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Understanding the Olympus auto Focus Bracketing Mode

 
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Troels



Joined: 15 Feb 2016
Posts: 504
Location: Denmark, Engesvang

PostPosted: Fri Feb 08, 2019 2:35 pm    Post subject: Understanding the Olympus auto Focus Bracketing Mode Reply with quote

I got my Olympus OM-D E-M5 II two months ago very much because I wanted the auto focus bracketing feature. I soon started experimenting and also got some satisfying results (see: this thread ).

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.
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davholla



Joined: 26 Apr 2016
Posts: 181

PostPosted: Thu Feb 14, 2019 4:35 am    Post subject: Reply with quote

Very interesting. I am thinking of going to Panasonic because of this feature.
I quite like this photographer's work.
https://www.flickr.com/photos/achimowl/albums/72157661755081246/page7

What were using before and do you think that you can get better photos now?
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Troels



Joined: 15 Feb 2016
Posts: 504
Location: Denmark, Engesvang

PostPosted: Thu Feb 14, 2019 8:32 am    Post subject: Reply with quote

Davholla,
Yes, the photos by Achim Kluck are outstanding. His collembolas are amazing.

Please, be aware that the techniqe used by him is different from mine. With the Panasonic camera Achim (and others) use the Post Focus feature while Olympus use Focus Bracketing.

With Post Focus the camera automatically takes a selection of pictures based on an analysis of the actual object and its depth distribution. The main idea beeing that you can select the picture with the sharpness you prefer and discard the rest. But of couse it also makes it possible to combine the pictures with stacking software.

The Olympus focus bracketing mode works with continous stepping unrelated to the actual object. You decide the starting point, stepsize and step numbers.

Unfortunately I do not know anything about the settings and the workings of the Panasonics concept. In the right hands it seems to work perfect.

Quote:
What were using before and do you think that you can get better photos now?

Before my lates purchase i used the Olympus E-620 (4/3 with flapping mirror) and the 50mm f:2 macro for field work. The model is now more than 10 years old, so the tehchical development since then is significant. But I don't think the difference will be visible at a 1024 pixels picture.

Most important for me is that I suspect it to be much easier to make handhold focus bracketing in the field (just started practicing). But of course the higher resolution and better sensor also add to the picture.

I also noticed that it seems to be more straightforward to save my favourite settings in My Mode libraries. That was a struggle in the older model. Sometimes it is important to make the changes in settings pretty fast.
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Lou Jost



Joined: 04 Sep 2015
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PostPosted: Thu Feb 14, 2019 3:46 pm    Post subject: Reply with quote

Nice work, especially comparing with the safe step size. I've always used step size = 1 and n=999, stopping the stack by pressing the shutter button when it appears to be finished. No fuss, no worry about f-stops or step number or step size. Now, with your result regarding safe step size, I may change my standard step size to 2 instead of 1. I would like to check though to see if there is more detail in stacks made from the smaller step size.
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zzffnn



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PostPosted: Thu Feb 14, 2019 6:27 pm    Post subject: Reply with quote

Very nice work, thank you, Troels.

What bracketing step size would you recommend, if I use (Oly E-M10 II), Panasonic 100-300mm at 300mm F/5.6 and Raynox DCR-150 or/and 250 to get about 1x-3x on sensor?

Beauty of this system is its compactness. It can be used for birding (telephoto) and super macro (with Raynox lenses on). If one already has the telephoto lens and is taking it for birding, macro capacity is easy and light to add on. I think its working distance is a bit longer than Oly 60mm macro as well.
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Troels



Joined: 15 Feb 2016
Posts: 504
Location: Denmark, Engesvang

PostPosted: Fri Feb 15, 2019 3:31 am    Post subject: Reply with quote

Lou,
Thanks!

Quote:
I may change my standard step size to 2 instead of 1. I would like to check though to see if there is more detail in stacks made from the smaller step size.

Always wise to check the theories. Please inform us about your results.

aaffnn,
Thanks,

Quote:
What bracketing step size would you recommend, if I use (Oly E-M10 II), Panasonic 100-300mm at 300mm F/5.6 and Raynox DCR-150 or/and 250 to get about 1x-3x on sensor?

I have no idea!
The next steps in my exploration are still: what is the effect of changing focal length? And what is the effect of adding a Raynox?

Afte a one week vacation from shooting endless series of focus bracketing and filling spreadsheets I might be ready to continue in a few days Smile

Unfortunately I don't have a long focal length lens capable of focus bracketing. The 50-200mm zoom of mine is from the old E-system and not compatible with focus bracketing.

But I plan to investigate the effect of using shorter focal length with the 12-40mm f:2.8 zoom lens. Possibly getting sharpness from close-up to infinity in one stack. Perhaps the results could be extrapolated to longer focal lengths?

However I seriously want to get hold of the newer Olympus 40-150mm f:2.8 before too long. It is still a little expensive, but looks perfect for my nature shooting: from butterflies and dragonflies (focus bracketing possible) to larger animals and birds (with the 1.4x teleconverter).
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Lou Jost



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PostPosted: Fri Feb 15, 2019 4:48 am    Post subject: Reply with quote

As far as I can tell, everything adjusts correctly for all focal length. Step size = 1 works well for all focal lengths. I am sure step size = 2 will also work for all focal lengths.

Troels, forum members have previously reported on focus bracketing with the 40-150 lens as tube lens; it works well but has a very shallow range of focusing

I regularly use focus bracketing with a 2.5x Optem objective on my 75mm Oly lens. Again only suitable for very shallow subjects, but works well. Sometimes there is a bit of red CA which usually stacks out.

I really like focus bracketing with the Oly 60mm + Century Optics closeup lens. Some very slight unsharpness in the corners but for many subjects the results are very very good. No CA. Raynox was much inferior.
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