Wednesday, April 14, 2021

M3 Bracketing: Now with manual focus bracketing

Those that have been following my Magic Lantern DOFIS developments will know that I introduced a so-called 'traffic light' feature into ML DOFIS, that tells you the depth of field (DoF) state of your current focus, relative to the last image captured. 

I'm pleased to say that this manual focus bracketing aid is now available in M3 Bracketing.

The feature is enabled via the menu:

In the above screen grab we see the new traffic light feature is switched to be on: it is either On or Off.

The background behind the M3 Bracketing implementation is as follows:

In the above we see the near and far DoFs of the last captured image. Until you take an image, when M3 Bracketing is running outside of ALT mode, you will not see any traffic light feedback.

In the second bar down we see the current focus being at the same position as the last captured image. In this case we see both the left and the right hand traffic lights are YELLOW. You can therefore return to the exact focus position as the last captured image by just refocusing until you see two YELLOW traffic lights.

In the third bar down, we see we have focused towards the macro end, thus the current near DoF is less than the captured image's DoF cover, but the current far DoF has a positive overlap with the captured image's DoF. In this case we therefore see a RED and GREEN set of traffic lights.

The fourth bar shows the situation where there is a near and far focus gap between the last captured image and the current point of focus. So we see a RED/RED display.

The fifth and sixth bars show what focusing towards infinity looks like.

Thus, all we need to do is take an image and then refocus in any direction, until M3 Bracketing info bar tells us when to stop to take a, perfect, focus bracket. That is, refocus until both go RED and then back off your focus until you see GREEN in one of the traffic light.

To illustrate how things look in the camera, the following screen grabs are illustrative. Note I have my style set to B&W in camera, to help me ‘see’ my composition clearer, rather than get diverted by colours in the scene,

In the above we see M3 Bracketing running in ALT mode. In this mode 'all' we can do is refocus and initiate auto focus (and exposure) bracketing, from near to infinity.

To use the new traffic light feature for manual focus bracketing, you first need to press the ALT button, to exit ALT mode. The script will continue working, although you are now outside of ALT mode. The screen will look something like this:

Here we see the M3 Bracketing info bar is still providing all the information, but now, as we are out of ALT mode, we can take images.

In the above I focused to an infinity blur of 10 microns, ie at about twice the hyperfocal: in this test the overlap CoC was put at 20 microns. We see I was using an 11mm focal length and we was focused at 154cm from the sensor, giving a near DoF of 554cm. 

Remember that M3 Bracketing uses a thick lens model and includes pupil magnification. It thus is about the best you can do, albeit we are still dependent on Canon giving us the 'correct' focus distance.

So let's take a picture:

In the above screen grab we see that none the info bar has changed, as I haven't yet refocused. But we now see the two YELLOW traffic lights, confirming I was indeed at the same point of focus as the last captured image.

As I’m at infinity, with a 10 micron blur, I needed to focus bracket towards the macro end, ie away from infinity, all the time recovering more near depth of field.

As I refocus, away from the position of the last captured image, we see the left hand traffic light immediately go red, but the right hand traffic light stays GREEN; telling us we have a positive focus overlap. All I needed to do was keep refocusing until I reached a point where I saw two RED traffic lights, telling me I now had a focus gap:

At this point, I could accept the focus, as it's only just gone RED, or refocus until I saw a GREEN traffic light on the right. If you set the overlap blur to slightly lower than your overlap requirement, you should be able to take an image as it just goes RED, ie not bothering to refocus to GREEN. Your choice.

The latest version of M3 Bracketing may be downloaded from the right. In this release I've also tried to address a few of the CHDK vs Canon UI issues. For example, when manually focus bracketing, outside of ALT mode, the info bar will disappear after an image capture. You can force the bar to return by doing a half shutter press, toggling ALT, or the script will automatically refresh after 3 seconds.

As usual I welcome any feedback on M3 Bracketing, especially from users of the script.

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