Thursday, March 18, 2021

EOS M3 update

With all my recent efforts being directed at my two Magic Lantern scripts, DOFIS and MUSIC, I feel I've rather neglected my 'third best' scripting camera: my EOS M3.

Although Magic Lantern doesn't run on the M3, CHDK does run on it.

I only have one script for the M3, namely my M3 Brackets Lua script, that you can download from the link on the right.

Like DOFIS, M3 Brackets is built on the DOFIS split/thick lens model, albeit assuming a pupil magnification of unity. The slight limitation with the M3 is that focus position feedback is to the nearest cm; but this is good enough for landscape focus bracketing, ie not macro bracketing.

M3 Brackets comes with the EF-M 11-22mm lens preregistered, but it is a simple matter to register other lenses, by just entering freely available data, ie minimum focus distance at maximum magnification, and the maximum magnification itself.

Obviously M3 Brackets only works with lenses that report focus and aperture etc and can be driven.

M3 Brackets allows you to add lens data on the fly by using the credit card method, as discussed here. Simply measure the distance from the sensor to the credit card, when it is 1/3rd of the image, and register this in the t variable. A t of zero means use a thin lens model, assuming a registered lens can't be found. But I recommend explicitly registering lenses.

M3 Brackets provides feedback in real time on the position of the focus bracketing.

#mode = 0 "Mode" {Off X2INF Min2INF}
#bracket = 0 "Exposure bracket?" {None +4Ev +3Ev +2Ev 2(+2Ev) 3(+2Ev) ISO1600 auto}
#infinity = 3 "Infinity focus" [2 4]
#overlap = 15 "Overlap (um)" [5 20]
#sleep_time = 2 "Delay (s)"
#bookends = 1 "Bookends?" {No Yes}
#screen_off = 0 "Screen off?" {No Yes}
#t = 0 "x for Thickness (mm)"
#hilit = 20 "% histo HiLit" [5 30]
#lens = 0 "Get lens name?" {No Yes}

Above we see M3 Brackets’ menu entries and that there are two focus bracketing options, ie from minimum focus to infinity or from the current position (x) to infinity, plus M3 Brackets gives you various exposure bracketing options.

M3 Brackets assumes you are exposure bracketing from an ETTR exposure, ie base exposure set for the highlights; you can then choose a single additional exposure at either + 1, 2, 3 or 4 Ev, ie for the shadows; or two or three additional exposures at a 2 Ev step; or a single ISO bracket; or let M3 Brackets auto exposure bracket for you, this time from a base exposure set for the shadows. In auto mode you need to ensure the hilit menu item is set, as this controls ensuring you capture the highlights, eg if set to 20, then bracketing will continue until the % histogram value of the top 20% is zero. Try the default first and adjust from there, ie between 5% and 30%. Once set hilit shouldn’t need adjusting again.

M3 brackets assumes it is focus bracketing from near to infinity, where infinity is defined as either at twice or three times the hyperfocal based on the overlap blur. That is an infinity defocus blur of either the overlap/2 or overlap/3.

If bookends are selected, M3 Bracket will insert a dark frame at the beginning and end of the bracket set.

Finally, you can get the name of your lens, for registering, by using the lens menu item.

Here is a single exposure test I just took in my kitchen, having focused on the nearest part of the toast tongs and setting an overlap blur of 15 microns. 

As usual I welcome any feedback on the M3 Brackets script or any of my posts.

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