Long Exposure Calculations

We'll be heading out for some night photography next month. Playing with long exposures is one of the thing you can't get around. Especially since I don't have any f/0.2 lenses in my bag.

Most lenses are sharpest (the so-called sweet-spot) around 2 stops from their biggest aperture (mostly around the apertures of f/8 and f/11) , but the problem is that these apertures kill the available light hitting the sensor of your camera. You could turn up the ISO (to 102400 if you have a Nikon D3s), but that gives you noise, and lots of it with most consumer dSLR's.

The only proper thing to do is adjusting the shutterspeed. Leave the aperture on its sweet-spot (f/9 or f/11), and the ISO at ISO100 or ISO200 (for the lowest amount of noise). But if you do that, chances are that you need exposures much longer than the available maximum on your camera, which is 15 or 30 seconds (depending on your camera). Which leaves you with the bulb setting on your camera. But how do you calculate the time needed for an exposure?

Bulb means that the shutter stays open for as long as you press the shutter release button.

All this isn't necessary when you're a pro dealing with these situation all the time. Because you'll 'see' the light and know instinctively what settings to use, but we're not all professionals. So we need a trick to get to those settings as fast, and as accurate as possible.

The following part is based on the settings and controls of a Nikon D300. Please consult your manual for the settings on your camera.

First we need to establish the camera settings with which the camera can calculate things itself (place the camera in Aperture Priority mode). This means cranking up the ISO as high as possible, and opening the aperture as far as possible. (e.g. ISO3200, and f/5.6).

Let's say that with these settings, the camera calculated an exposure of 5 seconds. You could take a picture, but with the aperture wide open and the ISO setting at static noise, the result can't be that good. We want crisp photos, so we need to adjust the ISO and aperture to a more pleasing setting.

From f/5.6 to f/11 is a 2 stops difference, and from ISO3200 to ISO200 is a 4 stop difference. Which results in a total of 6 stops. This means that we need to adjust the exposure by 6 stops in order to compensate for the aperture and ISO setting we changed.

5 seconds + 6 stops = 320 seconds exposure
(every stop doubles the exposure time)

This is way more than the maximum of 30 seconds the camera can produce, so we need to use the bulb feature on the camera. This feature is accessible in the manual mode. Just keep on dialing your selection'wheel' (or whatever control you use to adjust the exposure in the Manual mode), till you see a bulb setting in your display. Also, adjust the aperture to f/11 in Manual mode (these settings need to be confirmed, since you came from the Aperture mode on the camera), and the proper ISO setting of ISO200.
All you have to now is use a remote to hold down the shutter for 320 seconds.

The procedure described takes a bit of math (5 seconds * (2*2*2*2*2*2) = 320 seconds) and a whole lot of knowing your 'stops'. But if you're a bit lazy, can't be bothered with them, or just want to make sure, there are certain aids you can use. One of those aids is an iPhone/iPod Touch app called PhotoBuddy.

This application can do the calculations for you, and more, much more. The calculations are just a small part of the features available, and that for less than $2 USD.

The full set of features are:

• Calculate Sunrise/Sunset and the moon phase
• Determine the moon phase for any given date
• Measure distances with the built in camera (1st Gen. iPhone only)
• Built in exposure presets
• Calculate exposure changes (f-number / shutter / iso) based on the reciprocity law
• Calculate the Depth of Field (hyperfocal distance, near/far limit). Test the calculation online at http://iphone.ambertation.de/photobuddy/dof
• Find the maximal distance a flash can light using a given aperture / iso
• Calculate the minimal camera distance to fit objects of a given size on your photo
• Set the white balance of your camera by a list of color temperatures for common lighting situations
• Supports metric/imperial units
• Supports full, half and third stop
• Database of digital SLRs for simple setup (if your camera is not listed, you can simply choose the film size)
• Database of common motion picture cameras
• Custom Camera Settings (Film Size)
• PhotoBuddy remembers all your settings. You to continue right where you left
• Added Support for the new "Micro Four Thirds" Cameras as well as several new Models introduced at Photokina 2008
• CoC Calculator (based on desired print size, resolution and viewing distance)
• Change the CoC multiplicator for your cameras to fit your needs
• Support for ISO values up to 25600
• Bulb Timer: Allows you to take long exposure shots (up to 13 hours) in the bulb mode of your camera
• Interactive DoF graphic (change the depth of field settings with a flick of your finger)
• Bellows exposure compensation
• Camera presets for Red One and Hasselblad
• Angle of View Calculator
• HDR Bracketing Calculator (determine correct shutter speeds for multiple exposures used for high dynamic range images
• Quick camera selection

Needless to say that during these sort of photo shoots you need at least a good tripod and a remote (wireless/cable) shutter release.