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  • Daniel Knop

Diffusers for focus stacking

In addition to a low-vibration setup and high-resolution microscope objectives, an effective diffuser is essential for focus stacking images. Read here how you can get one easily and inexpensively.


Three white light diffusers are in front of a gray background, two of them are attached to a blue bellows device
A good diffuser isn't everything in focus stacking, but without it, everything is nothing!

Focus stacking with macro and especially microscope lenses requires a lot of light, either very bright light sources (flash units or powerful LED spotlights) or long exposure times (moderate LED lighting). In both cases, however, the even distribution of light is essential for good photos.

Detail reproduction and image sharpness alone do not make such a photo really attractive. I remember my first attempts with microscope lenses very well. At that time, I was able to achieve sufficient image sharpness with a gradually optimized focus stacking setup, as this article describes. However, my high expectations in terms of the aesthetic effect of the images were disappointed by strong, punctual light reflections on the objects, while other areas were drowned in a gloomy black-brown monotony and all interesting detailed structures were swallowed up. Looking at the fantastic photos of greats like Charles Krebs gradually began to frustrate me.


I searched the internet tirelessly for information on diffusers and found similar advice in many places: Grind a ping-pong ball on two opposite sides and put one of the resulting openings over the microscope objective. Then illuminate from the outside and the light will diffuse onto the object. It sounded good, but didn't achieve much, because the light was still far too concentrated on the subject.


Left: A table tennis ball ground on both sides is attached to a lens on a blue bellows device mounted on a camera. Right: A pingpong ball which has been ground on its entire underside is used as a light diffuser on a microscope objective
Left: First attempts 18 years ago: a sharpened table tennis ball as a diffuser. Right: Even other grinding variants could not improve the result – the light diffusion was too weak

Other net authors recommended fitting a yoghurt pot with a round opening at the bottom and sliding it over the lens as a diffuser. Or a white polystyrene cup. Or a plastic cleaning agent bottle. A DIY diffuser like this has helped here and there, but I always had the feeling that it wasn't the ideal solution. I took a lot of focus stacking shots with it, but I was never able to achieve optimum light diffusion on the object. The angle of the diffusion surface to the object was never ideal, the wall thickness and opacity of the material and the diameter of these diffusers were very different in this way and far from perfect.


Always bear one thing in mind with such materials: a thick opaque surface diffuses the light particularly effectively, but this has a decisive disadvantage: you need more light. And this is not just a question of light availability, because, as Kurz Wirz also points out (see below), a flash unit has to emit more light with very thick diffuser material, which inevitably means that the flash firing time is longer. The flash pulse never becomes brighter, but only longer, which results in more motion blur on the image, i.e. it costs sharpness: thicker diffuser material = better light diffusion + blurrier image!


With an LED spotlight, more light may mean a longer shutter speed, which can also cause more motion blur. Such everyday utensils were never really useful to me as diffusers. No wonder, as these objects are not designed for diffusing light, but for transporting yogurt, drinks or cleaning products. Frustration remained, and good advice was expensive.


The solution to the problem is actually very simple: if you want to diffuse light, then you should use materials that are specifically designed to diffuse light. There are simple solutions that don't cost a fortune and, with a little manual skill, really do provide a highly effective diffuser. And the specific tips I will give below are not even tied to a specific country such as Germany, as these materials are available almost everywhere.


The Godox diffuser

My first diffuser recommendation goes back to advice I received from Robert O'Toole in 2017. Robert recommended a very specific flash diffuser from Godox, which consists of a hemispherical plastic bell that has a metal plate on the open side with a round opening in the middle. This diffuser is actually intended to be attached with the metal opening to very specific Godox studio flashes and to diffuse their light. Robert recommended removing the metal plate with three screws and grinding a round opening in the center of the convex opposite side of the plastic dome, with which this plastic shell can be attached to a microscope lens.


Two hemispherical flash diffusers in front of a black background
Wide-angle soft focus AD-S17 from Godox (factory photo Godox)

I inserted an additional PVC fitting into this round opening and inserted three nylon screws to hold it in place. This makes it easy to attach the diffuser to the lens without leaving any marks. For the sake of simplicity, I dimensioned the opening in the diffuser so that the fitting fits in exactly and is firmly attached by friction, i.e. without any adhesive – but of course you can also glue it.


Left: A microscope objective rests on a white surface, with a hemispherical plastic light diffuser attached to it. Right: A microscope objective stands vertically on a white surface with a hemispherical plastic light diffuser attached to it
Left: Godox diffuser, without metal plate and with suitable opening for an objective Mitutoyo BD Plan Apo 10x. Right: No special holding device is required with a precisely drilled hole

Left: A round metal plate lies on a white surface, a small film scanner lens is screwed into the center, to the right of it is a hemispherical light diffuser made of plastic, which has a grey PVC fitting with three nylon screws in the middle. Right: A round metal plate lies on a white surface, to which a hemispherical plastic light diffuser is attached
Left: A universal holder for many objectives can be made with a PVC sleeve. Right: Three nylon screws allow attachment without leaving marks on the lens

The light distribution of this Godox diffuser is phenomenally good, especially in comparison with a pingpong ball diffuser or other everyday utensils. I supplemented this diffuser with a ring-shaped cardboard sleeve with a white and highly reflective surface on one side, which prevents light from shining past the edge of the diffuser, because depending on the shooting situation, this light could be reflected by any object and thrown back onto the object or directly into the lens.


Left: A hemispherical light diffuser made of white plastic has a circular cardboard sleeve on the edge. Right: A hemispherical light diffuser made of white plastic has a circular cardboard sleeve around the edge
Left: A round cardboard sleeve, which is attached to the front edge of the Godox diffuser, prevents light from passing by, which could cause undesirable effects as false light. Right: A white, high-gloss surface on the back ensures good reflection of the light

A focus stacking setup carries a camera with bellows, on which there is a hemispherical light diffuser at the front of the lens, around the front edge of which a circular cardboard sleeve has been pushed
In practice, this sleeve has proven to be very helpful because it aids in darkening the image background

Godox diffuser on the microscope

This diffuser improved the quality of my images enormously because I achieved a uniformity in the light distribution that was previously impossible for me. In order to be able to use this light scattering on my microscope, because vertical images are sometimes unavoidable, I opened such a diffuser in a strip from the central round opening to the outer edge. It could now be placed on the microscope stage by simply sliding it over the objective in order to illuminate it from the outside with flash units or LED spots.


Left: A hemispherical light diffuser with a hole in the middle was opened from the center to the outer edge through a wide slit; a microscope objective was placed in the middle of the diffuser for demonstration purposes. Right: A microscope carries a hemispherical, white light diffuser on its stage, which has been pushed around the objective with a suitable opening
Left: The modified wide-angle diffuser from Godox was opened at the side so that it could be slid over the objective from the front of a microscope. Right: With this diffuser, reflected light photos are possible, provided the objective has a sufficiently large shooting distance. This Godox diffuser is illuminated from the side with two flash units or LED lights.

This is the wide-angle soft focus AD-S17 from Godox, which is intended for the AD180 and AD360 flash units from the same manufacturer. The processing recommended by Robert O'Toole is very simple – unscrew the metal plate and create a round opening, e.g. with the help of sandpaper, on which the convex side is moved firmly back and forth. With this "Godox diffuser" I was able to achieve good results with all the microscope objectives I used at the time.


The IKEA diffuser

Another diffuser, which I now prefer to use even more and not only on the focus stacking setup, but also with a mobile double flash holder for macro photography with focus stacking (which will be presented in a separate article), is also based on a readily available product. It is intended by the manufacturer for light diffusion, but not for photography, but for lighting living spaces. This refers to a ceiling light from IKEA, a product called "Melodi".


A white ceiling light with a bell-shaped plastic lampshade hangs from the ceiling of a room
The 28 cm diameter shade of the IKEA Melodi ceiling light is ideal as a diffuser for a focus stacking setup (IKEA factory photo)

This light is available in two different diameters, 40 and 28 cm. For our purposes, only the smaller version with a diameter of 28 cm is suitable, as the larger one would probably be too wide.


This lamp, which is not only available directly from the Swedish furniture store, but also second-hand via online auction houses or classified ads, is simply gutted completely. However, it is essential to work carefully to avoid self-injury, as the material is very tough and stubborn.


Left: A bell-shaped lampshade that has been reworked into a light diffuser is located on a height-adjustable holder that sits on a CNC double rail made of steel. Right: A plastic lampshade that has been converted into a light diffuser is located on a holding element; you can see inside the diffuser and recognize a material deformation for mounting on a black, height-adjustable holder
Left: The lampshade of the IKEA ceiling lamp, reworked into a focus-stacking diffuser, attached to a height-adjustable holding element that sits on a sliding carriage for CNC rails. Right: This IKEA diffuser was cut wide at the bottom and shaped by heating the remaining material on the underside so that it can be screwed onto a holding element

The lampshade, freed from its inner workings, can be attached to the focus stacking setup in the desired position using various aids and diffuses the light sensationally; it is simply the perfect diffuser in which you also have plenty of space for the shooting object.


You will have to use your imagination when attaching it to the setup, as this largely depends on the requirements of your individual setup. As I have some experience in working with plastic materials and also have a small workshop, I personally made a cut-out on the underside of the diffuser for a very close object holder, and I heated the material so that I could shape it to fit a height-adjustable holder. Instead, you can also simply use a fixed block of wood or plastic, which you sand at an appropriate angle on the top to screw the lampshade onto it. However, be careful when heating the shade to deform it, as the material easily boils on the surface and blisters much faster than acrylic glass, PVC or polycarbonate, for example. It is best to practice heating with a cut-off piece first to get a feel for the material.


However, the use of this diffuser requires a certain distance between the lens and the object due to its design. For lenses with a very short shooting distance, e.g. film scanner lenses (e.g. Minolta or Nikon), it was necessary to place the lens holder directly into the diffuser. To do this, I created the above-mentioned cut-out on the lower side of the diffuser and created a suitable surface on the sliding carriage that carries this diffuser. In this case, the base is also equipped with a transverse slide rail and a corresponding slide carriage, so that the object can be moved transversely to the optical axis, for example to position it in the center of the image.


A focus stacking setup carries a camera with bellows and lens, in front of it is a large light diffuser, illuminated by two LED spotlights, in the background you can see a computer monitor on which the object can be seen: a prepared hornet Vespa crabro
This IKEA diffuser has a rectangular cut-out on the underside so that a lens holder can be placed directly on the designated base if required, so that lenses with a very short shooting distance can also be used

This IKEA diffuser has a rectangular cut-out on the underside so that a lens holder can be placed directly on the designated base if required, so that lenses with a very short shooting distance can also be used


The Beatsy reflective diffuser

The British microscopist and focus stacking expert Steven Beats ("Beatsy") has thought about how light can still be brought to the object with microscope objectives whose working distance is extremely short (see www.photomacrography.net, see below). In other words, he made the impossible possible.


On a focus stacking setup, there is a camera with bellows and lens, which has a paper sleeve around its front end, and in the center in front of the objective lens is a narrow strip of glass
Reflective diffuser based on an idea by Steven Beats

This requires a certain willingness to compromise when preparing the object, because it has to be very flat and small, so sometimes it has to be cropped all around, like a butterfly wing, for example. You can see this diffuser as a kind of focus-stacking basic research, but I find this idea so ingenious that I really want to cite it here.


Left: A narrow strip of glass lies on a white background, it is covered in black at the front end and carries a small piece of a butterfly wing, next to it is a strip of paper formed into a roll. Right: A schematic drawing depicts a lens illuminated from the front by a flash unit, and you can see with yellow arrows how the light is reflected at the lens via the paper diffuser on the outside to the subject, which is located on a narrow strip of glass in front of the lens.
Left: The material for the Beatsy reflective diffuser: a paper sleeve and a narrow glass strip that supports the object to be photographed. Right: Graphical representation of the light beam path with the reflections on the lens in the paper sleeve.

Steven's idea was based on the fact that the light reaches the lens along the optical axis, i.e. from a strong light source that shines directly into the front lens. This sounds absurd at first, but Steve blocked the exact area of the front lens in the light beam with a small piece of black, opaque cardboard, which he glued to a narrow strip of glass. And on the lens side of this cardboard was the object to be photographed, a flat, even structure, e.g. a small piece of a butterfly wing with wing scales. This glass strip was placed so close in front of the lens that the object was in the focal plane of the lens.


The light now passed the piece of cardboard (and object) and hit the metal front of the lens. This surface is usually slanted, so that the light was reflected back at an angle, but deflected outwards. Steven deflected precisely this light radiation with a paper sleeve, which he placed over the outside of the lens, so that part of it reached directly into the ultra-narrow gap between the lens and the object.


Left: A microscope objective has a strip of paper shaped into a round cuff as a diffuser, in front of which is a narrow strip of glass that has a small piece of a butterfly wing at the upper end directly in front of the objective lens. Center: A microscope objective is screwed to the front of a bellows device, in front of which is the narrow glass strip with the object to be photographed. Right: The front of a microscope objective can be seen from the side, in front of it the glass strip with the object to be photographed at a distance of only fractions of a millimeter
Left: The Beatsy reflective diffuser in use – this strategy makes it possible to get an amazing amount of light onto the object even with lenses that have a tiny working distance. Center: Without the strip of paper, the direct proximity of the object to the lens is recognizable. Right: When viewed from the side, it can be seen that the distance is only fractions of a millimeter, and yet a very bright image was produced in the experiment in this way

I recreated this constellation experimentally, and the result is indeed a brightly lit image on the camera sensor. This Beatsy reflective diffuser, as I named it, is actually suitable for capturing certain objects with microscope lenses whose working distance is close to one millimeter or even less. This is the case, for example, with numerous Nikon lenses, such as CF types ("chromatic aberration free") or M types (metallurgical), which do not have the specifications LWD or ELWD ("long working distance" or "extreme long working distance"). They are normally offered considerably cheaper, precisely because they are not normally suitable for focus stacking work.


This Beatsy reflective diffuser is certainly no substitute for the diffusers presented above, but it is very suitable for experimental work with some lenses that are otherwise not suitable for focus stacking and may be lying useless in a drawer.


The beer can reflective diffuser

Focus stacking legend Kurt Wirz took a different approach – as seen at www.Focusstackingforum.de (see below). He designed a diffuser that reflects the light directly onto the object without it first having to penetrate an opaque surface. This reduces the required luminance enormously. In addition, even with a good opaque diffuser, it must be assumed that the light still has certain gradients after penetrating the plastic surface, i.e. the inside of the diffuser is not completely uniformly bright everywhere.


Left: A camera with bellows and lens is placed on a focus stacking setup, and the bottom part of a beer can containing the subject is placed over the lens. Right: Inside the lower part of a beer can is a Eurocent coin with a piece of a butterfly wing glued to it
Left: The beer can reflective diffuser based on an idea by Kurz Wirz. Right: View into the beer can: Here, a Eurocent coin was used as a support for the object to be photographed

Kurz Wirz guided the light into a tubular structure that surrounded the lens and object and reflected it back to the center via a white wall surface. According to him, reflective aluminum, such as the inside of a beverage can, is even more suitable. In 2017, this gave me the opportunity to give an empty beer can a second life as a reflective diffuser for a Nikon M Plan 20x lens. The result was a sharp and equally colorful photo of butterfly wing scales (Chrysiridia rhipheus), despite the fact that the working distance of this lens was around one millimeter and I had previously always failed with conventional diffusers. It was precisely because of this tiny working distance that I was able to buy this lens at an online auction for a downright ridiculous price, despite its extraordinarily high image quality.


Left: A beer can stands on a white table surface, next to it a height-adjustable holder that carries the empty lower part of an identical beer can, inside of which is a butterfly wing piece, glued to a Eurocent coin as a carrier material. Right: A schematic drawing shows a lens illuminated from behind by two flash units, and yellow arrows show how the light shines past the lens into the bottom of an empty beer can, towards the bottom of the can, where a piece of a butterfly wing is the subject of the shot
Left: Incidentally, the beer can reflective diffuser does not only work with Franziskaner beer. Right: Graphic representation of the light radiation pattern with the reflections on the inside of a beverage can

For this reflective diffuser, you only need the bottom centimeters of the beverage can, cut off smoothly. For thin-walled cans, a pair of scissors will do it. You should make a V-shaped cut at the top to allow more light to enter. If, on the other hand, you are lighting from both sides, the cuts should be made there. Any holder will do as a support for the reflective diffuser, and the object to be photographed is simply glued to a coin or a piece of plastic, e.g. with an office glue stick, and the whole thing glued to the middle of the base of the drinks can.


The beer can reflective diffuser is therefore not only suitable, as Kurt Wirz notes in his forum post, for extremely even and loss-free light distribution, so that less light has to be used, which shortens the flash firing time and freezes movements better. It also helps to direct the light into the millimeter-narrow gap between the front of the lens and the subject, allowing lenses with an extremely short working distance to be used for focus stacking – at least for those subjects that are suitable in terms of shape and surface texture. This is great for practicing purposes and for using lenses with an extremely short working distance, but also, as Kurt Wirz points out, for achieving greater image sharpness with less light.


However, in follow-up dialogs on Focusstackingforum.de, he points out that this concept only works with a round body, not with a kind of mirror box with flat surfaces. In the latter, an unaltered image of the light pulse is projected onto the object, whereas inside a round hollow body, the curvature of the wall causes a strongly distorted light pulse image to be reflected. This represents the actual diffusion process and was ultimately the reason for me to call this device a "reflection diffuser".



Sources:


Beats, Steven, in www.photomacrography.net


O’Toole, Robert


Wirz, Kurt



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