Photographing insects: lenses

Last week I was asked about what objective I use when photographing live insects in the field. I do not always use the same objective, so I will describe the two I most frequently use. Neither do I use what would be the most suitable or state-of-the-art optics.

The camera (Olympus E-M1 mk I)

The camera I use is a mirrorless micro four-thirds camera, which has a smaller sensor than “full frame” cameras. The small sensor has two advantages for this type of photography: an objective of a given focal lens is equivalent to twice the focal length in a full-frame camera. This camera has also effective image stabilisation.

I am rather lazy so I almost never take a tripod to the field and hand hold the camera. Consequently, for macro photography I always enable image stabilisation and very frequently continuous focus. For handheld macro photography continuous focus helps even for static objects as it tracks the inevitable movements of the camera. This is because in many cases depth of (focus) field is only a few millimetres.

Insects can move fast on their own and/or the plants they are standing on may move in the wind. When magnification is high, a shift as small as a millimetre may ruin an image. The number of “hits” tends to be low even when working carefully. One needs to take hundreds of photographs and later select the few worth keeping.

Macro objective (50 mm prime)

The macro objective I own is a Zuiko Digital 50 mm f/2.0 macro for Four Thirds cameras, of Pro grade. Optically is very good, but its age and using it adapted means that focus is very slow. The focal length is equivalent to 100 mm. It can reach 1:2 magnification without use of a macro extension tube and 1:1 magnification with a 25 mm long extension tube, which I also own.

Being a prime (fixed focal length) and in its time being considered the sharpest lens available from any brand, it can produce incredibly good images… as long as the insects are tame and patient. With this focal length if the insects are rather small one has to get very near and and frequently wait for a couple of second until the camera locks focus.

Because of the rather low light level, it would have been difficult to take this image with an objective of longer focal length.

Image taken with the camera handheld, using the Zuiko 50 mm f/2.0 macro objective.
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Crop from the image above showing the detail.

The 50 mm f/2 macro works well together with the 2 x tele converter.

OLYMPUS DIGITAL CAMERA
Zuiko 50 mm f/2.0 macro + 2 x tele converter EC-20
OLYMPUS DIGITAL CAMERA
Detail from the image above.

The current M.Zuiko 60 mm f/2.8 micro-four-thirds macro objective with faster focusing and very good resolution would be better for this task. It also weights a lot less.

The advantage of working at close range is that one can hold a flash (or LED light) in one hand and the camera in the other. In the times of low ISO films this was one of the best ways of photographing insects, in which case one would focus by moving the camera very slowly and triggering the shutter some fraction of a second before perfect focus. If the light level is low (cloudy or sun low in the sky) the handheld use of a tele objective as described below becomes  very difficult if not impossible.

Tele objective (50-200 mm zoom)

The tele zoom I own is a Zuiko Digital 50-200 mm f/2.8-3.5 SWD for Four Thirds cameras, of Pro grade. Not up to current state of the start in focusing speed but more than fast enough for anything that is not flying. This is the objective I most frequently use for insect photography. To be able to reach high enough magnification, I use in combination with either a tele-converter or the 25 mm macro extension tube.

Image quality with the extension tube is excellent, although not on par with the 50 mm macro described above. With the Olympus 1.4 x tele converter image quality is what I consider good enough, while with the Olympus 2.0 x tele converter only marginally. This last combination is also difficult to use without a tripod. The tele-converter with less magnification provides a maximum focal length of 280 mm, which is equivalent to 560 mm in a full-frame camera. The tele converter with more magnification provides a maximum focal length of 400 mm, which is equivalent to 800 mm in a full-frame camera. The advantage of using a tele converter is that the minimum focusing distance is retained in spite of the increased focal length.

So, what I currently use most frequently for insects is the 50-200 mm tele zoom with the 1.4 x tele converter. It does not provide the best possible image quality but it allows me to obtain more good images as I disturb the insects much less, which allows more time for framing, focusing and for multiple takes of a given subject. This weights about 1.5 kg, has a minimum focusing distance of 1.10 m giving a field of view of xx times xx mm.

In full summer sunlight, the tele zoom plus a tele converter works well.

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Zuiko 50-200 mm f/2.8-3.5 SWD + 1.4 x tele converter EC-14
_6140578-10
Detail crop of the image above.
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Zuiko 50-200 mm f/2.8-3.5 SWD + 2 x Tele Converter EC-20
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Detail from the image above.

The tele zoom with an extension tube can be used to advantage, but the working distance is decreased.

If you have a camera with a bigger sensor, then if you want to be able to dispense of a tripod, you will need to use a shorter tele objective than the equivalent focal length of what I am using.


All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2017 Pedro J. Aphalo


 

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Dimming of LEDs

If you are interested in photography, and take photographs under illumination from LEDs, you need to be aware of how the dimming of LED lamps works. LEDs are becoming very popular, and dimmers are quite frequently used to adjust the light level. This applies to households, offices, commercial spaces, and the now ubiquitous special LED lamps sold for studio and on location video and photography.

There are two main approaches: constant current (CC) dimming and pulse width modulation (PWM) dimming. The later approach is much more frequently used, due to technical reasons.

Pulse width modulation, in everyday words means that the LEDs are very rapidly switched on and off. They alternate between maximum output and no output. When we plot the light output it is a train of peaks and valleys, and the average amount of light depends on the ratio between the light periods and the whole cycle period. The frequency remains constant, what changes is the width of the “square-wave” shaped peaks. Hence, pulse width modulation. The frequency used can vary very widely, from  100 Hz or so, the minimum that humans will not perceive as flicker, to much higher frequencies.

The figure below shows the irradiance variation in time, measured with a light sensor connected to an oscilloscope. (Irradiance is the amount of radiation energy per unit of receiving area and unit time. The relative units in the figure correlate with illumination.)

scope_fig_dim

The time axis is in milliseconds, and each cycle takes about 4 ms, corresponding to a frequency of 250 Hz.

Const current dimming, just limits the electric current driving the LED to a value that remains constant in time. By reducing the current flow, the irradiance is decreased.

This second figure, shows measurements from the same LED, and using the same measuring setup, but with the LED connected to a different driver circuit.

scope_fig_cc

Some LED lights meant for use as light sources in photography, avoid the effect of output fluctuations by using PWM an example, I measured dimming of an Amaran AL-H9 LED light source from Aputure. The manufacturer advertises it as using a type of dimming that maintains constant illumination. This is not completely true, as dimming used in this light source is based on PWM. However, by using a frequency of 40 kHz (0.025 ms) most of the problems of PWM are avoided. This is so because it is unlikely that anybody will use a shutter speed close to 1/40000 s, with a light source that has rather weak light output and still use the dimming function.


Caveat: An additional approach, mostly used when retrofitting LEDs for living and working space illumination, is the use of a phase shift dimmer, as earlier used for incandescent lamps, in the circuit that feeds the electronics driving the LEDs. I have not measured light in any installation using this approach.


All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2017 Pedro J. Aphalo


 

Fluorescence of glass filters

Until very recently I was not aware that optical glass filters can fluoresce when exposed to ultraviolet radiation. In this short post I use two filters from Heliopan that I own as examples of this. However, many other filters, and even the glass elements in some camera lenses can also fluoresce. The photographs below illustrate this, using a non-fluorescing filter from Formatt-Hitech as reference. The three photographs were taken with a UV blocking filter on the camera objective and ultraviolet-A illumination from LEDs with peak of emission at 365 nm (Led engin LZ1-10UV00-0000).

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Unknowingly using a filter that fluoresces can make us think that the object being photographed itself fluoresces. Photographs with default white balance are shown below. A photograph taken using the same exposure and set up but with the UV-A LEDs switched off is shown as reference.

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Same images with white balance set in Capture One 10 to the Novoflex Zebra reference grey card used as background. As reference we use a “normal” photograph taken under white light.

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Conclusion: If you intend to image UV-excited auto-fluorescence be aware that many other objects may also fluoresce, including the filters you may be using to block the UV radiation, or the camera objective itself.


All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2017 Pedro J. Aphalo


 

AC line frequency and shutter speed

The light output of many types of lamps varies at twice the frequency of mains AC line frequency. Alternating power follows a sinusoidal shape, alternating between positive and negative half cycles. This causes the doubling of the frequency, for each half cycle in the power supply there is pulse in the light output. Two different standard frequencies are used for household electric supply in different regions of the world: e.g. 50 hz in Europe and 60 Hz in USA. To ensure evenly exposed frames one needs to choose a suitable shutter speed: 1/100 s, or a multiple such as 1/50 s in Europe, and 1/120 s or A multiple such as 1/60 s in USA. The figure shows measurements done with an oscilloscope and light sensor, with three different lamps as examples (the LED is not the same as used in later examples, and LED lamps vary quite widely on how stable the light output is).

scope_fig_lamp

As can be seen in the figure above, incandescent bulbs, fluorescent lamps driven with electromagnetic ballasts, and even many modern LED-based light bulbs designed to be used as replacements of such bulbs, emit light that is pulsed. How deep are the valleys compared to peaks depends on the type of bulb.

Taking a burst of photographs at f/11, 1/200 s, or 0.5 light cycles per exposure we obtain successive photographs with different exposure. The histograms from nine consecutive frames showing variation in exposure by almost 1 EV.

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Taking a burst of photographs at f/13, 1/100 s, or 1 light cycle per exposure we obtain successive photographs with identical exposure. The explanation is that if the shutter speed matches the length of one whole cycle, or an exact number of multiple cycles, the exact moment when the exposure starts does not alter the amount of light reaching the sensor. The histograms from nine consecutive frames showing almost no variation in exposure.

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The phenomenon exemplified above with a digital camera also affects film cameras.

What I describe below, affects only digital cameras.


A further twist related to light sources which have a pulsed output, is the use of the electronic shutter built-in in some camera image sensors. The sensor can be enabled and disabled electronically to adjust the length of exposure while the sensor is continuously exposed to light . In contrast  to most modern mechanical shutters, which open and close extremely fast, this type of shutter does not enable and disable all pixels simultaneously. Consequently the whole image is not captured exactly at the same time. In this case, the fluctuation in the illumination results in banding: some rows of pixels receive more light than other rows during the time they are enabled. Olympus’s E-M1 (mk I) camera had this capability added through a firmware update (as “silent” shutter release modes). Sometimes it is said that this type of shutter cannot be used under artificial illumination. However, the solution to this problem is the same as discussed above: choosing a shutter speed that is a multiple of the frequency of the local AC mains line frequency ensures that all pixels are equally exposed. The next photo gallery shows a set of photographs taken under LED illumination (Osram 6 W, 220-240 V, 2700 K, E-14, driven by 230V AC at 50 Hz) using the “silent” shutter mode of the E-M1 (mk 1) camera. I took photographs of a white wall at each available shutter speed between 1/200 s and 1/8 s in shutter priority mode (edit settings used in Lightroom 6 to enhance visibility of banding: exposure +0.5, contrast +100, Black and White treatment).

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Least banding occurs at 1/100 s, 1/50 s, 1/20 s, 1/10 s. At shutter speeds slower than 1/20 s, there is slight banding in some cases but barely perceptible. The reason is that once the exposure comprises several whole light pulses, the contribution of an incomplete pulse to the whole exposure becomes smaller as the length of the exposure increases.


A caveat is that modern driver circuits for LEDs not always work at the AC line frequency. Constant current drivers and constant current dimming using well-regulated DC power supplies are alternative technologies that completely avoid light output fluctuations. Specially for dimming, in many cases use of frequencies higher than those of mains power, alleviate the problem. Frequencies can vary widely,  from about 150 Hz, which is enough to make the pulsing invisible to the human eye, to tens kHz in some LED light sources, which is fast enough for photography, except at extremely high shutter speeds.

In the case of fluorescent lamps, high frequency ballasts are becoming common and have in part substituted electromagnetic ballasts. These  ballasts use frequencies in the order of kHz, which combined with some delayed fluorescence of the “phosphors” can almost completely eliminate pulsing of the light output.


Nostalgia: You may have seen similar banding in photographs taken with film cameras of old TV screens based on CRT tubes. The explanation is that a moving electron beam in such displays also causes flicker, once again at 50 Hz or 60 hz depending the TV standard used in different countries. In addition, the mechanical shutter of some cameras was based on a curtain, and at high shutter speeds implemented as a narrow window or slit moving in front of the film frame being exposed.


Histograms were produced with RawDigger and are based on the actual raw data in .ORF raw files. RawDigger is available at https://www.rawdigger.com/

You may be surprised by the plots containing four histograms. Camera sensors do have pixels arranged in groups of four, with one red pixel, two green pixels and one blue pixel. Data for all of them are stored in raw files produced by digital cameras. The two separate green channels are not redundant, they contribute to the spacial resolution of the image.


All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2017 Pedro J. Aphalo


 

Macro extension tubes (lens mount)


This is the third of three instalments on the comparison of three sets of macro extension tubes for MFT cameras: Macro extension tubes (description)Macro extension tubes (glare), and Macro extension tubes (lens mount).


Disclaimer: I have no connection to any of the suppliers of the items compared in this test. I bought them from different on-line sellers. Although some of the products I bought have serious design flaws I have tested only one copy of each, bought in July 2017 (Kenko), June 2017 (COMIX), October 2015 (PIXCO). The items in production at the time you read this post may be of an updated design or quality. It is also necessary to be aware that in the case of some Chinese brands, cheap and expensive versions of an item may exist.


As yesterday’s tests showed that the 45 mm f/1.8 general-purpose prime lens had, when used at high magnification, rather bad performance in the corners and edges of the image, I used the M.Zuiko 60 mm f/2.8 macro for the two tests described here.

In the second and third tests, described in this post, my aim was to test the lens mounts, first for light leaks, and second for mechanical fit.

For the second test the two LEDs were positioned one on each side of the tubes at the position of the mount between the two tubes in each pair. The lens cap on the lens, and room in almost darkness except for the two LEDs. Camera set to ISO 25600 and exposure time 10 s (f/11, but this is irrelevant as the lens cap was on and no light passing through the lens). The results of this test were good. The mounts in all three sets did not allow light through.

For the third and fourth tests both LEDs were positioned pointing downwards towards the target. The camera was as before carefully levelled and I assessed whether any gross focus problems were visible in the images. To my eyes, there were none. It should be remembered, though, that with the lens pointing downwards, the possible effect of the weight of the lens on axial misalignment was minimised. I do see slight overall softness with the COMIX tubes, but it is likely to be caused by internal reflections, still degrading image quality in a situation where glare was not expected, as no strong off-axis light source was included in the setup.

Kenko 10 + 16 mm
M.Zuiko 60 mm f/2.8 and Kenko 10 + 16 mm automatic extension tubes. Diaphragm at f/2.8.
Pixco 10 + 16 mm
M.Zuiko 60 mm f/2.8 and Pixco 10 + 16 mm automatic extension tubes. Diaphragm at f/2.8.
COMIX 10 + 16 mm
M.Zuiko 60 mm f/2.8 and COMIX 10 + 16 mm automatic extension tubes. Diaphragm at f/2.8.

Differences in sharpness anyway are very small and not detectable reliably with this simple test. The slight softness due to glare, seems to be a small but consistent effect under this test.

In the last test I looked at the play in the mounts. Inserting two tubes between camera and lens, add three new possible sources for play and misalignment. Tubes are used to increase image magnification, and consequently also decrease depth of field. For this test I simply very gently push the lens while keeping the tripod as steady as possible.

I was not sure on how to show how play in the mounts affects the image when working at rather high magnification, so I recorded a short video (hosted at YouTube because of wordpress.com free account restrictions), while moving the objective by softly pushing it and moving in the mount while keeping the camera and the photographed SD card adapter still (there was slight vibration due to camera shake, but it was very little compared to how much the play in the mount displaced the image projected on the camera sensor). To give an idea of how much play there was, by lightly pushing the lens “nose” it was possible to move it by a few millimetres while keeping the camera steady.

While the Kenko and PIXCO tubes have little play in the mounts, not causing trouble, the COMIX set has an awful lot of play, so much as to make this set of tubes completely useless. The surface of the mounts in the PIXCO is rough, while in the Kenko ones smoth, and in the COMIX shinny and smooth. When a lens is mounted on the PIXCO adapter a gentle grinding vibration and noise are felt. The surface of the tube has developed some use marks, so it could potentially be a source of dust particles.


CONCLUSION: Extension tubes are hollow tubes, but they still differ dramatically in how usable they are! The amount of play in the mounts of the COMIX tubes is so extreme and the springs so weak as to make them unable to keep any objective at a repeatable position, which is a requirement for successful macro photography. Do take this into account when buying cheap pieces of optical equipment.


All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2017 Pedro J. Aphalo