Light leaks ruin more film than almost any other camera problem. But they're also among the most fixable issues—once you know where the light is getting in.
This guide provides a systematic approach to finding, identifying, and fixing light leaks.
Understanding Light Leaks
What They Look Like on Film
Light leaks appear as exposure where there shouldn't be any—fog, streaks, or bright patches in areas that should be properly exposed or completely dark.
Colour negative film:
- Orange, red, or yellow streaks and patches
- Colour varies because the orange mask affects how the leak renders
- Often most visible in shadow areas of the image
Black and white film:
- Light grey to white streaks and patches
- May look like overexposure but in wrong locations
- Clear on negatives where there should be black
Slide film:
- Dark streaks and patches (reversal process)
- Often renders as muddy red or brown
- Destroys contrast in affected areas
Why They Happen
Light sealing in cameras relies on:
- Foam strips that compress between body parts
- Felt or velvet light traps in film slots
- Precise mechanical fit of doors, backs, and holders
- Coatings and baffles inside the camera body
All these materials deteriorate over time. Foam becomes brittle and crumbles. Felt compresses and loses pile. Mechanisms wear and develop gaps.
Systematic Diagnosis
Before fixing anything, confirm where the leak originates. Random repairs waste time and may not solve the problem.
Step 1: Rule Out Development Issues First
Not all fog comes from light leaks. Before blaming the camera:
Check for:
- Fog across multiple cameras/rolls developed together → development problem
- Fog only in unexposed frames → probably light leak
- Even fog across entire roll including rebate → possible darkroom leak or old film
- Fog that varies frame to frame → likely camera leak
Step 2: The Definitive Test Roll
This test exposes the camera to light without exposing the film through the lens—any exposure must come from leaks.
Load film in complete darkness. Use a changing bag if needed.
Secure the lens cap firmly. For cameras with removable lenses, use a body cap if available.
Advance two frames. The first frame is often inside the cassette; give yourself clean test frames.
Take the camera into direct sunlight. Not just outdoors—direct sun.
Systematically expose every surface. Rotate the camera slowly to point the top, bottom, front, back, and all edges toward the sun. Spend 30-60 seconds on each orientation.
Advance one frame between orientations. This helps locate exactly where light entered.
Note which frame corresponds to which orientation. Write it down:
- Frame 3: top toward sun
- Frame 4: back toward sun
- Frame 5: bottom toward sun
- etc.
Develop the film and examine.
Step 3: Interpreting Results
Clear frames with no fog: No light leak. Your problem is elsewhere.
Fog on specific frames: Cross-reference with your notes. If frame 4 shows fog and frame 4 was "back toward sun," the leak is in the back seals.
Location of fog within frame matters:
- Fog at edge of frame: leak is on opposite side of camera body (light travels across to that edge)
- Fog at one end: leak at that end of the camera (top or bottom)
- Fog increasing toward sprockets: leak near film gate or sprocket shaft
Pattern of fog:
- Sharp streak: leak through small hole or gap
- Diffuse fog: leak through deteriorated foam (light scatters)
- Line along frame edge: leak at film channel or pressure plate edge
Common Leak Locations by Camera Type
35mm SLR Cameras
Door hinge (most common):
- Foam strip runs along the entire hinge side
- First to deteriorate on most cameras
- Creates fog/streak along one side of frames
Film door seal:
- Foam channel around the entire door perimeter
- Creates fog at edges of frame
- May only leak when camera is oriented certain ways
Mirror box foam:
- Baffles around the mirror that prevent light bouncing inside
- Creates overall fog or fog in centre of frame
- Can also cause internal reflections in images
Film counter window:
- Small windows showing frame count
- Usually sealed but can deteriorate
- Creates localised fog
Around sprocket shaft:
- Seals where the sprocket shaft enters the film chamber
- Creates fog at sprocket hole edge of frame
35mm Rangefinder Cameras
Back door seals:
- Similar to SLRs but often different construction
- Check entire perimeter
Viewfinder housing:
- Cracks in the housing
- Seals where viewfinder attaches to body
- Can create fog in corner of frames
Film pressure plate:
- Foam behind pressure plate in some models
- Creates fog at film plane
Self-timer/release shaft:
- Seals around mechanical shafts
- Can leak if seals deteriorate
Medium Format Cameras
Film back seals (modular backs):
- Light trap where back meets body
- Most common issue with removable backs
- Creates fog along edges
Dark slide slot:
- Felt or velvet that seals around the dark slide
- Compressed felt doesn't seal properly
- Can leak even with slide inserted
Bellows (folding cameras, TLRs):
- Pinholes in bellows material
- Cracks at folds
- Creates sharp, defined fog spots
Film chamber seals:
- Where the film spools seat
- Can create fog at ends of frame
Large Format Cameras
Bellows:
- Pinholes (most common)
- Cracks at corners and folds
- Creates distinct spots or streaks
Film holder light traps:
- Felt that seals film holder to camera back
- Creates edge fog
- May vary between holders
Ground glass frame:
- Seal between ground glass assembly and back
- Creates general fog
Lens board seal:
- Where lens board seats against front standard
- Creates fog across entire frame
Foam Seal Replacement
Once you've identified the leak source, replacement is straightforward for foam seals.
Materials Needed
Option 1: Camera-specific kit
- Pre-cut pieces for your exact model
- Usually includes adhesive backing
- Costs $10-20
- Best for popular cameras (Canon AE-1, Nikon FM2, Pentax K1000, etc.)
Light seal kits are available from USCamera, Jon Goodman (eBay), Aki-Asahi, and Micro-Tools. Expect to pay £8-15 for a camera-specific kit.
Option 2: Generic foam sheets
- Buy sheets of closed-cell foam in various thicknesses
- Cut to size yourself
- More economical for multiple cameras
- Requires measuring and cutting
Common thicknesses:
- 1mm: light baffles, thin channels
- 1.5mm: most door seals
- 2mm: hinge channels, mirror bumpers
- 3mm: deep channels, some mirror boxes
Tools Needed
- Wooden picks (toothpicks, bamboo skewers)
- Plastic spudger (guitar picks work)
- Cotton swabs
- Isopropyl alcohol (90%+) or naphtha/lighter fluid
- Small scissors or craft knife
- Tweezers
Replacement Process
Remove old foam. Use wooden picks to scrape out deteriorated foam. It may be gummy, crumbly, or tar-like. Take your time—rushing damages the channel.
Clean the channel completely. Soak cotton swabs in alcohol or naphtha. Wipe channels until swabs come out clean. Old adhesive must be removed or new foam won't stick properly.
Let dry completely. Wait at least 30 minutes for solvent to evaporate. Remaining solvent prevents adhesion.
Measure and cut new foam. For generic foam, measure channel width and depth. Cut strips slightly narrower than the channel width (foam compresses when door closes). Length should match exactly.
Test fit before applying adhesive. Place foam in channel without adhesive. Close door to verify it compresses properly without preventing closure.
Apply foam with adhesive side down. Peel backing and press firmly into channel. Use a pick to push foam into corners and curves.
Let adhesive cure. Wait 24 hours before loading film. Adhesive needs time to set.
Mirror box baffles often use very thin foam or flocking (fuzzy material). Self-adhesive flocking sheet is available and works better than foam for these applications.
Specific Locations
Door hinge: Long channel along hinge side. Usually requires 1.5-2mm foam. Cut one long strip or piece it in sections.
Door seal channel: Runs around door perimeter. Usually 1-1.5mm foam. Follow the channel around, mitering corners.
Mirror bumper: Cushion that stops the mirror at top position. Usually 2-3mm foam. Cut to exact size of original.
Film chamber baffles: May use foam or felt. Replace with same material. Felt is better for areas near film path (less lint).
Bellows Repair
Bellows leaks require different approaches.
Finding Pinhole Leaks
Flashlight test:
Set up camera in complete darkness (or use a changing bag large enough).
Open the back and remove any film.
Close the lens and put on the lens cap.
Shine a bright flashlight inside the bellows.
Examine the exterior of the bellows carefully. Pinholes appear as bright dots.
Mark each pinhole with a small piece of tape or chalk.
Sun test: Reverse the process—point the camera at the sun with back open (no film) and look for light points from inside.
Temporary Fixes
Liquid Electrical Tape:
- Brush onto pinhole location
- Dries flexible
- Lasts months to years depending on use
- Apply multiple thin coats rather than one thick coat
Black fabric paint:
- More flexible than nail polish
- Multiple coats needed
- Good for larger worn areas
Black silicone sealant:
- Very durable
- Less flexible—use only on areas that don't flex
- Good for corners
What not to use:
- Regular tape (falls off, leaves residue)
- Nail polish (cracks when bellows flex)
- Super glue (stiff, cracks)
Permanent Solution
If bellows have multiple leaks or are generally deteriorated, replacement is the best solution.
Options:
- OEM replacement bellows (if still available)
- Aftermarket bellows (available for many cameras)
- Custom bellows from specialty manufacturers
- DIY replacement with bellows material (advanced)
Film Holder Leaks (Sheet Film)
Film holder leaks are common with used large format equipment.
Testing Individual Holders
Load a sheet of film in complete darkness.
Leave the dark slide in.
Place the holder in direct sunlight for several minutes, rotating to expose all sides.
Develop the sheet. Any fog indicates a leak.
Test each holder separately to identify which ones leak.
Common Holder Leak Sources
Dark slide slot felt:
- Most common issue
- Felt becomes compressed and doesn't seal
- Replace with self-adhesive velvet ribbon
Light trap groove:
- Where holder seats against camera back
- Felt or foam in this groove
- Replace if compressed or missing
Holder body cracks:
- Particularly in plastic holders
- Check corners and hinge area
- Cracks usually mean replacement
Felt Replacement
Remove old felt with tweezers. Clean channel with alcohol.
Cut new felt/velvet slightly wider than channel so it compresses when dark slide inserts.
Apply with adhesive. Ensure smooth application—lumps catch on dark slide.
Test dark slide operation before using with film.
Verification
After repairs, verify the fix before shooting anything important.
Test Roll After Repair
Run the same test you used for diagnosis:
Load film in darkness.
Cap the lens.
Expose all surfaces to direct sun.
Develop and examine.
Clean frames = success. Your repair worked.
Still fogging = missed leak or inadequate repair. Repeat diagnosis to find remaining leak source.
Ongoing Verification
Even after successful repair:
- Run occasional test frames, especially if camera was stored for a while
- Inspect seals visually for deterioration
- If you see fog appearing on regular photos, don't wait—diagnose immediately
Prevention
Storage Practices
Do:
- Store cameras in moderate, stable humidity (40-60%)
- Keep cameras in cases or bags with silica gel in humid climates
- Occasionally exercise shutters and advance mechanisms on stored cameras
- Check seals annually on cameras in regular use
Don't:
- Store cameras in attics (heat accelerates foam deterioration)
- Store in basements (humidity damages seals)
- Leave cameras in hot cars
- Store with foam pressed against other materials (accelerates compression)
When to Replace Proactively
If your camera is more than 15-20 years old and has never had seals replaced, consider doing it before you see problems. It's much cheaper than wasted film and missed shots.
Signs seals need replacement even without visible leaks:
- Foam is sticky to the touch
- Foam crumbles when you touch it
- Foam has shrunk away from channel edges
- Visible light when you hold camera up to a bright light (back closed)
Costs
- Light seal kits: £8-15
- Replacement bellows: £50-150 depending on camera
- Professional CLA (clean, lubricate, adjust): £80-200
Summary
- Diagnose before fixing—confirm where light is entering
- The test roll method definitively identifies leak locations
- Foam seal replacement is straightforward DIY for most cameras
- Bellows leaks can be temporarily fixed with liquid electrical tape
- Film holder leaks are usually felt replacement
- Always verify repairs with a test roll before important shooting
- Prevention through proper storage extends seal life
Light leaks are frustrating but not fatal. A $10 foam kit and an hour of work can restore a camera to light-tight condition for years. The key is systematic diagnosis—fix the actual problem, not a guess.