Can exposure be bracketed by many sensor readouts during single exposure?

Question

I am getting into some HDR photography and was thinking of a new ways that camera manufacturers 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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.