How do rear gelatine filters compare optically with "regular" front-mounted filters? Where to buy them? How to cut them?

Question

I have a few lenses with with a rear filter holder so I would be curious to know how the rear "gel" filters compare with regular front-mounted filters in terms of optical performance.

Until recently, I had no idea about the variety of rear-filters available. I understand that it is not as easy to work with these filters and perhaps this is why they are a dying breed? (The Canon 17-40mm has a slot on the rear while the Canon 16-35mm II does not.) Not being able to easily mount filters on some super-wide and fish-eye lenses however (esp those with a large rounded front element) have prevented me from buying these lenses in the past. But perhaps I was wrong in assuming that a rear 10 stop ND filter was not a possibility without a $400 + $140 LEE solution.

I've read a few discussions in random forums (examples here and here) where some suggest that there are benefits (less vignetting) and others go as far as to say that rear mounted filters are superior to front mounted. Unfortunately none of these statements are supported by facts and hard evidence and there does not seem to be much information about rear filters in general.

It also appears to be difficult to find and buy these filters. B&H and Adorama offer mostly colour filters as well as some ND but I can't find a place to buy a 10-stop ND gel filter. I am most interested in the KODAK WRATTEN 2 Filters - filter number 3 and 4. (4 should be darker then the Lee big stopper) A link to a retailer (US, Canada) would be much appreciated.

I would also appreciate if someone could share how to handle, cut, and clean these filters as well as some other tips and tricks.

Answer 1

Gelatin filter were and are for the most part made of Gelatin and as such are very fragile and next to imposible to clean because their so soft and absorbent, a reason why some are glass mounted. Gelatin filters will fade with time and should be replaced occasionally and stored in their original packaging. Accuracy in colored glass is very expensive and difficult to control therefore it is one of the primary reasons for the gelatin filter. Filter placement was common knowledge among view camera users years ago. Internal filtering is the best placement, rear is second and frontal is third choice. This is for optical reasons but most viewed it as economical. Some lenses specially wide angels have glass and mounts that protrudes so far back within the camera that adding anything back there may interfere with the camera operation, this includes a thin glass mount filter or even the 2mm threaded bit for the filter itself. (one of the reasons some SLRs had a mirror lock up features) This has not stoped people from trying to cut and tape the thin gelatine filters back there and depending upon how you do it it should work. Placing the filter within the lens while better is more dificult and may present cleanliness issues if one cannot get to the glass that the filter slot exposed to the enviroment. Some older view lenses were made to come apart for filter insertion as some sheet film emulisions varied by batch and needed a constant filter pack for accurate color recording.

There is lots more to be said about gelatin but this is enough to get you started.

to confirm placement priority talk to a physics teacher or an optical engineer, some optomitrists opthomoligsts or opticians may also be able to tell you exactly why. You can also try Edmund Optics (http://www.edmundoptics.com/) and talk to an engineer for advice.

Answer 2

B&H sells the Wratten 2 filters ND #96 3 and ND #96 4 in 75mm X 75mm squares. They carry the other sizes listed on the Kodak chart as well. Since searching the B&H site doesn't always find them easily, I searched for them by googling "kodak xxxxxxx" where xxxxxxx is the number listed on the Kodak chart. You can also order them directly from Kodak using the Kodak Motion Picture Products Price Catalog.

Since B&H lists microfiber cleaning cloths as an accessory, I would assume they are adequate for cleaning the Wratten 2 filters. Before using a microfiber cloth, I would first try to use an air blower to dislodge any dust. I would recommend you handle them as little as possible and avoid touching them with your bare fingers so that you don't get skin oil/fingerprints on them. Tweezers or non-powdered latex gloves are two ways you could accomplish this. As to trimming, something like an x-acto knife should do the job cleanly and effectively.

From a comment by Stan:

Most all sources recommend sandwiching a gelatine filter between a couple of sheets of lens tissue and using a pair of scissors to trim to size/shape. A soft lens brush can be used to dislodge some dust that a "squeeze-bulb dust blower" leaves behind. Gelatine filters cannot be cleaned, per se. I have never tried microfibre cleaning cloths for delicate optics such as first-surface mirrors and gelatine filters; but, would not recommend them.