Pinholes in NIR and UVA

Using the E-M1 converted to full spectrum with the Pinhole Pro objectives is possible. Using a 58 mm NIR filter (Hoya R72) attached to the front of the 11 mm Pinhole Pro S11 worked fine, with no increase in vignetting. Using the StraightEdgeU 52 mm or Baader U-filter 2″  with a step-down ring blocked the corners of the image completely. The original 26 mm Pinhole Pro suffers a lot less from vignetting and can be used with these filters of smaller diameter than the front thread of the lens without problem.

Even if pinholes are simply holes, and consequently do not absorb radiation, their very small apertures mean that exposure times need to be very long in the UVA, even in bright light.

The photographs below were taken yesterday between 16:50 and 17:10.

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26 mm Pinhole Pro, f:173 60 s, StraightEdgeU filter.
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26 mm Pinhole Pro, f:104 0.6 s, Heliopan RG695 (Schott glass) filter.
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26 mm Pinhole Pro, f:104 1 s, Heliopan RG780 (Schott glass) filter.

The photographs were white-balanced in PhotoNinja on the clouds. None of them have been converted to monochrome, but using the long pass filter with longer cut-off wavelength resulted in extremely faint false colour.

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Pinhole objectives arrived today

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Thingyfy Pinhole Pro 26 mm MFT

Introduction

Pinholes need to be very small to provide a useful image. Consequently the corresponding f-values are small, in most cases f:100 or smaller. This results in either very long exposures, or requires the use of very high ISO values. As we will see in the example images this is less of a problem than what could be expected because as the resolution of the pinhole is low, the images tolerate very strong noise reduction processing without losing there character or mood.

Another consequence of the very small size of the aperture is that dust on the camera sensor assembly becomes much more visible than when using glass lenses and larger apertures. Dust that is not visible at all in normal use becomes very visible when using a pinhole, requiring extensive use the a “heal tool” during file editing.

The positive side of using a small aperture is that depth of field is very deep. Pinholes cannot be focused like glass objectives. In contrast there is an optimal pinhole diameter for maximum resolution, which depends on wavelength (colour) of the light, and focal length. As a consequence of this, macro extension tubes can be used to increase the focal length.

First impressions

The two Thingyfy pinhole objectives arrived today. They seem fairly well built and solid. They have a 58 mm filter thread and the MFT mount at the rear is part of the barrel. The finish seems to be black anodised inside and outside. They fit the camera mount just a bit tight, but nothing to worry about. The 26 mm focal-length Pinhole Pro with multiple pinholes, selected with a ring similar to an aperture ring with click stops, seems to me the most versatile and useful of the two. The range of pinhole sizes is broad (0.80 to 0.10 mm) and allows quite a lot of control over sharpness. It also has pinholes large enough to work nicely with “macro extension tubes” to obtain longer focal lengths. In this case the extension tubes are not used for closer focusing, but instead to increase the focal length.

The wide-angle 11 mm focal length Pinhole Pro S11 has a single pinhole, of a size slightly large for maximum resolution at its focal length, allowing its use with a short extension tube in addition to directly mounted on the camera.

First images

When I came back home from the post office there was still enough light to take a few test photographs of clouds in the sky.

Pinhole Pro S11

With this objective vignetting is very obvious as well as some colour shift towards the edges of the frame. A pinhole will never be very sharp and at this very short focal length (wide-angle) the geometry of the light path results in strong vignetting. Pinholes are used for the “character” of the photographs they produce and vignetting and low resolution are what makes them interesting. Their limitations are specially noticeable in small sensor size cameras like the micro four thirds camera I use.

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Pinhole Pro S11 11mm MFT, ISO 1000 f:79 1/15 s.

The Pinhole Pro S11 can be combined with a 10 mm extension tube to form a 21 mm f:150 pinhole.

Pinhole Pro

With a focal length of 26 mm, vignetting is much less than with the wide-angle pinhole. By selecting the pinhole size, it is easy to control the (lack of) resolution to one’s taste.

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With 16mm + 10mm Kenko DG macro extension tubes focal length is doubled to 52 mm.

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10mm + 16mm ext. tubes giving focal length 52mm, pinhole 0.25mm = f:208, ISO 1000, 1/3 s.

This combination works satisfactorily resulting in a moderate teleobjective. It became too dark before I could try using the extension tubes individually. These give 36 mm and 42 mm as focal lengths when combined individually with the 26 mm Pinhole Pro.

Camera objectives for UV photography

I have been testing some objectives for their UV transmission using LEDs as sources of radiation. I developed a protocol for such tests. Although used in this example to measure the spectral sensitivity of a camera sensor, the protocol can be easily adapted for the measurements of biological action spectra.

A draft report of these tests is available as an R notebook. This is an HTML file that can be viewed in a web browser. (The file is large, and takes some time to load.)

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Acknowledgements

I thank Lasse Ylianttila for comments on an earlier draft of the linked report, and for our many helpful and entertaining discussions about all things “UV to IR”, including photography.

MIDOPT filters

For those interested in photography “beyond the visible”, some of the filters available from Midwest Optical Technologies Inc. under the MIDOPT brand name should be very interesting. They are distributed in European countries by Stemmer AG. Both companies are specialised in the supply of machine vision equipment. What adds additional interest is that filters are supplied in very many different sizes (from M13.25 all the way to M105, mounted and unmounted, and even with mounts suitable for installation at the back of objectives with C-mount).

NIR photography

Not only long pass filters are listed in the catalogue, but also several NIR bandpass filters. Even more interesting are double and triple band pass filters. These filters on a broad-spectrum converted camera should tweak the sensitivity of the RGB Bayer-array channels in the sensors to narrow their spectral sensitivity or “move” one of the channels to the NIR. For example for B/G/NIR response the filter TB475/550/850. (The plot bellow is from the published specifications given with 10 nm steps, which results in “odd” shaped peaks.)

TB475-550-850

Also a narrow band-pass interference filter at the “red edge” of leaves’ absorption spectrum is advertised as suitable for diagnosing “plants’ health”.

Rplot01

UV photography

In this case the filters offered are less unusual. Mostly filters with the expected NIR leakage. On the other hand many of the VIS pass and UV and NIR blocking filters in the MIDOPT catalogue could possibly be very good for UV- and VIS-induced imaging of fluorescence in plants.

VIS photography

It is difficult to work-out from the spectra whether any of the UV/NIR cut filters would be good for improving the white balance of full-spectrum converted cameras when used in VIS light.

I have requested price quotes from Stemmer AG for the two filters whose spectra are shown above, plus for a NIR long-pass filter suitable for use with illumination from 940 nm LEDs. [Update 2018-01-23] I have received quotations some days ago, I just mention approximate prices as these may vary. Prices for 52 mm filters are in line with what one would expect for interference filters, in the range of 450 to 300 €.

 

Timelapse in the “dark”

Infrared (940 nm)

The night of a plant, 11 h 20 min condensed into a 10 s-long video. Only source illumination were two LEDs emitting infrared radiation at 940 nm (LED Engin LZ1-10R702). Camera: Olympus E-M1, converted to full spectrum, adapted Soligor 35 mm f:3.5 objective at f:8, Heliopan RG780 long pass filter on lens. Movement is most evident with the video at full screen resolution. Images converted to greyscale.

Ultraviolet-A (385 nm)

The night of a plant, 9 h condensed into a 8 s-long video. Only source illumination were two LEDs emitting ultraviolet-A radiation at 385 nm (LED Engin LZ1-10UA00). Camera: Olympus E-M1, converted to full spectrum, adapted Soligor 35 mm f:3.5 objective at f:8, StraightedgeU bandpass pass filter (UVROptics) on lens. Movement is most evident with the video at full screen resolution. False color, white balance set on white PTFE target.

Ultraviolet-A (365 nm)

The night of a plant, 12 h condensed into a 11 s-long video. Only source illumination were two LEDs emitting ultraviolet-A radiation at 365 nm (LED Engin LZ1-10U00), except during the last 90 min with skylight entered through a nearby window. Camera: Olympus E-M1, converted to full spectrum, adapted Soligor 35 mm f:3.5 objective at f:8, Baader U bandpass pass filter on lens. Movement is most evident with the video at full screen resolution. False color, white balance set on white PTFE target.

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Plant at the end of third session. Window light, Firecrest UVIR filter.

IR images edited in Capture One, UV images edited in PhotoNinja, videos created in ImageJ (Fiji distribution).

I will describe the rig I used in a separate post. The photograph below gives a preview.

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Camera “rig” as used for the three videos. Only the LEDs and filter were swapped between sessions and the objective re-focused at the start of each session. Photographs were taken on three consecutive nights, and the plant returned to a well lighted place during each daytime.