Does long exposure increase saturation?

I have noticed that some of my photos are more highly saturated than expected. These are taken without polarizing filters, often at extremely low light, with very long exposures. Is it just me or have others noticed this phenomenon. Is there an explanation for this? I notice that sometimes I get an unusual colour palette also. Usually the effect is quite attractive. Here is a shot I took earlier tonight. Exposure 5 minutes with 2 stop soft ND grad. No postcapture processing.

Here's another from last year. The water is almost electric blue! 2 minute exposure, 3 stop ND grad hard.

I also experienced strange(but interesting) colours with long exposures. My experience is , that at least there has to be a source of light to get colours. If there is no light - or not sufficient light, no colours are there -since colours come from object that reflect light. Ok, so far so good.
My guess for that saturation-effect is that the light (i.e. the colours) gets accumulated or amplified due to the long exposure and turn into something that the eye cannot see.

It is also curious that the images are tack sharp even with water and boats that constantly move with currents even with glassy smooth water. Looks like the harbor photo was taken during the 'magic hour' of saturated twilight & a small aperture used which turns points of light into starbursts. Nice work!

Pat

Pat, the marina shot was taken just before the night sky turned black. It certainly didn't look as vibrant blue to my eye. The water was quite still so the boats were not moving much. Once again I stress no saturation boost on this yet the blue channel looks on the verge of clipping.

Kah, I'd think it's exactly the opposite - longer the exposure, the lower the saturation. Hence a lot of oh-ah drop down shots involve underexposure.

What you are seeing is the effect of long exposure itself. An NPN member (forgot his name, sorry about that) has something to this effect in his signature: "Long exposure creates a scene that didn't exist". This for me is the intrinsic joy of long exposure.

I think a large part of it is that when we render these dark scenes with a camera, we oftentimes increase the exposure value of the scenes relative to how we perceive the real world - revealing the subtle colors with greater intensity. Also, we are usually compressing the low contrast lighting (giving the photograph true white and black points). Both things seem to oftentimes bring out colors more so than we naturally perceive them with the naked eye. So, I don't think its the long exposures as much as the difference of how the camera "sees" the world in this type of lighting.

Kah, I believe that it is partially a function of White Balance. The color temperature of the twilight sky is very cold and that is going to be much more apparent to your sensor (or film) than to your subjective eye. If you recorded with your WB set to daylight the cold blue resulting from the blue sky with no sunight to offset it will be really exagerated.

But I have a question ...
I'm hearing concerns about saturation these days that I never heard back in the day before digital. Your picture of the harbour looks like a Kodachrome slide and the saturation comes as no suprize to me at all. It looks like what we once considered normal. Is there now a trend away from that look? Are we now becoming more pre-occupied with "getting it right" just because with digital it's easier and we can?

Bob

Long exposures with film result in reciprocity failure, which may result in color shifts. I don't know if reciprocity failure affects digital sensors as well, but it seems to.

Digital sensors do not experience reciprocity failure on long exposures. They do get noisy though. What you are seeing is an accumulation of light over time that is not visible to the eye in any instant. The colors are there but they are just too faint for us to see.

I don't see how reciprocity failure can occur on a digital sensor. My guess is that colours & casts subtle to the eye are enhanced by the long-exposure. Robert brings up a good point about what is considered normal. I personally like the effect of the long exposure. Doing a curve correction for the blueness makes the shot look washed out. Often my photos have a magenta cast when I shoot during the magic hour. I never fully correct for these casts, which are there in reality, but accomodated for by our brains. Fabiola, the 2 stop soft was a necessity to keep everything correctly exposed, which I think you can see in the sky being slightly lighter that the water reflections. While an ND grad increases saturation, all the elements still need to be correctly exposed for. A light blue sky taken at night would not have been accurate either. Also the water, not effected by ND grad is incredibly blue, something the eye did not pick up at the time.

Well I use the Singh-Ray grads too and although the claim is that they are completely neutral, I am far from convinced. Here's a test. Pick a scene, make a composition, and shoot a scene at F16 and 1/100s at ISO 100. Then slide in a 3-stop hard step GND all the way down so that the dark part completely covers the entire frame. Then shoot the same scene at F16, and 1/12s (to compensate for the 3-stops of darkness) using the same white balance.

In theory (according to the neutrality claim), both images should look exactly the same barring the slower speed causing motion in the latter shot. I bet you won't. I've never managed to get the two images to look alike, the latter one always has a magenta cast.

Now, OTOH if you use a real 3-stop Neutral Density filter (not the graduated one) from Singh-Ray, then you will produce two images that look exactly the same.

I agree, AR, I have also noticed this. No NDG is completely neutral. Although some are more neutral than others.
BTW I arrive in Montreal Oct 2 from Calgary then drive to Quebec where I'll spend about 3-4 days, if you're still interested meeting up.

Interesting observation. I have no explanation for it... just ideas.

1) When shooting in low light (that's mostly why you're using long exposures, right?), it is more likely that you have different color temperature in different parts of the image (because there's no sun to overpower light sources), which will then create nice color contrasts & look brightly saturated (e.g. your blue sky and the reddish light from the lamps near the boats).

2) When shooting in low light, even the bright parts of the image are produced from "colorful" light - not like in bright light, where it often just looks "bright", but not saturated. Maybe a stupid idea... Maybe it's related to the dynamic range which in low light is often smaller than in bright sunlight, so that you won't have to worry about blowing harsh/small highlights & can therefore brightly expose for what would otherwise be midtones.

In any case, your image is great.

Basil

I too have often noticed just how vibrantly blue the sky appears at twilight.
I believe a lot of it is too do with the white balance setting used.
Nice shots too, btw.

I suspect much of the difference is due to the light gathering characteristics of the human eye, not the camera's sensor. The retina is composed of both rods and cones. The rods excel at light gathering ability, while the cones excel at color perception. As light levels fall, the cones become much less effective as they need brighter light to sense the color. At low light levels, the rods are doing the heavy lifting, capturing the vast majoity of the available light. The color is there, but the retina is less able to sense it.