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Learning to Evaluate Correct Exposure for Long Exposures

When I do long exposures, I don’t want to carry bag full of equipment. I’ll just grab my camera, put it on tripod and go. There’s no room for light meter of any kind. But how do I get exposures right? Here’s my tip.

Photo by jutei

At first, you have to have some kind of basic understanding what is aperture or f-stop. Here is an excellent article by amirulshahrom where you can gather all knowledge needed here.

The idea is use to your dSLR’s light meter as a learning tool. Take your dSLR outside and point it to different scenes. Try to memorize suggested shutter times in different scenes. That’s how you will slowly learn to evaluate different conditions without any light meter…you kind of become light meter yourself.

But before you go outside, here’s how you do it. First set your dSLR to aperture priority mode. This way your camera gives you correct shutter speed for your chosen f number. Next set your dSLR to match your film camera. With Diana Mini, shooting square format the lens is 30 mm, aperture on cloudy setting is f8, shutter time is 1/60s. (Remember that in some dSLR’s you have to multiply your lens’ focal length to get it match your film camera’s focal length. For example in my Nikon D80 I have to multiply my focal length with 1.5 to get Diana’s 30mm, I set my Nikkor 18-50mm to about 20mm.)

Put your dSLR on highest ISO setting it can do (for me it’s only 1600). Reason why we set ISO to highest setting and not to match your film’s IS is, that sometimes conditions can be too dark for your dSLR’s light meter. If it’s too dark my D80 won’t suggest any shutter speeds, it just says “low”. With higher ISO you can get a reading and then just calculate it to match your film speed. You can do these learning trips as often as you like or just point dSLR out of window just to get some rough numbers before you go out shooting

Here’s an example:
If your dSLR suggests you shutter speed of 4 seconds at ISO1600 and you’re shooting ISO200, your correct exposure would be 12 seconds. There is a 3 f-stop difference between ISO1600 and ISO200, so all you have to do is multiply suggested shutter speed by 3 (4s x 3 = 12 seconds).

But wait, there’s one more thing you should remember: that evil reciprocity! Basically it means that in long exposures the film will need more time than light meters suggest. I haven’t been too fussy about this, but if the time is more 5 seconds I might throw in couple seconds more, just to avoid underexposure.

I hope this helps evaluating your exposures.

written by jutei

4 comments

  1. marcus_loves_film

    marcus_loves_film

    Nice article and clever use for dslr if you don't have a dedicated meter! Easy tip for reciprocity failure: when the meter shows exposures greater than 1 second simply square(²) the number. For example 2²=4, 3²=9 4²=16, 5²=25. The reciprocity failure for each film is different but using this formula will get you in the ballpark.

    about 2 years ago · report as spam
  2. jutei

    jutei

    Thanks for the tip. I didn't know reciprocity failure affects that much!

    about 2 years ago · report as spam
  3. francoiswnel

    francoiswnel

    Hi, not to be rude but your exposure math is wrong! If the correct exposure is 4 seconds at ISO 1600, then the correct exposure at ISO 200 would be 32 seconds, not 12 seconds. Stops of light are not linear, so a difference of 3 stops is actually 8 times the amount of light. 4 seconds x 8 is 32 seconds. Another way to look at it - the exposures below are all for the same amount of light (assuming aperture stays constant) 4s @ ISO 1600 8s @ ISO 800 16s @ ISO 400 32s @ ISO 200 64s @ ISO 100
    about 2 years ago · report as spam
  4. jutei

    jutei

    Ok, thanks for correcting me :)

    about 2 years ago · report as spam

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