Film speed, how does it work in a digital camera?

52Caddy

Hi all,

Can anyone explain how a digital camera accomplishes changing the film speed?

Isn't the physical characteristics changed on regular film, different grain sizes?

Does it use more pixels for each point of light?

Thanks for any explanation, or references if it's already been covered!
Eric

Shay Stephens

Changing the "film speed" on a digital camera is much like turning up the volume on a radio when a very soft song is playing. The sensitivity of the radio is enhanced and you can hear the softer audio much better.

You typically make a trade off however in that you will hear more pops and noise in the audio, the frequency response goes down, etc.

With a digital camera, you can increase the sensitivity of the sensor at the expensive of more visual noise (film grain), decreased dynamic range, etc.

52Caddy wrote:

Hi all,

Can anyone explain how a digital camera accomplishes changing the film speed?

Isn't the physical characteristics changed on regular film, different grain sizes?

Does it use more pixels for each point of light?

Thanks for any explanation, or references if it's already been covered!
Eric

ziggy53

In digital cameras, the imager actually records at one rated speed. The speeds are rated as ISO and do roughly correspond to film ISO. (ISO used to be called "ASA", for those of us who are "age challenged".)

The digital imager records a RAW image which has to be interpreted by the firmware and processor in the camera. Part of that processing can include transposing the taken ISO image to a different ISO. Higher ISOs require "stretching" or "amplifying" the exposure, much like turning up the volume on a radio/TV. Similarly, any noise in the image also gets stretched and amplified.

The larger the imager, all other things being similar, the less the noise, and the greater the opportunity for amplification to a higher ISO. Technology is starting to make a big difference in noise level as is the firmware processing.

I hope this helps,

ziggy53

ziggy53

Shay can obviously type much faster than I can. I think he has a higher typing ISO than I have.


:giggle

Good answer, BTW.

ziggy53

erich6

I think there's also another way that manufacturers build selectable ISO in digital cameras. CCD's and CMOS sensors measure light through the photovoltaic process (photons are converted into a voltage based on how many electrons are generated by the imager). How many electrons are mapped to how much voltage is a function of the gain of the sensor. Some of the sensors have selectable gains that allow you to get better sensitivity at the expense of dynamic range and more noise. Generally, making the sensor more sensitive prior to digital conversion results in better noise performance than with processing/stretching after digitization. In many cases, cameras use the selectable gain for changes at the lower ISO levels and processing for higher ISO. You can achieve the same effect by post-processing with Photoshop or the like. It's like "pushing" film in development.

Erich