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  • Daniel Knop

LED lighting for focus stacking

Aktualisiert: 25. Apr.

LEDs have now conquered large parts of the lighting market, and they are also helpful and popular for focus stacking shots with high microscope lenses. However, they also have disadvantages compared to flash units. Which ones are suitable, what is their light output and what is the best way to use them?


Two weevils inspect LED light-emitting diodes
Even weevils seem interested in LED lighting

Continuous light has advantages over short flashes of light. For example, the image effect can be assessed much better in advance, including the formation of shadows. There is also far less wear and tear on the light source, as numerous individual series with many hundreds of shots put quite a strain on flash units, whereas an LED light can cope with this effortlessly.


On the other hand, LED light also has a downside, as the slightest vibrations caused by camera functions easily find their way into the image in the form of blurring, and even minimal stability deficiencies in the entire setup are mercilessly revealed. Also, the color rendering of some LEDs is often not really satisfactory, very much depending on the light source used.


Not all the desired color spectral components are actually present in the desired quantity for every mixed light color (color temperature) that appears suitable. Not all white light is the same. Daylight color between 5000 and 6500 Kelvin (K) can, for example, contain a lot of blue and red, or relatively little of both – but the mixed light color could be quite similar in both cases. Only a spectral diagram can provide clarity here, but hardly any manufacturers offer one for their products.


Spectral diagram with colored column representation of the individual light spectral components

Spectral diagram with colored column representation of the individual light spectral components
Two spectral diagrams, one of natural sunlight and one of artificial light; both are fictitious and in no way representative, but illustrate the harmonious spectral balance of sunlight on the one hand and an unbalanced spectral distribution in artificial light on the other, in which some components can be very weakly represented, which can then also be seen in photos

Of course, this applies technically to flash units to the same extent, but there we are dealing without exception with units whose light source has been specially designed for photographic purposes, so that the spectral composition can largely be relied upon, at least within certain limits of variation. The situation is different with LEDs. Of course, there are also special photo LED lights, as well as studio devices and smaller video and photo LED lights whose spectral composition actually meets our requirements. However, light-emitting diodes from room lighting are often used for focus stacking, e.g. LED strip lights or square LED recessed panels (whose light source is usually also strip lights).


Recessed LED light with connection cable
Focus stacking "starter model": LED recessed panel from room lighting

Of course you can experiment here, but in any case you are dealing with two variables that are difficult to assess in advance: Luminance and spectral composition.


Theoretically, the matter is simple: red, green and blue together produce colorless light that appears white. However, if there is a lower proportion of red and blue, for example, the light can appear slightly greenish and still be offered as a daylight color. However, red and blue colored areas on our objects then appear rather dull in the photo, simply because there is not enough radiation of the relevant wavelength to reproduce them. If, on the other hand, the light contains a high proportion of red and blue, the corresponding colored areas on our objects will also appear correspondingly colorful. Here you can only experiment to find suitable LEDs with test shots. However, you should do this before you build a lighting system with certain LEDs.


LED recessed panel is disassembled, you can see the frame with LEDs, the rear panel and the diffusion plates
If you disassemble a built-in LED panel, it becomes clear that the light source is an LED diode strip that has been laid in a ring and shines through a diffusion surface

View of the LEDs in the LED installation panel, you can see four square, yellow illuminated areas
One advantage of such recessed LED panels is that no additional diffuser is required

In room lighting, you often have to choose between warm tone and daylight color, but sometimes it makes sense to combine both, e.g. two daylight colors and one warm tone. But here, too, you can only make progress by trial and error because the spectral composition of these light sources is not standardized and their wavelengths are not usually specified in detail. Information on the CRI index (color rendering index) is also usually provided in vain.


It is even more difficult when it comes to luminance, as this must be sufficiently high for our purposes. If the light emission of your LEDs is not strong enough, there is little point in trying, because you would have to drastically extend the exposure time to compensate for the light deficit, and in doing so, camera-internal vibrations would increasingly enter the image. This costs image sharpness. In principle, the shorter the exposure time, the less vibrations are depicted.


You must also bear in mind that our human eye only ever perceives light brightness in relative terms, not in absolute terms. If the LED appears bright to you, this may be because your eye is currently dark-adapted.


LED spotlight

A lux measurement is better. That's why I carried out such a measurement with my two studio LED spotlights, each with 60 watts of power, in order to have concrete measured values (each value read x 100).


A 60-watt LED spotlight at 100% setting, without diffuser, measured in front of the lens, i.e. where the subject would normally be: 85,000 lux


The same LED spotlight with identical setting, but with diffuser: 30,000 lux (you can see how much light a good diffuser costs).


Two of these 60-watt LED spotlights at 100% setting, measured with diffuser: 60,700 lux


And to have a comparison value, spring sunlight under a blue sky on the same day at 14:00: 95,000 lux


A lux meter is located in front of a large light diffuser, which is illuminated by two LED spotlights
The lux meter records more than 60,000 lux with two 60 W studio LED spotlights

With these two LED spotlights, I usually have more light than absolutely necessary, i.e. I don't have to work with a high ISO value, but can set values around 200 to 320 and still reduce the LEDs to around 80 %, so I still have reserves. The amount of light actually required varies greatly anyway, for example depending on the background: am I using a black felt layer that absorbs an enormous amount of light or a white paper surface that reflects a great amount? It's always good to be able to go one step further here, and with a slightly higher ISO setting (e.g. 800), 50 or 60 % is usually enough for me.


The light tunnel

If you want to use standard LEDs from room lighting, you will usually end up with LED strip lights. You would have to place these behind a diffuser, as otherwise unsightly strong reflections could occur on the subject. It would be easier if you used square LED recessed panels with a surface area of approx. 10 x 10 or 15 x 15 cm. Light strips are already installed in these panels behind a diffuser surface. You can also open many of these panels with a few screws and then gain access to the diffusion surface to change it as required, to increase or decrease the light scattering. Combining three square panels to create a light tunnel in the shape of an upside-down letter "U" is particularly ingenious. To do this, simply screw the three panels together using four metal brackets from the DIY store.


Three LED panels are positioned next to each other in a U-shape and have been screwed together to form a unit using four metal brackets
Three LED recessed panels can be easily connected to each other by screwing them together with four metal brackets

To be able to compare recessed LED panels with my studio LED spotlights, I also carried out a measurement here. My panels measure 105 x 105 mm illuminated area, two of them daylight color and one warm tone.


Measured value per panel: 5650 lux


The use of three such panels, arranged as a light tunnel, provides around 17,200 lux


Three LED panels are connected to form a U-shape, with the measuring probe of a luxmeter underneath
The lux meter records a good 17,000 lux at the LED light tunnel

Advantage

The elements emit light over a large area, so they have their own light diffuser


Disadvantage

The amount of light is rather poor. In terms of brightness, it is certainly possible to take usable pictures, but you have to compensate for the relatively low amount of light by using a higher ISO value and/or longer exposure times. Many modern mirrorless cameras allow you to use quite high ISO values without image noise, so it would be advisable to work with ISO values around 800 and keep the shutter speed as short as possible. You must bear in mind that with scanner or microscope lenses, you do not have an aperture, so you have no variable here.


Three LED panels are connected to form a U-shape, underneath is a prepared hornet that is to be photographed
If you adjust the setup and camera adjustment to the lower amount of light, you can take good pictures with this type of LED light tunnel

Like a little more?

An alternative to these or similar LEDs from normal room lighting would be special video lights or the aforementioned studio LED spotlights, which emit significantly more light. The latter usually have the advantage of being adjustable so that you can adapt the brightness to your requirements, e.g. the lens type, the focus distance (the distance between the sensor and the light-emitting lens), or simply the brightness of the subject or the background color. All these are variables that require the amount of light to be adjusted, and a simple control dial makes life much easier. What's more, with video lights or studio LED spotlights, you generally don't need to worry about the spectral composition of the light, unlike with room lights diverted from its intended use.


Two studio LED spotlights shine onto a large light diffuser
Two studio LED spotlights with 60 watts of power each provide an abundance of light, and if this is distributed via a good diffuser, hardly anything is left to be desired

Advantage

Special video lights or studio LED spotlights provide suitable spectral distributions and usually have plenty of power. Devices for stationary use have a mains connection and often a collecting reflector that bundles the light into a soft beam and prevents light loss into the room.


Disadvantage

Apart from some small, battery-operated video lights, video lights and LED spotlights produce such concentrated light that you need a good diffuser. In addition, two powerful studio LED spotlights emit so much heat at 100 percent operation that the microscope objective can heat up considerably during prolonged exposure. I am concerned about the risk of delamination of laminated lens pairs (thermal expansion of the glass and softening of the lens epoxy). That's why you should switch them off between the individual image series if possible.


Hold still please ...

In general, when focus stacking with LEDs, you must always bear in mind that all movements that occur during the entire set shutter speed are relentlessly captured in the image and cause blurring. Some photographers sneak out of the room at the start of the automatic series of shots to avoid any vibrations caused by footsteps on the floor. This can help, but does not completely solve the vibration problem, as some micro-movements are also caused directly in the camera or by the motorized feed and too short pauses before the shutter releases. Such loss of focus would not occur with flash. With LED light, however, you need to develop strategies to avoid this.


The most important thing is to eliminate the vibration caused by the shutter movement. Ideally, you should use a mirrorless camera and use its electronic shutter, because then no shutter moves at all. The second best option, actually hardly any worse, is to use an SLR camera that you have set to "silent shutter release". The following happens: The first shutter curtain is opened before the picture is taken, but the sensor is switched off. By the time the picture is taken, everything has calmed down again, and functionally this is not much different than if the shutter curtain was still closed. When you press the shutter release button (or get the electronic shutter release signal from the control unit), the sensor is energized and the exposure begins according to your set shutter speed. It is ended by the falling second shutter curtain. Although this produces a vibration, this will only have an effect when the shutter is fully closed, and this is the moment when no more light reaches the sensor. This is why no more vibrations should actually enter the image than with an electronic sensor.


However, you should bear in mind that with this option, general micro-vibrations in the room, e.g. caused by people walking in the next room, road traffic or other factors, will have a much greater effect on your image sharpness than when using flash units. So you need to have everything much better under control here. Of course, this also applies to any instability of your entire set-up, including the table on which it stands.


The lighting characteristics

The LED light appears very evenly distributed on the subject. Flash light, on the other hand, appears somehow bolder, livelier and more imaginative in its effect. Whether this is an advantage or disadvantage depends on your expectations and taste on the one hand, and on the surface texture of your subject on the other. The matt black body of a blowfly reacts completely differently to the red/green/blue/yellow-silvery back of a cuckoo wasp Chrysidae.


Section of the wing of a butterfly, you can see a good 20 individual scales image width in very different colors, illuminated with LED spotlights

Section of the wing of a butterfly, you can see a good 20 individual scales image width in very different colors, illuminated with LED flash units
Wing section of Chrysiridia rhipheus, once illuminated with LED (2 x Studio LED spotlights 60 W), once with flash units (3 x Godox TT350); both images have not been cleaned of dust particles and have not been processed in any other way; the lighting characteristics are very different

The colored, metallic-looking wing scales of the Chrysiridia rhipheus butterfly are a good example here, as they show these differences very clearly at 20 x – although I personally couldn't say which I like better. This differs from scale to scale, and sometimes I find a particular wing scale in the LED version and also the corresponding scale of the flash version attractive, but in different ways – two beautiful variations on the same theme.

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