[HDRI] Re: CanonHDRcap exposure determination

[Greg Ward]

Hi Mark,

I thought I had explained this at some point, but I guess not....

CanonHDRcap works as you surmised, but only looks at the image  
thumbnails, not the whole image (which would take too long).  As a  
result, it doesn't always pick the right exposure to start with -- if  
your bright spot takes up less than a pixel in the thumbnail, it may  
choose a shutter speed that's slower than it should to start.

The application actually keeps the exposure right before the one that  
was blown out, so sometimes you can capture this by setting a  
*larger* search step.  This also makes the capture sequence faster.   
You may end up with a bit of a jump between the shortest exposure and  
the next-longest, but you're more likely to capture that peak.  I  
wouldn't recommend a search step greater than 4, in any case.

I could be persuaded to add a parameter for the starting shutter  
speed.  If set to some positive value, CanonHDRcap would then start  
from the specified speed rather than doing its search.

-Greg

> From: "Mark Banas (lists)" <listmail at mab3d.com>
> Date: February 2, 2006 7:08:41 PM PST
>
> Here is one for Greg (of course):
>
> How does CanonHDRcap determine the "minimum" exposure that is  
> relevant for an HDR capture dataset?
> From my observations it seems to "search" for the darkest exposure  
> where a min. % of pixels are "blown out" and then start marching up  
> the exposure scale from there based on your spacing and # of shots.  
> I'm sure this is an over-simplification of things, but I tried (in  
> my "tethered-to-a-G5-tower-in-a-windowless-office" tests) to find  
> that threshold of minimal % where the limits of my camera's shutter  
> speed would be hit.
>
> Since I'm stuck inside with my workstation, the "best and  
> brightest" to focus on is a 22w halogen bulb on the ceiling track  
> lights. It seemed that under certain "ideal" camera conditions (f/ 
> 8, ISO 100) the "searching" would stop at 1/500 sec., regardless of  
> the step amount for searching (and 1 stop search takes a while). If  
> I shot additional images I could march down to 1/2500 sec. and  
> still have nearly the same # of blown out pixels in the image  
> (after that I started to resolve the bulb and reflector).
>
> I then checked to see if the captured image resolution has anything  
> to do with it (a strict # instead of %) and found no difference.  
> When I tested it under tungsten bulbs, it started the series at the  
> point where *no* pixels were blown out, but perhaps this is due to  
> my camera settings and the dimmer bulb. Again, this is without a  
> laptop to go out and sample some serious range, but I'm working on  
> that...
>
> BTW, I shot 13 HDRs in 2 hours, including a disappointing panorama  
> of the dusty floor of my office (without being able to "lock" in  
> the settings after a "search," multi-shot panos are impossible -  
> the range keeps adjusting).
>
> -Mark