Creating a light Probe

[Matt Sugden]

It's a lovely day hear today, the sun is out for a change, and the sky is looking interesting and cloudy. So....

 

I reckon I might pop out and try my hand at creating a HDRI light probe, I have a mirror ball, and a lovely new Nikon DSLR, can anyone offer any tips on doing this? I believe I need to make several photos at different stops, but how many is deemed enough?

 

I wondered if there was any value in trying to line up the sun, so lining it up in max later would be easier and more predictable? any hints on this perhaps?

 

Cheers.

 

Matt

[SandmanNinja]

http://www.dhimaging.com.au/3D/downloads/A Simple Technique to Align a Light for the Sun in an HDRI Image.pdf

 

It's a PDF I made after watching some tutorials - FANTASTIC!!

 

Congrats on using the finest brand of cameras in the world! (yeah, I like Nikons). I've seen people zoom in on a christmas tree ball (heck, even a ball bearing rolled into a doll's house!) and as long as you get 2-3 angles on it, you should be in business.

 

I'd do as many stops as you can. I do a few test shots at slow shutter speeds and when the view finder is almost completely white (i.e. you can see a few things poking thru the white), erase the other test shots and start there. Use every f/stop or shutter speed (which ever one you want to use) until it's completely black. On my D200, that's about 15 to 20 stops.

 

Hope that helps - and post links to it!

[Matt Sugden]

hi thanks for the tutorial... and the pointers of the stop settings.

 

with regard to referencing the sun, what I was wondering is if there s a good default position to take the photographs of the probe from, so that the position of the sun is very, how can I put it, predictable in it's placement, at least on the horizontal axis.

 

also, with regard to the 15-20 isn't that a bit overkill? wouldn't I be able to get away with about say 8? as long as the range between light and dark is covered?

 

have you managed to capatilize on the nikon raw image format for using the hdri? as I assume there is a certain amount of higher dynamic range exposure info in there already?

[JamesTaylor]

The more stops you use the better end result you will get. Using 15 of 20 stops, if done well, should allow you to gain sharp shadows from the HDRI as opposed to just diffuse light.

 

there is a higher range of info in a raw file, however i would describe a RAW file as being of Medium Dynamic Range

[SandmanNinja]

What James said.

 

I think if you shoot RAW/NEF files and take them in, i think technically you'll have a better source image to build your HDR. I haven't done comparisons to confirm this, so it's just an opinion.

 

Sun position. Hmm.... A couple hours on either side of noon I reckon. Unless you're after a specific lighting (i.e. sunrise, sunset)

[Matt Sugden]

heh! your not getting what I mean about the sun position at all..... and it's my own fault for not being very clear.

 

I'm referring to if people bother to line up the sun position on the steel ball (while taking the photos), so that when they load the image into max they have a clear understanding and reference point to where they would expect it to be casting light in the scene, so that positioning it is easy and predictable to understand.

 

I often find when loading hrdis that I have to move the sun around a bit until it's casting where i want it too, but this is often a case of trial and error, though I have used helpers in the past.

[SandmanNinja]

I'd imagine it would be the brightest source of light in the image, no?

 

Using the method in that PDF, I move my spotlight/sun to match the brightest point of light in the HDRI.

 

If you're shooting a spherical image, the sun should be visible.

 

I guess I'm confused. It's Friday and I hear a bottle of grog calling my name...

[Matt Sugden]

couple of tips I've discovered already from making this light probe...

 

1: You need to have the camera in complete manual mode, as aperture automatically adjusts when the shutter speed is lower/raised, maybe obvious to some, but it took me a moment to realize this.

2: My camera has auto iso and auto white balance, over riding these both means that the camera is giving neutral and referenced image each time.

3: you've got to take the stopped images very quickly, unless it is clear sky. I can see the movement of the clouds as I took each stop.

4: when taking the photos from a mirror ball I took one set of exposure shots then moved around the ball by 90 degrees. By doing this by a measured amount it has meant that when I take the images in to HDRshop to remove the tripod and camera from by re-aligning, it was much much easier, no messing around with alignment points, just used re-align by 90 degrees, much more successful results.

5. For each set of exposures for each view point, I loaded the set into photoshop and created a layer for each photo, then when I applied the cropping it was was exactly the same for each photo of the mirror ball in the set.

 

Last thing I have learn't, this is a very involved and time consuming process, not for the faint hearted!! I'll let you know how I get on with the finished results.