Long Exposure Sensor Testing. I shoot a lot of nightscapes and aurora’s. It’s not uncommon to see my camera in the 3200 to 6400 ISO range doing 30 second or longer exposures for extended periods of time. So for me one of the main performance criteria for any camera is long exposure noise. If your a keen photographer you would be aware that there a many sites offering reviews on new cameras, but few (none to my knowledge) test the sensor with a long exposure with no lens to get the base performance for sensor noise.
Most tests performed use a relatively short exposure (1 sec) at various ISO ratings, and it’s assumed that the ISO to TIME relationship is linear i.e. a one second exposure at 3200ISO is the same as a 16 second exposure at 200 ISO. The only real way to see how a camera is going to perform at 3200ISO for 5 mins is to test it. A while back I started to tested sensors using this basic test:
Three exposures are taken from the camera. The exposure times for these shots are 1 second, 30 seconds and 300 seconds (5 min).
The camera is configured as follows:
Camera body with body cap only.
View finder shut or covered.
ISO 3200.
Long exposure NR off.
Standard NR off.
RAW recording with the max bits per channel (normally 14).
Normally in M or B mode.
Post processing:
Most of the shots out of the camera are very good, particularly the later model’s, so to bring out the underlying noise I use the following workflow:
Images are converted using Adobe DNG Camera raw with all settings at ‘0’ except for +2 exposure.
Crops of the images are taken from the center covering an area or about 300x300px.
No re-scaling, filters or manipulation is applied to the images.
The results speak for themselves, however it is interesting to note that some cameras hold up very well having hardly any more noise at 30 seconds than compared to 1 second. Since most of the work I do is normally in the 15-30 second range this gives me a good idea how quickly the sensor deteriorates and it’s usefulness, and of course the 5 min exposure is the real leveler. So what does this test?
Well assuming that the camera is totally light sealed it should, in theory be showing nothing more than sensor noise. Cameras that have a long exposure NR feature actually create this image to deduct from the previous exposure, which in theory leaves you with a clearer exposure. However the noise pattern is never the same, so the subtraction method is never perfect, and it also requires you to wait just as long as the first exposure to calculate the noise. For more about this see my long exposure NR page.
A few points to keep in mind when reviewing the results:
The age of the camera.
The MP size of the sensor, and subsequently the different in pixel cell size.
There will be some variation (tolerances) from camera to camera.
