Determining acual size from historic photo

[AdminGuy]

Determining the size of 1911 TT number plates from photo. A little math to help you out - like me, you may be a little rusty.

Tire size = 28x2.5". Therefore rim size = 28-2.5(2)=23"

Relationship algebra: (*my photos are bigger and a little more accurate than the ones posted)

#26 Godfrey
          Photo      actual
Rim    1.505        23
         _____       ____
Plate   0.75    =    x


1.505x= 0.75(23)
x=17.25/1.505
x=11.46"

#17 Franklin
1.951x=0.972(23)
x=11.46"

#31 Moorehouse
2.045x=1.025(23)
x=11.53"

So we can figure that the number plates were approx. 11.5" in diameter. That's ahhh....292mm.
This same formula can be used for many things when looking at a photograph. Drawing a grid overlay on blown up photocopy is a great way to help establish relationships. Hope this help's someone... sometime. Figure something out.

[inrustwetrust]

scaling photos

Scaling from photos is a great technique. If you have actual dimensions to campare to, such as a frame it is easy to make a multplier to determine actual size. It is best to scale from a straight on shot. If other pictures are available, make a multplier for those too and compare dimensions. It helps to use a couple of references with their own multpliers to compare results. With a pair of verniers and a calculator you can get surprizingly close. It's easy. Take the actual measurement from the piece you have and divide it by the measurement from the photo. If a dimension on a frame is 25'' and the dimension on the photo is 5'' , the multplier from the photo measurement is 5. The more references you use will increase the accuracy as you will see that there will almost always be differences in different measurements. It helps make a part that may be nonexistant

[AdminGuy]

Another great explanation. Thank you! And yes, wide angle lens create odd distortions. If your using a digital make sure zoom is at middle position.

[charlie]

im trying to get some info on a 1950 indian 250cc brave that was givin to me and is in bad shap. would you know some ware i might go to get help? thanks chalrie