ISO and dynamic range (digital)

I understand generally how ISO relates to aperture and shutter-speed;

that high ISO means better performance in low light, etc. My question

is: in a scene with a wide dynamic range (bright sun + dark shadows in

the frame), does lowering the ISO increase your dynamic range. Or

stated differently, are my chances of capturing the whole scene

without blowing out the highlights better at ISO 100 than at ISO 1600?

This is an excellent question. I wish I had the answer to it myself actually. I've been doing alot of reading some photograhy and have 3 books on the way and I am curious as to this as well.

In a simplify view, available system dynamic range depends on two functions, the sensor resolution (number of bits) and (the absent of) noise. You could increase Dynamic range in the same two ways. #1 Shoot RAW instead of JPEG (12 bits in most DSLR verse 8 bits with Gamma for JPEG). #2 Lower ISO decrease amplification noise there by increase dynamic range. Use ISO 100 instead of ISO 1600 (+ND filter).

Yes, lower ISO settings provide images with a wider dynamic range.

Dynamic range of a DSLR is a function of the bit depth of the system and the sensitivity of the sensor. A sensor with good "low light" capture has the ability to capture more shadow that a sensor with poor "low light" capture. The limiting factor is the noise floor, which will determine, how good the shadow areas will be captured.

 

"Bright light" capture is limited by "blooming", or the spill into adjacent photo site, which occurs when the photo site is overloaded.

 

The current technologly increases the ISO by increasing the gain of the sensor amplifiers.This is why noise is a problem using high ISO's as the noise is also amplified as well as the desired signal.

In theory, if the gain of an amplifier is increased, dynamic range also increases till the amplifier reaches overload, as it has the ability to increase the level of a smaller signal.

 

Therefore in theory, higher ISO's should give greater dynamic range.

In reality, the sensor noise floor and the available bit depth of most DSLR's means that the increase in dynamic range is not able to be used. Some file compression has to be used to make sure a full range of tones is able to be captured.

 

It is now thought that 32bit colour capture approximates the range of the human eye, so we have a long way to go yet, as most DSLR's only use 12bits!

Thinking about my last post, are you talking about dynamic range or latitude? Using our current technology, decreasing ISO increases latitude due to the same restrictions. The lower the ISO the lower the amount of data collected by the sensor/amplifier/processor chain. It is then easier to fit this data into the 12bits available in the file structure.

 

It's easy to be confused between dynamic range and latitude when talking about digital technology.Most DSLR's have a latitude of 5 stops, though this is probably more a limitation of the data processing capability.

 

One technique to increase dynamic range is to use is the "combined image in photoshop" technique. Taking two exposures, one for shadows and one for highlights, will give an image with a larger dynamic range than is possible with a single exposure.

 

Another way would be to use (dare I say it!) a high quality negative film and scan it with a quality scanner. Both methods have a greater dynamic range than a single digital exposure.

 

It all depends on your favoured method of working. As always, don't get hung on the digital v film argument, pick the best tool for the job that suits your method of working.

 

In the end it's the image, not the method that counts.

The answer is that you will get the best dynamic range using the sensor at its "native" or base ISO. The reasoning is that each pixel has a maximum amount of light that it can capture before it is overexposed, which sets the upper limit, while there are other considerations that set the noisiness of the shadows. These include the sensor's own sources of electronic noise and noise in the amplifier and analogue/digital converter. Several of the in camera sources of noise are inversely related to the strength of the signal, as is photon shot noise (an inescapable consequence of the physics of light), and these tend to overwhelm sensor dark current noise (at least for reasonably short exposures - though night exposures of several seconds and more can suffer from dark current noise more than from other sources, as it depends on the length of the exposure). When you use a higher ISO, the camera meters so that the brightest areas of the picture make use of less and less of the capcity of the pixel as the ISO is increased, so the brightness range is represented by fewer electrons. For every doubling of ISO the camera is capturing half the light during exposure. Meanwhile, the sources of noise are increasing relative to the size of the captured signal. The result is that the maximum signal to noise ratio is decreasing as ISO is increased.

 

Some cameras offer a lower than base ISO (ISO 50 on several Canons) - and here, there is more of a tendency for highlights to blow out. In this case, ISO 50 is useable if the picture deosn't contain bright highlights.

Garry, I do not fully understand the distinction between "latitude" and "dynamic range." I gather from the answers so far that on a bright sunny day, shooting "RAW" at ISO 100 is the best bet (assuming no tripod, no ability to bracket, no HDR techniques in image editor). What I am concluding is that digital camera makers should put more energy into increasing the dynamic range (bit depth?) of sensors rather than the pixels. I am sure the math is more complicated than I understand, but, for me, I would much rather the new Canon 5D sensor (for example) had only 8 megapixels, but added a couple of extra stops of sensor latitude (or dynamic range).

 

Thanks to all for the input.

For an excellent discussion of dynamic range in digital photography, take a look at Roger Clarke's web page. http://clarkvision.com/imagedetail/index.html

 

The posters above are on the right track, but the discussion isn't quite as thorough as Clarke's.

 

...Dave Hachey

Ilkka wrote:

 

"Yes, lower ISO settings provide images with a wider dynamic range."

 

Really! This is a revelation for me. I've posted a question regarding avoiding blow-out, and nobody mentioned this. I wonder if the difference is significant. I'm going to experiment with this. Thanks.

This shot was taken at ISO200 1/60 at f5 on my 20D 17mm (17-85). It shows blown highlights and I should have taken two shots at different exposures to get the full dynamic range I wanted. I doubt it a different ISO setting would have helped.

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