Category: Accident Reconstruction

(Note: This post incorporates several important fundamentals along with the discussion of controlling sun stars. As always, it’s all summarized in the Takeaways at the end.)

A previous post illustrated how the number of aperture blades affected the number of rays produced in a sun star. As discussed, sun stars are created from bright spots of light in an image when the lens aperture is stopped down. While prominent sun stars might be desirable for landscape or creative photographs, they are usually unwanted in forensic photos. In fact, there have been instances where photographs with distinctive sun stars have been limited or disallowed. Here’s how to control—or even eliminate—their prominence.

There were three small, distinct bright light sources in the following night scene. A series of images was made at every whole aperture from f/16 through f/2. (As a reminder, there are six stops up from f/16: f/11, f/8, f/5.6, f/4, f/2.8, f/2.) Only four of the seven total images—each two stops apart—will be shown below. The other three intermittent images fit in the progression as you’d expect from what you’ll see below.

This first image was made with the aperture stopped down to f/16. As expected, the sun star rays were most distinct at this aperture. [Click on image to enlarge, then click on back arrow to return to this post.]

Opening up two stops to f/8 noticeably reduced the sun star effect. [Click on image to enlarge, then click on back arrow to return to this post.]

Opening up two more stops to f/4 almost completely eliminated the sun star effect. This should be acceptable for any foreseeable use. [Click on image to enlarge, then click on back arrow to return to this post.]

Finally, opening up yet two more stops to f/2 eliminated any trace of sun star rays. [Click on image to enlarge, then click on back arrow to return to this post.]

As you can see, the wider open the aperture (which means the lower the f-number), the less pronounced are the sun stars. But you can’t just adjust your aperture to either intensify or to eliminate sun stars. Whether it is daytime or nighttime photography, changing your aperture affects your image in two main ways: exposure and depth of field (DOF). 

Recall that the smaller the f-number, the larger the lens opening. Just like 1/2 of a pie is twice as large as a 1/4 of a pie, a lens aperture of f/2 is twice the diameter of f/4. Here’s how changing aperture changes exposure and DOF:

Exposure: Opening the lens aperture lets in more light, which brightens the exposure. To keep the overall exposure the same, you must compensate by the same number of stops by using a faster shutter speed, lowering your ISO, or using a combination of both. Since all the above images were already at my Nikon Z 8’s lowest ISO of 64, my only option was to select a correspondingly faster shutter speed every time I opened up the aperture.

This means that for this series of four images, every time I opened up the lens aperture by two stops, I had to use a shutter speed that was two stops faster. As you can see from the captions under the images, at a constant ISO 64, the f/16 image required a 2.5 second exposure. Opening the aperture two stops to f/8 required a shutter speed of 0.6 seconds, which is two stops faster. Likewise, f/4 required 1/6 second and f/2 needed 1/25 second to keep the same overall exposure. Each were two stop increments of aperture and shutter speed.

As a side note, since the camera remained on a tripod throughout all the photographs, the shutter speed had no effect on the sharpness of the images. But changing shutter speeds will definitely affect the appearance of any moving elements in the image frame.

Depth of Field (DOF): DOF is how much of the scene—from near to far—is in acceptable focus for a given focus point. DOF is controlled by aperture. The more open the aperture, the less DOF. Conversely, stopping down the lens aperture increases DOF.

A deeper DOF is more critical in daytime photographs where almost everything is visible and, in most forensic photography, should be acceptably sharp. At night, a more shallow DOF can be perfectly acceptable, especially if the background and foreground are mostly black, as in the photos above.

As you can see on the enlargements by clicking on the photos above, f/4 would likely produce acceptable DOF. Depending on your case, even f/2 (or an aperture between f/2 and f/4) might give you all the DOF you need. As the photographer, you need to (and you get to!) decide on the tradeoff between DOF and sun star prominence.

Takeaways:

-1- You can control the prominence of sun stars from small, bright light sources (day or night)  by your choice of aperture. The more stopped down your aperture, the longer and more prominent the rays. Opening up the aperture will shrink the rays until they essentially disappear at the most open apertures.

-2- Opening up or stopping down your aperture will also affect the DOF in your image. You must decide how much DOF you will need. That amount would likely be different for daylight versus night photos—even of the exact same scene.

-3- Just like with any other photography, opening up or stopping down the aperture will require that you correspondingly adjust your shutter speed, ISO, or a combination of both, to maintain the same overall exposure. Make sure your shutter speed is sufficient for your image, especially if there are moving objects in your frame. Also, setting your ISO as low as possible minimizes noise and maximizes dynamic range, both of which are even more important in night photography.

Many lenses have an odd number of aperture blades, but several have an even number. For most photography, there is little to no noticeable effect.

There is, however, a noticeable difference if you have a “sun star” in your image. A “sun star” is the name given to noticeable rays—day or night—emanating from the sun or from an artificial light source.  A sun star is created when the lens is stopped down. The more the lens is stopped down, the larger the rays of the sun star.

An even number of aperture blades results in the same number of sun star rays. As the image below shows, a lens with ten blades (the TTArtisan 50 mm f/2 lens), stopped down to f/16, resulted in ten pronounced rays from a street light at night. [Click on image to enlarge, then click back arrow to return to this post.]

An odd number of blades results in twice the number of rays. Using the same camera as above, but switching to a lens with nine aperture blades (the Nikkor Z 50 mm f/1.2 S lens), also stopped down to f/16, resulted in eighteen sun star rays. [Click on image to enlarge, then click back arrow to return to this post.]

Takeaways:

-1- Day or night, “sun stars” are often created from small, bright light sources in a photograph when lenses are stopped down.

-2- An even number of lens aperture blades creates the same number of sun star rays. An odd number of blades results in double the number of rays.

-3- An even number of blades creates more pronounced rays than does an odd number of blades.

-4- Sun stars are usually undesirable in forensic photographs. A future post will describe how to control the prominence of those rays. But if you must stop down for depth of field, an odd number of lens aperture blades will create more rays, but they will be less obtrusive.

One kind of forensic night photography requires you to accurately capture the appearance of a scene as closely as possible under lighting conditions similar to what they were at the time of an incident. Of course that results in an overall image that appears dark since, after all, it is a night scene.

Using fully automatic exposure modes will result in the scene being significantly (and obviously) overexposed. This is because the camera is trying to record the scene as a mid-tone (frequently referred to as “middle gray” even when the subject isn’t gray). This automatic brightening of night scenes usually results in noisy, overexposed images that don’t resemble the actual scene at all.

Almost all consumer and prosumer cameras have a fully automatic exposure mode denoted by a green camera icon with or without the word “Auto”. The image below shows the full Auto mode icon on a Nikon D5600 and Canon T7i. (Professional cameras like my Nikon Z 8’s often do not have this mode.) [Click on the image to enlarge. Then click on back arrow to return to this post.]

In Auto mode, the camera sets the aperture, shutter speed, and ISO, and you cannot change or override any of them! Because the photographer cannot affect exposure in any way in full Auto mode, when teaching I refer to it as the “Green Mode of Shame” to drive home the point that you as the photographer are left without any ability to control your exposure (or a number of other important settings). While this mode might work for ideal lighting during the day, it just doesn’t—and can’t—work for ambient light night photographs. (Note: Semi-automatic exposure modes Program, Aperture Priority, and Shutter Priority will be addressed in a future post.)

I made the following two images in a lighted parking lot that had a white, a black, and a silver vehicle in the scene. (Note: both images were captured in the camera’s raw NEF format.)

For this first image, I used a Nikon Z 5 in Auto mode (its Green Mode of Shame). As discussed above, the camera tried to create a mid-tone image, and I couldn’t do anything about it. [Click on the image to enlarge. Then click on back arrow to return to this post.]

Note the camera chose a wide open aperture to let in as much light as possible (f4 on that lens), and an ISO of 25,600(!), while setting the shutter speed to 1/50 of a second. (A shutter speed of 1/50 of a second is considered handholdable with a near 50 mm focal length. The thinking is that if someone is going to use a camera in Auto mode, the camera will most likely be handheld. Consequently, the camera will open the aperture and boost the ISO to keep the shutter speed handholdable. But as usual, I had this camera on a tripod, like I almost always do in any light. )

Even though the original image is quite noisy (the great reduction in size and the JPEG compression of this posted image reduced the appearance of noise from the original raw), that’s not the biggest issue with it. The real problem is the scene was nowhere near this bright; it didn’t look anything like this at the time I made this image.

For the image below, I switched to manual exposure mode (the only exposure mode I ever use). I kept the aperture at f/4, but then set the ISO to the camera’s minimum of 100 to minimize the image noise. Because the camera was on a tripod, I wasn’t too concerned about shutter speed as long as it stayed within reason. [Click on the image to enlarge. Then click on back arrow to return to this post.]

With the aperture and ISO set, I adjusted the shutter speed until the image on the back of the LCD looked like what I was seeing with my naked eye. As it turns out, the Auto image was four stops lighter than this more accurate one.

Note 1: By default, I have the camera’s Picture Control set to Neutral with reduced contrast and saturation so the LCD closely matches my computer monitor.

Note 2: For an actual night photograph case, I start by tethering my camera to a laptop that is calibrated to match my desktop monitor. There’s much more to the actual process than I did for this demo, but it’s close enough to illustrate the point that you need to take manual control of the camera.

While this second image is close to showing the scene as I saw it, to view it properly, you would need to view it with a black surround in a darkened room with your monitor brightness set to 140 cd/m² to match what I see. Regardless of these technicalities, comparing the two images—in whatever light you’re in or computer you’re on—it is obvious that the upper Auto exposure image doesn’t look anything like the more accurate lower manual exposure mode image.

In a number of cases where I’ve done both the night photography and explained why photographs submitted by other experts or photographers were improper and misleading, all of those inaccurate photographs have been stricken and disallowed by the judge.

While it’s a little bit off-topic, being able to explain how and why photographs were made and why they accurately represent what they purport to show—and why other ones don’t—is critical in almost all ambient light night photography cases.

Takeaways:

-1- Fully automatic exposure modes (like Auto) are not designed to—and cannot—accurately capture ambient light night images.

-2- You need to shoot night photographs using manual exposure mode with a low ISO to minimize noise, with the appropriate aperture for the depth of field you need, and with the shutter speed set to match what the scene looks like to the naked eye. There’s much more to the whole process, but that’s the essence of it.

-3- While the lower photo looks much closer to what the parking lot looked like when I made the photographs, it was not made with the procedure I use for case work. It is close enough to make a quick demo comparison against a demonstrably incorrect Auto method that yielded a demonstrably incorrect image.