Low ISO

[blakemcguire]

Just out of curiosity, if you use an ISO sensitivity lower than the base ISO of a sensor, are there any disadvantages?

[inkista]

Reduced dynamic range, or so I've been told about using iso 50 on my 5D Mark II.

[Colin Southern]

Hi Blake,

Normal range ISO settings are done by altering the gain (amplification) of the signal after it's read from the sensor, but the "high" and "low" settings (with low being ISO 50 on a Canon) are done by digital manipulation, so it's a "smoke and mirrors" approach as opposed to a true ISO.

As Kathy points out, it's all about dynamic range. DR is greatest at your base ISO (100 for Canon, which typically gives around 11 to 12 stops), but you lose 1 stop of DR for each doubling of the ISO (because the noise floor stays the same, but the signal captured is less and less). ISO 50 is a bit different, but you still lose a stop.

In reality it's not a big deal though - I use it without hesitation if I need to; the advantges nearly always outweight the penalties.

[dje]

This thread raises a touch of nostalgia for me - it must be close to a year to the day that I posted my first thread on CiC in relation to ISO standards - and if I recall, Kathy and Colin were the respondants to my thread.

But in relation to this question, I have to say that I haven't heard of this concept before and have to ask what is it for ? Does it effectively de-sensitise the sensor so that you need a higher exposure ? (eg slower shutter speed to get a particular effect ?)

Dave

[Colin Southern]

This thread raises a touch of nostalgia for me - it must be close to a year to the day that I posted my first thread on CiC in relation to ISO standards - and if I recall, Kathy and Colin were the respondants to my thread.

OK, I feel a year older

But in relation to this question, I have to say that I haven't heard of this concept before and have to ask what is it for ? Does it effectively de-sensitise the sensor so that you need a higher exposure ? (eg slower shutter speed to get a particular effect ?)

Sorta/kinda. The problem is that you can't decrease the sensitivity of the sensor any more than you can increase it (it's a physical property of the sensor); all you can do is "something" later. In the case of ISO 50 they actually over-expose 1 stop (cuts into safety margin) and then digitally halve the result.

And yes - it's done for getting a longer exposure (or just less of an exposure if studio lights are too bright and one is too lazy to adjust them) (assuming aperture is at a minimum).

[inkista]

In other words, think of it like having a built-in 1-stop ND filter.

[dje]

Colin and Kathy thanks.

Dave

[John C]

I think that ISOs less than 100 or whatever the 'native' ISO is for the sensor may have higher dynamic range. It seems to depend on the camera. Take a look at the DxO Image Quality site where they provide actual test results for different cameras. One of the measurements they make is dynamic range (Ev) vs. ISO. For my Sony Alpha 77, they show the dynamic range is HIGHER for ISOs less than 100, at least according to DxO. This seems to be borne out by photographers reporting high image quality for ISO 50. I looked at some other cameras and some went up and some were more or less the same (I only looked at a few).

[blakemcguire]

Thanks guys. I haven't been in the situation where it's really useful to me yet (I've actually just acquired a step up ring for my polarizer AT LAST), but this does confirm my suspicions. Now I know for if I do need it at some point.

I think that ISOs less than 100 or whatever the 'native' ISO is for the sensor may have higher dynamic range. It seems to depend on the camera. Take a look at the DxO Image Quality site where they provide actual test results for different cameras. One of the measurements they make is dynamic range (Ev) vs. ISO. For my Sony Alpha 77, they show the dynamic range is HIGHER for ISOs less than 100, at least according to DxO. This seems to be borne out by photographers reporting high image quality for ISO 50. I looked at some other cameras and some went up and some were more or less the same (I only looked at a few).

On DXOmark it seems that they're using a "measured ISO." For example, for the D90, the lowest ISO they show is listed as "measured ISO 145" and "manufacturer ISO 200." It seems that they didn't actually test it at the "Low-1" setting.

[Colin Southern]

From the "archives" ...

When to use ISO 50 on Canon SLR Cameras

[Colin Southern]

1 more reference from the Canon Digital Learning Centre:

http://learn.usa.canon.com/app/pdfs/...QuickGuide.pdf

[IMG][/IMG]

[Shadowman]

I have an older model Gossen incident light meter and it has an ASA speed between 6-3200. I read that the evolution of film speed measurements coincided with the development of manufacturers film, such ASA 6 for Kodachrome film, ASA 50 for Fuji RVP and Kodak Vision2, ASA 100 Kodacolor Gold. So sensitivity of film (ASA) evolved into sensitivty of digital sensors (ISO) with another standard in between (DIN).

When I used a point and shoot film camera it was suggested I use a higher ISO film (400-800) during the evening. These films usually had shorter expiration dates and were also affected by airport X-ray machines.

[Colin Southern]

I'd kill for a native ISO 6 mode

[blakemcguire]

I'd kill for a native ISO 6 mode

And I'd kill to not have to use it :P (me and my hand holding)