MOS to JPG Converter

Convert MOS to JPG: What This Tutorial Covers

MOS is the raw capture file written by Leaf medium-format digital backs — the Aptus and Aptus-II line that Leaf shipped as a division of Scitex, then Kodak, and finally as a subsidiary of Phase One. It holds untouched sensor data in a TIFF-derived container, which is why a .mos opens in Capture One or Leaf Capture and nowhere else. This tutorial walks a Leaf shooter (or anyone handed a stray .mos) through turning that raw into a JPG anything can open, and explains what the converter actually does to the sensor data on the way — because it is doing real raw development, not a file-extension rename.

How to Convert MOS to JPG

1. Upload Your MOS File: Drag and drop your .mos onto the page or click "+ Add Files". You can queue a whole shoot and convert every frame with the same settings. Medium-format raws are big, so the wait you notice is the upload, not the conversion.
2. Pick a Quality Preset: Under Image Compression, Quality Preset starts on Very High (Recommended), which writes JPEG quality 95. Move up to Highest (quality 100) for a print or archive master, or step down to High / Medium for something you can email.
3. Set Image Resolution (Optional): Image resolution starts on Keep original, preserving the back's full pixel dimensions. Use Preset Resolutions, Width x Height, or Resolution Percentage to bring an 80-megapixel frame down to something a website or a client proof can use.
4. Convert and Download: Click "Convert" and save the JPG. Files upload over an encrypted connection, are processed on our servers, and are deleted automatically after a few hours. No sign-up, no watermark.

Walk-through: What "Developing" a MOS Actually Does

A raw file is not a picture yet. The sensor behind a Leaf back is covered by a colour filter array, so each photosite records one colour channel only — the file is a mosaic of single-channel readings plus the metadata needed to interpret them. Our pipeline hands the .mos to LibRaw, the same open-source raw library that sits under darktable and RawTherapee, and LibRaw performs the three steps that turn sensor readings into an image:

• Demosaic. Neighbouring photosites are interpolated so every pixel gets a full red, green and blue value. This is where the megapixel count you see in the JPG actually comes from.
• White balance. The as-shot neutral recorded by the back is applied so greys come out grey instead of green or magenta.
• Tone curve and gamma. The sensor's linear response is remapped to a display-referred curve, then encoded to 8-bit sRGB for the JPEG.

That is genuine raw development, and it is worth being precise about what it is not: there is no HDR tone-mapping stage in the pipeline. Highlight rolloff comes from the standard raw tone curve, not from a scene-referred tonemap operator, so a frame you deliberately exposed to hold a bright sky may still clip where your own Capture One recipe would have recovered it. The developed result is a reasonable, neutral render — it is not your custom recipe, and it never will be.

Quality Preset Ladder

Each preset maps to a fixed JPEG quality value, so you can pick by number rather than by adjective:

If you need to land on a specific size rather than a specific quality, switch from Quality Preset to Specific file size and type a target — it starts at 8 MB. Leave Auto Scale on and the converter is allowed to reduce dimensions as well as quality to reach the number; turn it off and it must hit the target on compression alone, which on an 80-megapixel frame means a much harsher JPEG.

Which Leaf Backs Write .mos

The first-generation Aptus backs — Aptus 17, 22, 54S, 65, 65S, 75 and 75S — write .mos as well; LibRaw lists all of them as supported. The later Leaf Credo backs moved to the Phase One IIQ format instead, so if your files end in .iiq rather than .mos you are on a Credo and this is the wrong page.

Common Errors and How to Fix Them

• "The JPG is enormous." That is the medium-format pixel count, not a fault. An 80 MP frame at quality 95 is a genuinely large JPEG. Drop Image resolution to a preset such as 2160p, or set a Specific file size, or run the finished JPG through the Image Compressor.
• "Highlights look clipped compared to Capture One." The converter applies a standard raw tone curve, not your highlight-recovery recipe. Do the exposure and highlight work in your raw editor, export a 16-bit TIFF, and convert that instead — see TIFF to JPG.
• "Colours are slightly off." The developed image is encoded to sRGB. If your Leaf workflow lives in Adobe RGB or ProPhoto, the shift you are seeing is the gamut conversion, and it is expected for a web-ready 8-bit file.
• "A file was rejected." Leaf backs can record .mos in an uncompressed variant and a smaller compressed variant, and not every raw engine reads both. If one file fails while others from the same shoot succeed, open it once in Leaf Capture or Capture One and re-save it uncompressed, or export a 16-bit TIFF and convert that.
• "I want the alpha channel preserved." JPEG has no alpha channel at all, and neither does a raw sensor readout — there is nothing to preserve here. If you later composite the shot and need transparency, export to PNG instead.

When This Doesn't Work

The one case no converter can rescue is a partially-written or truncated .mos — a card that was pulled mid-write, or a backup that copied incompletely. LibRaw will refuse the file, and there is no repair path; re-copy from the original card or another backup. The other case is a workflow question rather than a failure: if the point of shooting medium-format raw was the editing latitude, do not treat any JPG as your master. Develop in Capture One, keep the .mos, and use this converter for the delivery copy. If you want a delivery file that still carries 16-bit depth and richer metadata, go to MOS to TIFF rather than JPG.

Frequently Asked Questions

What exactly is a MOS file?

MOS is the raw format written by Leaf medium-format digital backs — principally the Aptus and Aptus-II series used on Mamiya and Hasselblad bodies. It stores unprocessed sensor data in a TIFF-derived container along with the metadata a raw converter needs to develop it. Because the pixel data is a colour-filter-array mosaic rather than a finished RGB image, a general-purpose viewer cannot render it even though the container looks TIFF-shaped from the outside.

Does converting to JPG lose quality?

Yes, in two distinct ways, and it is worth separating them. The larger loss is editing latitude: developing the raw bakes in white balance, exposure and tone, so the recovery headroom that justified shooting medium-format is gone from the JPG. The smaller loss is JPEG compression itself, which at the default quality 95 is not visible at normal viewing distances. Keep the .mos as the master and treat the JPG as delivery.

Which preset should I use for a print master?

Highest, which writes JPEG quality 100, with Image resolution left on Keep original. That gives you every pixel the back recorded and the least compression the format allows. Be aware that quality 100 is still lossy — JPEG has no truly lossless mode — so for an actual archival master, MOS to TIFF is the better target.

Should I choose the JPG or the JPEG file extension?

They are the same format; the File extension control only decides what the downloaded file is named, and neither option is preselected for you. .jpg is the safer default because a few older Windows tools still assume three-character extensions. .jpeg is equally valid and is what the ISO standard's own naming registration prefers. Nothing inside the file changes either way.

Why does the converted image look flatter than my Capture One preview?

Capture One is showing you Leaf's own camera profile plus whatever recipe you have applied. This converter applies a neutral standard development — demosaic, as-shot white balance, standard tone curve — with no profile of yours attached. For colour-critical work, develop in Capture One and convert the exported file rather than the raw.

Can I convert a whole shoot at once?

Yes; queue as many .mos files as you like and they all convert with the settings you chose. The practical ceiling is upload size and time rather than anything on our side — a folder of 80 MP raws is tens of gigabytes, so it is usually faster to convert in batches of a few dozen and let each upload finish before queuing the next.

Does the JPG keep the EXIF and shooting metadata from the raw?

Standard EXIF fields that survive raw development — camera and back model, exposure time, aperture, ISO, capture date — carry through into the JPG. Leaf-specific maker-note blocks and the raw processing instructions do not; they describe how to develop the sensor data, and once the file is developed there is nothing left for them to describe.

What happens to my MOS file after I upload it?

It is uploaded over an encrypted connection, developed and converted on our servers, and the JPG is returned to you. Both the uploaded raw and the output are deleted automatically after a few hours. There is no sign-up, no watermark, and your images are never shared or made public.