how much noise is normal in long exposure

[Charliedelta]

I noticed that there is a lot of noise in most of my long exposures (I use a tripod, radio triggers, and mirror lock-up, of course). To prevent noise I use ISO 100, on my D7100. For the lens I use either the Tokina 12-28mm or the Nikkor 50mm 1.8g. most of the times I also use an ND filter. When I shoot at dusk, and want to do a long exposure, I oftentimes find a lot of noise in my pictures (usually I like to shoot 15-20 second exposures).

Is it somehow normal to have noise present in these conditions? In other words, does it come because I am trying to get everything darker with the ND so that I can keep the shutter open longer, trying to get more light on the sensor? What would you do to avoid it (or to reduce it, at least ) ?

Thanks

 

[Overread]

Two thoughts;

1) Have you exposed your photo properly to start with? Ergo have you got the histogram up and most of the bulk is as far over onto the right side as possible without clipping the far edge (and thus overexposing). If you've any areas which are dark in a photo then they will hold more noise (even at a lower ISO) especailly so if you brighten them up in editing

2) The longer a sensor is on the hotter it gets - and the hotter a sensor gets the more noise it generates. This is where there is a "long exposure noise reduction" mode in most DSLRs. This mode takes a second shot after the first with the shutter blades closed for the same length of time as the first. The result is that any detail on that second frame is heat-generated noise so the camera can deduct that from the first frame to give you a final shot with less noise.
This tends tobe something you find in astrophotography a lot where they might be exposing for several minutes.

 

[Tinderbox (UK)]

Make sure you cover the viewfinder when doing long exposures.

John.

 

[TCampbell]

Make sure you cover the viewfinder when doing long exposures.

John.

BTW, I recently ran into an article that questioned whether this was really valid. I'm sure it is on a great many cameras, but modern cameras have come a long way. So they decided to put the advice to the test.

The "test" they performed involved leaving the lens cap on, wrapping every part of the body to ensure no light leaks could come from anywhere EXCEPT the viewfinder. Then they deliberately shined a very bright light into the viewfinder while taking a very long exposure.

If the camera has no light leaks, one would expect a black image (apart from the normal amount of noise a digital sensor will create).

After performing several tests with their camera, they were unable to create any signs of light leaking in through the viewfinder.

I did not perform the test, but I did remove my lens and put the camera into mirror-up cleaning mode so I could inspect how the mirror folds up and covers the focusing screen and realized that if actually folds up quite nicely and I can well imagine how it would be extremely difficult to get a light leak through that.

I'm guessing that there may be a number of newer camera for which covering the viewfinder isn't really necessary.

 

[Tinderbox (UK)]

I suppose if your dslr comes with an viewfinder blank cover as some of mine have done i would normally use it, when we see reports of lens mounts with light leakage where you would never expect there to be any, it teaches you to never take anything for granted.

John.

Make sure you cover the viewfinder when doing long exposures.

John.

BTW, I recently ran into an article that questioned whether this was really valid. I'm sure it is on a great many cameras, but modern cameras have come a long way. So they decided to put the advice to the test.

The "test" they performed involved leaving the lens cap on, wrapping every part of the body to ensure no light leaks could come from anywhere EXCEPT the viewfinder. Then they deliberately shined a very bright light into the viewfinder while taking a very long exposure.

If the camera has no light leaks, one would expect a black image (apart from the normal amount of noise a digital sensor will create).

After performing several tests with their camera, they were unable to create any signs of light leaking in through the viewfinder.

I did not perform the test, but I did remove my lens and put the camera into mirror-up cleaning mode so I could inspect how the mirror folds up and covers the focusing screen and realized that if actually folds up quite nicely and I can well imagine how it would be extremely difficult to get a light leak through that.

I'm guessing that there may be a number of newer camera for which covering the viewfinder isn't really necessary.

 

[Tailgunner]

I would add that I believe 200 is the Native ISO for the D7100. I never shot mine below that with mine.

As mentioned above, ark spots tend to show noise when brightened up. Also, you could try switching on "Noise Reduction." This helps eliminate noise but it increases the time for taking each exposure.

 

[KmH]

Native ISO for a D7100 is 100 - 6400 ISO.

 

[wfooshee]

The "cover-the-viewfinder" advice is for metering, not for light leaks. The raised mirror covers the focusing screen, blocking any light into the body of the camera from the pentaprism.

The metering system is in the pentaprism and could be fooled by light into the eye window. Since you're shooting manual mode, metering is irrelevant.

A remote-controlled exposure, with the viewfinder open, could get more light into the metering system than it shoud with the eye window unblocked.