If you have recently made the jump from 35mm to medium format, you already know the stakes are higher. The cameras are heavier, the math for your depth of field is totally different, and every single click of the shutter costs a lot more money. So, there is nothing quite as heartbreaking as getting an email from your lab, opening up your scans, and realizing half your shots are ruined by thick, blurry streaks of white or orange light spilling across the edges of the frame.

You frantically check your camera. The seals look fine. The bellows do not have any pinholes. The dark slide is perfectly straight. So what exactly happened?

Chances are, you fell victim to the dreaded "fat roll." If you shoot medium format long enough, it is going to happen to you at least once. It happens to people who have been shooting for two weeks, and it happens to people who have been shooting for twenty years. But once you understand what a fat roll is and why it happens, it is actually pretty easy to banish it from your workflow forever.

What Exactly is a Fat Roll?

To understand the problem, we first have to talk about how 120 film is packaged. Unlike 35mm film, which is safely hidden away inside a light-tight metal or plastic cassette, 120 film is essentially naked. It is just a long strip of film taped to an even longer strip of opaque backing paper, all rolled tightly around a plastic core.

When you load 120 film into a camera, you are pulling that backing paper across the film plane and threading it into an empty take-up spool. As you shoot, the film unwinds from the full spool and winds onto the empty one. The only thing keeping the unexposed film safe from ambient light is the backing paper and the physical tightness of the roll itself. If the paper overlaps the edges of the plastic spool perfectly, light cannot get in.

A fat roll happens when the film and paper do not wind tightly onto the take-up spool. Instead of feeling dense and rock-solid when you pull it out of the camera, the roll feels squishy. The paper might bow out slightly in the middle, or it might extend slightly past the flat plastic rims of the spool. Because it is wound loosely, the thick roll of paper acts like a loose sleeping bag. Ambient light sneaks in between the layers of paper right at the top and bottom edges, causing localized light leaks.

Fat Roll Leaks vs. Camera Leaks

Before you blame your rolling technique, you want to make sure the issue is actually a fat roll and not a problem with your camera's light seals. Here is how to tell the difference.

Camera light leaks are generally consistent. If you have a bad door seal on your camera, the light leak will normally show up in the exact same spot, on the exact same side of the frame, roll after roll. Because the light is hitting the film from a specific physical hole in the camera body, the leak often looks like rigid stripes or sharp geometric shapes.

Fat roll leaks are much messier. Because the light is creeping in through the edges of the paper roll while it sits on a desk or in a bag, the leaks are wavy, blurry, and erratic. They will almost always start at the very edge of the film border and bleed inward toward the center of your shot. On color negative film, these leaks usually look orange or red because the light is passing through the protective backing paper and striking the backside of the emulsion physically. If the damage is really bad, you'll see fogging covering the entire edge of the film strip, sometimes completely washing out the edge numbering.

The Culprits: Why Your Film Isn't Winding Tight

So, the roll is loose. But why did it happen? There are normally three main culprits behind a fat roll, and they range from simple user error to failing vintage hardware.

1. Loading Without Proper Tension

This is by far the most common reason. When people thread the paper tongue into the empty take-up spool, they sometimes let go of the supply spool with their thumb. Without tension, the paper unspools a little bit on the supply side, introducing slack into the system right from frame one. If you start with a loose core, it will only get sloppier as you advance through the roll. By the time you hit frame ten or twelve, the paper isn't winding flush against the plastic edges of the spool anymore.

2. A Bad Take-Up Spool

In medium format, your empty take-up spool is always just the leftover spool from the previous roll you shot. Not all plastic spools are created equal. Some budget film stocks use slightly flimsy plastic spools. If a spool gets warped in your pocket, dropped on the pavement, or even slightly squeezed out of alignment, the film will not wind straight. It will bias to one side, causing the paper to bulge out on the opposite end.

3. Weak Springs in Your Film Back

If your loading technique is flawless and you are using good spools, but you still get fat rolls, your camera might be aging out. Inside almost all medium format film backs, there are flat metal leaf springs that physically press against the paper spools to keep them from free-spinning. Over forty or fifty years, this metal can lose its springiness. If those metal tabs go flat, they stop applying pressure against the roll. The camera will effortlessly advance the film, but without the slight resistance provided by those springs, the roll ends up winding loosely.

How to Prevent Fat Rolls

Saving your medium format films comes down to a few basic habits that you just need to bake into your shooting routine.

• Keep your thumb on the supply spool: When you are feeding the dark paper tongue into your take-up spool, always keep your left thumb pressing gently on the fresh roll of film. Thread it, turn the advance crank a few times until passing the start mark, and only remove your thumb when you are ready to close the camera back. The goal is to keep the paper as taut as a guitar string the whole time.
• Don't squeeze the roll when you take it out: When you open the back of the camera after your last shot, do not grab the exposed roll by the middle. If it is a little loose and you squeeze it, or worse, if you try to pull the paper tight to fix the slack, you will cause "cinching." Cinching happens when the layers of film rub aggressively against each other, causing physical friction scratches across your negatives.
• Unload in the shade: Never open your camera back in direct, blinding sunlight. Turn your body so your shadow falls over the camera, or step into a doorway or a car. The less intense light hitting that fresh roll of paper, the better.
• Wrap it up immediately: Most modern 120 films have a lick-and-stick band or an adhesive strip. Fold the lip of the paper under tightly, lick the stamp, and seal it shut. But don't stop there. A lot of medium format shooters carry a handful of thick rubber bands in their pockets. Wrapping a rubber band around the middle of the sealed roll guarantees it won't unspool while rattling around in your bag.

Dealing with Hardware Issues

If you have tried holding tension, changing brands, and sealing your film tight, but your camera is still handing you squishy rolls of film, it might be a mechanical problem. Flat tension springs inside vintage camera backs sometimes can be carefully bent back into shape by a knowledgeable technician, but it is risky to try yourself without the right tools. Often, the easiest fix is to just retire the problematic film back or insert and pick up a working replacement.

Getting your gear sorted out changes the whole shooting experience. You stop worrying about whether your images will survive the trip home, and you get to focus purely on the photos. If you think it is time to upgrade or find a replacement setup that functions smoothly, you can always check out some incredibly reliable options through a medium format camera search. And remember, storing those extra rolls in the dark is just as important as how you load them, so tossing them straight into a dark, padded compartment of a good camera bag immediately after shooting is always a good habit.

Medium format is too beautiful to let simple light leaks ruin the magic. Take your time, keep things tight, and enjoy those massive, detailed negatives when they finally come back from the lab.