How long custom hood for 100-400 @ 400 on a crop?

Anyone know how long the hood should be on a 100-400mm lens on a 1.6x crop body at 400mm? I find myself shooting towards the sun every now and then, most often at 400mm and would like to prevent flare as much as possible. I'm thinking of making a hood extension by woodturning. Any idea exactly how long/big I should make the extension to work only at 400mm with minimal vignetting?

Just use the hood that comes with it.

The hood is made for 100mm on FF, not big enough at 400mm on a crop. The point is I often have to shade the lens with my hand or something else when shooting close to the sun at 400mm. Oh well I guess I'll just have to try it out and start out big and just make it shorter until it's perfect.

Just something to consider, the new Sigmas are coming with a lens hood extension for crop sensor cameras. At least with the 85mm f1.4 lens it was supplied for that use and IDed that way. Sometime in the future Canon might see the flare-light and provide not only a hood but a crop sensor lens hood extension as well. After dealing with the aftereffects of the seismic disasters that don't know when enough is enough that is. The US and the rest of the world will soon be feeling the ripple effects from the tsunami that caused Japans situation. Camera gear is just one small tip of a huge iceberg of affected industries.

Xcelx wrote in post #12307837
The hood is made for 100mm on FF, not big enough at 400mm on a crop. The point is I often have to shade the lens with my hand or something else when shooting close to the sun at 400mm. Oh well I guess I'll just have to try it out and start out big and just make it shorter until it's perfect.

Experiment with an extension made of black construction paper and masking tape. This is extremely simple and inexpensive.

You could even make a petal-shaped extension if the place where the original hood attaches does not rotate with either focal length change or focus change.

Once you've established the optimum shape, then you can create a permanent solution. I don't recommend using wood, though. I would make something that is collapsible (able to fold flat) and could be applied when needed. By being collapsible, it will store more easily in your camera bag.

SkipD wrote in post #12308116
Experiment with an extension made of black construction paper and masking tape. This is extremely simple and inexpensive.

You could even make a petal-shaped extension if the place where the original hood attaches does not rotate with either focal length change or focus change. .

Thanks for the tip! Hadn't thought about making a petal shape, the lens doesn't rotate. I will have to experiment with it