I need some help

[CSIJon]

Hello all, this is my first post. I am a Forensic Specialist (basically CSI). I use a camera for many different scenarios. I have had a good bit of training on photography but most of it is crime scene photography. One of the things we have to do is to photograph fingerprint and shoe impressions. We do this in RAW on a tripod at 90 degrees to the subject. My question is that I have been told different things on this. I was told that to get a true 1:1 photo your lens had to be set at 55mm, then told 55mm with and extension tube; B & H told me not a 60mm lens will give you a true 1:1 ratio. I have also seen that some macro lens say 1:1 with 100mm focal length. We have to have it at true 1:1 so it can be compared. I believe (not 100% sure) that you can calibrate with Photoshop if close, but I just want to get to the truth without using Photoshop. If this is the wrong thread I am sorry. If anyone can help it would be great. Thank you and look forward to exploring this forum much more.
CSIJon

 

[clanthar]

Hi,

1:1 typically means life-size magnification in macro photography. It can be achieved using different focal length lenses. With shorter lens focal lengths the camera (front of lens) is closer to the subject. Macro photographers will select longer focal lengths in order to be able to move back from the subject. Many lenses sold as macro lenses can't achieve a 1:1 ratio as is and require the use of an extension tube to move the lens further away from the film/sensor.

You can define 1:1 macro like this; the recorded image of the object is the same size as the object. Using film then for an example, if you photograph a dime at 1:1 then you could place the actual dime on the developed film and it would be a perfect size match. To achieve 1:1 with a digital camera then would mean that the object couldn't be physically larger than the sensor. So I don't think that's what you're trying to do. If it is and you're photographing shoe impressions then you've got one seriously extreme digital camera and a 55mm lens unlike anything the world has yet seen.

I'm going to guess that what you're trying to achieve is a finished print in which the image of the object is the same size as the real object. This can be done with any lens focal length -- you just need to do the math. The easy way would be to measure the original object and then produce a print with the object the same size in the print. If you can't measure the object you could still produce a to-size print by knowing the size of the sensor, the lens focal length, the distance between the camera and object (focus distance) and some basic trig. Lens focal length in this case will still play a roll if what you're photographing is 3D. The rendition of perspective in a photo is a function of subject to camera distance. Given that lens focal length crops the image it can influence camera placement which in turn determines the rendition of perspective.

Take Care,
Joe

 

[Big Mike]

Yes, 1:1 reproduction ratio in photography usually means that the size of the object is the same size as the image as it appears on the film (or in this case, the camera's sensor). So a finger print would probably fill the whole frame if you were shooting at 1:1...and I'm guessing that is not what you're after.

I'd think that an easier way to go about this, would be to also shoot something with a discernible size...the obvious being a ruler or tape measure, but anything standard could work, like a dollar bill or a coin etc. Then when you get the photo into your software, you can resize the image so that the object of know size, matches the 'actual size'. You can then measure other things (foot print etc) to determine their size.

And yes, it will be quite important that you shoot at 90 degrees.

 

[CSIJon]

Thanks for the info. Yes, we are trying to print to 1:1 without having to resize it. For court purposes they cannot be manipulated. We do you an "L" scale in the photo or just a ruler.

 

[table1349]

This may get you started: Forensic Photography Information Guide

Also you might have your Chief or the head of your training division get in contact with the FBI about sending a person or two to their Forensic Collection School. It will cover all the bases of basic Forensic collection and recording of evidence including photography.