Although I have two infrared-converted Nikon DSLRs, I sometimes am out and about only with my iPhone and see a shot that would look good in infrared black-and-white. There are numerous reports phone cameras of phone cameras being used with a 720 nm cut-off filter held in front of the lens. The best guide I have found is that by Rick Shea, described here and here. However, he was unable to use the ultra wide lens/camera on an iPhone. That’s because there are problems with the design of any attachment used to hold the infrared filter.
During the winter I was determined to see if I could get usable infrared photographs from the ultra wide lens (equivalent to a focal length of 13 mm in a full-frame 35 mm camera), since it offers pictorial possibilities many people do not have in their range of lenses for their DSLR or mirrorless IR-converted camera.
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| iPhone 11 Pro with 720 nm filter. Wide (26mm equivalent) Lens/Camera |
Having produced attachments that work, I would not want the reader to think that a iPhone is in any way a substitute for an IR-converted camera. There are problems that are inherent in the way the cameras operate, in using the standard Camera app, in the size of the sensor and with lens hotspots:
- Using the Apple Camera app, it would be logical to think that the standard settings of 0.5x, 1x and 2x would engage each of the three lenses (respectively 13, 26 and 52 mm focal length in 35 mm equivalents. By holding a finger over the lenses to see which one is producing the image, 1x and 2x magnifications are both produced by the 26 mm lens. Only above x2 (and then not consistently at higher digital magnifications) did the 52 mm lens come into play. I have therefore abandoned the Apple’s standard Camera app for infrared. Instead I use the camera function of Adobe Lightroom. There, selecting UW (13 mm), W (26 mm) or T (52 mm) does actually engage the respective cameras. I will use UW, W and T to denote the three cameras below. Incidentally, I have the file format set to DNG. However when on UW, DNG is not available and the format reverts to jpg.
- There is some sort of algorithm operating in the iPhone 11 Pro that interlinks the three separate cameras. If one camera is obstructed there can be times when the UW will not focus or get the correct exposure.
- The presence of a 720 nm filter needs an increase in exposure of approximately 12 stops or E.V.s. With the fixed wide aperture of the three lenses (f/2.4, f/1.8 and f/2.0) and optical image stabilisation for W and T, the phone can be hand held for subjects in sunlight. However there is no stabilisation with UW and some form of support may be necessary depending on the exposure and how steady you are in holding a phone camera.
- All the lenses have an infrared hotspot. Ways of dealing with them by making presets for Adobe Lightroom or Photoshop are described by Rick Shea.
- The phone has a tiny sensor. That couple with the 12-stop increase in exposure will tend to produce a noisy image. I have define Topaz DeNoise AI excellent in dealing with infrared images from the iPhone.
- Forget any notion of obtaining faux-colour infrared images. Yes, it can be done but the colour range that can be achieved is not worth the effort.
Filter Holder Requirements
In view of the above I set out to cover all three lenses with a 720 nm IR filter. My first attempt using the clip-on filter mounts that hold a 37 mm filter was completely unsuccessful. I realised there were two problems to overcome, that had not been envisaged when these clip-on plastic mounts were produced for a single phone lens. Although I removed all of the thin plastic plate from the inside of the filter ring, I could not find a position for the clip which allowed all three cameras to be covered by a filter at the same time. There was vignetting from the filter ring either in the UW and/or the W image according to where the clip was positioned. In short I could not find a position where a 37 mm filter could ever work with all the lenses of the iPhone 11 Pro. This problem is, of course, caused by the extremely wide fields of view of the UW and W lenses arranged as they are on the rectangular camera plate of the phone. With diagonal fields of view of 118° and 80° it is only too easy for the filter mount to come into the side or corner of the frame.
A second problem was that in moving the clip around light could get enter the space between the filter and the lens and reflect back into the lens, causing characteristic light marks on the image. To that end the light within the phone’s camera cluster must be turned off.
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| The effect of light leaking between the IR filter and the phone |
Therefore, it is clear that any attachment has to be simple enough to be used by any of the three lenses without physical adjustment and has to prevent light entering behind the infrared filter.
With it not being possible to use a 37 mm filter, the next common larger size is 52 mm—and I had a 52 mm 720 nm filter to hand. There was no problem finding a position in which a 52 mm filter could be used without vignetting. However, the phone itself is not wide enough and needs a plate into which the cluster of cameras will fit and of sufficient width to hold a circular filter mount.
Filter Holder Construction #1
My first attempt to make a filter attachment is shown below. It was made from 1.0 mm styrene sheet and bars. The width of the plate is that required for a 52 mm filter holder to be fitted. The supporting bars allow a close fit to the phone and the lengths have to avoid the buttons on the sides. I first made the olate (cutting out the round-cornered rectangle was the most difficult bit). Then I bought a metal 55-52 mm Step-Down Ring. The 55 thread on the outside extended from half-way down the ring while the 52 mm thread inside went the whole way through. Therefore to get a broad flat surface I cut off the part of the ring with the 55 mm outside thread using a cutting disc. When smoothed down, I stuck it to the styrene plate with cyanoacrylate superglue. To reinforce the joint and to make sure light was excluded I ran black Sugru around the edge. Before gluing the ring in place, I checked its intended position in order to make sure there was no vignetting from any of the cameras..
To be used this plate has to be held tightly to the phone in order, again, to prevent light reaching the back of the filter. The large, soft-sprung clip shown in the photograph is the best I have found so far. The spring has to be gentle in strength because one side presses on the glass front of the phone.
After a spray of matt black paint I found this style worked fine. The attachment, filter and clip would fit in the pocket. Anything similar could easily be built using a 3D-printer.
The use of this filter holder is not confined to an IR filter. Standard 52 mm Polarising and neutral density filters could be used for ‘normal’ photography and video.
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| The Step-DOWN Ring used to hold the 52 mm IR Filter |
Filter Holder Construction #2
With the design of the iPhone lens cluster, a circular filter is not ideal. A rectangular one would be easier to deal with. I therefore made a similar plate to #1 for taking a rectangular piece of filter material. That could be narrower. However, finding a piece of rectangular 720 nm filter material is another matter. I was not keen to buy a large circular filter and cut it down on the chance I could do so without breaking the glass sandwich. Industrial quantities were out of the question but I did find somebody on eBay selling what seemed to be 720 nm plastic material of the sort used in electronic infrared controllers. I soon cut a piece down and sealed it to the plastic plate. Unfortunately, while the initial cut off may be around 720 nm, the supplied spectrum showed there was a dip in transmission at some longer wavelengths and the attachment was unusable. That was a pity because for a purely infrared filter attachment of small size the approach seems ideal. If anybody can get hold of material used to make photographic 720 nm IR filters, or has a go in cutting down a large circular filter, then making such a small attachment would be worthwhile. Incidentally, I have found no other cut off filter, other than 720 nm, works with the iPhone. Lower wavelengths let too much ordinary light in; higher wavelengths so little that the camera tries to take a photograph of the inside of the filter. Again though, that design with suitable filter material could be made by 3D-printing.
Filter Holder Construction #3
The last design uses a SmallRig cage (2776) and therefore avoids having to have a clip to hold a filter holder in place. I realised that I could use a filter mount (Kinqwon Flycoo 52 mm Filter Adapter for GoPro Hero 10/Hero 9 Black) since the rectangular lens bulge of the GoPro is similar in size to the lens plate of the iPhone. I found that the height above the plate of the cage caused vignetting. I therefore cut the back of the filter attachment leaving the rectangular opening surrounded by a 52 mm filter mount. That was glued to the rubber covering of the lens plate. However, there were holes where light could enter around the edges. I therefore sealed the whole thing with black Sugru. That arrangement works perfectly most of the time. However, sometimes when the phone is inserted in the cage there can be a very small light leak between the phone and the plate of the cage (to the right of the logo in the photograph below). I insert a piece of 1mm styrene card into that gap. I suppose I could glue card to the inside of the plate but the quick fix works fine.
Like #1, #3 can be used with other 52 mm filters.
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| The 52 mm filter holder designed for a GoPro |
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| Black Sugru used to fill gaps |
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With the changing of models and phone sizes, there is of course no permanent solution to producing a filter mount for iPhones. Already the iPhone 11 Pro is two models behind. For those who do not want to have to make a holder like #1 and #2 every time they change their phone, then the cage option with some sort of filter holder of the type that fits a modern GoPros may be the way to go. There must surely be a commercial opening here for iPhone filter mounts that cover all the cameras and not just attach to the cage to cover one (W) as at present.
Finally, I have been surprised about the quality of the infrared black-and-white photographs from the IPhone, but more on that topic in a later post. Suffice it to say at this stage that the UW (13 mm equivalent) lens/camera produces some very strange images with periperal area of mush - just as if some noise reduction process has gone crazy locally. There creative possibilities since anthing towards the centre of the frame is not affected. All-in-all impressive black-and-white IR photographs emerge from the W and T cameras.
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