Sony A7 - Full Spectrum Mirrorless Conversion
By 2014 I had gotten rather tired of big & heavy DSLRs (Nikon D3). With full frame mirrorless cameras beginning to emerge I decided to sell all of my existing cameras and switch to the Sony A7. I also sold an infrared converted DSLR, but I didn't want to give up on IR, quite the opposite. I wanted my main camera to be able to shoot IR and colour, so I decided to try a full spectrum conversion.
When I look back on this decision it sounds like quite a risky one considering how untested this was, but it has actually been the most interesting and successful photography project that I've made...
Two freestylin filters, the exterior being full spectrum ('F' series Nikkor 24mm f/2.8 pre-AI)
WHAT IS A "FULL SPECTRUM" CONVERSION?
Off-the-shelf digital cameras are restricted to visible light sensitivity [roughly 400 - 650nm (nanometers)] artificially, to improve colour accuracy. This UV and IR wavelength blocking comes from a filter ("hot mirror") that sits in front of the sensor. Converting a camera to "Full Spectrum" involves removing the hot mirror. Opening up the camera to a much wider range of the EM spectrum (~50-1100nm).
MIRRORLESS ADVANTAGE - THE EVF
DSLRs make some sense for straight IR conversions, where external filters aren't needed, but even then mirrorless would be better. For a full spectrum conversion a mirrorless camera is an absolute game-changer! Unless you have no other choice I would not recommend a DSLR for full spectrum conversions any more! Here are the benefits of a mirrorless camera for full spectrum (compared to a DSLR):
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No focus calibration needed for any lens, or in any wavelength
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Viewfinder works with visibly opaque external filters
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Viewfinder shows the wavelength you're shooting in / no guessing what you're looking at
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Viewfinder can show white balance settings (needed for IR)
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Viewfinder has no colourcast with external hot mirror (visible light)
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No operation speed or auto-focus penalty in live-view
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Viewfinder shows the correct exposure (no chimping required)
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On sensor metering is considerably more accurate
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Viewfinder can show live histogram, focus peaking, levels, highlights etc.
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Viewfinder shows cropped lenses full size
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Access to almost all vintage 35mm SLR lenses (often much cheaper & better for infrared)
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Viewfinder can punch in (zoom) to focus manually more accurately
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No light leaking through the viewfinder
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Reviewing images inside the viewfinder in strong sunlight
Although most modern DSLRs have live-view, which gives some of the above features on the rear screen, this is not the same as having an always on, shaded, high resolution viewfinder. Since DSLR focusing works by bouncing off the under side of the mirror, thus no longer functioning in live-view (since the mirror is flipped out of the way). This means that live-view uses a backup focusing system which usually isn't very fast.
IR - AUTO EXPOSURE
Mirrorless cameras meter via the light hitting the sensor. This means whatever wavelength you're using your metering is accurate. even if it wasn't you can see the image you're capturing in real-time so shooting full manual is more reliable and faster than any auto mode anyway.
The exposure value can often looks like underexposed with IR on mirrorless (most notably for mixed wavelengths, like 590nm). However, this is mostly because the red channel is recieving a lot more light and the metering is trying to preserve the highlight detail in every channel so this is a very good thing.
IR - AUTO FOCUS
Mirrorless autofocus is all done on the sensor plane. Just like the metering - contrast detect AF is done by looking at the image (which is always being processed with mirrorless anyway), but this is also true for phase detect system, which is nestled in between the pixel photo-sites. This means that there are almost no focusing issues on the mirrorless system. Front and back focusing / calibration is just not a thing here. More importantly for a full spectrum conversion the AF does not need to be calibrated for different lenses when shooting in different wavelengths. Focus is just as fast and accurate with any infrared wavelength as it is with visible light.
Autofocus is such a game-changer on mirrorless I would never recommend anyone convert a DSLR to full spectrum ever again, even if you shoot manual focus or at f/11 all the time.
IR - FILTERS
With this conversion you'll need filters to shoot anything except Full Spectrum itself (which isn't classically very desirable). You'll also want those filters for every lens you have with a different filter size (I will talk about the ways around this later), so I advise planning this carefully. Don't do what I did here if you want to save money, this was just for science...
The image (above) shows most of the options you have when owning a full spectrum camera. If the variety of filter wavelengths and manufacturers aren't overwhelming enough, the post processing techniques are endless, so it can be hard to know how to approach the art. My time with Full Spectrum so far has taught me that you can get away with most effects by using only three filters. If I had known this from the beginning I could have saved myself a lot of money. These are the most versatile filters from my experience:
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590nm - for colour infra-red photography (red filter)
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IR Chrome - a multi-band colour IR filter (by Kolari) that doesn't require a channel swap
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830-850nm - for high contrast b&w infra-red photography
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Hot Mirror - for normal colour photography (this is not the same as a UV/IR cut filter, see below*)
The Red filter (590nm) can be bought for reasonable prices (budget brand or second hand). The Hot mirror and IR filters have cheaper Chinese alternatives, but to my knowledge there is no budget version of the IR Chrome filter. Reducing the filters you need to just a handful can save a lot of money. This takes careful consideration based on the lenses you intend to use, so I made a separate page for that HERE.
THE 'HOT MIRROR'
One of the best things about a full spectrum mirrorless camera is that it can be used for visible light photography with very little difference to normal camera. This means you only need a single camera to shoot any wavelength and there are very few limitations or down sides. All you will need is a hot mirror.
My biggest mistake with Full Spectrum was assuming that a 'UV/IR Cut' filter would return the colours back to factory default. Many poor purchases and several months later I finally realised that what I really needed was a proper 'hot mirror' filter. These work by absorbing invisible light, rather than reflecting it.
This made a huge difference for me when trying to achieve realistic colours. Some people say that their camera works fine with a UV/IR blocker, but my guess is they likely did an auto WB and got close enough (with the values heavily skewed). If this is true then it's bad for a few reasons. It ruins your dynamic range, reds/greens are still not shown accurately and the colours will differ closer to the edge of the frame. It's worth noting that an external hot mirror will likely not quite be perfect either, but it will get you 95-99% there, rather than about 60% with a UV/IR cut.
comparing a UV/IR Cut & Hot Mirror filter to emulate visible light
The above hot mirror was bought from Kolari Vision. I suspect this is a rebranded Schott BG38 filter, but I can't confirm that. Neither of which are cheap anyway. I have recently discovered the Chinese QB21 filter, which provides indistinguishable results for a fraction of the cost. The only down side to that is that I could not find them in every size that I wanted.
IR CHROME - FILTER
This is probably the most interesting development for full spectrum photography. Traditionally to get results similar to the classic Kodak Aerochrome IR film you would use something like a red filter (590nm), swap the red & blue channels (if you can get your WB low enough) and then hue shift the foliage towards red. This is a bit of a black art and even with an EVF you won't be able to see the correct colours in the viewfinder.
With the IR Chrome filter (Kolari Vision) you can now get pretty close to the Aerochrome look without a channel swap. This multi band pass filter can you show you orange/red foliage live (in an EVF) and you barely need to change the WB values.
As an added bonus - this filter dramatically reduces any hotspot issues you might have had (90%), has no focus offset and can even show you a wider variety of other colours in the scene (anything that doesn't reflect infrared light). To see more about this filter you can see my full review here.
ADAPTING - MANUAL LENSES
There weren't many lens options (especially reasonably priced ones) when I first got into the Sony A7 platform, so I fell back on adapting vintage SLR lenses. This has been largely a very positive experience. All of the good IR performing lenses (accept one) have not been native FE autofocus lenses. Using older manual lenses has been fun, mostly pretty cheap and provided clean IR images, but there are some caveats to that. Some people won't like the manual focusing, but for me the bigger issues is sharpness in the corner of wider lenses (anything wider than about 40mm). This is a much bigger issue when shooting colour IR, since anything above 830nm is only dealing with IR (rather than red as well), which focus as different distances. Some lenses are better than others here, but none I've found so far have been good.
The manual focus Canon FD 85mm f/1.2 L lens, adapted to the Sony A7
ADAPTING - AUTO SLR LENSES
The first generation A7 didn't initially support phase detect (PD) AF on the few AF adapters that were available back then. It was added in a firmware upgrade later on, but it was still pretty slow. From the A7RII onwards, autofocus speed and ability significantly and consistently improved (this benefit also helped 3rd party lenses). AF adapters are now available for Canon EF, Nikon AF-S/G, Contax G and even Leica lenses (yes, manual lenses).
NATIVE FE-MOUNT LENSES
Several years have now passed since I started using native full frame E-mount autofocus lenses and I have been able to test quite a few. There are some really great options for fast, sharp and light weight lenses, but image quality for infrared is often marred by some pretty horrific issues. There is one exception to this and that's the Zeiss FE 55mm f/1.8 lens. This is not a cheap lens but when you realise it's also extremely good for infrared you could even see it as a bargain.
HOT SPOTS
IR "hot spots" are a bright circle that collects in the middle of the image, but I use the term to describe any unwanted light bouncing around the frame. It ruins contrast and is usually impossible to properly fix. This issue occurs when the lens internals reflect (rather than absorb) IR, or the glass elements reflect (rather than transmit) IR light. The issue is present in almost all lenses to some degree, is worse (smaller and more intense at) at smaller apertures and can vary at different focal lengths with zoom lenses.
Colour infrared (mostly lower than 750nm) usually has slightly less intense hot spots (because it's mixed with more visible light which doesn't have the issue). This can be more noticeable with colour IR since the problem is concentrated in the blue channel, damaging the colour balance and the contrast.
This is not the sun, or sun flare (or the moon). It is not helped by using a lens hood. What you're seeing is the brightness of the objects (in the frame) reflecting across the lens and collecting in the middle. This is often worse on modern, fast or wide lenses, but there is no hard rule for telling how I lens will cope.
There are some online lists that try to help with this, like Kolari and Life-Piixel, but they are full of errors so I wouldn't recommend putting any stock in their results. To have a large database they let anyone report. Most people don't know how to properly test for this issue, so many times I have bought a lens because someone says it's fine and it ahs been terrible. For example, the above lens is listed as "good"!!! This is why I started my own tests, which you can see here - HOTSPOT LENS DATABASE.
MIRRORLESS SENSOR ISSUES
I was quite lucky with the first generation Sony A7 because later ones started to cause banding from the more advanced PDAF sensors. From what I can tell the mkII Sony A7's suffer a little, the mkIII's are the worst and the mkIV's are slightly improved. This issue carries over to the Nikon mirrorless line, since the Z6 and Z7 inherit the Sony mkIII's sensor. This issue does not affect Canon or Panasonic's mirrorless range of cameras.
FINAL WORDS
As far as Sony's are concerned the original A7 seems to be the best. The A7R (which I now own) is a close second as although it has a better sensor it does suffer from shutter shock, which will make some of your images blurry. If I was to choose a camera system right now (without being invested in lenses) it would probably be the Canon R6 or maybe the R5. however I would be concerned that they probably don't have a lens that can compete with the stunning Sony/Zeiss FE 55mm f/1.8, which I think it THE best IR lens you can get for any system. Many of the below images are taken with this lens.
To see more infrared images from the Sony A7 check out my Flickr Album.
