ISO and Time Exposure

[ggoodroe]

One of the areas I plan on getting back into is Time exposures...

 

regarding digital cameras and ISO...

 

1) Would a lower ISO camera (like the D200 ISO 100 vs D70s ISO 200)

be more sensitive in metering time exposures?

 

2) are these cameras sensitive enough to meter Time exposures at all

or should I just go get a Luna Pro and say the heck with it?

 

Any help appreciated.

 

George

St. Petersburg, FL USA

[ellis_vener_photography]

Long exposures with digital cameras can be problematic. The sensors get warm fro mthe

long application of electrical current and this results in random bits of image "noise". The

best results I've gotten with exposures longer than a few seconds have been with the top

of the line Canon DSLRs (1Ds & 1D mk.2 versions -- and here the 1d mk. 2 seems to be

better than the 1Ds mk.2) and the Nikon D2X.

[phule]

A number of digital SLRs (and many point and shoots) offer the option to do a Dark Frame Subtraction. This is where the camera takes a second exposure without opening the shutter or flipping the mirror out of the way and then subtracts the noise in the second image from the first image.

 

This can give you very clean images with long exposures (to a point), but (of course) it doubles the exposure time for any one image.

 

For /very/ long exposures (I'm thinking 5-6 hour night photography exposures), where the temperture may change significatly between the first exposure and the dark-frame subtraction exposure, this method probably will not work very well.

[SCL]

Long time exposures aren't going to be effectively metered by either a DSLR or handheld meter. The advantage with a DSLR is that you can do a test exposure and immediately review the results.

[fourfa]

are you shooting while tethered to AC power? If not, battery life on multi-hour exposures will be a big problem.

 

this is one area where film has a distinct advantage. I've been highly frustrated by Nikon on this account, as I like to to do long nighttime shots quite a bit. The AF cameras drain the batteries quickly (like a single frame), and the MF cameras can't use the G lenses I'd prefer to use (12-24, 70-200, 18-70, 24-85, 17-55 etc etc)

[steve_muntz]

Lower ISO's are less sensitive than high ISO's. One trick to get you in the ballpark is to crank it up to ISO800 or 1600 to get a reading, then lower back to the ISO you'll shoot with and convert the reading you have (ISO800 being three stops more sensitive than 100, etc). It won't be perfect, but a good starting point and you can use the histogram to adjust from there.

 

How long are you talking about? 30 seconds looks pretty good on my Fuji S2 at ISO100, but 5-6 hours isn't going to happen on a DSLR.

[phule]

<< but 5-6 hours isn't going to happen on a DSLR. >>

 

As Andy pointed out, with dedicated power (i.e. AC input) it could.

 

After all, astronomers have been using CCDs for long exposures (multiple hours) for many years now. Of course, these sensors are cooled with liquid nitrogen to keep the noise levels as low as possible.

 

Still, a really cold (precipitation free) winter night with dedicated power and a DSLR could make for an interesting experiment. :)