The Epson Stylus Photo R2880 offers benchmark quality. It prints up to A3+ size in professional quality. It costs around 600 euros. The main difference compared to ‘standard’ ink jet printers is that it uses pigment ink. This means the prints are waterproof and have light fastness, unlike the water-soluble inks found in standard models. It also produces black-and-white prints that are truly black and white, not mixed-colour prints, including beautiful grey tones. Heavyweight papers are no problem. There are more expensive models at the top end, such as the 3880, which has larger ink tanks and is therefore cheaper to run.
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I know you only want to print up to A4, but I’m not aware of any model on the market that achieves this quality and uses pigment ink. Professionals need the larger formats too, and you can also perform processing using rolls. The result is genuine fine art prints that are displayed in museums and have good light fastness.
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https://www.epson.de/de/de/viewcon/corporatesite/products/mainunits/overview/2733