With such a powerful tool I want to understand the working and the effect of the many different setting possibilities. That was surprisingly difficult.
The Olympus manual just explains how to reach the different settings in the menu but says nothing about the optimal settings and why you should choose one and not another.
The manual offers no answers to questions like this:
• How do I know that my step size is below the safe step size for stacking?
• How should I change the settings if I alter the aperture from 2.6 to 8?
• How should I change the settings if I change the magnification from 1:1 to 1:4?
• What happens if my stack is so deep that the magnification changes from 1:1 to 1:2 during the shooting?
• Is the actual step size constant or does it change according to the changing focus distance?
• Can I continue a bracketing session if it seems to be too shallow at the end?
This forum offers many posts about the topic (mainly the long thread
running from mar. to sep. 2016) with many nice examples but only little detailed information or explanation.
My general search on the Internet offers surprisingly little considering the function has been available on the O-MD E-M1, E-M5 and E-M10 and some Pen models for more than a year. I have found many nice examples of successful shootings but most authors conclude that you have to do your own experiences by trial and error.
One exception is Richard Turton.
Turton comes a little closer to an understanding of the mechanism. He has done serious experiments and measurements including extracting EXIF data about focusing. Many interesting diagrams, but unfortunately (for the close-up photographer) he chose to cover the distance from 0 m to 10 m resulting in very poor resolution in the macro/close-up area.
He presents a formula behind the in-camera calculations of the size of the Bracketing Focus Differential (BFD). But it has not been possible for me to use it to calculate the precise bracketing step size from combination of focal distance, f-number and BFD settings. Possibly my fault.
I am also a little confused about his terminology about internal, mechanical Focus Step (from EXIF), Bracketing Focus Differential step (menu setting) and the physical depth of the focus change on the object. Somebody else might explain it to me.
However Richard Turton has an interesting diagram showing the ratio between calculated safe step size and the resulting step size from using BFD=1 at different apertures. That is the core of the problem because we want the biggest steps possible without un-sharp banding to minimize the number of shots and reduce the risk of object movement. In other words we want the ratio to be a little greater than one. Preferably with a comfortable safety margin.
For magnifications close to 1:1 he gets a value of this ratio around 3 suggesting a safe BFD-value of 2. It is worth noticing that the curves for all f-values are identical for shortest distances. That means that this relation is not influenced by changing the f-number. An important observation.
But again the resolution of his data in the close-up range is very small.
So I decided to add to the picture with some experiments and more calculations.
My main goal was to be able to decide with certainty how to do the settings in the Focus Bracketing menu when working in the field with the Olympus 60mm f:2.8 macro lens taking pictures in the close-up range of 1:1 to 1:5 (giving field of view from 17,4 mm to 87 mm on micro 4/3 sensor). A better understanding would also make it easier to decide between shooting handheld or using a tripod. As a consequence of this rather pragmatic approach I use Working distance (WD) instead of focus distance (FD) in my measurements.
Here is the first part of my results.
On bracketing Focus Differential (BFD) and working distance (WD).
With the use of mechanical focus rails we are accustomed to look at step size as a constant measure which should be changed for different magnifications or changes of aperture.
It seems that Olympus has made a design that compensates for changes of magnification in the close-up range.
Explanations:
Bracketing Step Size: Average of nearest 10 pictures (WD of every 10’th picture measured).
Working Distance (=WD): Measured distance from front rim of the lens without sun shade to focus point. To get the Focus Distance (FD = distance from sensor plane) you should add 102 mm.
BFD = Bracketing Focus Differential. A value from 1 to 10 in the camera bracketing menu.
Safe Step Size: To get some tested values in my distance range (adapted to my camera sensor) I chose to use the calculator on the website of Johan J. Ingles-Le Nobel. It is claimed to have an incorporated safety margin of around 75%.
Each dot in the diagram (except for the Safe Step Size!) represents 10 pictures in the stack. The horizontal distance between two dots shows the difference in WD between the focus point of first and last of the ten pictures. The vertical distance shows the average growth in focus distance between each of the 10 pictures (=Average step size).
I have added simple fitting lines to compensate for measure inaccuracies.
Interpretation:
A)
At f:4 the setting of BFD=3 (Focus Differential 3 in the Bracketing menu) gives you focus steps very close to the optimal step size at WD 83 mm to 127 mm (from 1:1 to 1:2).
BFD=2 gives you a comfortable smaller step all the way from 8,3 cm (1:1) to 30 cm (around 1:5).
That makes it much simpler to produce deep stacks at lower magnifications (f. i. going from 1:2 to 1:4). Very nice to know.
A comparison between diagrams with other f-numbers shows that
B)
For any given distance (WD between 8,3 and 30 cm) and f-number the depth of a single step in a bracket is proportional to the chosen BFD number.
At f:2.8 and 1:3 magnification we get:
BFD=2 gives Step size = 0,6 mm
BFD=4 gives Step size = 1,2 mm
BFD=6 gives Step size = 1,8 mm
On bracketing Focus Differential (BFD) and f-numbers
Light conditions or time constrictions often affect our possibilities to choose between apertures.
Luckily the camera automatically adjusts the actual used focus steps to the altered DoF resulting from changing f-numbers.
Take a look at this nice diagram: (Sorry for the BDS in the heading. It should have been changed to BFD)
C)
For a given BFD value and distance the actual bracketing step size is proportional to the f-number
That means that if your optimal BFD value is 2 at f:4 you will still get the optimal safe bracketing step size no matter how you change the f-number from 2.8 and until diffraction limits you. That is also extremely convenient knowledge.
And since every focus step in a bracketing session apparently is determined separately on basis of actual focus distance, BFD-setting and f-number:
D)
Bracketing sessions can be prolonged by simply pressing the shutter release again without doing any adjustments to settings or focus
For any Focus Bracketing setting there is no difference between the pictures from one bracket with 200 exp. and the collected pictures from 4 successive brackets with 50 exp. each without any adjustments of focus or bracketing menu settings in between.
And just a reminder:
E)
Any bracketing session can be interrupted by pressing the release button
Conclusion (almost too good to be true?):
You can do most of your field close-up hunting from 1:1 to 1:4 with any f-number you prefer without doing any bracketing menu adjustments apart from setting your BFD-step size to 2 units and the picture count to 100 (or another number if you prefer).
In good light a high f-number will give you more DoF and short and fast sessions. If you reach the remote part of your object before the 100 pictures (visible on the screen or the EVF) you simply stop the session.
In bad light where you must use a lower f-number you might need to repeat the shutter release a few times after the first session to add more DoF (or change your setting to f. e. 200 pictures).
Sounds too easy?
These nice results leave one big question: Why on earth has Olympus given us a menu with so many non-relevant possibilities? And no really useful manual?
The answer must be that the focus bracketing mode is designed to be used for many other applications than just the 60 mm macro lens in the 83mm to 300 mm range: Wide-angle, long tele, product photography, architecture, landscape and – very relevant on this forum - high power photomicrography using an Olympus tele lens as tube lens with microscope objectives (yes, it has been done with a 150mm).
Seems like we have got a very flexible and usable tool. Still many details to be uncovered.
