One question which I have been pondering for some time is: do I need to have a digital camera converted to full-spectrum for UVA photography? and are there any modern objectives that are good accidental UVA-objectives?
This is not a question of cost alone. Although a converted camera can be used for VIS photography, obtaining good colour reproduction requires effort. A suitable filter is used on the objective to replace the one removed from the image sensor unit during conversion. As it is not possible to find a perfect match to the filter removed, one or more colour profiles of the camera need to be created and applied instead of the one used automatically by the camera and/or raw file converters. So, in many cases, for best results one would need to carry two different cameras to any field trip. In addition a conversion voids the camera manufacturer’s warranty and even access to official service facilities. Continue reading Digital UVA-photography with M43 equipment
[updated 2019-02-13] [I will update this post again after testing the sensor]
Rather recently Vishay announced a miniature sensor under the name VEML6075 with two channels nominally centred at 365 and 330 nm. The peak width at half maximum is 20 nm. So, in practice it is a sensor measuring two regions within the UV-A band with the tail of one of the two channels extending into the UV-B. It is not a sensor capable of separately measuring the UV-B and UV-A bands. However, under sunlight it collects enough information to obtain a reasonable estimate for the UV Index (see the application note from Vishay for deatils).
It is not just a sensor but instead a sensor module with a digital interface. It has all the electronics for temperature compensation and for converting the analogue signals from the sensor into digital data with a rough calibration applied. The package of this sensor is 1 mm thick and 2 mm times 1.25 mm in area. The sensor itself is much smaller and it follows reasonably well the cosine law without any diffuser. Price? Less than 2€ as a component… and between 4 and 7 € for a breakout board.
A breakout board is a small printed circuit board usually containing a single, or very few components. The components included are only those needed for a single complex integrated circuit or sensor module to function, and given the small size of the components, the board allows easier soldering by hand of wires. I have bought two different breakout boards with the same VEML6075 sensor. They differ in size, the smaller boards has components on both sides, while the larger one only on one side. (Drag the slider to see the bottom of the boards.)
The images above cover an area of 23 mm × 17 mm. I took a pair of photographs at higher magnification, and as it fits a UV sensor, I photographed it both in visible light and in UV-A radiation. The whole image is 3 mm tall by 4 mm wide.
Technical information about the photographs
All photographs were taken with an Olympus E-M1 digital mirrorless camera, tethered to a laptop computer and controlled using Olympus Capture 2 software. A camera converted to full spectrum was used.
For the images at lower magnification I used a modern M.Zuiko 60 mm f:2.8 Macro objective, a Sunwayfoto FL-96 LED light source. I took focus-bracketed stacks of between 15 and 35 images, depending on the depth of the electronic components on each side of the boards. I merged the stacks of raw images using Helicon Focus 7 and edited and converted the images to compressed JPEG format with Capture One 12.
For the higher magnification photographs I used a Zuiko 38mm f:3.5 macro objective (Olympus OM-System ca. 1972-1975+, single coated early version). The visible source was the same, and the UV-A light source was a Convoy 2+ 365nm UV-A flashlight filtered with a visible blocking filter. For the UV-A photographs a used on the objective a zwb1 2mm thick filter.
All the images are slightly cropped from the full frame, most to better align them for the slider. The photograph below shows the nearly 50-years-old objective. It is very small and its mount is the same as used for microscope objectives.
The “Ultimate Lens Hood” seems like a good tool. It is still to be seen if it is stiff enough and/or a bit sticky so as to easily stay in place on the glass surface. It has the potential for being very useful but how easy it will be to handle with different lenses is still to be seen.
I just made my pledge for one ULH at Kickstarter. If you want to get your own, be aware that the campaign is about to end.
See my earlier post to learn how I have been managing until now with normal collapsible lens hoods made of rubber. I recently uploaded a gallery of photos from an ongoing project where I am taking photographs through train windows.
Aputure’s Amaran AL-M9 and Sunway Foto’s FL-96 are very small and handy LED light sources. The Amaran AL-M9 has been relegated to a second place in Aputure’s catalogue by the Amaran AL-MX, but I haven’t bought this newer and three times as expensive version. Both light sources compared here are roughly within the same price range.
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.
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.
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.
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).