DSLR ISO Settings

[larry_s1]

What does a DSLR do when the ISO setting is changed? For example,

when the ISO is changed from 400 to 800, since the sensitivity of the

chip doesn't change, does the camera process the "darker" image to

lighten it and produce an acceptible file? Or is something else

going on.

 

I ask because I often shoot film at 3200 (by need not choice) and the

Nikon D70 only has ISO settings to 1600, no boost setttings like the

D100. Nikon told me underexpose by a stop and lighten later "would

be essentially the same as if we had included the higher ISO settings

in the camera."

[mormegil]

I don't think so. I think the camera is changing the CMOS or CCD sensitivity. I say this because ISO 100-1600 covers 5 stops. But if you blow out a highlight at ISO 1600, you can't rescue it by applying Exposure Compensation on the RAW file down 4 stops. So that data is gone.

 

Maybe the "boost" is cheating a way around the physical limit of the camera.

[Greg M]

I use exposure compensation alot with my Digital Rebel. If you shoot at ISO 1600 and underexpose by 1 stop, I would think the result would not be the same as shooting at ISO 3200. The sensor gives me much the same results as actually shooting film at the various ratings, so it seems like the makers have some way of making the sensor more or less sensative based on the ISO setting. I am in no way knowledgable enough to know this for a fact, but this has to be right.

 

It also sounds like some of Nikon's "technical support" personel are as bad as some at Canon.

[pascalharvey.com]

It is the same, in fact that exactly what the camera does, under-expose at 200 ISO and push up the gain to get 400, 800 or so. But to get the best from your shot, it is recommended to shoot in RAW, because JPEG is only in 8bit.

 

Of course, the noise will be quite high.

 

Pascal H

[tlsohl]

Yes, you are affecting the sensitivity...specifically, the "gain" of the sensor.

[carl smith]

It simply turns up the "gain." The sensor can't collect photons any more efficiently, so it's similar to underexposing and pull processing.

[jakob_norstedt_moberg]

Some speculations based on my general knowledge of electronics:

 

The sensor is digital, and you can not adjust the sensitivity in the same way you adjust the sound level from an amplifier. The sensor delivers digital data, ones and zeros only.

 

As far as I know, the sensor in DSLR:s is actually a 16 bit sensor (bit 1 to 16). The raw file delivers 12 bits. If you pick out the 12 highest bits of the 16 (bit 5 to 16), you will cut away all random noise which resides in the lower bits. If you pick bit 4 to 15 you will get some more noise and get 1 step higher sensitivity (doubling the ISO). You can pick bit 3 to 14 which doubles once more. Bit 1 to 12 will give you 2x2x2x2=16 times higher ISO, and all the noise the sensor originally picked up. You would go from ISO 200 to ISO 3200 by simply select digitally which bits you use from the sensor.

 

This is how I should construct the electronics around a sensor to allow for ISO adjustments.

 

/Jakob

[jespdj]

Jakob, the sensor is a digital-to-analog converter. The camera just turns up the analog gain if you choose a higher ISO setting. It's not done digitally.

[jespdj]

Ofcourse I meant the sensor is an <b>analog-to-digital</b> converter, not the other way around. *When* are we getting better forum software here that allows you to edit posts?!

[luke_kaven]

The CCD charges are passed from the sensor through a readout register as an analog signal and passed through an amplifier stage before A-D conversion. The ISO number controls the analog gain on the amplifier stage. When you amplify the signals, you elevate the noise floor further into the visible signal region. Thermal noise, power supply fluctuations, and internal sensor properties contribute to the background noise.

 

Luke