is F*(1-1/p), where p is the pupil magnification, which for most wide angle lenses, ie retro focus lenses, will be greater than unity. Thus, even if p is high, the most we will be out will always be less than the focal length of the length.

As for a capture workflow here is the one I follow:
  • After visualising and composing the scene, I assess the dynamic range of the scene and decide if I can 'get away' with a single exposure capture throughout, or if I need to adopt a +4Ev approach, ie ETTR for the sky and adjust the exposure for the land by up to 4Ev. If the DR is greater that 4Ev, then consider two exposures for the sky, at the ETTR and one between the ETTR value and the exposure value for the land;
  • Estimate the hyperfocal and if the nearest object of interest (x) is closer (to the entrance pupil) than half of this, then prepare to take some focus brackets. If not then focus slightly beyond the hyperfocal by, say, at least 2*H and take a single focus capture (with or without an ETTR as required);
  • If the number of focus brackets, using H/(2x), is less than say, 5, divide the lens throw (T) by that number and take the indicated focus brackets; for example in the case of H/(2x) being 4, at the quarter throw points, ie at the nearest focus point and at 1/4, 1/2 and 3/4 of the throw and, finally, at infinity - giving 5 focus brackets in total;
  • If the estimated number of focus brackets are more than 4, then assess if it’s still ok to just take 4 focus brackets. For example, if the C is, say, 12, ie I’m using a 12mm lens, then I can afford to double the bracketing C to 24 microns, assuming a full frame camera, thus, in this case, I can still divide the throw by 4, even if H/(2x) is up to 8. But note that the acceptable value for C will depend on the size and how you display your image, eg social media vs a print in a competition;
  • Capture the focus brackets at the exposure for the ground and, when at infinity, adjust the exposure for the sky and take the sky capture(s).
Although the above may appear complicated, I can assure you it is not and, after practicing, you will soon feel at ease with the workflow.

I’ll discuss post processing in a future post. 
As usual I welcome any comments on this post or any of my posts.

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