An Investigation of E-M1 Focus Bracketing

Started Aug 30, 2016 | Discussions
Richard Turton Regular Member • Posts: 177
An Investigation of E-M1 Focus Bracketing
29

In keeping with their usual practice, Olympus tells us absolutely nothing about how the focus bracketing function recently added to the E-M1 actually works. It would be useful to know how to adjust the focus bracketing parameters to obtain a specific depth of focus. I have now done some tests with the E-M1 and the mZD 60 mm f/2.8 macro lens to determine how the camera settings affect the range of the focus bracketing function.

There are four factors that determine the size of the focus bracket:

1. Set number of shots in the Focus BKT menu. Within a sequence the focus moves in equal size steps from near to far focus, so the size of the focus bracket is linearly proportional to Set number of shots.
2. Set focus differential in the Focus BKT menu. The size of the focus bracket is also linearly proportional to this parameter.
3. The lens aperture. This plot shows how the focus step size varies with lens aperture, expressed in internal Focus Count units (EXIF tag 0x0301), with the lens focused at infinity.. The size of the focus bracket in counts is linearly proportional to the lens f-Number.

4. The Focus Distance, defined by Olympus as the distance from subject to sensor. The distance from the front of the 60 mm lens to the sensor is specified as 82+19.25 = 101.25 mm (3.99 in). This is not a nice linear function like the other three. Focus Bracketing sequences were shot for a range of distances from the focusing target. For each frame the Focus Count was read from the EXIF data, producing the following two graphs:

For both cases a curve of the form

count = a (D + FL) / D² + b

was fitted to the measured data, where D is the Focus Distance, FL is the lens focal length (60 mm), and a and b are constants. Combining these two to eliminate the intermediate count variable produces the following result:

Or for those who prefer, here is the same plot in inches.

Except for short distances, the focus step is very nearly proportional to the square of the focus distance. Unfortunately, neither of these graphs is very readable. A reasonable approximation is given by the expressions

Focus Step (metres) = f-Number x (Focus Distance (metres) / 21.4)²

or, in inches

Focus Step (inches) = f-Number x (Focus Distance (inches) / 133)²

The total size of the focus bracket is equal to the focus step for the appropriate distance and aperture, multiplied by the Set number of shots and Set focus differential parameters.

To confirm that all the mathematical curve fitting has maintained its link to reality, the accuracy of the focus step prediction was spot checked at three focus distances (only three because it is a lot of work). A 10-frame focus bracketing sequence of a linear scale was shot at a 45 degree angle and the point of best focus in each frame was estimated by eye. The estimated distances were fitted to a straight line with the following results

Focus Distance  Predicted Focus Step  Measured Focus Step  Error
0.5 m                  1.57 mm                        1.54 mm                        -1.9%
1.0 m                  7.26 mm                        7.55 mm                         4.0%
1.5m                   17.1 mm                        16.6 mm                        -2.9%

Everything so far has been about the change in focus. The other factor that needs to be considered is the variation of depth of field with distance and aperture.

Searching for "depth of field calculation" brings up a number of DoF formulae and DoF calculators. Comparing a few of them showed that they produce slightly different results for the same inputs! This plot was produced using the formulae from Wikipedia and the commonly accepted Circle of Confusion for Four-Thirds of 0.015 mm.

More interesting is this plot showing the ratio of DoF to Focus Step for a focus differential of 1.

It appears that Olympus intended the depth of field to be 3 times the minimum focus step. I don't know if the rise in DoF at the ends of the curve is real or a consequence of the approximations made in the analysis. In any case, it is safe to assume that the depth of field is at least 3 times the step size divided by the focus differential.

Here is an example showing how the focus differential affects the image. These are 100% crops of some plant thing, all made from the same 16-frame focus bracketing sequence shot with a focus differential of 1. They were assembled using Zerene Stacker, taking every frame, every second frame, every third frame, and every fourth frame respectively.

To my (not very good) eyes there is a barely perceptible loss of quality as the focus differential goes from 1 to 3, but a more noticeable difference between 3 and 4.

Olympus E-M1
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estarkey
estarkey Contributing Member • Posts: 782
Dear Sir, Utter Brilliance!
3

Olympus needs to publish this as a white paper on their website, and then cut you a big fat check!

My bet is you are certainly an engineer or scientist, and this investigation you've conducted brings a smile to my face!

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Okapi001 Senior Member • Posts: 2,393
Much appreciated - nt

No text.

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Skeeterbytes Forum Pro • Posts: 12,369
Re: An Investigation of E-M1 Focus Bracketing
1

Wish we had "sticky" posts as this belongs adjacent to a "Check your snap-ring focus" thread. Well done!

Cheers,

Rick

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glassoholic
glassoholic Senior Member • Posts: 1,483
Re: An Investigation of E-M1 Focus Bracketing
1

My head hurts....

But well presented!

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Achiron
Achiron Regular Member • Posts: 320
TL;DR?

No text.

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OP Richard Turton Regular Member • Posts: 177
Focus Bracketing Tables
8

To make this information somewhat practical to use in the field, I have prepared two tables presenting the data used to plot the graphs. They are JPEG's, which is a very strange way to present text data, but I don't know how else to include it here.

E-M1 Focus Bracket Size as a function of Distance and Aperture

E-M1 Focus Bracket Size as a function of Distance and Aperture

OP Richard Turton Regular Member • Posts: 177
Re: Dear Sir, Utter Brilliance!
5

estarkey wrote:

Olympus needs to publish this as a white paper on their website, and then cut you a big fat check!

My bet is you are certainly an engineer or scientist, and this investigation you've conducted brings a smile to my face!

I am a retired engineer, which means that I have the time to work on this stuff, and I find the technical side of photography less difficult than the artistic side.

Gary from Seattle Senior Member • Posts: 1,936
Re: Focus Bracketing Tables

Richard Turton wrote:

To make this information somewhat practical to use in the field, I have prepared two tables presenting the data used to plot the graphs. They are JPEG's, which is a very strange way to present text data, but I don't know how else to include it here.

E-M1 Focus Bracket Size as a function of Distance and Aperture

E-M1 Focus Bracket Size as a function of Distance and Aperture

This looks very good, Richard. I would be most likely to use focus bracketing for macro. So, if I read this last chart, as an example: If I am 8" @F11 each step should be .026 inches? Is this correct? Then I would choose the number of steps by the depth of the subject matter I want to be in focus? Correct?

Also, since a lot of macro is at minimum focusing distance of about 4", what would happen, then?

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knightmelodic
knightmelodic Senior Member • Posts: 1,647
Beyond Awesome
1

Wow! Absolutely brilliant work. Thank you very much for sharing. And to echo another poster: Olympus should indeed cut you a nice big check as well as retain your services. Long-winded technical stuff is a drag to read but I force myself because it's inevitably informative. Such is not the case with this work. It was an easy, understandable read from start to finish and actually made me think I could follow along. I can't of course, but my point is that that is the beauty of the article. And not only  is it informative, it is also immediately applicable. Very well done, sir. Very well indeed. I know know I'm going to buy an EM-1 very soon. I haven't referenced it but I'm used to using focusing rails on my DSLRs so I wonder how much difference there would be in the results. Thanks again!

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jeffnsue New Member • Posts: 3
Focus Bracketing

Really interesting Richard, thank you.

Having no scientific or engineering background I’m struggling to keep up with some of it, can you help me on one specific issue please.  Your photos of the "plant thing” show stacks of every second image and every third and you make the point that the depth of field is three times the focus step. Using bracketing on my EM5ii your table suggests that at 300mm and f4 the focus step at 1 is 0.66mm. This would give depth of field of 2mm. Is it logical therefore to say that I could set a focus differential of 3 and get a good result?

I photograph wild orchids in the field with wind movement being the major problem so if I can get away with fewer shots so much the better, the alternative being to use differential 1 and discard any shots where the plant has moved too far for Helicon to be able to handle. I actually use the Panasonic 45mm macro, not the Olympus 60mm but I guess if I use your 60mm data I’ll be more than OK for the 45mm

Jeff Hodgson

OP Richard Turton Regular Member • Posts: 177
Re: Focus Bracketing Tables

Your interpretation is correct.  The total size of the focus bracket is equal to the number from the table multiplied by the focus differential and number of shots settings.

The minimum focusing distance of the 60 mm macro lens is 8 inches.  But that is the distance from the subject to the sensor ( manufacturers seem to agree on this definition).  But the sensor is 4" behind the front of the lens, so at minimum focus the front of the lens is only 4" from the subject.

OP Richard Turton Regular Member • Posts: 177
Re: Focus Bracketing

jeffnsue wrote:

Really interesting Richard, thank you.

Having no scientific or engineering background I’m struggling to keep up with some of it, can you help me on one specific issue please. Your photos of the "plant thing” show stacks of every second image and every third and you make the point that the depth of field is three times the focus step. Using bracketing on my EM5ii your table suggests that at 300mm and f4 the focus step at 1 is 0.66mm. This would give depth of field of 2mm. Is it logical therefore to say that I could set a focus differential of 3 and get a good result?

I photograph wild orchids in the field with wind movement being the major problem so if I can get away with fewer shots so much the better, the alternative being to use differential 1 and discard any shots where the plant has moved too far for Helicon to be able to handle. I actually use the Panasonic 45mm macro, not the Olympus 60mm but I guess if I use your 60mm data I’ll be more than OK for the 45mm

Jeff Hodgson

Theoretically you should be able to use a focus differential of three with no loss of quality, but depth of field is somewhat subjective and varies from person to person.  You should probably experiment with the focus differential setting to find out what is acceptable for you.  I am currently using a focus differential of 2 because I can't see any difference between 1 and 2 and it cuts the number of exposures in half.

I am very familiar with the problem of subject movement and spend a lot of time staring through the view finder waiting for flowers to momentarily stop swaying in the wind.  Due to camera movement the success rate for handheld focus stacking is very low.  I have tried sorting through the shots and discarding the ones with excessive movement, but it is very time consuming and doesn't seem to produce enough improvement to make it worth doing.

The information presented only applies to the Olympus 60 mm macro because that is the only lens I have that supports focus bracketing.  I bought this lens specifically to take advantage of this new feature in the E-M1.  All my other lenses are original Four-Thirds lenses and the focus bracketing option is greyed out.  I can't say anything about how other lenses behave.

I use Zerene Stacker instead of Helicon Focus because it is less expensive.  It produces much better results than the in-camera focus stacking.

jeffnsue New Member • Posts: 3
Re: Focus Bracketing

Thank you Richard, it seems my powers of logic are not totally dead!

You say focus bracketing is greyed out for your other lenses, is that really so?  I know the in-camera stacking only works with three Olympus lenses but can you not use Focus Bracketing and stack in Zerene?  My EM5ii doesn't have the in-camera function but my focus bracketing function has the facility to take up 999 shots at 10 different focus differentials. Photographing orchids I've used up to 25 shots with varying degrees of success.

Jeff Hodgson

OP Richard Turton Regular Member • Posts: 177
Re: Focus Bracketing

jeffnsue wrote:

Thank you Richard, it seems my powers of logic are not totally dead!

You say focus bracketing is greyed out for your other lenses, is that really so? I know the in-camera stacking only works with three Olympus lenses but can you not use Focus Bracketing and stack in Zerene? My EM5ii doesn't have the in-camera function but my focus bracketing function has the facility to take up 999 shots at 10 different focus differentials. Photographing orchids I've used up to 25 shots with varying degrees of success.

Jeff Hodgson

That's what I thought too, but when I checked yesterday with a Four-Thirds lens focus bracketing was not available.  But I just checked now using a micro FT lens (the two kit lenses are the only mFT lenses I have) and saw that focus bracketing is available.  So it appears that focus bracketing works for micro Four-Thirds lenses but not for original Four-Thirds lenses.

Gary from Seattle Senior Member • Posts: 1,936
Re: Focus Bracketing Tables
1

Richard Turton wrote:

Your interpretation is correct. The total size of the focus bracket is equal to the number from the table multiplied by the focus differential and number of shots settings.

The minimum focusing distance of the 60 mm macro lens is 8 inches. But that is the distance from the subject to the sensor ( manufacturers seem to agree on this definition). But the sensor is 4" behind the front of the lens, so at minimum focus the front of the lens is only 4" from the subject.

Ok, Richard. I read the thread but missed where your distances are to the sensor. This is a great table and I'll be sure to save it. Thanks.

I agree with others that Olympus would be interested and should publish this on their website or send it out by e-mail!

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Olympus E-M1 Olympus M.Zuiko Digital ED 75-300mm 1:4.8-6.7 Olympus M.Zuiko Digital ED 60mm 1:2.8 Macro Panasonic Lumix G X Vario 35-100mm F2.8 OIS Olympus 12-40mm F2.8 +1 more
OP Richard Turton Regular Member • Posts: 177
Re: Focus Bracketing Tables

Gary from Seattle wrote:

Richard Turton wrote:

Your interpretation is correct. The total size of the focus bracket is equal to the number from the table multiplied by the focus differential and number of shots settings.

The minimum focusing distance of the 60 mm macro lens is 8 inches. But that is the distance from the subject to the sensor ( manufacturers seem to agree on this definition). But the sensor is 4" behind the front of the lens, so at minimum focus the front of the lens is only 4" from the subject.

Ok, Richard. I read the thread but missed where your distances are to the sensor. This is a great table and I'll be sure to save it. Thanks.

I agree with others that Olympus would be interested and should publish this on their website or send it out by e-mail!

Olympus could provide much better information than I have been able to discover by testing, but it seems to be their policy not to share this type of information with their customers.

OP Richard Turton Regular Member • Posts: 177
Re: TL;DR?

Achiron wrote:

You are right.  I see now that I should have gone straight to the results and skipped over the process of getting there.  I misjudged the audience.  On the hand, if I had posted this on the Photographic Science and Technology forum all the mistakes and unjustified assumptions would have been pointed out.

Pedagydusz Veteran Member • Posts: 5,526
Re: An Investigation of E-M1 Focus Bracketing

Brilliant! Excellent technical work.

Bookmarked, thank you.

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Pedagydusz Veteran Member • Posts: 5,526
Olympus, please listen!
2

I feel that it is appropriate to repeat what I have written about this very subject, in answer to another very good post, by lester11 about 4 months ago:

I wish that Olympus, in the next version of this excellent process, would add just a bit more practicality:

First, we should set the closest focus point.

Then the more distant point to be still in focus.

Then indicate the desired number of steps.

A further refinement (but I would be satisfied with only those I mentioned above) would be to have the camera calculate the number of steps from the DoF corresponding to the f stop.

Then it would be extremely useful, no more guesswork, or very little, bull's eye each time!

Please listen, Olympus!

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