Another Example of Using a Polarizer at an Accident Site

In a post last year, I showed an example illustrating the importance of using a polarizing filter when photographing skidmarks at a wreck site. As you rotate a polarizer while looking through it, you can watch the glare either move or disappear entirely, depending on the subject and the angles involved.

Below is a photograph of two lanes on an Interstate that don’t appear to have any tire marks. (Click on photo to enlarge, then click back arrow to return to post.)

No polarizing filter used. Skidmarks aren’t visible. (Made with Nikon D850 with ZEISS Milvus 50 mm f/2 macro lens.)

No tire marks were visible to the naked eye either, but they could be seen through polarized sunglasses.

Just adding a polarizer on a lens might not have any effect until it is rotated. It cannot be said enough: You must rotate a polarizer before every shot if you’ve moved the camera even a little bit.  Fortunately, the effect is easy to see looking through the filter before putting on to your lens, or through the viewfinder or when using Live View once it’s on the lens.

Here is the same view using a polarizer rotated for maximum effect (Click on photo to enlarge, then click back arrow to return to post.):

Polarizing filter rotated until glare removed from skidmarks. (Made with Nikon D850 with ZEISS Milvus 50 mm f/2 macro lens.)

In this example, if I hadn’t used a polarizer, I couldn’t have captured the skidmarks shown here. Also notice there is now enough contrast that you can see the police paint marks in the tire marks.

Most polarizers lose between one and two stops of light. Because polarizers are dark filters—like polarized sunglasses—you must compensate for that loss of light by increasing your exposure by opening your aperture (losing depth of field) or slowing your shutter speed (making it harder to handhold) or raising your ISO sensitivity (adding noise and reducing dynamic range). A one-stop polarizer costs more money, but it is money well spent—especially when light levels are low (like on overcast days and near sunrise or sunset). Note that since I always use a tripod, I always choose a slower shutter speed, since the camera will remain steady no matter how slow the shutter speed is.

Next to a camera and lens, a tripod and a polarizer are your two most important tools for outdoor photography, especially accident sites and buildings.

Full-Size Pickup Dimensions 25-Years Apart

I bought my beloved 1995 Ford F-150 XLT SuperCab Flare Side 4×2 pickup 25 years ago as of February 2020. I’ve put more than 360,000 miles on it, and it has been bulletproof. Its original two-tone paint still looks great, and it has never needed a valve job nor had the heads off. I did, however, have to get both fourth and fifth gears replaced in the five-speed manual transmission because they wore out!

1995 vs 2020 F-150: 25 years apart. (Click on image to enlarge, then click on back arrow to return to post.)

I’ve noticed that the latest generation of F-150’s seemed larger than my 1995, but I hadn’t made a direct comparison until recently.  While the 2020 model in the photo was a SuperCrew four-door Lariat, my 1995 was a SuperCab two-door XLT.  (The dealer didn’t have a white 2020 SuperCab XLT.) But the size difference is evident.

For the table below, I compared the dimensions, curb weights, and tires sizes between the 1995 and 2020 model years for a Ford F-150 XLT SuperCab 4×2 Styleside with 6.5-ft bed and a 5.0L V-8 engine. As summarized at the bottom of the chart, the 2020 model was larger in every dimension, had larger wheels and tires, and a notable increase in curb weight. (Click on table to enlarge, then click on back arrow to return to post.)

It’s not only the Ford F-150 that has grown over the years, of course. Many, if not most, cars and light trucks have gotten bigger and heavier over time. When reconstructing an accident, it is important to get the specs for the actual model year of the vehicles involved, and not just assume they are a “standard” half-ton pickup or a “standard” car.

Use a Polarizer at Accident Sites

Except when making night photographs, I almost always use a polarizing filter (polarizer) when photographing accident sites. Rotating a polarizing filter removes glare and increases saturation relative to the angle of rotation.

Fortunately, to determine the amount of rotation you need for the effect you want, simply look through the polarizer (through the threaded side if you’re holding it, or through the viewfinder or rear LCD once it’s mounted on your lens) as you turn the outside ring. There are no settings or calculations or other analysis you have to do to get the effect you want. But, like polarized sunglasses, polarizing filters decrease the light, so you will have to adjust your exposure accordingly. Most polarizers require an additional one-to-two stops of exposure to compensate.

Speaking of sunglasses, make sure you remove your sunglasses—especially if they are polarized—before looking through your polarizer. Otherwise, you won’t be seeing the actual effect of your filter. In fact, if they line up, you won’t see anything; it will all go black.

While a polarizer will take the glare off of roads, grass, and trees at accident sites, it has its greatest effect on tire marks. In this first image, you can tell there are multiple tire marks on the road, but they lack definition. The photograph is properly exposed, but glare obscures any detail. (Click on an image to enlarge. Click on back arrow to return to this post.)

Tire marks without polarizer. (Nikon 24-70 mm lens on Nikon D3s.)

For this second image, I attached a polarizer to the front of the lens, and rotated it until I got the maximum detail in the tire marks. I had to brighten the exposure to compensate for the light lost with the polarizer. This is rarely an issue when your camera is on a tripod, but if your polarizer loses two stops of light, it might be difficult to keep the shutter speed high enough to handhold the camera.

Tire marks with polarizer. (Nikon 24-70 mm lens on Nikon D3s.)

This photograph obviously shows much more detail than you could ever extract from the first image. This increase in detail from reduction of glare is what makes a polarizer one of the three most important accessories for automotive forensic photography, along with flash and a tripod.

It’s important to remember to rotate the polarizer between each shot, and to compensate for any light loss by adjusting your exposure. It is also important to remember that a polarizer is the only filter whose effects you can’t replicate in post-processing.

Use a Polarizer and Fill Flash for Vehicle Photos

Many times, we can’t choose when we will inspect vehicles or accident sites. That means you’ll have to deal with whatever light conditions you encounter. It’s up to you as a professional to come back with good photographs, despite the adverse light.

One example was a black BMW in a tow yard field on a blindingly bright day close to noon. As you can tell from the hard edge of the shadow under the car, the sun was almost directly overhead. Any details in the upper body panels were obscured by glare, while the shadows were too dark to show any details. (Click on an image to enlarge. Click on back arrow to return to post.)

BMW in sun at tow yard. No flash, no polarizer. (Nikon 24-70 mm f/2.8G lens on Nikon D810. Exposure: f/13, 1/60 sec, ISO 160.)

Even though this image was properly exposed, between the glare and the blocked shadows, you can’t get much information about the damage to the car. That defeats the purpose of even making the photo.

To counteract the glare, I used a polarizing filter and rotated it until most of the glare was gone. To open up the shadows, I added a flash in the hot shoe of the camera to create fill flash under the hood and down the left side.

BMW in sun at tow yard. Fill flash and polarizer. (Nikon 24-70 mm f/2.8G lens on Nikon D810. Exposure: f/10, 1/60 sec, ISO 160.)

This second photograph was also properly exposed, even though it was 1/3 stop darker overall—by choice. This image demonstrated that the combination of the polarizer for the glare and fill flash for the shadows yielded a much more balanced image which showed details that just couldn’t be recorded without those tools.

It should be noted that I use a one-stop polarizer. This means you only lose about one stop of light when using it. Many (most?) polarizers cost you about two stops of light.  Also note that fill flash, being fill and not the main light source, did not affect the overall exposure; it just brought out detail in the shadows.

Besides using a tripod, regular use of both a polarizer and fill flash for vehicle photos ensure you’ll consistently capture all the data you’ll need to show details in your vehicle photos.

Photography for Accident Reconstruction, Product Liability, and Testing Class Outline

From August 12 through 14, 2019, I will be teaching the third Photography for Accident Reconstruction, Product Liability, and Testing class for SAE. This time it will be at Southeast Toyota Technical Center in Jacksonville, FL. We’ll cover a lot of material in the three days, and you’ll come away making better quality, more professional photographs from that point on, regardless of the location or lighting conditions. After all, your photographs are at least as important as any other part of your work. You’re a professional, and your photographs should reflect that professionalism. After this class, they will.

Here’s a detailed outline of the class: SAE Forensic Photography Class Outline_2019-07.

Please contact me if you have any questions or would like more information about the class.

Suggested Gear for Accident Reconstruction, Product Liability, and Testing Photography

Even before I started teaching Accident Reconstruction, Product Liability, and Testing Photography classes, I’ve often been asked about what photo gear works the best for those areas.

In response, I’ve created two wish lists at B&H Photo Video: one for Nikon Nikon Gear Wish List and one for Canon shooters Canon Gear Wish List. I’m a Nikon shooter, so most of my direct experience is with Nikon equipment. Here are some notes on the lists:

-1- I currently use the Nikon D850. It’s arguably the best all-around camera on the market, but I recommend the Nikon D750 for Nikon shooters for several reasons:
– Its files are more manageable in size, but are still plenty large.
– It still has the manageable body size and shape, and even has the really useful flip up and down LCD screen.
– It has a built-in flash to use to trigger the Nikon 4804 R1 macro flashes.
– The built-in flash isn’t terribly powerful, and can’t be rotated or removed, but can be used in a pinch.
– Right now, it is on a fantastic sale—especially with the 24-120 mm lens. You save $1,200 instantly.
– It’s been out for a while, and is tried and true.

There are similar advantages for Canon shooters with the 6D Mk II vs. the 5D Mk IV. If you have the budget, the Nikon D850 or Canon 5D Mk IV can’t be recommended highly enough. But they are not necessary for the work we do.

-2- The lists show both the ZEISS Milvus 50 mm and either Nikon 60 mm or Canon 100 mm macro lenses. I use the ZEISS, but also have the Nikon. I use my ZEISS Milvus 50 mm lens for most of my work photography, since it has a normal perspective. I also use the ZEISS Milvus 100 mm lens when I need to fill the frame with a macro shot, but can’t get close enough.
Advantages of the ZEISS are: Precise manual focus; amazing micro contrast; and, 50 mm is accepted “normal” lens that I use for almost everything.
Disadvantages of the ZEISS are: Manual focus only (but that is my preference); and, only enlarges to 1:2 (or half life-size).
Advantages of the Nikon: 1:1 (life-size macro); autofocus (but see note below); close enough to “normal” focal length; and, less expensive.
Disadvantages of Nikon: Not as easy to manually focus.
Note: When shooting macro images, you’ll most likely have to manually focus anyway, so having a more precise manual focusing ring is a real benefit.

Speaking of ZEISS lenses, four ZEISS manual focus prime lenses make up my work kit: the classic ZEISS 25 mm f/2 Distagon; the ZEISS Milvus 35 mm f/2; the ZEISS Milvus 100 mm f/2 macro; and, the aforementioned ZEISS Milvus 50 mm f/2 macro. I use the latter for 90 percent of my work. ZEISS makes lenses for Nikon, Canon, and Sony mounts. They have unrivaled sharpness and micro-contrast, and such smooth and accurate manual focusing that you’ll forget autofocus exists! Continue reading “Suggested Gear for Accident Reconstruction, Product Liability, and Testing Photography”

Eliminate Traffic with Neutral Density (ND) Filters

It is often difficult to photograph highways without traffic blocking part of your shot. But you can effectively eliminate moving vehicles from your photo by using a dark neutral density (ND) filter. Neutral density filters are neutral gray gel or glass filters that require longer shutter speeds for the same exposure with the same aperture and ISO, depending on how light or dark the filter is.

This first image was made on a mostly sunny day without any filter over the lens. The vehicles are all slightly blurred, but are still recognizable. (Click on image to enlarge, then click on back arrow to return.)

Traffic barely blurred with no ND filter. (ZEISS Otus 85 mm f/1.4 lens on Nikon D850.)

At my Nikon D850’s lowest numbered ISO of 64, I chose an aperture of f/9 on my ZEISS Otus 85 mm lens. This yielded a shutter speed of 1/125 second. If the vehicles were traveling at the 65 mph speed limit, they would have traveled 0.76 feet during that 1/125 second. That small movement explains the minimal blur of the vehicles.

To blur the vehicles by increasing the shutter speed, I screwed a 3-stop Formatt Hitech Firecrest Ultra ND filter onto the front of the lens. Keeping the f/9 aperture and ISO 64 settings with the same lens, this reduced the shutter speed to 1/13 second (which was actually 3 1/3 stops less).

Traffic blurred with 3-Stop ND filter. (ZEISS Otus 85 mm f/1.4 lens on Nikon D850.)

At 1/13 second, vehicles at 65 mph would travel 7.3 feet. This rendered the vehicles as no longer recognizable, but they still blocked sections of the roadway.

For this final image, I replaced the 3-stop ND filter with a 10-stop Formatt Hitech Firecrest Ultra ND filter. I again kept the aperture at f/9 and the ISO at 64 while using the same ZEISS Otus 85 mm lens. This yielded a shutter speed of 2.5 seconds, which was 8 1/3 stops slower. (Note: the edge of a cloud resulted in a slightly underexposed image, so I increased the exposure by one stop in post-processing, so this final image was 9 1/3 stops less than the original without the filter.)

Traffic gone with 10-Stop ND filter. (ZEISS Otus 85 mm f/1.4 lens on Nikon D850.)

At the 65 mph speed limit, vehicles covered 238.3 feet in 2.5 seconds. Assuming the dashed lane lines plus spaces were at the standard 40-foot increments, the length of the roadway shown in any lane in this photo was much greater than 238.3 feet. Consequently, every vehicle in both directions crossed the image frame in a fraction of the time the shutter was open. This meant vehicles would never register, but would leave only faint blurs across the image.

Here are some factors to consider:

-1- This only works when traffic moves fast enough not to register while the shutter is open. It won’t work at all in slow or stopped traffic, since slow or no movement would allow the vehicles to register in the frame, even at longer shutter speeds.

-2- Bright sunshine complicates the problem since your shutter speed will be quite high because of bright ambient light. The amount of ND needed will depend on the ambient light levels and the speed of the traffic.

-3- Dark ND filters will likely fool your Auto White Balance (WB) like it did for the 10-stop filter. I corrected it with one click in Adobe Camera Raw. This wouldn’t have been the case had I initially set the WB to a specific value.

-4- If I had exposed the darkest for another stop (that is, 5 seconds instead of 2.5 seconds), I would not have had to brighten it by a stop in post, and there would have been even less chance of a blurred streak from passing vehicles. When in doubt, it is always better to slightly overexpose (as long as you’re not blowing out your highlights) to get the slowest shutter speed possible for the least perceptible blurred streaks.

-5- You can stack multiple ND filters to get the amount of neutral density you desire. Alternatively, variable ND filters allow you to rotate part of the filter to darken or lighten the effect of the filter.

-6- The numerical value of an ND filter is unimportant. You just need one dark enough to get your shutter speed slow enough to reduce the vehicles to imperceptible blurs. That will often be in the eight-to-ten stop range.

-7- I always work with a tripod, but even if you don’t, a tripod is absolutely necessary for the long exposure images. There is no way to handhold a camera steady for 1/13, much less 2.5, seconds.

Benefits of Fill Flash

While may seem counterintuitive, adding fill flash even on a bright, sunny day brings out details that better document your subject, which can make your photos even more useful. In this first image, the car and measurement rod were properly exposed, but the high contrast from bright sun resulted in little detail in the deep, dark shadows. (Click to enlarge image, then back arrow to return.)

High contrast from bright sun with no fill flash. (ZEISS Milvus 50 mm Macro lens on Nikon D850.)
Adding light from a flash resulted in a more balanced image, with details now visible even in components on the frame, suspension, and under the hood. The car and measuring rod remain properly exposed.
Excessive contrast reduced with fill flash. (ZEISS Milvus 50 mm Macro lens on Nikon D850 with Nikon SB-910 flash.)
Determining the amount of fill flash needed will be a topic for future posts. In the meantime, it’s worth experimenting with both manual and TTL flash settings until it becomes second nature.

Focus on Your Subject

Especially when handholding a camera, autofocus points might land on an object closer or farther away from your intended subject. This can result in an unimportant element in sharp focus, while your subject isn’t sharp.

There are three main ways to avoid this:
-1- Use manual focus. This allows you to choose what will be sharpest in your image, even if you recompose or your camera moves, as long as you remain the same distance from your subject.
-2- Put camera on a tripod, then move your autofocusing point until it’s on your intended subject.
-3- If handholding, autofocus on your intended subject, press your focus lock button, and recompose.

Your intended subject should be in focus. For this pair of photographs, the camera was on a tripod. In this first image, the autofocus point latched onto the black cable shield in the foreground. This caused the axle and wrapped tire behind it to be soft. (Click on image to enlarge, then back arrow to return.)

Focus on black cable wrap in foreground, not on axle or wrapped tire. (Nikon 24-70 mm f/2.8E lens at 50 mm on Nikon D850.)
For this second image, the focus point was moved off the cable wrap and onto the axle and wrapped tire.
Focus on axle and wrapped tire, not black cable wrap in foreground. (Nikon 24-70 mm f/2.8E lens at 50 mm on Nikon D850.)
Which photograph is correct? Depends on whether the subject is the black cable wrap or the tire and axle. Make sure you focus where you intend.

Watch What You Focus On

If you use autofocus lenses, make sure the focus point is where you want it. I almost exclusively use ZEISS manual focus prime lenses, so I always have full control over where and what I focus on. (The only exception is when I use the Nikon 18-35 mm FX lens for pole photography. It’s a nice, small, light wide zoom lens with autofocus, which I can control using my CamRanger. This will be the subject of a future post.)

While I used my ZEISS 35 mm f/2 manual focus lens to purposely create these two photos, they illustrate an issue I’ve seen when I get photographs from either police or other experts. This car was inspected in a small, crowded area of a tow yard. To photograph the entire front end, I had to use a wide angle lens, and crouch down in bushes and weeds. In this first photo, the weeds are in sharp focus, but the car is blurry. (Click on the image to see a larger version, then click on back arrow to return to the post.)

Focus on weeds in front of wrecked Taurus. (ZEISS 35 mm f/2 lens on Nikon D850.)
This can easily happen if the autofocus point happens to pick up a weed or anything else between the lens and subject. Unfortunately, no amount of sharpening or other post-processing can restore detail to the car itself.

This second photo has the car in proper focus, with the weeds out of focus in between the lens and car.

Focus on front of wrecked Taurus. (ZEISS 35 mm f/2 lens on Nikon D850.)
While you’re in the field shooting, it can be difficult to tell if your subject is in focus by looking at your camera’s LCD, unless you zoom in to 100% and scroll around the image. If you have unimportant elements between your lens and your subject, it’s best to either manually focus, or make sure you move your autofocus point onto your subject.