N90 Exposure time when bigger than 30 seconds.

On a Nikon N90, in aperature priority mode in very low light, I

sometimes get the "low" reading on the meter (meaning that the

longest exposure time-30 seconds- is not enough to properly expose

the photo) Hower, it will still let me take the picture. My

question is, when I do take this picture (with lo showing in the

viewfinder), how does the camera determine the shutter speed--does it

just "guess" or pick something or does it hold open until enough

light has entered to make a proper exposure? Any ideas?

When the camera indicates "low", it means that 30 seconds

shutter speed isn't enough to achieve the proper with whatever

f-stop you've combined it with. Unless you manually overide it,

the camera opens the shutter for the 30 seconds because that's

the maximum shutter speed that it can automatically work with. If

you need longer times, you'll need to manually set of the shutter,

preferably with a cable release, at the bulb setting. many old

mechanical cameras could only be set at the longest of 1

second before the photographer had to result to the bulb setting.

Note also that no camera will never compensate for the film's

reciprocity law failure. It can't, because the

reciprocity characteristics are different for each film, and

the camera doesn't know specifically which film you're using.

<p>

Reciprocity law failure will almost certainly cause a significant

exposure error at the kind of shutter speeds you're talking

about. With many films, significant correction becomes

necessary around 1 second, but check your film's data

sheet.

<p>

In other words, autoexposure isn't particularly helpful at

such long speeds, regardless of how well the camera tries to do it.

To further add, if you open the aperture to f4, and the meter

indicates you need 20 seconds, if you then stop down to f5.6, the

shutter speed the meter needs will be 40 seconds to

compensate. But since the camera can only automaticcally go

up to 30 seconds, it will indicate "low" on the LCD screen and

then by default set the shutter to 30 seconds. So the only

solution would be to manually set the shutter at the 40 seconds

at the bulb setting.

But, as pointed out, you still haven't taken into account the

reciprocity characteristics of the film, which will require you to

compensate by adding time to what the meter has already

indicated if it requires more than 1 second (assuming you have

not set the camera's exposure compensation [EV] buttons to

increase exposure by, say, half a stop or more). But that's a lot

more complicated topic.

Ok, I understand that better now. Thank You for responding and wow, I was impressed with the speed that I got an answer. Look forward to using this again, and hopefully someday being able to contribute answers. Thank You

MIke