Sony A7 - Full Spectrum Mirrorless Conversion
The original Sony A7 just turned ten years old and so I want to revisit why this is still such a fantastic option for full spectrum conversion! Back when I bought the A7 I had been using a Nikon D3 for 6 years and wanted something much smaller and lighter, but I also wanted to shoot more infrared photography. Mirrorless offered the possibility of switching spectrums with a simple filter swap and maintaining a usable viewfinder. Not many had tried this by early 2014 (at least not on a full frame mirrorless), so it was a risk back then and it really paid off, but today (2023) it is still an amazing option due to newer Sony's having strong banding in infrared.
A set of good lenses for IR that share the same filter thread
At 474g the A7 was over two and a half times lighter than my previous camera. It wasn't as sturdy, didn't have anywhere near the battery life and the AF performance was nowhere near as good, but the image quality was far better, I could carry it with me all day without hurting my shoulder and evn shoot video if I wanted to. I didn'tdo that much, but I did send it away to be converted to full specrum after only 2 weeks. This invalidated my warranty, but since I got it as a grey import that didn't make much difference...
If you're wondering what a full spectrum converion is... Off-the-shelf digital camera sensor's sensitivity is restricted to roughly 400 - 650nm (nanometers) by a filter that sits in front of the sensor. This is done to improve colour accuracy becase this is what we see (visible light). In a "Full Spectrum" conversion this filter, called a "Hot Mirror" is removed, opeing the camera up to roughly 350-1100nm. This allows for shooing UV, Visible and various Infrared wavelengths by adding filters, either in front of the sensor or the lens.
DSLRs make some sense for straight IR conversions, where external filters aren't needed. However, they have focus offset issues with IR. You can have the focus calibrated, but that is only accurate for one specific lens and focal length, so you can't even do it for one zoom lens. This is one reason why mirrorless are a better option, but there is another even more compelling reason, the viewfinder. Especially for a Full Spectrum conversion a mirrorless camera, the EVF is an absolute game-changer! Here is a list of benefits that an EVF provides (compared to a DSLR):
No focus calibration needed for any lens, or in any wavelength
Viewfinder works with visibly opaque external filters
Viewfinder shows the wavelength you're shooting in / no guessing what you're looking at
Viewfinder can show white balance settings (needed for IR)
Viewfinder has no colourcast with external hot mirror (visible light)
No operation speed or auto-focus penalty in live-view
Viewfinder shows the correct exposure (no chimping required)
On sensor metering is considerably more accurate
Viewfinder can show live histogram, focus peaking, levels, highlights etc.
Viewfinder shows cropped lenses full size
Access to almost all vintage 35mm SLR lenses (often much cheaper & better for infrared)
Viewfinder can punch in (zoom) to focus manually more accurately
No light leaking through the viewfinder
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.
Mirrorless cameras meter via the light hitting the sensor. This means whatever wavelength you're using your metering is more accurate, but since you can see the image you're capturing in real-time anyway, shooting full manual is more reliable and faster than any auto mode anyway.
IR metering can look underexposed 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 and thus can be a very good thing if you process your images from RAW and want the get the best detail in the highlights.
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.
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:
590nm - for colour infra-red photography (red filter)
IR Chrome - a multi-band colour IR filter (by Kolari) that doesn't require a channel swap
830-850nm - for high contrast b&w infra-red photography
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 and there is an alternative to the IR Chrome filter from TrueColorInfrared. 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 only a slight impact on the colour accuracy (although they may need more tweaking in post (auto WB was less reliable on the A7, although it was fine on the RX1 so YMMV). 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, although you might need to be careful with most 'UV/IR Cut' filters since they tend to reflect light rather than absorb it and the difference can be quite noticeable...
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, but that might change over time.
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. I was also a rather poor after buying the camera and getting it converted, so I fell back on adapting vintage SLR lenses. This has not only been an extremely fun photography experience, since there are so many interesting and reasonably priced vintage lenses around, but it has also provided me with access to much higher quality infrared performance.
Most of the good IR performing lenses have been vintage manual lenses. Using older manual lenses has been fun, mostly pretty cheap and provided much cleaner IR results, although there are a few caveats to that. Some people won't like the manual focusing, some won't like the sharpness (especially with wider lenses). This is a much bigger issue when shooting colour IR, since anything below 830nm is dealing with a mix of IR and red, which focus as different distances. Some lenses are better than others here, but none I've found so far have been great.
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). There are some lenses that focus quite adequately using AF adapters, but I find this the least interesting aspect of adapting lenses to the Sony platform.
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. Sony and Zeiss have a bunch, plus there are a ton of 3rd party native AF lenses that offer great quality for the money. However, for infrared most of these lenses are marred by some pretty horrific hotspot issues. There is one big exception to this and that's the Zeiss FE 55mm f/1.8 lens. This is not a cheap lens, but it's nice and sharp with some pleasant OOF rendering. Second hand prices for it are pretty decent now and when you factor in how extremely good it is for infrared you'll realize that tit's worth its weight in gold!!
Lenses are designed to avoid extra light bouncing around the lens and sensor box, but as these methods have improved over the years it has tended to make the issue worse for invisible light like infrared. This means that a lot of newer lenses can look fogged, washed out, low ontrast and in the worst cases bright spots in the middle of the frame. Light doesn't always collect in the middle of the frame however, it depends on the lens and what you're shooting. It's almost always caused by light from inside the frame, not something that's fixed by using a lens hood. So, if you have a bright IR area in the bottom left of the image the issue can occur as a haze in the top right. Although this is more lilke ghosting, it usually just adds to the general mess of all the other contamination.
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, which you can see very clealry in the example below.
Worst & Best InfraRed Hotspot Performance
(all taken at f/8, iso 100)
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. From what I hear this issue seems to be true for most modern mirrorless cameras unfortunately, but I don't have any hard data to back that up.
As far as Sony mirrorless are concerned the original A7 seems to be the best infrared performer. The A7R (which I now own) is a close second. Although it has a better sensor it has horrific shutter shock, which will can be a real pain. If I was to choose a camera system right now (without being invested in lenses) it would probably be the Canon R6 or R5, in hopes that they avoid the PDAF banding issue. 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 remains the best IR lens I've seen. Many of the below images are taken with this lens.
To see more infrared images from the Sony A7 check out my Flickr Album.