Kolari's Answer To The Hot Spot Issue

Kolari Vision are a popular choice for IR conversions on many camera types. Their prices are reasonable and they have many wavelengths on offer. Recently they've added an option to have any of these internal filters coated with a special anti-reflective (AR) coating to reduce hot spots. When I heard about this I had to give one a try because a lot of native lenses for the Sony FE cameras (A7 series) suffer from hot spots and many of them quite badly. Normally there is no fix for the hot spot issue because it's due to the lens, but let's put it to the test to see if it's as good as they advertise...

Anti-Reflective coated external filters are nothing new, so it's been possible to use them on a full spectrum conversion for a while. Filters like the 093 MRC, from B+W (an 830nm IR filter) have this coating. You can spot these because instead of being an opaque (black) colour they reflect a blue/purple. The same is true of the Kolari internal filter (see below).

Full spectrum conversion (left) vs the AR coated 720nm conversion from Kolari (right)

The problem with these external filters is that they don't do anything for hot spots. I have tested this on several different types and there is no discernible difference at all. Because of this I was a little dubious as to the effectiveness using a similar trick on the internal filter replacement. Fortunately that concern was very short lived because this internal coating does indeed work. It doesn't completely remove the effect, but it does take it down by 1-2 stops in intensity. What I mean here is that; Let's say a hot spot normally starts to show up on a lens (using a normal internal conversion) at f/8, it will start closer to f/16 on the coated conversion, and that can make all the difference.

Results

Here are some samples, comparing a few different AF lens options for the Sony FE platform. All of these options are known to produce a hot spot to varying degrees. These will be compared between a standard (uncoated) 720nm filter and the Kolari Anti-Reflective Coating conversion. From left to right, these lenses are:

• Sony FE 50mm f/1.8
• Zeiss Batis 25mm f/2
• Sigma 35mm f/1.4 Art (with MC-11 adapter)

Note: In all the below samples Kolari's coated version will be on the right.

Sony FE 50mm f/1.8

▲Uncoated Filter▲ -  f/1.8 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/4 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/8- ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/22 - ▲Kolari AR Coating▲

The IR hot spot on the FE 50mm is not bad at all. It's not as clean as theSony/Zeiss 55mm f/1.8, but then not many lenses are. What little hot spot there is here on the smaller apertures is almost non-existent on Kolari's coated filter. Let's see how it does with a worse performer...

Simga 35mm f/1.4 Art

▲Uncoated Filter▲ -  f/1.4 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/2 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/4 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/8 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/16 - ▲Kolari AR Coating▲

The difference here, between the standard and coated filter is much more noticeable. The difference at f/8 being the stand-out one for me. This lens really starts to get annoying at f/8 on a normal filter, whereas the coated version is considerably better.

Zeiss Batis 25mm f/2

▲Uncoated Filter▲ -  f/2 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/4 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/8 - ▲Kolari AR Coating▲

▲Uncoated Filter▲ -  f/22 - ▲Kolari AR Coating▲

Some lenses provide terrible results when stopped down and this is frustrating true of native wide Sony Lenses. Although the coatings clearly help, it won't be enough to save the day in these extreme cases. It'a worth noting that 720nm is also pretty forgiving. The hot spot will be intensified with the stronger wavelengths like 830nm or higher.

At 720nm the Batis lens is generally acceptable at f/4, when using Kolari's coated conversion. With a standard 720nm filter the chances of getting usable results on the same lens and settings are heavily against you, producing a large amount of failures. This is where the coated option shows it's true value in my opinion. Common middle apertures see improvements that often make the difference between a good image and a bad one.

Vignetting

You may have noticed that there is significantly more vignetting on the normal filter. In some small part this may be due to the improved hot spot, but there is another big reason why this is happening. As well as the AR coating this conversion is also using a special 'thin' filter. This has the effect of greatly reducing vignetting on the Sony A7 due to the normal filter cluster having very thick glass on the filter. You can read more about that here. This service can be applied to normal Sony A7 series cameras as well as full spectrum or IR conversions. I'm so impressed by this that I want to have it done to all of my A7 cameras and I will if I can.

Summary

I don't really need to write much here as the pictures speak for themselves. Kolari's 'anti reflective' coating is doing a great job in my opinion. It's not going to fix any lenses hot spots entierly, but it will help and can make the difference of certain apertures being acceptable or not. I wouldn't necessarily suggest upgrading your current conversion for this, but that's totally up to you how much you're affected by this problem and that will depend on what lenses you're using. However, if you're thinking about getting a camera converted then I highly recommend going for this.

Future Tests

I will continue to post results comparing lenses on these two cameras. next up will be the Sony/Zeiss 35/2.8, this was another bad performer and could become not too bad with the Kolari camera. I want to do more testing on the Sony/Zeiss 55/1.8 because although that lens doesn't suffer from hot spot it did suffer from some issues with external filter, which I assume would be gone here. I'm also curious whether the thin filter on the Kolari conversion would adversely affect corner sharpness, so all those tests will be coming soon...