Friday, November 16, 2018

Depth of Field Bar

Way back I introduced the idea of a focus bar.

My first attempt was, in retrospect, too ambitious; as I attempted to cover everything to do with focusing. In other words in became too complicated and complex.

In this post I’m releasing a new ‘focus bar’ that is aimed at one thing, and, I believe, does this well. That is providing the ‘best’ focus info to inform our photography. The script is called DoF Bar and only runs in LV.

Here is a screen shot of the new DoF Bar running on my 5D3.

The script’s menu just has two sub-menus. The first simply toggles the DoF Bar on and off. 

The second allows the user to tell the script what he/she wishes the minimum focus to be shown as: note this doesn’t need to correlate to the actual lens focus minimum. By default the minimum focus is 50cm.

The DoF Bar has three zones.

 * Zone 1 (white) is from 0 to the minimum DoF.
 * Zone 2 (green) is from the minimum DoF to the hyperfocal point.
 * Zone 3 (white/black/white) is from the hyperfocal point to 4x the hyperfocal distance.

Note that the black and white bars provide the following info: the green-back transition is at the hyperfocal; the black-white transition is at twice the hyperfocal; the white-black transition is at three times the hyperfocal; the end of the bar is at four times the hyperfocal. 

In blurs, assuming that an ML blur criterion of 30 has been set and (sic), for simplicity, that diffraction aware is off, these transitions occur at 30, 15, 10 and 7.5 microns.

The total required blur, ie ML set circle of confusion (CoC), is set in the normal ML menu. The user can have diffraction aware on or off.

For a full frame EOS, I suggest you set the ML blur (CoC) to 30, and for a crop EOS at 20; as, using the DOF Bar, you can achieve whatever infinity blur you need; but note that, because of the granularity of the Canon lens control, as you approach infinity the focus steps, and hence distance feedback, become rather coarse. Note that if diffraction aware is on, the defocus blur at the hyperfocal will, obviously, be less than the ML set (total) CoC, composed of defocus and diffraction.

As the DoF Bar shows DoF info, you should switch off the ML DoF info display.

The left hand text, above the bar, shows the near depth of field distance.

The right hand text, above the bar, shows one of two pieces of information.

If the focus point is less than the hyperfocal, this box shows the far depth of field distance.

If the focus point is greater than zero, this box shows the infinity blur in microns. Obviously, when the focus is at infinity this will be zero; and when the focus is at the hyperfocal, this will be the defocus (sic) blur or CoC.

Note that infinity blurs less than, say, twice the size of a sensor pixel are rather wasted. So infinity blurs (FF) between 30 (ok quality) and, say, 10 (high quality) are where you should be aiming for; according to you image presentation needs, ie screen or a print being reviews by a judge in a competition.

Finally, below the bar, we see three hemispheres (dots). These dots correspond the current near depth of field, the focus distance and the far depth of field.
Thus in the example above we see the following: the near DoF is at 54cm; the far DoF at 1.4; the focus is at 71cm (from the lower ML menu); and the dots visually show this relative to the minimum focus, ie the white-green bar transition and the hyperfocal, ie the green-black bar transition.

But the DoF Bar has final feature that is the ‘killer feature’. Namely, if you take an image, dots above the bar record the near DoF, the focus point and the far DoF of that captured image.

Thus the screen would look like this for our example above:

As we have not yet refocused, the upper and lower dots coincide. So let’s now refocus to, say, 1.51m (as shown in the ML menu):

Here we see our new DoF info relative to the previous image and we see we have correctly focus bracketed, as the current near DoF is less than the last image’s far DoF.

So let’s take a second image here. The DoF bar now looks like this, with the upper dots having been reset to the latest captured image.

Finally, we keep refocusing and finally arrive at the position where we have a far DoF, infinity blur of 11 microns, ie a very high quality: as seen here.

Thus, in three images, we have focus bracketed from 54cm to infinity, and achieved an infinity blur of 11 microns.

I hope some find my latest script of value. I’m releasing the script at the beta level, as I hope I may get some feedback to help refine it before I release it at version 1. The DoF Bar script may be downloaded from the link on the right.

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