Long exposure problem

Steve M · 22 Jun 2014 at 09:07

Recently bought a D5200 and a 35mm f1.8 lens and took it out last night to get some long exposure shots. I'm having an issue with the images when using bulb mode.

At a glance this one looks ok but when you zoom into the image there are dots all over it..

These dots are not present when using shutter priority on exposures of up to 30 seconds and only appear only on bulb mode on shots of a minute or longer.

This one was a 6 minute exposure, ISO 100, f22

Zoomed in on the water. These dots are all over the image:

Any ideas?

JBuk · 22 Jun 2014 at 09:24

Reflection of the stars? Heh

Did you long exposure noise reduction turned on?

Steve M · 22 Jun 2014 at 09:28

Not reflections of the stars. Completely cloudy, plus the dots are all over the image.

No I didn't. Is it simply noise/hot pixels?

JBuk · 22 Jun 2014 at 09:31

It's a possibility

Steve M · 22 Jun 2014 at 09:33

The NR hammers the battery, especially on a 6 minute exposure.

Will try the shot again next week with the NR on.

Wilhelm · 22 Jun 2014 at 09:33

Even at ISO100 you'll get noise if the exposure is long enough, so it could well be that.

Also, and this may sound silly, but you could check/clean your lens with a blower brush in case it's dust fragments.

Wilhelm · 22 Jun 2014 at 09:34

Wilhelm said:
Even at ISO100 you'll get noise if the exposure is long enough, so it could well be that.

Also, and this may sound silly, but you could check/clean your lens with a blower brush in case it's dust fragments.

...On both sides of the lens glass!

Steve M · 22 Jun 2014 at 09:38

Wilhelm said:
Even at ISO100 you'll get noise if the exposure is long enough, so it could well be that.

Also, and this may sound silly, but you could check/clean your lens with a blower brush in case it's dust fragments.

The lens and camera are brand new, so both very clean.

I will try again with the NR on. Still learning and this is the first time I've ever taken a shot in bulb mode on a camera.

Anyone have any experience of how effective the NR is in a shot of this kind?

squerble · 22 Jun 2014 at 11:04

Nikon's noise handling seems to get more favourable views than others so it may turn out quite well.

Derek W · 22 Jun 2014 at 12:07

Steve M said:
The NR hammers the battery, especially on a 6 minute exposure.

Will try the shot again next week with the NR on.

A lot of people just leave the noise reduction off for that reason. If you take a 6 minute exposure then the noise reduction after will take the same amount of time.

The hot pixels that appear on the image will (or should) remain in the same places whenever you take a shot so some people just create a photoshop action to remove these pixels in PP rather than worrying about them when taking the photo.

Andy90 · 22 Jun 2014 at 16:58

The longer your exposure the more chance this will happen. Basically the sensor heats up. Each pixel will react to light hitting it differently, you get a 'leakage' between the pixels and they read different values resulting in 'brighter' pixels.

You can either live with it or turn on Long Exposure Noise Reduction. The trade off is that as Derek W states above, the camera will make another exposure for the same time period that the shutter is open i.e a 30 second exposure will have another 30 second one created in camera, so the camera will remain 'frozen' for 1 minute until it sorts itself out. One frame is then subtracted from the other removing the noise.

You also cannot rule out stars from the equation on cloudless nights, but they would all rotate/move along the same path. 30 seconds is more than long enough for star trails to appear with wide angle lenses.