How To Determine Long-Exposure Settings For a Night Photograph

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Although the noise performance of modern camera sensors at high ISO settings has improved dramatically over the past decade, photographers who demand clean, noise-free images must still wrestle with the problem of eliminating high ISO noise. One strategy is to simply avoid shooting at high ISO settings whenever possible by lowering the ISO and extending the exposure time. The question that is often raised is – “what settings should I use to obtain a proper exposure”? This is not a particularly difficult question to answer, however the answer becomes infinately more useful and valuable if one understands the reasoning behind the solution. This article is my take on solving this problem.

It is important to understand what a proper exposure actually is in the context of low-light or nighttime photography. A proper exposure is NOT determined by how the image looks when reviewed on your LCD screen after exposure. Your LCD display will fool you every time – it is not determinative of a proper exposure. What IS determinative is what is shown on your histogram, as this creates a graphic representation of the data actually captured and recorded by your camera sensor. So let us begin by learning what the histogram is, and how it can be effectively used in the context of our night photography. The following graphics explain the concepts we need to know in order to determine a proper exposure.

NOTE – Click images to view full size (Tablet or PC)

DETERMINING LENS APERTURE (f STOP)

So now that we know what a proper night exposure histogram looks like, how can we determine the settings to create an image with a good histogram, which also minimizes high ISO noise in our image?

The first exposure setting I prefer to establish is the lens aperture. This is a balancing act- the wider the aperture, the shorter the exposure time needs to be, however, the wider the aperture, the shallower the depth-of-field (DoF) that is produced. My usual procedure is to select the widest aperture that still gives me the necessary DoF my composition requires. To do this, I refer to one of the readily-available Hyperfocal Distance Calculator apps that resides on my smartphone. A test shot taken with the lens focused at the calculated hyperfocal distance will allow me to review the exposure to be certain the my subject matter is entirely in focus. If not, I will reduce the aperture (thus increasing the depth-of-field), refocus at the new calculated hyperfocal distance, and try again. When I am satisfied with the focus at the selected aperture, I can then move on to determining ISO and exposure time.

DETERMINING ISO AND EXPOSURE TIME

Ideally, I would like to shoot my long-exposure at my camera’s native ISO, which in the case of the Pentax K1 is ISO 100 (this is a common native ISO, however your camera may vary). Shooting at the native ISO will minimize random noise in the image. the following image comparison shows the extreme noise difference between an ISO 6400 shot and an ISO 100 shot, which is the whole point of doing a low ISO, long-exposure in the first place.

Noise Comparison ISO 100 vs ISO 6400

So let us figure out what exposure time we need in order to get a proper histogram if we were to shoot at ISO 100. We could do this with guesswork or trial and error, but that would be frustrating and wasteful of our precious shooting time. Fortunately, there is a quick, simple technique that can be employed to help us figure out our exposure.

Step 1

Set your ISO to 6400. Set your exposure time to 15 seconds. Take a test exposure and check the histogram in your image review. If the histogram is too far to the left, increase the exposure time. If the histogram is too far to the right (possible, but unlikely in most circumstances) decrease the exposure time. Take another test shot and evaluate the histogram. Continue adjusting the exposure time and taking test shots until you are satisfied that you have a proper histogram. Make note of these exposure settings.

Step 2

You will now use the previously noted exposure settings to calculate an equivalent long-exposure time at ISO 100. There are two ways to do this. One way relies on any of the readily-available smartphone Exposure Calculator apps. There are many available, you may already have one at your disposal. You just enter the test exposure information, enter the desired ISO, and the app tells you how long the new exposure should be at ISO 100 (or any other ISO you choose).

A second method utilizes the “Six Stop Rule.” Simply stated, this rule says that the number of SECONDS of exposure at ISO 6400 will equal the number of MINUTES of exposure at ISO 100. For example, if the test exposure that produced a good histogram was 15 seconds at ISO 6400, the equivalent exposure will be15 minutes at ISO 100.

FINAL THOUGHTS

While shooting a long-exposure at native ISO is the ultimate goal, it is not always absolutely necessary to shoot at that low an ISO. Perhaps, like me, you prefer to shoot long-exposures using the Long-Exposure Noise Reduction (LENR) function available with your camera. In that case, the total camera time will be double the exposure time (since the camera is taking a second exposure with the shutter closed in order to accomplish a dark frame subtraction, which cancels out hot-pixel noise). Therefore, a 15 minute exposure actually takes 30 minutes of field time, which may be unacceptable to you. Perhaps you are shooting a panorama or mosaic, in which case you may simply not have enough time to capture all of the long-exposures at ISO 100. Under these circumstances, you might well consider shooting your long exposures at ISO 200, ISO 400 or even higher. The choice is yours, and your decision will be informed by your knowledge of the characteristics of your camera and the circumstances you are faced with on any given night. but now you have a framework for quickly determining what exposure settings to use when shooting a long-exposure night photograph.

Pocket Softbox App For Night Photography

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Pocket Softbox is an Android app (available from Google Play) that turns your smartphone or tablet into a nice low level lighting source when photographing at night. In the photograph shown above, I used Pocket Softbox for two purposes. First, I used the app to light the trees during the course of several long exposures by walking up and down the roadway, holding my phone in a manner that cast light up into the treetops. Second, whenever the ubiquitous fireflies settled down and ceased their display, I found that by walking around for a minute or two with the Pocket Softbox app set to 3200°K, the fireflies would resume their flashing display. Simply clicking on the Pocket Softbox icon (above) instantly brings up the app in the last mode that was used. This is what the app look like in use. I have selected a lime green color by sliding my finger in a vertical direction on the screen, and the brightness was selected by sliding my finger horizontally on the screen. The settings can be locked, which prevents inadvertent changes in color or intensity. In RGB color wheel mode, any possible color in the RGB color space can be selected, so if you want to light paint with color, this is the mode to use. If you select the Kelvin Color Scale mode, the app produces white light at the selected color temperature. The app comes with 7 commonly used Kelvin presets, as shown above, but you can save any number of your own commonly used settings as a preset. As an example, I named and saved two – Pinkish Red and Bluey Blue. Saving a preset is as easy as selecting Save Preset from the menu and entering the name you want to use in the dialog box. You can choose to use Eye Light mode from the menu, which produces a round shape as seen above. I have not found anything useful about Eye Light mode for my photography, with the exception that it is also used in Strobo mode. When Strobo mode is selected from the main menu, you are presented with the screen above, where you choose the duration of the light and dark cycles. The app will then flash your chosen color on and off through the Eye Light circle, using the parameters you have set on the screen above. This is useful for helping control the amount of light being added to the exposure by counting the number of times the light flashes. Not enough fill light? Up the number of flashes. Too much fill light? Reduce the number of flashes. This makes your light painting controllable and repeatable. I recommend this app without reservation. It is free, has no advertising, is small, and requires no special permissions. A very nice simple light painting tool to have in your bag of tricks!

[Sold Quickly] FOR SALE – Ocean Kayak Frenzy & Seaquel Oar – $150.00

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This heavy-duty Ocean Kayak Frenzy solo sit-on-top model is an ideal blend of design concepts that make it well suited for various kayaking conditions. From ocean kayaking, surf riding, river floating and moderate white water, the Frenzy does it all. The shorter length of the Frenzy compared to other models allows for excellent maneuverability, while the tri-form hull and keel design creates a kayak that tracks in a straight line without undo fuss on the paddler’s part. The wide stance of this kayak makes for a very stable kayaking experience. It is easy to board from in the water, and actually takes an effort to capsize this boat. This is a particularly fine kayak to scuba dive from due to its outstanding stability. There is a tank holder molded into the stern of the kayak, and heavy duty straps to hold all your gear. Here are additional views of this kayak. It is an older design than is produced today, lacking a molded-in cup holder and side handles, otherwise, the design is essentially the same as the current Frenzy model from Ocean Kayak. I have added on the Scotty Bait Caster/Spinning Rod Holder pictured above. I have also added a bungie cord style oar holder on the side opposite the rod holder, so fishing off this kayak is a breeze, especially with the stable, wide stance this kayak provides. Also included in the sale is a fine quality Aqua-Bound Seaquel oar. This oar separates into two sections for easy storage and transport. This is an aluminum oar, which Aqua-Bound has discontinued, but I paid approximately $70.00 for it when new. This is truly an ocean worthy kayak. I used it with much enjoyment at California’s Channel Islands National Park while living aboard my trawler in Southern California. You can’t go wrong buying this kayak and oar for the bargain basement price of $150.00. You will have to come pick it up in the Omaha, Arkansas area. Cash only – first-come, first-serve! E-mail me at hal@mitzenmacher.net if interested.