Suggestions re Crime Scene Photos Please

[acroreef]

I am a police officer and do some evidence photography work. Recently I had to photograph a footwear impression on a door. (A suspect kicked in a door leaving a faint impression of his shoe). I was using a Canon XTi. I set the camera on "P" - program mode. I used 100 ISO, tried different white balance settings between daylight and cloudy, and also used both spot metering and partial metering.

Most of my photos came out too light even though I tried to shade the image from the sun. I used no flash but did light the image obliquely using a flashlight, from various angles. The "point" light source may have been too bright even though I dimmed the light and moved the light away from the image. (I also used a tripod).

Any other suggestions to "darken" the faint image in a bright environment? Should I have set the camera to aperture mode and tried a higher fstop to let in less light? Are there any lighting "tricks" to pull off this type of shot.

Thanks in advance to all who respond. This is a terrific site. I appreciate the education it provides.

Dave

[jiro]

You could try lifting the impression using a gel or tape lifters then shoot it on an ultraviolet light, Dave. I think the ultraviolet light still has to be set obliquely to record the impression. From what I saw in the crime scene TV programs, they use a small aperture (for sharper images) and set at a low ISO then they drag the shutter for an exposure of about 2 - 3 seconds. Since the shot would be reversed in ultraviolet light, the impression would become lighter thus giving a clearer image of the impression. I must have watched too many CSI programs to remember this. Hahaha!

[Malcgeorge]

I am a police special and so have an interest in 'getting the evidence'. A ND filter or even a polarising filter could help in such a situation. Or some kind of shade large enough to cut down the directional sunlight ( impractical and you would never find it in the back of a police car!). Also use a tripod and a small aperture (means slower shutter speed) is another good solution as this will increase depth of field to make sure the image is sharp. If the door was painted white or with gloss paint I reckon the polarising filter to cut out reflected sparkle/shine is a good bet.

[revi]

I'm not a native English speaker, so:
by "impression" do you mean a print on the door (in different colour, from dust, mud, etc.); or do you mean a physical impression like in a bas-relief, where the footprint shows up as height differences?

If the latter, you might have to direct your light almost parallel to the surface, to make the relief stand out by creating shadows. In that case, if you have sunlight, you might need a rather powerful flash to overpower the ambient light.

If the print is formed by deposition of material, colour filters could help to make the print stand out more. The angle of the light is less important in this case (apart from the risk of reflections of course).

I don't think your flash light would have been too strong, compared to the output of a decent cobra flash (and from about 1m onwards, your camera handles those just fine, without over exposure).

Hope this helps,

Remco

[Malcgeorge]

PS even present police budgets could run to a polarising and ND filter I reckon!

[MiniChris]

Many moons ago, in film (primarily 35mm Plus X), I used either a polarizer or a red filter combined with a polarizer (like you would use to heighten the contrast between sky and clouds) to enhance this same kind of imagery. As others have suggested, since this type of evidence rarely moves (unless it rains or the sprinklers come on), it is recommended to use a longer exposure combined with a smaller aperture (f:22). For a dirt impression, I've never seen a better method than a plaster cast. do be careful when shooting at an oblique angle that you do not over-extend the shadow and create a "non-existent" secondary pattern which could quite easily be challenged by the attending defense attorney (Barrister?).

To get an infrared image, you could purchase an inexpensive digital camera and have it reconfigured to only shoot in infrared. While it does not exactly reproduce a film infrared image, it does serve to enhance faint edges (however, see above as to double imaging).

While the red filter would have no effect in the actual shooting, using the red (or other filters) in post production, you could quite readily produce the same effect. All you really need to do is "enhance" the original image without doing anything to the original image that would create something not 'there."

[acroreef]

Thanks folks, I am going to experiment with longer exposure time/smaller aperture and also a polarizing filter. Thanks for the suggestions.

Dave

[Colin Southern]

Hi Dave,

To me, it seems like a simple metering issue.

In a nutshell, ...

- Yes, use Av mode. Most photographers use Av mode 95% of the time

- Changing the aperture (F-Stop) won't alter the exposure because the camera will change the shutterspeed to compensate (that's pretty much the whole idea of the automatic exposure mode)

- All camera metering is based on the assumption that what it's seeing is a medium gray (which most scenes average out to). Often this does a good job, but it's none-the-less easily fooled - especially by scenes that have large areas that are predominantly NOT a medium grey (think "black cat on a black rug" or "Polar bear on snow"). Fortunately, the solution is easy - all cameras have a feature called Exposure Compensation ("EC"); all you need to do is dial in a little +EC or -EC so that your exposure is what you want it to be.

PPS: Changing the aperture in this situation will only affect your depth of field. Normally in this type of situation I'd expect to use a very narrow aperture (ie "large F-Stop number") (eg F16).

PS: Spot-metering isn't what most people think - basically what it means is "expose the bit I'm pointing at as a medium gray". If it happens to be a medium gray (in terms of brightness, not colour by the way) then it'll expose correctly ... if it ISN'T a medium gray, it WILL be by the time the camera takes the shot unless you again add or subtract some exposure compensation.

[Malcgeorge]

Dave,

You don't say where you are a police officer; like which country are you in even?. If your not from the UK you may not know what is a 'Special Constable', but I guess if you are you know. anyway for everyone who isn't in the UK essentially 'Specials' are volunteer officers who do all the things a regular officer does, but only does duty part-time and when it is convenient to them and does NOT get paid. Usually there is a minnimum amount of time a month they must do; 16 hours plus training is a usual sort of amount.

I do TPT ( i.e. emergency response to 999 or what in US is 111 calls) as a Special and we have a digital camera in the car to collect evidence when needed. It is more of the 'point and shoot' variety than the all singing dancing DSLR type so some of the advice may be off the mark if you are refering to just having this kind of routine kit in a patrol car. I think I and others have assumed you were using something more elaborate than this and if you were then the advice seems pretty good. I am doing a night turn tomorrow, so I will take a close look at the camera in the car and report back.

[Peter Ryan]

My first question was 'what was the colour of the door' and that actually lined up perfectly with Colin's comments above. It is an exposure issue.

[Malcgeorge]

I can report back that despite a very hectic duty I managed to get out the camera that is supplied for us to collect evidence from its bag and take a look. The camera we were provided with was a Vivitar Vivicam X225 digital which is a straight forward point and shoot camera with 10.1 MP and 4x digital zoom. Otherwise it is about as basic as they come, but they are cheap so there is one in each car. So not much latitude for getting fancy and technical - but then on a hectic night duty with grade 1s stacking up no one had the time anyway.