About a year ago I wrote an article about the Astronomik clip-in filters that I used to shoot infrared and colour photography on my full spectrum Sony A7. ultimately I wasn't able to recommend them due to the odd wavelengths and reflection/ghosting issues, but I've been curious to try other brands. Internal filter offer some clear advantages over external ones, so I decided to try out the 'STC clip' filters from Cyclops Optics next. I purchased one 850nm infrared filter to see how I got on with it.
These filters come in: Sony E, Nikon F, Canon EF, Pentax K and Fuji X mounts. Plus the first four come in APS-C and full frame versions, covering a large portion of the camera market. They also come in all the wavelengths commonly associated with internal IR conversions - 590, 720 & 850nm. By comparison the Astronomik options are 642, 742 & 807nm (more for astronomy, apparently).
Internal filters are always going to be fiddly to put in and take out. These are slightly harder to deal with than the Astronimik filters since there are no supplied tools. Also unlike the Astronomik filters - there's no foam backing to protect the cameras sensor and assist with fit. All the corners of the STC filters are sharp metal and this is made potentially worse by the filter not actually "Clipping" in to position out of the box with the one I had. It rested in the sensor well, falling out if no lens was attached.
After asking Cyclops for advise on this for over a month they eventually got back to me and sent a link to a video that shows how the top tabs (the two either side of the center one) can be adjusted. This worked and made the filter finally "Clip" into place. It's a shame that this was not mentioned on any of the supplied paperwork or the website (at the time). Now that this has been made clear to me I can highly recommend the STC clip filter.
The shape of this filter is slightly bigger and more rectangular than the Astronomik. This makes it very slightly better in the corner with longer focal length lenses. However it starts to vignetting around the same point (just above 50mm). 85mm starts to be effected in the very corners, albeit very slightly. 200mm is where it starts to be more obvious.
Taken with the Cyclops Optics STC filter using a 600mm lens
Taken with the Astronomik filter using a 600mm lens
Another problem you'll notice above is the light leaking from the STC Clip, which is related to its poor seal / fitting before I figured out that you could customize the fit. This didn't noticeably affect mid to wide-angle focal lengths, as you'll see below. The thing that most disappoints me about the STC is the little clamps on the sides. They cut into the image more than anything else, which seems unnecessary since they hold the frame together, not the filter itself.
I have been extremely impressed by the image quality from the Cyclops STC. Here are some samples using various focal length prime lenses:
Konica Hexanon AR 21mm f/2.8
Konica Hexanon AR 24mm f/2.8 II
Konica Hexanon AR 28mm f/3.5
Konica Hexanon AR 35mm f/2.8 II
Konica Hexanon AR 135mm f/2.5
All of these lenses are from the Konica Hexanon AR range due to them having very low amounts of infrared hotspot. Compared to external filters the images from the wider lenses have noticeably less vignetting due to the angle of light coming into the lens.
This is another image from the Konica 135mm f/2.5 lens. If you look to the top corners of the uncropped image you will just see the frames of the filter starting to come in at the sides. Hopefully you can see from the cropped image how good the sharpness can be from this lens.
Bokeh & Flare
The below image shows an 85mm lens vignetting more than usual on the left (f/1.2 in this case). The image on the right shows a much more severe issue when lens flare casts a shadow from the filter edges on to the sensor. These were taken for a panorama and it's worth noting that in this case these issues did not destroy the end result:
Despite the Konica lenses being great for IR because they share the same filter thread size, using these internal filters still help out here. If you use external filters then the lens hoods would need to go on over the top of them and that causes vignetting on the wider lenses. Removing the need for external filters fixes that issue, but here are the other benefit of internal filters:
No need to swap the filter when switching lenses
Only one filter of each type is needed
These filters can be used with super-wides or fish-eye lenses that can't attach external filters
Removes ghosting issues from harsh lighting reacting from flat glass surfaces
Protects the sensor when switching lenses
This is another big win for the Cyclops filters is internal reflections. I never once saw the below issue with the images taken on the STC filters. This happened between 5-10% of the time when shooting high contrast scenes on the Astronomik filters. I really tried to make it happen with the STC filters but could not.
The reflection issue from the Astronomik filter
The available mounts and wavelengths of the STC Clip filters are great for infrared photographers. The lack of ghosting / reflection issues is very much appreciated and the vignetting is better than the Astronomik also. Image quality from the STC Clip is very good (tested to 21mm) and now that I've fixed the fit issue I can highly recommend this filter! For full spectrum users who either want to avoid external filters or can't use them (super wide lenses) this is a great option!