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Do You Actually Need to Cover the Viewfinder for Long Exposures?

If you’ve ever wondered what the point of the little viewfinder cover that comes with most cameras is, or the viewfinder shutter that some higher end cameras have, you were probably told that it helps prevent light leaks. And then, if you’re anything like me, you probably dismissed that information as totally useless, since what are the odds that light would come through the viewfinder and affect your shot? But if you’re seeing weird purple or green flares in your long exposure shots, it might be time to revisit that idea.

For years, I had absolutely no issues with light leaks. Even when I was shooting on a tripod and didn’t have the camera to my eye, there was never any problem with my photos. However, this was before I picked up an ND (neutral density) filter.

Flare in image of waterfall

Having read up on good filter brands, I picked up a good midrange B+W 10 stop filter. At over $100, it wasn’t a cheap knockoff, so I was shocked to see major flaring in some of my images taken with it. The photo above is a 5 minute exposure, showing the flare well. I figured there was nothing I could do, that must just be how these things work, and I took care to avoiding shooting into the sun and shaded the lens.

Frustratingly, the flare was completely unpredictable. Sometimes I took all the care in the world, and still ended up with a ridiculously huge flare in the centre of the images. Other times the sun would be right in the corner of the frame, and no issues.

I’d put the filter back for a few months, then pull it out for a day and be reminded of why I hate using it. It was such an incredibly frustrating experience that I was on the brink of springing for a much more expensive filter when I decided to do some serious testing to see if I could figure out how exactly the flare was being caused.

Going down to a nearby river, I started snapping shots with the filter on. No flare was visible, even when shooting into the sun. After a several test shots at different angles, I decided to pull out a tripod and take an image of the river.

Rapids in a river with sensor flare

Boom! Massive flaring, despite the sun being nowhere near the frame. In fact, the sun was behind the camera. This is when I clued in to the possibility of sensor flare. With the other test shots, my eye was to the viewfinder. But on the tripod, the viewfinder was exposed to the sun. So I took another shot, this time covering the viewfinder with my hand.

Rapids in a river, viewfinder covered

And violà, flare solved. Not only is there no ugly flare artefact, but the overall colour and contrast across the whole image is far better. So it wasn’t a filter issue after all, it was light coming in through the viewfinder! But why did I only notice it with the ND filter on?

What causes sensor flare, and how do you avoid it?

With the ND filter, light coming in from the lens is 10 stops darker, but light coming in from the viewfinder is unaffected. Ordinarily there’s more or less the same amount of light on both sides of the camera, and very little light actually manages to get from the viewfinder to the sensor - so little that it’s invisible relative to the much brighter exposure from the lens. But by darkening the lens so much, the viewfinder’s light leak is comparatively much more visible. Since the light causing the flare isn’t coming through the lens, this is called sensor flare instead of lens flare.

This can happen without ND filters too, any significant variance between the lens’s light and the viewfinder’s light can cause sensor flare. For example, if you’re shooting stars at night, and you blast a speedlight or strobe into the viewfinder you’ll almost certainly encounter issues. Heck, this could even be a problem if there’s a streetlamp behind you. Thus it’s most likely that this’ll be an issue with an ND filter, but you could see this viewfinder flare in other rare cases, and it’s worth being aware of that.

So what’s the solution? Simply cover the viewfinder when shooting long exposures with ND filters, or if there’s a bright light behind the viewfinder. This is very convenient with some high end cameras like the Nikon D810 or D5, that have a little switch beside the viewfinder to shutter it. Other cameras may come with a plastic viewfinder cap that you can slide over it, or you can just hang your camera strap in front of it.

In the photo below, the sun was right behind the camera, but by using the viewfinder cover there was absolutely no flaring.

A small waterfall

Note that this is not a problem with mirrorless cameras, since the electronic viewfinder does not allow light to reach the sensor.

Some cameras have built in viewfinder shutters, like the Canon 1DXII, Nikon D5, Nikon D810, and Nikon D500. Others come with an accessory piece to slot over the viewfinder. If you’ve lost your cover or maybe you bought used and the camera didn’t come with it, you can buy the Nikon DK-5 on Amazon at a fairly cheap price (the DK-5 seems to be used by all Nikon cameras that don’t have a built in cover). Canon cameras tend to have the eyepiece cover attached to the camera strap, and I can’t seem to find it sold separately.

Hopefully this can save some of you from the frustrations I went through trying to figure this out! Of course, you might still encounter run of the mill lens flare, but if you’re doing everything in your power to shade the lens and still having problems then it’s probably a light leak from the viewfinder, and you now know how to avoid that with ease.

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Lauchlan Toal is the creator of UnlockCreativePhotography.com, and a Halifax based food photographer. Outside of food photography, he enjoys most genres, finding fun in any kind of photography challenge.

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