Sony A7 mkIII - Another Full Spectrum Conversion

Converting the original Sony A7 (back in early 2014) convinced me that mirrorless cameras are superior to DSLRs for infrared in nearly every way. With a full spectrum conversion you get a highly-capable system for both colour (visible light) and infrared (as well as UV) in single, much lighter camera. Compared to a DSLR this conversion has many benefits and almost no down-sides.

Fast forward to 2020, I have now had the mk3 Sony A7 model as a full spectrum conversion for a few months. On paper this camera has many improvements over the first generation, but unfortunately for infrared there are now a couple of asterisks against the Sony system compared to others. Here's how it panned out for me...

The full spectrum converted Sony A7III, with Zeiss FE 55/1.8

A stellar performing IR lens & B+W 093 MRC filter

MIRRORLESS ADVANTAGE w/EVF

Firstly let me outline the benefits of a full spectrum mirrorless camera, over a DSLR:

 

  • No focus calibration needed for any lens in any wavelength

  • Viewfinder works with visibly opaque external filters

  • Viewfinder has no colour-cast with external hot mirror (visible light)

  • No Live view penalty for speed or focusing

  • Viewfinder shows the correct exposure

  • Viewfinder shows the wavelength you're shooting

  • Viewfinder can show white balance settings (needed for IR)

  • On-Sensor metering is considerably more accurate

  • Viewfinder can show live histogram & level

  • Access to almost all vintage 35mm SLR lenses (often much cheaper & better for infrared)

  • No light leaking through the viewfinder

SONY A7 mkIII FEATURES

The mk3 Sony adds many features over the original model. Matching many aspects of high-end DSLRs, while adding even more:

  • 15 stops dynamic range

  • 4k video, downscaled from 6k (plus crop 4k)

  • 2.2 times more battery life (710 CIPA rating)

  • Sensor Stabilization (up to 4.5 stops)

  • Stronger body design. More metal & weather sealing

  • Dual memory card slots (for backup or various file types)

  • Quieter mechanical shutter + Silent Shutter

  • 10fps with both mechanical and electronic shutter

  • Considerably faster auto-focus + Eye AF

  • Uncompressed RAW option

  • Better EVF optics (although not high resolution like the A9 / A7rIII / A7rIV

  • Much larger buffer (more than double)

  • Lower noise / dual gain

  • Touch Screen (for focusing)

  • Better video button layouts

  • Joystick button for easier focus point selection

  • More comfortable shutter position

SONY A7III INFRARED DISADVANTAGE

Two issues for infrared photography have emerged for modern cameras and unfortunately the Sony A7III suffers from both of them:

 

  • Internal diagnostics LED lamp fogs long exposures in any wavelength of a full spectrum conversion

  • PDAF "striping" issue occurs much more often in infrared, is worse and unfixable in post

Although I managed to find someone to fix the first issue (Alan Burch), I didn't realize how much of an issue the second one would be until I started to use the camera. Compared to visible light the striping issue is much worse with infrared. It happens more often and cannot be fixed in post (see below):

FILTERS

Here's a video example of the A7 mkIII showing a few of my favourite filter options (using the Olympus OM 100mm f/2.8 lens). The quick swapping of filters was done using the Mafrotto Zune magnetic filter system. Also note the changes in white balance:

The Red filter can be picked up for very little money as it's quite easy to find them second hand. The others, although normally much more expensive, will be worth their weight in gold for a full spectrum camera. So if you just spent all your money converting a camera I suggest considering your filters carefully. See this list for more advice on the economy of filter options.

 

ADAPTING LENSES

I intended to use non-Sony lenses with my original A7 from the start as a stop-gap, but only until I could afford Sony lenses. I didn't expect that it would become one of the best features of this camera (system) for me and that's still true of the mkIII. The image stabilization, stronger mount high dynamic range sensor and silent shutter of the A7 mkIII makes using older metal lenses even more compelling. Also the much improved autofocus speed of the mkIII (as well as features like: Eye-AF) make using Canon & Sigma lenses (on the Metabones and MC-11 adapters) almost as good as a Canon DSLR. 

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For those purely interested IR performance I should mention that the Canon FD 135mm (left) is very good for infrared and the Sigma Art 135mm (right) is very bad. The reason for this difference is likely due to the age of the lens. Older lenses tend to have less advanced coating that play nicer with infrared light transmission. Longer focal lengths are also "usually" better, but these are trends and can never be relied on. For more info check out my infrared hotspot lens list.

 

HIGH PERFORMANCE INFRARED LENSES

Over the past two years I have discovered a few lenses that have proven to provide very good results with high contrast scenes. Here are a few lens, of various types, that have proven to be very good for infrared light and avoiding light contamination from the frame.

These lenses are, clockwise from left:

  • Carl Zeiss FE 55mm f/1.8 ZA - This auto focus lens (for the Sony A7/A9 series) is the best quality lens for IR I have found for the mount so far. Since it's a generally very good lens for sharpness and bokeh it makes the rather high price actually very reasonable if you have an infrared or full spectrum Sony FE camera.

  • Carl Zeiss Planar 85mm f/1.4 ZF-IR - This Nikon F mount lens was one of the three Zeiss lenses made with special coatings for infrared transmission. This minimises infrared reflectivity issues inside the frame to a negligable level, the lowest of all three here. Unfortunately these lenses were expensive when they were available (2008-2015) and are now extremely rare. The version shown here is a prototype of that version (hence the low serial number), so is even more rare.

  • Carl Zeiss Jena S0-3.3 75mm f/3.5 - This is one of four lenses for the German Stasi, made to spy through small holes. These were corrected for infrared light and used special coatings for infrared transmission.

NOTE: The last two of these lenses are specially made with infrared in mind. The other is a bit of a fluke however, just don't expect any Zeiss lens to be this good with infrared light, it's usually not the case.

LENSES - INFRARED ISSUES / HOTSPOTS

Many lenses have issues with transmitting infrared light. This can show up as a concentration of light in the middle of the frame (traditional hotspot), or brighter parts of your scene mirrored across the frame. All of these issues are reflections caused by the specific lens you're using and its coatings. I have tested a bunch of lenses for this, which you can read more about here. Here's a quick list of Sony AF lenses and how they cope with IR light (scored out of 10):

 

FINAL WORDS

As good as the A7 mkIII is, as a camera, I feel like the stripping issue makes it difficult to recommend as a IR or full spectrum conversion. If you do any long exposures as well then I highly recommend looking for someone who can fix the internal LED light bleed issue (which Life-Pixel and Kolari cannot). If you'd really like to convert a Sony system (perhaps because of the amazing Zeiss FE 55/1.8 lens) then I recommend either going for one of the first generation models or the 4th.

 

Alan Burch (who converted my camera) recently converted his first A7 mkIV and tells me that it does not suffer from either the striping or the IR light bleed issue. Obviously that's a much more expensive camera, so if you're looking for something a bit more affordable - keep your fingers crossed for the cheaper A7 mkIV to come out soon and be just as good...

Edward Noble

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