This post is triggered by a few comments I got over on a DPReview forum. Namely, that I have been rather lax in my terminology and therefore some of my posts were confusing in places. Many thanks to those on the DPReview forum for pointing such things out to me.
In tweaking/correcting things, I've also taken the opportunity to change the M3 Brackets UI (see the screen shots and video below).
The latest version may be downloaded from the right.
So let's start with the (tidied up) Depth of Field Information Script (DOFIS) model, which is based on a split/thick lens model; that I believe, although not perfect, is a better model than assuming a thin lens.
The objective with the CHDK M3 Bracketing Lua script is to provide in-camera feedback, and carry out focus and exposure, auto bracketing, for deep focus photography, ie near to far/infinity in focus.
The script's first role is to help the user assess if focus objectives can be achieved in a single capture, ie focusing to ensure infinity and the nearest point of interest are in focus. I personally follow a simple strategy, namely: 'focus for the foreground and set (defocus) blur for the background'.
Thus M3 Brackets is designed to tell you your point of focus, an estimate of the near depth of field (DoF) and the infinity blur in microns.
The absolute near DoF expression, ie as measured from the sensor plane can be found from the information in the DOFIS model above). Note the focus distance from the front principal, u, as used in the M3 Bracketing code, is s in the DOFIS model above:
Which, after manipulation, gives us this form of the expression for the near DoF, as measured from the sensor, that I use in the M3 Bracketing code:
Where f is the focal length, p is the pupil magnification, u the focus distance from the front principal, t the hiatus between the two lens principals, and h is the 'short form' of the hyperfocal distance, ie (f*f)/(N*C), where N is the infinity aperture number and C the circle of confusion or the blur criterion at infinity or at the overlap point when focus bracketing.
As for the UI change, because I'm a Magic Lantern user, I've put all the focus feedback into an 'info bar' at the top of the screen. This bar is accessible in CHDK ALT mode or outside of it. Thus you always will have information available.
In ALT mode, as before, you can initiate focus stacking by pressing the SET button. If you come out of ALT mode you can take images as normal, but still have real time access to the DoF info.
Whether you are in or outside of ALT mode, if the M3 info bar disappears then refocusing or pressing the RIGHT key in ALT or doing a half shutter press outside of ALT, will recover the bar.
M3 Bracketing will keep running until you do a full shutter press in ALT mode or capture a focus/exposure bracket set. When running, you can freely move in and out of CHDK ALT mode.
To show how M3 Brackets runs, the following screen captures and a video are illustrative. Note in the captures the CHDK overlays are skewed to the left relative to their actual position. Also note that peaking was fully functional as M3 Bracketing was running, although the HDMI capture, for some reason, doesn’t show the peaking.
In the above we see a typical M3 LCD view before CHDK is enabled. By repeatedly pressing the INFO button, we have arrived at the histogram screen. This is a good starting place to compose and set exposure. However, you should move away from this view before staring M3 Brackets, as the histogram will cause the screen to flash in ALT mode.
In the above we see CHDK has been enabled and that M3 Bracketing is ready to run, ie by doing a full shutter press.
In this screen capture we see M3 Bracketing is now running, as we see the console area, the single line of text above the CHDK ALT, is telling us that we are at a focal length of 12mm and that we have two options. If we change focal length we need to press the RIGHT button to reset M3 Bracketing. Or we could press SET to start a bracketing sequence, eg a focus bracket set or an exposure bracket set or a combined focus and exposure bracket set.
We currently don't see the info bar at the top. So let's refocus.
Now we see the info bar, which has three areas of information.
The left hand group of information tells us we are focused less than the hyperfocal (<H) and at 280mm from the sensor. M3 Bracketing uses the Canon lower focus value.
The middle group tells us how many brackets we will take from the current focus position, when auto bracketing, to the hyperfocal. When auto bracketing the script will take at least this number of brackets plus one or two more, ie to cover to the infinity bracket position you have requested.
The right hand group shows the near depth of field from the sensor, 219mm in this case, and the infinity defocus blur in microns: 63um in this case.
In the above we see we have refocused to beyond the hyperfocal (194cm), where obviously the script is telling us the number of brackets is one. We also see that the near DoF is at 530mm and that we have a defocus infinity blur of 7um: a good place to be, ie better, than being at the hyperfocal, which, in this example, was based on a CoC/overlap blur of 20um.
In the above we see we have refocused and changed focal length, from 12mm to 22mm, and have thus reset things by pressing the RIGHT button. If we did not do a reset, the script would think we were still at 12mm.
In the above video clip we see M3 Bracketing running outside of ALT mode. In the console area, M3 Bracketing tells us the script is running, which is a useful reminder if the top bar should 'disappear' and needs refreshing with a refocus or a half shutter press. As we can see, the M3 Bracketing script is giving us real time access to the focus position, the number of brackets we will need, the near DoF and infinity blur.
In this mode it is easy to focus on a near object of interest, eg using peaking or zooming in, take note of it and then refocus for the background blur, checking to see if the near DoF covers the near field object we first focused on. If we have covered focus we can simply take as many shots as we like, after which we can return to ALT mode and carry on using M3 Bracketing. Or we could return to Alt mode without taking a shot and reset focus to carry out auto focus bracketing.
In the final video clip we see M3 Bracketing taking a focus bracket set, finishing with a focus bracket at three times the hyperfocal. Note the clip starts with me exiting M3 Bracketing and then restarting the script.
Once the script has taken a focus bracket set, or a full shutter has been pressed in ALT mode, the script will need to be restarted by doing a full shutter press in Alt mode.
Bottom line: if you have a Canon M3 and love deep focus photography, the M3 Bracketing script is well worth looking at: IMHO ;-)
As usual I welcome any comments on this post or any of my posts.
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