Roving Thoughts archives

How long an exposure can my camera meter?

A friend and I were talking about long exposure photography today, which brought up the issue of determining how long an exposure you needed. Like most DSLRs, my Nikon D90 will only automatically expose out to 30 seconds; if you need a longer exposure than that, you have to time it by hand in Bulb mode.

(In my opinion this is a pointless limitation, probably reflexively carried forward from the days when there was only so much space on a physical shutter speed dial. Since cameras are computers these days, you could even limit automatic exposures to no more than 30 seconds while still letting the camera count the time for longer manual exposures.)

But that's just how long an automatic exposure the camera will do, not the limit of what it will meter for you. If the exposure is at the 30 second limit and the viewfinder is still reading 1 EV of underexposure, you can fix that with an exposure that has 1 EV more time, ie a 60 second bulb exposure. So how long an exposure can I actually meter with my camera?

• in my normal settings, the viewfinder will show up to 2 EV of underexposure. If I change the camera to use 1/2 EV steps for exposure compensation instead of 1/3 EV steps, this increases to 3 EV of underexposure.

• I can extend the range of the viewfinder meter by adding negative exposure compensation (a 1 EV underexposure with -1 EV of exposure compensation will read as a correct 0 EV exposure). My camera allows up to +/-5 EV of exposure compensation, pushing metering to an effective 8 EV of underexposure.

  (The allowed range of exposure compensation doesn't changed based on the step size used.)

Since every EV is a doubling of the exposure time, 8 EV of exposure past 30 seconds is 128 minutes; call it a two hour exposure. If this isn't enough, I can start pulling the viewfinder meter back by metering with a high ISO but actually photographing with a low ISO. My camera has a base ISO of 200 and is easily raised to ISO 3200, giving me another 4 EV of metering range (a total of 12 EV); that takes me to an exposure duration of over 30 hours.

So the real answer here is that my camera battery is going to die long before I get to an exposure that I can't meter. (Probably the sensor would overheat from continuous use, too.)

(In theory the D90's meter is only specified to work down to -1 LV. In practice this isn't even moonlight and I've metered without problems in much darker conditions.)

PS: since I'm unlikely to sit still for a one hour exposure, much less a two hour one, I don't even need to change the exposure compensation step size. My normal settings still give me 7 EV of exposure past 30 seconds, which is already just over an hour. Mind you, using 1/2 EV steps makes the eventual time calculations somewhat easier.

Swallowing a whopper: Giant Robo and the Tragedy of Bashtarle

I tweeted:

The only way I can swallow Giant Robo's Tragedy of Bashtarle is to believe that Daisaku had the most sheltered childhood ever.

This is the kind of thing that only fits in 140 characters if you already know what I'm talking about. So, a blog entry. There are spoilers for the second episode of Giant Robo.

In the first one and a half episodes of Giant Robo, the Tragedy of Bashtarle is presented as a big mystery. Daisaku, our 12 year old protagonist, has never heard of it and has no idea what it is, while both Big Fire and the adult Experts of Justice seem all too familiar with it; mention the Tragedy and everyone shuts up and looks sad. We learn various bits about it over the first episode, such as that it happened ten years ago, that it's allegedly been scrubbed from the history books, and finally that it was some epic Shizuma Drive related disaster involving one of the scientists who invented it going too far. So far, so good; this is all believable and builds tension.

Then partway through the second episode we find out the details. To wit, that the Tragedy of Bashtarle not only destroyed an entire country (the eponymous Bashtarle) and killed one of the five scientists who shared a Nobel prize for inventing the Shizuma Drive, but its seven days of aftereffects also killed 2/3rds of humanity. All of this happened only ten years ago.

Daisaku was two years old then, and he grew up in the Tragedy's immediate aftermath (and he's interested in and knowledgeable about the Shizuma Drive). Yet somehow he has never heard of the Tragedy, not just by name but at all. The massive death toll (and disruption of life) had so little effect on his childhood that he could be completely oblivious to it. Hence why I say that Daisaku has to have had the most sheltered childhood ever in order to make his (and the audience's) lack of knowledge even vaguely plausible.

(Yeah, yeah, Giant Robo isn't the kind of anime where you're supposed to think like that.)

(This has come up now because I'm (re)watching Giant Robo as the first show in my vague 'watch some classic anime' plans for this year.)

PS: the Tragedy of Bashtarle demonstrates that Giant Robo's setting is a lot nicer than the real world (or was, before the tragedy's effects hit). In this world, I'm pretty sure that losing all electricity, machinery, and vehicles for seven days wouldn't come anywhere near that sort of death figures because a significant portion of humanity isn't well enough off to depend on those things in the first place.