How many shots needed?

[mikerault]

Ok, I want to do a photoshop merge of a wreck, I have 100 foot visibility and the wreck is 400 feet (plus or minus) long, how many shots will I need with a 18-55mm Canon lens on a Canon 20D? I figure to hover slightly above the wreck about 20 feet above the deck and about 80 foot out and take several shots along the length.

I don't think a wide angle will do what I want since it tends to bend straight lines making a merge difficult if not impossible.

Anyone done something like this?

Mike

 

[DuboisP]

http://www.plongeur.com/forums/showthread.php?t=26535

Welcome in France !

 

[Soggy]

Talk to Chris at www.chris3d.com. He did this with the U-352. I think he used a wide angle, but only used the center portion of the shot. Programs like autostitch will correct lens distortion, though.

 

[victor]

At 18mm I believe the angle of view is 75 degrees
If you are 50ft out this would let you take about 80ft of the boat at every shot.

If visibility is 100ft then 80ft is too far away as that is over 100ft at the edge of the picture.

I would try for at least 100% overlap so at 18mm and starting with the stern of the boat in the centre of the picture I would attempt to take a picture every 40 ft ending with the bow in the centre of the picture, so at least 10 pictures.
As I took each picture I would look for a marker on the edge of the current shot and use that as the centre of my next shot.

The hard part will be keeping the distance from the boat and the relative angle the same for the whole period.

I wish you luck with the project, post your picture when you are finished.

 

[mikerault]

I'll do my best! I have a hand held sonar unit to help with keeping distance and depth constant.

Thanks!

Mike

 

[Soggy]

At 18mm I believe the angle of view is 75 degrees
If you are 50ft out this would let you take about 80ft of the boat at every shot.

Keep in mind the crop factor, so an 18mm lens would have an efl of ~29mm on a 20D. I don't know if you considered that in the 75 degrees or not...just a comment.

 

[victor]

Keep in mind the crop factor, so an 18mm lens would have an efl of ~29mm on a 20D. I don't know if you considered that in the 75 degrees or not...just a comment.

I know what you mean, it is very confusing these days with the various crop factors(Nikon 1.5 Canon 1.6 Oly 2.0).

I found the angle of view on a canon web site for that lens on a digital camera.

I am not sure what effect being underwater has on that angle of view, I think it increases it slightly.

For that reason I would go for the 100% overlap using the edge of one shot as the centre for the next. That way you don't have to worry about exact angles or distances.

Another advantage with this level of overlap is that using the centre of each image rather than the edges reduces any distortion from the lens.

 

[mikerault]

Good tips...now if I can just implement them!

 

[morayp]

Stay close and take a Lot of pictures. You have to fight viz, parallex and scale. It will be much easier to piece the pictures together if they are of a larger scale, ie, taken close up.

 

[victor]

Mike
Have a look at www.autostitch.net
A fantastic program, we have been playing around with it on above water pictures.
It is seriously good at stitching a number of photos into 1 big picture.
You need to insure that you have a significant amount of background around the subject.
I suspect with this product you could do it as 10 picts of the bottom of the boat, 10 of the top and let the product sort them all out.

Tell us how you get along.