Rodney Campbell's Blog

A couple of days ago I posted about a light leak issue I was having through the viewfinder with long exposures. I got a few responses from people I know and trust including

the purpose of the eyepiece blind is to prevent light entering the viewfinder to affect your image’s exposure settings, but when you’re making the exposure, the mirror is raised up against the focusing screen, preventing any light that might be entering though the viewfinder from entering the mirrorbox, and thus affecting the actual image that you’re shooting. That’s why the viewfinder blacks out during the actual moments of exposure

So I’d planned to investigate this further by doing some more test shots (with the screw in ten stop ND to exclude any possible light refraction and other issues with the mess of Lee filter stuff I usually shoot with – Lee filter holder with Bigstopper, GradND and the 105mm CPL on front) when another friend suggested a much simpler test 🙂

So…

I setup the test as follows – I waited till it was dark outside (night) and then setup my camera on my desk with the following settings:

– Lens cap on
– Camera body set to manual focus
– Manual in bulb mode with an Aperture of f/8 and ISO 100
– Used a remote interval timer set for an exposure of 3 minutes
– Long Exposure Noise Reduction OFF

All of the images below are straight out of camera RAW images converted to JPEG – no other adjustments made

This first exposure is the baseline – the room was darkened but I used the viewfinder cover to completely block out the viewfinder for the shot – the shot is completely black as expected – the only interesting fact is that the shot appears to have no noise (even at 100%) – nice (and only one hot pixel to be found)

NIKON D600 + 85.0 mm f/1.8 @ 85 mm, 180.00 sec at f/8, ISO 100

This second shot is the same camera position, etc – the only difference is that I removed the viewfinder cover and shone an LED torch at the viewfinder from a slight angle from about 30cm away – draw your own conclusions…

NIKON D600 + 85.0 mm f/1.8 @ 85 mm, 180.00 sec at f/8, ISO 100

I decided to follow up with a few more tests. I took two shots with the room lights on – the first with a dimmed setting; the second with the lights (LED downlights) up higher and with the camera angled at about 30-40 degrees so the back of the camera was pointing a little more up towards the ceiling so that some of the light might spill into the viewfinder but not be pointing directly into the viewfinder. I’ve included the second of those tests here but essentially they were both black like this

NIKON D600 + 85.0 mm f/1.8 @ 85 mm, 180.00 sec at f/8, ISO 100

I then pointed the camera straight down on the desk so that the ceiling downlight shone down into the viewfinder – again I should be expecting a completely black image – you can see how that turned out…

NIKON D600 + 85.0 mm f/1.8 @ 85 mm, 180.00 sec at f/8, ISO 100

Lastly to exclude the effects of things like the sensor circuits heating up, etc (this was my sixth consecutive 3 minute exposure with almost no time between each) I took another shot with the room lights on but the viewfinder cover in place (in fact the camera was still in exactly the same position as the last shot – facing downwards on the desk with the downlight shining directly onto the back of the camera) and as you can see completely black again – and again even at 100% I see no evidence of noise (dark current, etc) and only just the one hot pixel

NIKON D600 + 85.0 mm f/1.8 @ 85 mm, 180.00 sec at f/8, ISO 100

So seems reasonably conclusive to me. I do however intend on doing a further test – I want to see if the light leak might be happening only during the moments the mirror is being raised or lowered and so I’ll try some tests like:

– dark room, start the exposure with the viewfinder exposed but don’t start shining a light at the viewfinder till after the mirror flips up and also stop it before the mirror flips down
– as above but start the light before raising the mirror but stop before lowering
– ditto but start the light after the mirror raises and stop after the mirror drops