Monday, April 26, 2021

M3 Bracketing: Now supports a HAMSTTR based workflow

In this post I'm pleased to introduce another useful feature into M3 Bracketing, directed at those that need to capture high dynamic range scenes.

As we know there is a difference between shooting film vs shooting digital. The bottom line being that film responds to light in a similar (non-linear) way to our eyes, whereas silicon does not. Rather than repeat what someone else has said, a good read on the differences may be found here:

Thus, if one can, ie shutter speed and ISO is not an issue, then to capture the highest quality image data. eg tonal quality in both shadows and highlights, you should 'Expose To The Right' (ETTR), at the 'base' ISO, AND ensure you not blow out the highlights.

Many, including myself, adopt an ETTR strategy: some call this approach Histogram And Meter Settings To The Right, or HAMSTTR.

Knowing the dynamic range of the scene is of critical interest: 

  • can I use one image to cover the shadow and highlights?
  • if I need more than one image, then how many?

Although you can use external spot meters to scan the scene, many these days 'just' rely on their camera for exposure information, eg shutter speed and/or looking at the histogram (in realtime on a mirrorless camera vs after captured in a DLSR without a live view histogram capability.

Of course knowing the shutter speed of an exposure is all well and good, but this alone won't tell you the working dynamic range of the scene. For this you need to compare the exposure that satisfies/captures the highlights, with the exposure that addresses the shadows. This is not only scene dependent, but also user dependent, ie what satisfies you. 

In the latest version of M3 Brackets, I've added a feature that helps you find the DR of the scene. 

In the above screen grab we see a typical situation. The (Canon) live view histogram is showing that there are clearly clipped highlights: so we need to first set the (ETTR) exposure for the highlights, eg by adjusting the shutter speed.

In the above we see the shutter speed has been reduced to 1/30s from the starting 0.3s setting. The histogram now gives us confidence that we have no blown out highlights and that the exposure is a reasonable ETTR setting. But, we know the shadows are likely now under exposed: but by how much?

Let's now get M3 Bracketing up and running and first make sure focus is OK:

In the above we see I've set focus beyond the hyperfocal at an infinity defocus blur of 10 microns, ie at about twice the hyperfocal (as I used an overlap CoC of 20 microns in M3 Bracketing).

Because we have switched off CHDK ALT mode, we also see the new exposure feature. Namely, in the middle info field, we see 0.0ev, rather than the focal length which is normally displayed in this field, and still is in ALT mode.

The 0.0ev tells us that this is the reference exposure, from which we will explore the dynamic range of the scene. So let's adjust exposure for the shadows:

In the above screen grab we see the exposure for the shadows has been set using the histogram, resulting in a shutter speed of 1s. On my camera we would also see the M3 Bracketing top bar, but unfortunately the HDMI capture gadget I've got can't show the histogram and the bar at the same time, so let's switch off the Canon histogram and look at the M3 Bracketing bar:

In the above we see that M3 Bracketing is telling us that the exposure difference from the highlight exposure is 5.0ev. In other words, once we had set the exposure for the highlights, we would need to take additional exposures, to cover 5ev, to address the shadows. If we were exposure bracketing, say two at 3ev, or whatever bracketing scheme you wished to do.

To be clear, you might get away with one ETTRed image, eg risking not seeing details in the shadows: but to ensure you capture the shadow data for post processing, some form of bracketing is indicated, at around 5ev.

Once you know the ev offset that is required, eg to address the shadows, you can also use this value to set the sky bracketing offset in M3 Bracketing.

In conclusion, the M3 Bracketing script gives you complete access to both focus and exposure information. Canon M3 owners now have no excuse when capturing deep focus and high dynamic range scenes.

No comments:

Post a Comment