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Exploring the Limits of Lens Focus Bracketing

Last Updated: 9th Nov 2019

By Volker Betz

Introduction

Since my first use in in micromineral photography ( Betz, 2005), focus stacking has made significant progress and industy has developed equipment to use the technique . This includes stacking software, camera rails and control equipment and setups. In Germany stonemaster.com is one of the pioneer companies in this field.

Also the camera and lens producers made new cameras which support focus stacking. One of the features is called focus bracketing. This function allows to change the focus of the attached lens without extra equipment. At present this is available mainly at MFT-cameras with a 13x17,3 mm sensor in connection with a macro lens limited to a reproduction scale of 1:1.
Not expensive extra equipment like close up lenses and extention tubes expand the reproduction scale further and the author was interested to find out the useful limits. Focus stacking on the camera using a macro lens has been reported before by the author using a 30 mm macro lens:

https://www.mindat.org/article.php/2709/Focus-Stacking+in+the+Camera+using+a+Macro-Lens

Due to the low working distance the use of a close up lens and extension tubes in this report was not possible. Now with a 60 mm lens the lowest limits go down to less than 2 mm, if a digital zoom of 4x is used..


Methods of Focus Stacking


Stacking (or multi layer composition) started with a mechanical rail (e.g. a micrometer sliding table) allowing to vary the distance between a focused lens on the object. Either the camera optics-combination, or the object is moved. First versions used manual adjustment, but soon stepper motor driven (automated versions like stackshot or stackmaster) have been used. This method can be called rail stacking.

The second method is focus stacking. In this method the distance between object and camera optics combination is fixed, only the focus of the lens is varied. First applications used a macro lens with a focus motor which was driven by a tethering software like Helicon remote which supports Nikon and Canon Cameras. In more recent versions camera manufacturers integrated such functions into the camera software. A couple of cameras can do this. A informative book of Groß & Crassman (2019) gives details. This method can be called lens focus stacking.

Testing lens focus stacking


Equipment

For my experiments I built and used a camera stand ( based on earlier setups) which is shown in the drawing. It was assembled to be stable, versatile and transportable. Made from aluminium profiles it has a wight of ~ 16 kg and folded a dimension of 36x30x80 cm. It includes a 20 cm diameter darkfield ringlight, home made, with Film Grade ® White LED strip light ( 5600 K, CRI 95, waveformlighting.com).

02297600015730557868491.jpg
New setup for macro lens


The stand can also used in inclined and horizontal position.

02194350015730575798491.jpg
Versatile stand in horizontal position


As camera a Panasonic Lumix DC-G9 (electronic curtain mode) with a Olympus M. Zuiko Digital ED 60 mm f 2.8 Macro was used. This lens has its highest resolution at about f =4.5. In order to enhance magnification a Raynox M 250 close up lens and automatic extension tubes (10+21 mm) have been used. Lens focus stacking can be made without extra equipment, but is easier to use a tethering software (Lumix tether) installed on a powerful computer. The stacked pictures are life transferred to the computer and stacked with Helicon Focus pro and processed with Helicon filter. The disadvantage of cameras with many functions as focus bracketing is that it takes some time to get familiar with it. Once familiar with the complex menu, the most important functions can be linked to a function key and are then easy to use.

Testing the low limits

The low limits have been tested with a 31 mm (10+21 mm) extension tube between camera and lens. Experiments with 37 mm extension tube failed for an unknown reason. In front of the lens a Raynox M-250 close up lens was mounted. The working distance is then ~40 mm.

As test object an object micrometer was used in transmitting light mode. Additional the camera built in digital zoom mode was tested and the focus of the lens set near 1:1. The results are shown in the following picture.

09470510015730587428491.jpg
Resolution test with object micrometer (full view)
Resolution test without digital zoom, 2x and 4x digital zoom

08058180015731192388491.jpg
Objectmicometer resolution test in the pixel range
Resolution test without digital zoom, 2x , and 4x digital zoom, cropped to show the pixels.

Results of lens focus stacking
The resolution test shows with and without digital zoom a resolution of ~2 µm. It also shows no significant loss in resolution if extension tubes and close up lens are added to the 60 mm macro lens and the resolution is rather limited by the pixel size than by the optics.

With a digital zoom of 2x the resolution of sensor and optics match. So a digital zoom of 2x does not reduce picture quality significant and can be used.

A digital zoom of 4x provides an excess of pixels and the picture quality drops. But is still useful at least for any web application.

Compared to rail stacking, lens focus stacking needs more pictures/stack. This is most likely due to the fact that the pictures differ in size from near to far significant (up to 20 %). So the stacking software must correct picture size. This correction fails if the number of pictures is to small. In practical use step 3-5 have been used with 10 to 120 pictures per stack. Stacking is very fast, compared to rail stacking, the limiting factor seems to be the transfer from camera to computer via USB 3.0. A stack of 120 pictures takes about 45 seconds.

Application sample
02368530015731387128491.jpg
The working range of lens focus stacking

One lens can be used over a wide range of reproduction scales.



Of course it is also posssble to photograph coloured minerals. Click on the picture for more details.

Conclusion
Using a modern camera with built in focus bracketing and a 1:1 macro lens combined with a close up lens and automatic extension tube provides the possibility take pictures over a very wide range ( from about 1:10 to 8:1) with one lens if (low cost) extension tubes and a 2x close up lens are used. With some less resolution also 10:1 can be matched, but rail stacking with a 10x Plan Apo Lens, tube lens and bellows is significant better in this range. But lens focus stacking is also possible with a 10x Plan Apo Lens if a digital tele lens is used as a tube lens. A test picture made with a USMCO 10 x a=3.0 ∞ 200 lens mounted on the front of a Tamron 14-150 F3.5- 5.8 Di III resulted in a resolution of > 1 µm. This promising technique needs further investigation and is not very variable in reproduction scales.

04970900015731442718491.jpg
Resolution of a 60 mm macro lens at f=4.5 ( top) compared to an m plan apo NA = 0.30 (below)
Using a M plan apo lens mounted on a digital tele lens and used with lens focus stacking ist a interesting altenative for very small objects. Distance between the lines is 10µm.

The advantage of Lens focus stacking is, that with one lens a wide range of reproduction scales are possible and an expensive automated stacking rail is not necessary. As a mirror less camera and a electronic curtain are used, a less stable versatile stand can be used. Lens focus stacking does not cause significant vibrations on the setup and is much faster than rail stacking.


Rererences

Betz, V. (2005) Micromineral Photography with Multifocus Processing, Mineralogical Record, 36, 365-369

Groß, S. & Craasmann, O. (2019) Fokus Stacking. 295 pp. ISBN: 987-3-9820474-1-6




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