Identifying the length of the object on the image

[Erol2]

Hello

First of all, I was told this is a physics problem. If it's incorrect, I apologize.

I have an image

I want to know the length of the black ship (the ship which is carrying a ship). Let's assume that I know the length of the text on the ship (I have a reference object).

Is this related with camera lenses? How is object becoming smaller on the image (bigger distance) and at what rate is the size decreasing? I was doing some tests and it seemed to me that it's not decreasing by some constant. I understand that it's related with angles (bigger angle means that the end of the object with be the furthest and the smallest on the image).

Can somebody please help and give me the right directions?

 

[mfb]

The ship is far away compared to its size. I would ignore perspective and assume the (length of ship)/(length of text) ratio in the image (along the visible side - taking into account edges in some way) is the same as for the actual ship.

 

[russ_watters]

If you have a higher resolution image (the original?), you may be able to identify the draft (depth) markings on the barge and use that to scale the length of the barge.

For long distance photography with a lot of zoom, perspective issues mean you can't use an object for scale that isn't the same distance as what you are trying to measure. By that I mean you have no way of knowing how close the people are to the barge, so you can't use them for scale.