How To Calculate Long Exposures
Your camera’s meter does a pretty decent job of giving you an accurate exposure for shutter speeds up to 30 seconds. But what if you want or need an exposure time of more than 30 seconds or perhaps several minutes? How do you calculate long exposures, the proper ISO and shutter speed combination, to avoid having the image be over or under exposed?
It’s easy. First, establish a base exposure by using 30 second as your shutter speed and ISO as the exposure variable for preliminary calculations. In Manual exposure mode, select 30 seconds for the shutter speed, an ISO of 100, and the appropriate f-stop for depth of field considerations (This is all done after you’ve focused and finalized your composition, obviously). During late twilight, early dawn, or if using a strong neutral density filter, the meter should show the image as being underexposed at this point.
Now, simply increase the ISO in full stops until the meter indicates that the scene is exposed correctly. Count the number of increased stops. If it was two stops, for example (ISO 100 to 400) then you just add those two stops to the shutter speed (30 seconds to 2 minutes) after resetting the ISO back to 100 and the exposure mode to Bulb. These are reciprocal exposures (30 seconds and 400 ISO equals 2 minutes and 100 ISO).
An important consideration when doing long exposure photography in the early morning and late evening is that the light is changing very quickly. If you are doing a 3-minute exposure, for example, the intensity of light at the begining of the exposure will be very different than when you finish. That means there’s a risk of overexposure during the fast-changing light of the mornings and underexposure in the evenings. This should be factored into the information offered above in order to get the correct exposure during these times of day.
For additional help, Lee Filters Stopper Exposure and ND Timer are two phone apps that can help calculate long exposures as well.
Nice Info, Thanks
Thank you so much for this detailed blog on long exposure. This is SO valuable for not only hobby photographers like myself but beginners and those struggling with the technical aspects of setting up their cameras … I especially appreciated this so much because when I get out there to shoot I forget these settings. So thank you!! We appreciate the time you take to post this for us.. now I just need to practice and apply it.
Great read! A very interesting concept, first time I’ve learned about this technique… will definitely be trying it soon
Math = me crying in the fetal position in the closest corner!
Thank you for your tip. I’m still trying to wrap my mind around it, though. I use the app Photo Pills to do my long exposure calculations. However, I’m having a difficult time determining how you determined that the two stop increase from ISO 100 to 400 equals 2 minutes. Can I assume that for every 1 stop increase in ISO, I should add 1 minute to the 30s? So let’s say it looks properly exposed at ISO 1600. Would that be a 4 stop increase, so when I shoot at ISO 100, I would dial in 30s + 4minutes? Thank you for your help with this!
Floyd, you have to double the exposure time for each stop of light. So 30 seconds to 1 minute equals one stop and 1 minute to 2 minutes equals the second stop. If we needed three it would be 4 minutes, etc.
Richard: great explanation for Breakthrough Photography Filter Systems. I have started to invest in this system and you have definitely answered my query’s as to which applies best to my interests.
I was investigating ‘LEE ” and like the glass concept of B/T beter.
Thank you for sharing your experience.
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Best Regards,
Thanks! Nice and simple! Defo one tip to log away in memory for that special moment.
I have read professional photogs who seemed to decide that a 30″ exposure is what they want from the start. I am very confused as to how they go about doing this. Your example says that the initial settings should include a 30″ exposure and then use the ISO to increase the exposure since the 30″ will be underexposed. Continue until properly exposed. Just how dark does it have to be for 30″ to be underexposed? Can this technique used in brighter circumstances??
Great information! Long exposure photography is a lot of fun and it is handy to have pointers available to make the exposures work. Thank you for all you do and the knowledge you share!