Long Exposures

I mentioned in my last post that I was practicing/experimenting/trying long exposures recently. I mean the stuff outside of the comfort zone of the camera's metering and automatic exposure possibilities. The things where the camera just says "LO" in the display because it's too dark to meter. Exposures that are longer than 30 seconds so you have to set the camera to "Bulb". I want to share my observations here.

But first... why the hell would you do that?! To make abstract photos of things in motion, of course. Those who know me know that this means water, mostly. But it's also clouds. Or cars on the Freeway. Or star trails. With water, a 30 second exposure of the surf crashing onto a beach is not long enough - the water will still show some remainders of structure most likely. But with exposure times beyond 2 minutes, the water really turns into a surface without structure... and then have a look at the reflections of rocks and such. Sigh. So beautiful. Photos like that have an otherworldly, tranquil quality.

Now, my pathetic examples shown below are a shame compared to some of the gorgeous works on the net but hey... I'm practicing. And on that particular evening Flat Rock at Torrey Pines State Beach was "a zoo" as a fellow photographer on Google+ put it. People everywhere! It's clearly the sort of photos that are easier to make in Winter. :P


Star Destroyer Flat Rock (NIKON D700, 278s @ ISO 800; f/11, 48 mm (in 35mm)

So, how do you approach this type of photo? How do you determine the correct exposure time when it's so dark that the cameras meter doesn't work? Praise the benefits of digital photography, of course. You simply crank up the ISO all the way to the max, leave the lens wide open, make a quick and dirty test exposure, and then check the histogram.

Correct the settings and make more test exposures until things are looking good (expose to the right but don't overexpose), and then start doing the math: 1 second at ISO 25600 means 2 seconds at ISO 12800, means 4 seconds at ISO 6400, means 8 seconds at... wait a minute. That's annoying. Yes, I already mentioned the Android app "PhoForPho" in my previous post - it contains an exposure calculator! You enter the settings of your test exposure and your desired settings for the final exposure, and it will tell you the correct exposure time. It also has a setting to compensate for the use of neutral density filters, and you can pass the exposure time directly to a timer - awesome! (I'm pretty sure that there's a myriad of similar apps available for the iPhone, too.)

For the photo above, it told me that my exposure time would be something like 18 minutes at ISO 200. That's pretty darn long of course, and the reason why I switched to ISO 800 (yes, it was very dark already). My choice of f/11 was rather stupid of course, looking at the photo f/8 would've been enough easily. But believe me... with all the things you have to take care of, forgetting to check if the aperture setting makes sense is excusable. :)

There's some obstacles when it is so dark: one is composition (yes I know, the photo above shows more a lack thereof but again... I was just practicing). It's easier to frame before it gets dark, of course, but it's helpful anyway to bring a flashlight. The other obstacle is focusing - and a flashlight is a good helper for that, too.

Now, the D700's auto focus is awesome even in low light, but the tiny AF helper light often is not enough. With a flashlight, you can either help the camera's AF, or at least illuminate the distance scale on the lens when you focus manually. :) The most annoying thing in that situation btw. are lenses that focus "beyond infinity"  (ie. all of Nikon's modern AF-S lenses), rendering everything slightly out of focus at that setting. While I'm pretty sure there's a wonderful technical explanation why these lenses have to do that, it still sucks. It's best to test it during the daytime so that you know what to do when it's dark.


Solana Beach Tidepools (NIKON D700, 142s @ ISO 100; f/8, 16 mm (in 35mm)

But there's yet more obstacles. The closer you are to the equator, the quicker dusk and dawn happen. I was making these photos at dusk, and by the time I was finished typing the values into the PhoForPho app, it had already gotten considerably darker (yes, really - no kidding), and a 4 minute exposure turned into an 8 minute exposure! Remember... from 4 minutes to 8 minutes is just one stop longer. (which also means: it really doesn't matter if you accidentally close the shutter some 10 or 15 seconds later.)

The next obstacle is noise, or more precisely, hot pixels. With the sensor being "on" for such a long time it heats up, resulting in hot pixels (or subpixels) that do not record the actual light anymore, but are more like micro-overexposures on a pixel level that appear in the photo in the form of a tiny white or red or green/cyan dot.

I've never had any problem with hot pixels on the D700 when I stay within reasonable ISO limits and do not exceed 30 seconds exposure time. There just are no hot pixels. (my Fuji S5pro was quite different in that regard, ahem.) Because of that, I usually had the "long exposure noise reduction" turned off. It just doesn't make sense for "normal" long exposures within the cameras automatic exposure range.

Nikon's long exposure noise reduction is a dark frame subtraction: after the actual exposure, the camera makes another, equally long exposure with the shutter closed, recording a map of the hot pixels, and then removing them from the actual exposure with that mask. That means: making a 5 minute exposure means a total wait of 10 minutes. Now you know why I prefer to keep it off for the exposures up to 30 seconds. :P

But with these really long exposure times, especially when it's necessary to use higher ISO (like in the first photo above), it's absolutely necessary to use it. Darker areas in the frame are more prone to show hot pixels. The first photo was made without long exposure noise reduction, and it looked horrible. And yeeeeeees, it definitely takes longer to clone/heal the hot pixels in post processing than simply waiting for the camera to do it automatically. (you can of course move the camera to a new/different shooting position and begin composing your next frame already while the camera finishes the exposure with the noise reduction job.)

Which yields the last obstacle: time. You can't make these photos when it's pitch black (exposure times would become waaaaay too long because you most likely need to stop down the lens for some depth of field if you're going for the seascapes and some rocks and stuff, like me), so it's either dusk or dawn. You can make two, maybe three exposures within 30 minutes, but that's it. You better know what you're doing then. That's why I practice. :)

[and for the sake of completeness, here's the list of the other, small obstacles: after focusing the lens, switch to manual focus. Make sure VR is off. And Auto-ISO of course. You need a cable release with a shutter lock feature obviously (or one of those fancy programmable cable releases, in that case you don't need the timer). Make sure you get the camera strap out of the way. And keep the dog away from the tripod! That sort of thing.]