HDR, EXR Question

[GWestphal]

Why do you need multiple exposures? Doesn't one long exposure contain all darker exposure values intrinsically? So figure out what the highest exposure should be, take that image, then just integrate less time to get the darker exposures? I could perhaps see that each pixel might saturate at some accumulated light value, but couldn't you then just read a differential say every 10 nanoseconds and then just cumulatively sum each pixel vector to achieve the desired effect. Perhaps reading the sensor isn't fast enough yet, but that seems like something that could easily be achieved using smaller sensors or just putting effort into making faster read out.

 

[corkyg]

False premise. I don't need multiple exposures.

 

[Throckmorton]

I don't even understand why digital cameras have shutters.

 

[TanisHalfElven]

I don't even understand why digital cameras have shutters.

thats actually something that confuses me for phone cameras. The image on the view finder is sometimes just perfect, and then instead of capturing that, it goes ahead and take a "shot" and messes it up.

 

[Throckmorton]

thats actually something that confuses me for phone cameras. The image on the view finder is sometimes just perfect, and then instead of capturing that, it goes ahead and take a "shot" and messes it up.

I found an explanation though it doesn't explain cell cameras http://www.steves-digicams.com/knowledge-center/why-digital-cameras-have-mechanical-shutters.html

Here too http://www.npr.org/blogs/alltechconsidered/2009/05/do_digital_cameras_need_shutte.html

So apparently you can have pixels that you electronically control exposure time for.

 

[ElFenix]

you need different exposures to cover larger dynamic range. the dynamic range of the sensor is fixed, there is a certain number of photons it has to collect before it registers any signal, and there are a certain number of photons beyond which it cannot collect additional signal. with just one exposure, your dynamic range is fixed at that of the sensor. with multiple exposures, you can shift that dynamic range up and down around the common proper exposure so that you can add dynamic range through software.

in your typical exposure, you'll have areas that are too bright to collect much useful information from (the sensor gets full and beyond that, added light is just discarded), and you'll have areas that are too dim to rise above the noise floor of the sensor. to get information from the bright areas, you would underexpose the main scene, thereby avoiding filling the sensor for the bright areas and gaining information. to get information from the dim parts, you'd overexpose, thereby lifting the signal out of the noise floor and gaining information.

 

[xj0hnx]

In post you use different aspects of each exposure to get the effect right. Even when using a single image you make copies of it and create layer mask to expose different pieces.