Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

submit your photo


Hall of Fame
View past winners from this year

Please participate in Meta
and help us grow.

Take the 2-minute tour ×
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It's 100% free, no registration required.

I am getting into some HDR photography and was thinking of a new ways that camera manufactures could help us in capturing HDR pictures.
Say you want to capture three image and you have a constant Aperture and ISO with shutter speeds of 1/2, 1 and 2 seconds (for -1, 0 and +1 exposure compensation).
You set the shutter speed to 2 seconds and press the shutter release button.
1/2 seconds later, the camera takes a readout from its sensor and saves the output to a file.
Another 1/2 second later (for 1 second exposure) it takes another readout and saves the data.
1 second after that (for the 2 second exposure) it takes another readout of the sensor and saves the data.

So for a single shutter count, we get 3 images at different points in time.

I am not sure if any cameras does that but is it technically possible to do it?

share|improve this question
    
There's a couple of similar preset settings on my Casio EX-FH100 that take a series of shots at different exposures right after the other and add them together into one photo simulating HDR. As it's only a P&S though this method doesn't seem very accurate, although it does take the edge off the extremes –  Dreamager Sep 21 '11 at 16:53
add comment

4 Answers

up vote 3 down vote accepted

This is not really my area, but I believe that readout is achieved by discharging the charge that builds up at each photosite due to arrival of photons. This would mean sampling pixels during the time the shutter is open would be equivalent to taking a series of separate exposures using an electronic shutter.

Now in this scheme you could then average several images in the shadow areas to reduce noise and hence increase dynamic range. But that wouldn't really be much different to shooting a series of images using a traditional HDR approach, plus you'd have to deal with rolling shutter issues on CMOS chips.

It's a good idea, I'm just not sure it's possible. The best you could hope for would be to have a rough way of detecting when a photosite was going to saturate. You could then sample the photosite at that point, and multiply the value post digitization by the proportion of exposure time it had. That would prevent overexposure, increasing the amount of light the sensor could handle before saturation hence increasing dynamic range. However, CMOS chips read line by line, so I imagine it would be very hard to read individual pixels ahead of time.

You'd also get some weird effects with moving subjects with this scheme, or indeed any time based HDR approach. The best idea in my opinion was Fuji's superCCD R which had an extra set of low sensitivity pixels (giving two readings in each location). This effectively allowed two different exposures to be recorded at the same time.

share|improve this answer
    
CCDs are indeed read by discharging the pixels. CMOS pixels, however, are usually read in a non-destructive way, through a read-out transistor. –  Edgar Bonet Sep 21 '11 at 15:33
    
The main reason I wanted to know if multiple readouts is possible for a single exposure is because I was excited that It could open up a whole world of possibilities for post processing. Imagine having a single RAW file that can contain multiple exposure values for each pixel and if some of them get overexposed, you should be able to bring them back to a previous point in exposure time –  Vivek Sep 21 '11 at 17:54
    
@Vivek: Fully reading a multi-megapixel sensor takes some time, and the pixels you read last will end up being exposed for a longer time than those you read first. That is, unless you use some kind of electronic (rolling or global) shutter. But if you use a shutter, you may as well clear the charges that you read as there is little benefit in keeping them in the pixels. And then you end up doing a “traditional” multiple exposure, only with an electronic shutter replacing the mechanical shutter (which can be good). –  Edgar Bonet Sep 22 '11 at 8:29
add comment

It is kind of already done but requires a special sensor. Fuji's SuperCCD EXR and EXR CMOS sensor are wired so that they can read one half of the photosites separately from the other half. When set to EXR-DR mode (Dynamic-Range priority) half of the pixels are read partly through the exposure and the other half at the end. Fuji combines both halves with various ratios to increase the DR by up to 4 stops (1600% as Fuji labels it). The consequence is that you get image of half the resolution.

There are technical issues doing this with a normal sensor. CCD would give the most trouble since they are read by transferring the charge from one row to the next. You will notice this as a vertical line if you point a CCD camera a bright light. CMOS sensors can be read any time but causes the photosite to discharge. This is why CMOS cameras show a 'jello' effect while filming video, since it takes time to read the whole image, although things are improving, so we may get there.

Once there results would require special processing to account for motion artifacts. It gets more complicated with flash which is why Fuji turns it off in EXR-DR mode.

share|improve this answer
add comment

Total amount of light after 2 seconds in one photo can be buffered and saved to a medium. Indeed you could take many different points of time than 3 or 5 and have more subtle transitions. I assume video cameras work like this and probably also video mode in DSLRs. Comparing to conventional HDR technique, I see benefit of prolonging lifetime of a shutter mechanism.

On the other hand your approach would probably ruin quality of images because in my case, out of 3 exposuress, at least first two photos are tax sharp and overexposed one is "ok" because of the camera movement. long shutter speeds are not always possible to achieve and preserve image quality.

share|improve this answer
add comment

As Matt Grum already explained, cameras cannot do this, but you can approximately imitate such behavior by setting your camera underexposed, shooting a series of photos and incrementally add them together (e.g. as layers blended using "Linear Dodge" in Photoshop or "Addition" in Gimp) to obtain HDR source images.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.