Basics of RA-4 Colour Printing with the AGO Film Processor

RA-4 is the process for making colour prints from colour negatives in a darkroom. With a colour enlarger, a sheet of RA-4 paper, and the AGO Film Processor, you can develop your own colour prints at home without a professional lab. This guide covers everything you need for your first successful print.

A big thank-you to Mario Mazzotta from Colla Lab, Italy, for sharing his deep knowledge of RA-4 printing. His experience has helped shape this guide.

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How the Process Works

RA-4 printing involves two stages: enlarging and developing. First, you project your colour negative onto a sheet of RA-4 paper using a colour enlarger and expose it for a set amount of time. Then you develop the exposed paper in chemistry to reveal the print.

The chemical process uses just two baths: a Color Developer and a Blix (a combined bleach and fixer), followed by a water rinse. After that, the print is fully processed and ready to dry.

The AGO Film Processor handles the developing stage. You load the exposed paper onto a REEL Paper 5 reel, place it into a Paterson tank, attach the AGO, and run the process in daylight. There is no need to stay in the dark after the paper is loaded and the tank is sealed.

Once you see your first result, you adjust exposure time, lens aperture, or colour filtration on the enlarger and repeat until you are happy with the print. Expect to make several test prints before everything comes together. That is part of the process.

What You'll Need

Practical Theory

Exposure and Making a Test Strip

Getting the exposure right is the foundation of a good print. Rather than guessing and exposing a full sheet, always start with a test strip. A test strip is a single sheet of paper divided into bands, each exposed for a different amount of time. This shows you the full range from underexposed to overexposed in one go, so you can choose the best time for your final print.

To make a test strip, cover part of the paper with an opaque card and expose the first section. Then move the card to uncover the next section and expose again. Repeat until you have five or six patches across the strip. Use 3-second intervals and set your enlarger lens to f/8 as a starting point.

After developing, look for the patch with the best balance of tone and detail. Use that exposure time for your next full print.

Important: more light means a darker print. It feels counterintuitive at first, but a longer exposure or wider aperture produces a denser, darker image, just like a longer shutter speed in camera makes a brighter exposure, which prints as a darker negative.

Thinking in F-Stops, Not Seconds

Once you have a base exposure, it is tempting to adjust in seconds. But seconds are linear and photographic exposure is not. Adding 2 seconds to a 5-second base exposure is a big jump. Adding 2 seconds to a 20-second base is almost nothing. This makes fine-tuning unpredictable.

F-stops work on a doubling and halving scale, which matches the way your eye and the paper actually respond to light. Each full stop doubles or halves the exposure, no matter where you start. If your base exposure is 8 seconds:

• +1 stop = 16 seconds
• -1 stop = 4 seconds
• +2 stops = 32 seconds

Think in stops when making adjustments and your test strips will become much easier to read. Also try to keep exposure times above 20 seconds when possible, as longer times give you more precise control over small adjustments.

Colour Correction

After your first test strip, the colours will probably be off. This is completely normal and part of the process. Colour correction in RA-4 printing is done using the yellow and magenta filters on the enlarger head. Cyan is not used in RA-4 printing.

Good starting point for your colour head:

From there, adjust based on what you see in your print. The colour wheel below shows how each direction maps to a filter adjustment:

Early on, make bold changes. Adjust yellow and magenta by 30 units or more. This helps you clearly see which direction you are moving and makes confident decisions on the next print much easier.

How to Judge Whether the Colour is Correct

The best indicator of correct colour balance is skin tones. Human skin is very sensitive to colour shifts, and our eyes immediately notice when something looks wrong. Regardless of a person's ethnicity or skin shade, healthy skin should appear natural and warm without any strange colour cast. If the skin tones look right, the rest of the image usually does too.

Enlarging: Step-by-Step

Preparing the Negatives

Start with well-exposed C-41 colour negatives with good density. Well-exposed negatives make focusing, colour correction, and printing much easier. Avoid negatives with a transparent film base, such as Kodak Aerocolor (Santacolor, Flic Film Elektra 100) or Harman Phoenix. These films lack the orange mask that standard C-41 film has and will not print correctly with normal RA-4 filtration.

Place the negative into the film holder with the emulsion side facing down: dull, matte side down; shiny base side up. Use a dust blower to remove any particles before inserting the holder into the enlarger. Dust will show up clearly in your final print.

Set Up the Print Area and Focus

Take out a sacrificial sheet of RA-4 paper in the dark and place it under the enlarger. Turn on the enlarger light and adjust the head height until the projected image is slightly larger than the paper. This ensures full coverage. If you have a masking frame, use it to create a clean border. If not, tape guides onto the enlarger baseboard to help you position paper accurately in the dark.

Next, focus sharply on the film grain using a focus finder. Focusing on the grain ensures your print will be crisp. Once focus is set, turn off the enlarger light before handling any unexposed paper.

Make a Test Strip

In total darkness, take a fresh sheet of RA-4 paper and close the paper box. Using the sacrificial sheet as a mask, expose the test strip in steps. Move the mask to uncover a new section every 3 seconds. Aim for 5 to 6 exposures on the same sheet.

Roll the exposed paper with the emulsion side inward and load it onto the REEL Paper 5. Close the Paterson tank with the funnel, then turn the lights back on. Now develop the strip to see how your exposure looks.

Developing: Step-by-Step

Default Process Times

We deliberately run chemistry a few degrees below the official 35 °C. This slightly slows development and gives more consistent results. The AGO Film Processor runs its standard 35 °C program, so you do not need to change any times in the AGO.

Preparing the Chemicals

You can use any common RA-4 kit designed for home users, such as Arista, Bellini, or Adox. These typically come in 2.5-5 L kits. Mix according to the manufacturer's instructions and store working solutions in 1 L bottles.

To reach temperature, place the bottles in a bucket of warm water and heat to around 32 °C using a sous-vide heater. This is the most reliable way to hold a stable temperature without constant monitoring. Once the chemicals have reached temperature, you are ready to develop.

Developing with the AGO Film Processor

Pro Tips

Chemical Replenishment

With the AGO and REEL Paper 5, the minimum volume per step is 250 ml. Discarding this after every print would be very wasteful. In practice, you can reuse the same 1 L of developer for at least 15 prints without noticeable quality loss.

For even more consistent results, use chemical replenishment: instead of discarding used chemistry, replace a small amount with fresh chemistry after each print. This keeps the developer and blix active across a whole session and ensures results are uniform from the first print to the last.

The replenishment amounts below are slightly higher than some manufacturer datasheets to give you a reliable margin, especially for home use:

Dodging and Burning

Dodging and burning let you control the brightness of specific areas in a print without affecting the rest of the image. Dodging means blocking light from part of the paper during exposure to make that area lighter. Burning means adding extra exposure to a specific area after the main exposure to make it darker.

For dodging, hold your hand or a small card between the lens and the paper during part of the main exposure. For burning, cover the rest of the image with your hand and uncover only the area you want to darken for additional time. In both cases, keep your hand moving slightly to avoid hard edges in the print. Start with small adjustments of around half a stop and write down your steps so you can repeat them reliably.

Holding your hand higher above the paper creates a larger, softer shadow for broader areas. Lower gives more control over smaller spots.

Pre-Exposure

Pre-exposure (also called flashing) means giving the paper a very small amount of light before the main image exposure. This raises the baseline density just enough to bring out shadow detail in high-contrast negatives, without significantly affecting midtones or highlights.

To try it, use the enlarger with no negative in place (or a blank area of film) and give the paper 1-3 seconds at f/11 or f/16 before making your main exposure. Be conservative, as too much pre-exposure will fog the paper and flatten the contrast too far. Just a touch is often enough.

Your First Colour Print

RA-4 printing rewards patience. Your first few prints will probably need adjustments to exposure and colour balance, and that is entirely expected. Each iteration teaches you something, and after 5-10 prints the process starts to feel natural.

Make notes as you go: exposure time, filter settings, lens aperture. Good notes mean you can replicate a successful print reliably and understand exactly what to change when something is off.

Few things beat the feeling of seeing your photograph come alive in colour, by your own hands. If you have questions about getting started with RA-4 printing or the AGO Film Processor, we are always happy to help.