Ana sayfa Lighting Taming the light – Internal reflections and contrast issues in macro shooting

Taming the light – Internal reflections and contrast issues in macro shooting

tarafından Güray Dere

A long time ago I read an article written by the master John Hallmen, originally titled “Att tämja blixten” in Swedish. It meant “taming the light” and contained many tips on using flash. What I want to talk about this time is not about flash, but I will use the same title to name the article: Taming the light.

The following topics may seem exaggerated at first glance. But the truth is different. These are the “details that make the difference”, the little-known tricks.

Internal reflection problem

In a macro photograph, we put many parts together. The light from our subject passes through them and reaches the sensor. But there is a small problem: Does all the light reaching the sensor come from our subject? Unfortunately no! We’ll see where and how problems arise, but first let’s see the negative effect.

İç yansıma sorunu yaşayan bir fotoğraf
A photograph suffering from internal reflection

The photo above was taken last year with the lens and tube system that our friend Irfan sent me for testing. After a short examination, it turned out that the lens was in good condition and the source of the problem was internal reflection in the tube. Here is a photo of the same lens with a setup that takes care of internal reflections:

Aynı lens ile çekilen, iç yansıma sorunu çözülmüş bir fotoğraf
A photo taken with the same lens, with the internal reflection problem solved

Let’s not confuse the reflection on the insect with the internal reflection. Since these are test photos taken with flash, the bright parts of the insects are a bit too bright. But in the second photo, the blacks are black and the details are sharper. The internal reflection problem has been corrected.

Let us examine the sources of internal reflection and solutions in a little more detail.

Problem: Front ambient light

When we focus on an object, we only see the part of the image that enters the frame. But there are lights coming from outside the frame, hitting the front glass of the lens from the right and left. Although these are not visible in the photograph, they enter the lens. They fall not on the sensor but on the surfaces inside the lens or on the inner surfaces of parts such as tube or bellows. From here, they reach the sensor by making reflections and cause problems such as haze, loss of detail, and gray appearance in places that should be pitch black.

We will do something about the interior surfaces that are prone to reflection, but first we have to deal with the unnecessary light coming in.

Solution: Lens hood

Do you use lens hoods on your lenses? It is common to use lens hoods, especially on tele lenses. The purpose here is to get only the light coming from the subject into our lens. To block the environmental light. It’s like horse blinkers! You need to adjust the angle of these horse blinkers according to the frame the lens sees.

When photographing landscapes with wide angle lenses, our frame is very large. These lenses are not often equipped with a lens hood. There are usually leaf-like parts. Because a lens hood that extends forward immediately enters the frame. Such a lens hood that opens like a flower would look a bit funny. In tele lenses, we have the chance to use long lens hoods because the angle we look at is narrower. As the shade extends forward, light from the side is prevented from entering. We get a high contrast image.

In macro it’s more interesting. Like tele, we’re looking from a very narrow angle. We have the chance to use long lens hoods. But we’re too close to the subject! We can’t extend the lens hood too far. If we do, we will have problems such as scaring the creature, blocking the frontal illumination, casting shadows.

Fortunately, the working distance changes with the magnification. And we have the chance to change the lens hood design as the magnification increases.

At high magnifications the frame is physically so small that it covers a much smaller area than the front glass of our lens. This brings a big advantage. We can design a cone-shaped lens hood that narrows towards the front to effectively cut out ambient light, but without compromising the illumination of the subject.

I use a lot of enlarger lenses, which are used reversed, so the mounting sides are facing outwards. And since almost all of them have standard m39 mounts, we have a great convenience here: M39 extension tubes!

m39 uzatma tüpünden parasoley
Lens hood from m39 extension tube

In the photo above, on the left there are 2 pieces of (British made) m39 extension tube. In the center, again 2 pieces (Russian made) of m39 extension tube connected as a lens hood to a Nikon El-Nikkor 50mm f2.8N enlarger lens. On the left is a one-piece m39 extension tube connected as a lens hood to a completely different lens, the Meyer Diaplan projection lens.

You can find these prehistoric tubes for almost free. They are usually sold in sets of 4 pieces. I like the Russian made ones better, they are robust and the interior design is quite successful. By attaching 1 or 2 pieces to the outside of the reversed lens as above, we get a very useful lens hood.

Lomo mikroskop lensi ile el yapımı parasoley kullanımı
Handmade lens hood on Lomo microscope lens

While modern lens coatings play a big role in blocking out ambient light, it is still possible to see the benefits of a lens hood. The vast majority of lenses I use are not modern. Especially microscope lenses are the same age as me. Many of us use similar models of Lomo etc. brand microscope lenses. Therefore, we should not say “no way” and use a lens hood if possible in microscope lenses. Since our working distance is a few cm, we keep the length of the lens hood small so that we can illuminate.

Problem: Incorrect diffuser / lighting position

Whether handheld or studio, there is a common mistake we make. We bring our lighting devices too close to the subject in order to use them effectively. This can be caused by a flash held a little bit in front or a paper cup placed in front of the lens as a diffuser. This illuminates the insect very well, but the front glass of our lens sees these illuminating surfaces around the edges. That strong light comes right through the lens. Sometimes it paints the whole photo white, sometimes it creates a crescent-shaped flare on the opposite side of the light source.

Solution: Lens hood and the proper lighting

This is what the lens hood is all about. If we use this, we can be more flexible when using our lighting devices because we protect the lens from light. However, if we use a very short lens hood, such as a microscope lens, or if we cannot use it at all, we need to pay attention to the diffuser length.

For example, if we take a portrait of a fly at a distance of 2cm, there is no point in illuminating the back of the fly. It is enough to keep the diffuser length at 3cm. The side of the fly facing us should be well lit. If we used a 10cm diffuser, we would also illuminate the back of the fly in vain, it would send light into the lens.

Problem: Tube/bellows/helicoid reflective inner surfaces

When the budget is limited, we have to turn to cheap solutions. Chinese adapters and handmade tubes are often the main cause of internal reflection problems. In fact, regardless of the price, it is better not to rely too much on these devices and take precautions.

Ic_Yansimali_Tup
Tube with internal reflection problem

The reflection problem is quite easy to detect. We simply hold the tube up to the light, look through it and examine the walls for reflections. The next photo is of the tube that took the problematic photo at the top of the page. When I examined the tube, it looked like this.

The inside of this tube with a lens attached to it, is so bright that the garden landscape, the trees, with all their details – just like a projection machine – fall on the inner surface of the tube and are reflected from there and reach the sensor (our eye). This environmental light pollution affects the whole image. There is light leakage all around the inner surface.

Solution: Light trap

Devices such as tubes/blinds/helicoids usually have a grooved inner surface to reduce internal reflection. In a good quality material this is sufficient, but in Chinese products, even if there are grooves, they are not matte enough to prevent brightness. In this case we place light traps inside the tube and at the entrance and exit. You will read more about this process in a moment.

There is one more point to be considered with bellows. Old and worn bellows may have cracks and tears. In order to prevent light from leaking through these holes, the bellows should be thoroughly inspected and these holes should be repaired.

Problem: Adapters that do not fit well

Due to poor quality control, the adapters that connect the parts sometimes don’t fit properly (sometimes they fit so tightly that we can’t unscrew them!) This can manifest itself as a slight wobble, which can cause light to leak in through the gap.

Solution: Robust adapter

Since I haven’t encountered this situation myself, I haven’t had to solve it yet. I think temporary solutions such as black tape will cause problems in the long run. It will also disturb aesthetically. These parts also carry a load. We want to entrust our precious cargo to a sturdy adapter. My recommendation would be to replace adapters that do not hold well.

Problem: Light reflection from the sensor

Strange as it may sound, the sensor of our camera is also a light source. The image projected there illuminates the sensor and this illumination illuminates the whole system from the inside, the back glass of the lens facing in, the inside of the tube and so on. So it reflects. And this reflection happens over and over again in a reciprocal way between the elements inside.

This is more or less the case with all sensors. But in some models it is more pronounced. In extreme cases, such as long exposures at night, we hear complaints of sensor reflection. You can see the effects of sensor reflection in the two photos below (photos not by me).

Sensör yansıması
Sensor reflection

We see how the lamps on the bridge create layers and layers of reflections. This is what I call reflections over and over again. Sensor-lens-sensor-sensor-lens-sensor… This is how the light loses its power each time, making internal reflections. It creates ghost images. They look like they’re behind several layers of glass. This reflection applies to every pixel in the image. Of course, bright spots are the real problem. Manufacturers are not idle. They are taking measures to reduce this problem. Sony A7 and Sony A7II are a good example for this. I know that both cameras do very good work and I’m already using the Sony A7II.

Sensör yansıması
Sensor reflection

Here are photos of the same frame taken with two models of bodies. In the Mark II version, the sensor reflection of the street lamp has been significantly corrected. Now I have opened another topic for you to research when buying a new camera 🙂

Solution: Light trap

I’ll keep the mystery of the light trap going for a while longer! The panacea light trap 🙂

There is a place that is illuminated by the sensor reflection and creates a problem. In macro, I almost always use reversed enlarger lenses and extension tubes. There is an interesting problem with the reversed lens. Around the front glass of the lens is usually the brand model information. And they write them in bright white letters. Now when we turn these white surfaces with the reversed lens to the sensor, guess what happens? Internal reflection! These white letters strongly reflect light coming from the sensor. So they need to be covered with a light trap.

Problem: Body defects

This problem is very rare, but there are interesting cases. It needs to be investigated when renewing the body. There is always the possibility of some problems with new models. I am writing just to mention it.

For example Nikon D300, there are complaints that the light used in the focusing system leaks through the body and causes internal reflections. Another example is the Fujifilm X-T1, where light leaks through the HDMI port on the body. It also degrades the photo.

Solution: None!

If we’re lucky, we’ll get a warranty. Fortunately, these are very rare problems. We can forget about them for now.

LIGHT TRAP

Finally, we come to the light trap, and don’t mistake the name for space technology. Light trap means a surface that’s as matte as possible, that doesn’t reflect light. Of course it has to be black. A surface that absorbs all light, like a black hole!

Pentax SMC-K 135mm f2.5 lensi iç yansıma azaltıcı çerçeve
Pentax SMC-K 135mm f2.5 lens internal reflection-reducing frame

Manufacturers use different designs and materials to reduce internal reflections. The Pentax SMC-K 135mm f2.5 lens in the photo is famous for its sharpness. On the mounting side of this lens we see a rectangular frame. This creates an opening just enough for the sensor to see through. The design avoids leaving the glass bare and cousing sensor reflections. This frame is a light trap.

Olympus 65-116 değişken uzunluklu tüp iç kaplama
Olympus 65-116 variable length tube inner coating

The Olympus 65-116 variable length tube is a legend among macro-lovers. Look at the inner surface covering of this tube, which shows its quality in every detail. This velvety, fabric-like structure, with no reflections whatsoever, is a light trap.

Last year I found a paper sold under the name “light trap”. A seller in the UK was selling A4 size paper for 6 pounds. Curious, I immediately bought one and used it for tube connections. But then I realized that there was a completely identical material sold in stationery stores under the name “black velvet paper”. It was a waste of money and postage. Now you should visit the nearest stationery store.

Light trap for reverse mounted lenses

agrandizor-isik-tuzagi

Above you can see a light trap made for the Rodagon WA 40mm enlarger lens. In the center is the lens alone, on the right is the light trap built into the reverse mount adapter, and on the left is the lens and adapter used together.

We adjust the hole in the center of the trap to the appropriate size so that only the glass of the lens is exposed. The light trap is bidirectional. Both the inward and outward facing part is black. Otherwise there would be no point. In this way, we block the reflections of the lens’s brand model inscription, if any, and the light that will hit the glass from the edge and enter.

Light trap for microscope adapter

RMS_Adaptor_Lomo_Microscope

I use a flat microscope adapter, the RMS-M42 adapter you see in the photo. The outward facing part of this is not of much interest to us at the moment. But we want the inside, the part facing towards the sensor, to be completely matt. Metal adapters, even if they are black in color, give a considerable reflection.

RMS_Adapter_Light_Trap

We cover the inward facing surface of the adapter with the light trap as it is. As you can see, I have left the opening too wide. The shiny metal part of the microscope lens is visible. You need to make the hole a little smaller.

Light trap inside the lens hood

Parasoley içi ışık tuzağı
Light trap inside the lens hood

I mentioned earlier about using m39 tubes as a lens hood. Instead of using them in their bare state, we can use them by placing a light trap between the two parts to reduce internal reflections. The hole diameter is important here. It is necessary not to close the glass too much. If we are not going to use the aperture wide open, we can also cover the glass very little. In the photo above, a light trap is placed between the two parts of the tube in the center.

You can use this logic exactly the same way in devices such as tubes, bellows, etc. that you normally use. If you can’t make an inner coating, you can use light traps in this way to significantly reduce the internal surface reflections reaching the sensor. But I have to remind you again, if you leave a hole too small, you can cause corner darkening in the image. It is not difficult to adjust the appropriate size with a few trials.

We are luckier with APS-C cameras in terms of vignetting, since the sensor is smaller, the hole for the traps in the tube and bellows can be left a little smaller.

Covering the inner surfaces of lens adapters

This part mostly concerns those of you who use mirrorless bodies. After I switched to the Sony A7II body, I started buying various adapters to use the equipment I had. Since I found it strange to pay over $100 for a small hollow metal bracelet, I turned to Chinese adapters around $15-$20. I still avoided the cheapest ones, of course. Not that much 🙂

When the adapters started arriving, I found horrible internal reflection even in the best crafted ones. My predictions were spot on. Fortunately, I had prepared all the material in advance: Velvet paper, pencil, ruler, scissors. All but one of them was completed in one session. But one of them was a bit challenging. That’s why I chose it for the tutorial.

Adaptörde iç yansıma
Internal reflection in the adapter
adaptor_isik_tuzagi_plani

Unfortunately, these very well crafted adapters fall down in the internal reflection test. When we hold it as above and look at the inner surface, it is immediately seen that it has the potential to ruin our photo. This surface needs to be dealt with immediately.

After the first attempts, it quickly became clear that it was going to be a challenge. Unfortunately, the inside of the adapter is not shaped like a cylinder, but like a truncated cone. So when we cut a rectangular piece of paper and insert it into the adapter, it doesn’t fit on the surface. I had to try 3 times and finally design a truncated cone shape on the computer. After cutting out the printed samples and finding the right size, I drew and cut two pieces on velvet paper to cover the inner surface.

adaptor_isik_tuzagi_acilim

The cutout on the main part is for the diaphragm pin to fit through. The thinner one will cover the smaller diameter part. The measurements are so precise that there is no need to use glue when we insert these papers into the adapter. When the two side ends of the paper meet, they will clamp tightly end to end.

isik_tuzagi_kenar_parlamasi

By the way, I realized that the side surfaces of the cut paper will be in a position to be seen by the lens and the sensor. And white paper is quite bright, even if it is very thin. So I exaggerated the precision a bit and colored the side surfaces of the velvet paper with a black pen so that there were no white gaps.

İç yüzeyi ışık tuzağı kaplanmış adaptör
Adapter with inner surface coated with light trap

We put the velvet paper inside and here is our adapter. Compare this photo with the first one, taken in exactly the same light and at exactly the same angle. We’ve created an interior surface that would make the manufacturer jealous. Our light trap is complete.

Important Note

As you can see, the material from which we create the light traps has a velvety, hairy surface. After you cut around it, some of these hairs are released. After cutting, you need to shake the piece well and blow it out. Even so, some of it may remain or some other fibers may be released from the tissue after a while.

When one of them lands on your sensor, it shows up in the photo as a wire-shaped dust speck. If they do, they are very easily removed with an air pump. I haven’t had a problem so far (for more than 1 year). If you have been using DSLRs for a long time and are already used to sensor dust, you won’t even notice additional dust from light traps. The purpose of this note is to remind you to have a pump.

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