IR with the Fuji IS Pro and lenses that don’t accept filters pt. 2

Wratten gelatine filters

Since then, I did buy some Wratten gelatine filters on eBay, but unfortunately they were all mouldy. I bought another lot of Wratten gelatine filters, and these seemed to be okay, except for a piece of dust on one that seemed to have become integrated with the gelatine.

The second lot of filters I bought didn't contain any red filters though, I bought it because it contained a couple of ND filters.

Reading the instructions for the gelatine filters (and also reading user experiences on the web), you need to handle them very carefully. The oil from your skin can easily leave a smear on them, and they are difficult (maybe impossible?) to clean.

Looking on the web, it seems nowhere sells gelatine filters except old ones available on eBay. Eventually, I found that Lee produce a set of polyester filters, which are meant to be similar to gelatine filters. (In that they are thin and you can cut them to size).

Lee polyester filter and making a filter mount

So I purchased a Lee Infrared polyester filter. Really I wanted the Lee Tricolour Red Filter, but I thought it would be better to get the true infrared filter first so I could compare the optical quality against my Hitech CR-39 resin IR filter.

I bought the filter from Warehouse Express, it was about £20 including postage. Cheaper than a Wratten 87 IR filter for £40 off ebay.

To mount the filter on the lens, I cut a circle of cardboard out to act as a mount. At first I used white card, then I realised black would be better to minimise any reflections.

With that done, I trimmed the circle down a couple of times until it fitted nicely on the rear of the lens. I left a tab area untrimmed to give an area to grab the filter from for removing from the lens. Then I cut out the centre of the circle, making sure the hole was big enough.

Next I placed the Lee IR polyester filter between two sheets of paper. I placed the card mount I'd made on top of the paper, then cut around it (not particularly carefully). I used double sided sticky tape to attach the polyester filter to the card mount, and then trimmed the polyester filter to fit the mount better.

I am still using double sided sticky tape on the lens to attach the filter to the lens rear. It seems to be working okay so far.

I'm not sure if there is any actual advantage of making a card mount like this, instead of just cutting a piece of polyester filter and attaching it directly. But it seemed like a good idea at the time.

I also made a little cardboard packet to keep the filter in. This should stop it getting bent, scratched, and dirty. I did notice that the filter seems to attract dust very easily, like most filters.

Filter tests

We've not had very much sun, but on Sunday we did get some periods of sun amidst the clouds, so I managed to take a few test shots. I compared the Lee Infrared Polyester filter (rear mounted), The Hitech Infrared CR-39 resin filter (front mounted on Cokin P filter holder), and the Lee 182 Light Red lighting filter (rear mounted).

All the shots were done on my Tokina 10-17mm/3.5-4.5 fisheye zoom lens, at 17 mm. When I first did some tests, I set the lens to focus between 0.4 m and infinity (nearer the 0.4 m mark than the infinity mark). However, I then found that the images were mostly out of focus.

So I took some more shots later in the day, and this time used 4 focus settings for each shot. The focusing scale on the lens is very short between 0.4 m and infinity, so the focus wouldn't be exactly the same for each test, but around the same.

On the computer I checked the shots for the largest depth of field. For all of them I found the ones where focus was set on the right half of the infinity symbol were best. (This is the same as I found for visible light use of the lens with my 5D2).

I used an aperture of f/7.1 and adjusted the shutter speed to try and get a similarly exposed shot for each filter. This was quite difficult as most of the image just looked white on the LCD. When converting the RAW files I used exposure compensation to try and get the images better matching.