[_MB_]

I've recently come into possession of a lovely Sigma 14mm prime. It has a holder of sorts at the rear of the lens for cut gel filters. I've never in my life used anything that didn't screw on the front or drop into a slot so am at a loss as to where to find a supplier. Cokin, Lee etc seem to have nothing along these lines and google has found a few mentions but no online shops so far as I can see.

What kind of price do they fetch? Do they cut easily? I'd assume a cutting mat and a scalpel/boxcutter would do the trick. I'm concerned that a poor cutting technique would result in shavings/dust from the filter getting into the lens or onto the sensor.

Any help would be greatly appreciated

[Stormbringer]

I 'Goggled' Photography Gel Filter and got the following sites amongst others :-

gel filter Photography/Darkroom Equipment - Electronics - BizRate - Compare prices & buy - Discount - Price

Digital Photography Lighting Accessories

Gel filters would seem to be used more in a studio setting than anything else. Somebody here will give you better advise as I'm still a rank noveice at this but it might give you a start.

[Dabhand16]

Try SRB at Luton. I can't give you a link as my work computer can't multitask in the same century, but it ought to come up with Google. Might be SRB-Gittens.

I'm sure they will be able to help.

[Les Meehan]

OK here goes, brief tutorial...

Gel filters come in two sorts, camera quality and lighting gels. For camera use only buy optical quality filters. The most famous camera filters are the Kodak Wratten series which are CC (colour correction) filters. Other makers, Lee, SRB etc., also produce CC camera filters. The Kodak filters are made from high quality optically flat gelatin, whereas some others (Lee) are acetate (which is not quite as good but is more durable and can be washed clean). Gelatin camera filters are VERY fragile and easily damaged either with finger prints or scratches and are very difficult to clean without causing damage.

CC filters were used all of the time by pros needing to neutralise the slight colour casts inherent in different batches of colour film and colour imbalance introduced by processing variability. This was usually done with a test of a new batch of film prior to a job requiring accurate colour (i.e. all jobs). The colour cast would be assessed on a daylight balanced light box and the necessary CC filter chosen for use on the camera. The Wratten filters were either 3"x3" (or a bigger size which I forget) and were placed in a filter holder that was then placed on the front of the lens. CC filters make subtle changes to the colour of an image on film and come in a range of 'densities', e.g. CC5Y, CC10Y, CC20Y for the Yellow colour. Since the filters are in 'density' values they can be combined, e.g. CC5Y + CC10Y = CC15Y. Each filter has a 'filter factor' which is used to compensate the exposure due to loss of transmitted light e.g. CC5Y needs plus one-third of a stop. With TTL metering this is taken care of when you meter with the filter(s) in place.

The Wratten range also includes filters for correcting different light sources and for other things, such as contrast control in BW (i.e. Red, Yellow, Green, etc.). These latter filters are known as 'colour compensating' filters as they are intended to make a dramatic change to the light entering the lens, e.g correcting fluorescent lighting.

Perhaps best known are the ND (Neutral Density) filters which are often used on camera to allow changes to the shutter speed or f/No., usually for creating movement in an image or for long time exposures (e.g. creating an empty street devoid of moving people in daylight.) ND filters are measured in density values, e.g 0.3ND, 0.6ND etc. Each 0.3 value is the same as one-stp of exposure so a 0.6ND blocks two stops of light and requires a x4 increase in exposure. Again, they can be used together for combined effect, e.g. 0.3 + 0.6 = 0.9 (or three stops reduction of light).

NOTE: if using a digital camera you don't need CC filters!

Lighting gels, as they are known, are usually made from heat resistant acetate and come in a wide variety of colours and strengths. These are used to create 'mood' when using lighting in a studio (or location). The commonest are the warm filters used to make us look pretty in a portrait (the same effect is achieved with gold brollies and flash). Widely used in the film industry, they are great for correcting mixed lighting, i.e. daylight from windows and tungsten light inside. By taping the necessary filter over the window glass you can make the daylight the same as tungsten in terms of colour temperature for a particula film type or, in reverse, filter the indoor lights to match the daylight.

ND filters are incredibly useful for studio and location lighting to fine-tune the lighting setup. When you want to quickly balance the intensities/contrast of a lighting setup it is usually easier to add ND filters to the brighter lights rather the move them back or reduce intensity with dimmer switches (with flash you would use power ratio switches).

Filters are what used to separate the snapper from the pro when it came to serious studio or location work. Now, with digital cameras and RAW files this difference has almost vanished for the 35mm format. Obviously, larger format film users still need to understand the use of filters if they are to achieve the best results (on the film). Of course, if the film is to be digitised then, again, the use of filters is less of an issue.

MB, if you have a digital camera forget about filters and do everything with the RAW file (if possible).

Remember, no matter how good the filter it WILL reduce the image quality of your fine lenses!

Hope this helps.

Regards

Les Meehan

[_MB_]

Many thanks Les and others - I should have clarified I'm well aware of how filters work and that the majority of the correction types are not required with the advent of raw processing. It was specifically gel filters I was asking about.

Will look up Wratten.