Forensics Tutorial
Focus Magic is used by multiple law enforcement agencies to recover detail from images that is not visible to the naked eye and has been used in multiple court cases including a murder trial. For court cases we have been asked how Focus Magic works so that the jury is happy that the image has not been “doctored” and is valid as evidence.
Example for Out of Focus Blur
Original image focused using a Blur Width of 20.
Example for Motion Blur
Original image focused for horizontal blur using a Blur Width of 20.
The Effect of Resolution
For Focus Magic to recover detail, the Blur Width needs to be an absolute minimum of five, but should preferably be between ten and twenty. If the Blur Width is less than ten then it is usually beneficial to upscale the image by a factor of two before focusing the image.
Original image focused using a Blur Width of 20. The license plate is quite clear to read. This image is within the “Forensic Recovery Range”.
Original image reduced to half its size and focused using a Blur Width of 10. The license plate is still quite clear to read. This image is also within the “Forensic Recovery Range”.
Original image reduced to one quarter of its size and focused using a Blur Width of 5. The license plate although a little clearer, can no longer be read. This image is no longer within the “Forensic Recovery Range”.
Resolution for Scanned Photos
For scanned photos, scan them in using a resolution which is high enough to have a blur width between 10 and 20. If for example the photo was scanned in at 1200 bpi (bits per inch), and the blur width is 6, then scan it in again using 2400 bpi so that you can use a blur width of 12. If the image was scanned in at its highest resolution then increase the image size as for a digital camera (next). Not all scanners produce sharp scans. If you are scanning in an image with quite a low blur width, it is vital to use a good scanner which doesn’t contribute to the blur of the image. After scanning the image in, compare the real image with what you see on the screen and make sure that it hasn’t contributed any blur to the image. If you have the negative of the photo, then you could make a large print of the photo before scanning it in. Make sure that the photo lab understands the importance of getting a sharp image.
If the photo is from a digital camera, then increase the image size so that the blur width is in the 10 to 20 range. It is probably best to increase the file size by a factor of 2 or 4 so that the interpolation is kept as simple and hence as accurate as possible.
If the blur width is greater than 20 then scale the image down so that the blur width is in the 10 to 20 range. Focus Magic can process a maximum blur width of 20. If it could go above 20 then you wouldn’t gain much (or anything) anyway.
The Effect of Noise
A grainy image won’t focus as well as a smooth clean image.
Image with a normal amount of noise (grain).
Noise (10 % uniform noise using Photoshop) was added to simulate a grainy image. The re-focusing is not nearly as good.
If you already have an image, there may not be a lot you can do to remove noise or grain. If you have a negative, you might be able to reduce the noise by developing the print again using better equipment. If the image has dust on it, then you can gently wipe the dust off.
Focusing Accuracy
The focusing Accuracy should be set to Highest. This will focus the image more accurately but it will also take a lot longer to process the image.
