Why no ISO 25 for DSLR's?

Hello,

When I used film cameras I would always carry around a few rolls of Fuji 50 slide film and kodachrome 25, they were really handy when I was after the best quality and speed was not an issue, and also when I wanted a long shutter speed in bright sunlight, for example, when I wanted to blur a stream in a landscape picture.

I was wondering if there was any technical reason that DSLR's do not offer lower ISO settings, or whether the manufacturers don't bother with lower ISO's because they don't think photographers want/need them.

I'd love to have a DSLR with such a feature, if nothing else it would be nice not having to carry around ND filters all the while.

I'd love to hear other peoples comments on this,

--
Julie

Julie Sinar wrote:
[snip]

Julie Sinar said:

I was wondering if there was any technical reason that DSLR's do
not offer lower ISO settings, or whether the manufacturers don't
bother with lower ISO's because they don't think photographers
want/need them.

I'd love to have a DSLR with such a feature, if nothing else it
would be nice not having to carry around ND filters all the while.

I'd love to hear other peoples comments on this,

Click to expand...

There is a technical reason.

Sensor have something called "nominal sensitivity." This is their "natural" sensitivity -- the sensitivity where they have their best dynamic range. ISO changes from that are made by either boosting or damping the signal from the sensor. Both decrease dynamic range: boosting amplifies noise and decreases DR in the shadows. Damping, however, doesn't affect the rate at which the photosites saturate: that is, the highlights will blow out once the "wells" fill up no matter what's done to the signal afterwards.

Therefore, if you have a sensor whose nominal sensitivity is ISO100 and you damped it to ISO25, you would get blown highlights about as often as if you'd overexposed by two stops. This is unacceptable for most uses.

The EOS-1D at least does implement a "damped" ISO100 -- the nominal sensitivity of its sensor is ISO200. This effect is quite clear -- significantly reduced dynamic range in the highlights.

BTW, if the scene doesn't have too much dynamic range, you can always shoot in RAW and "expose to the right" -- that is, overexpose as much as you can without losing the highlights. Then pull down the tones. You will get better shadow tonality and lower noise (as if that matters) -- IOW, all the advantages of a "damped" sensor.

Petteri
--
[ http://www.prime-junta.tk ]

Petteri Sulonen said:

There is a technical reason.

Sensor have something called "nominal sensitivity." This is their
"natural" sensitivity -- the sensitivity where they have their best
dynamic range. ISO changes from that are made by either boosting or
damping the signal from the sensor. Both decrease dynamic range:
boosting amplifies noise and decreases DR in the shadows. Damping,
however, doesn't affect the rate at which the photosites saturate:
that is, the highlights will blow out once the "wells" fill up no
matter what's done to the signal afterwards.

Therefore, if you have a sensor whose nominal sensitivity is ISO100
and you damped it to ISO25, you would get blown highlights about as
often as if you'd overexposed by two stops. This is unacceptable
for most uses.

Click to expand...

Many thanks for the explanation, I guess there's going to be a need for ND filters for some time to come,

Julie

Many thanks from me, as well, Petteri.

Julie Sinar said:

Petteri Sulonen said:

There is a technical reason.

Sensor have something called "nominal sensitivity." This is their
"natural" sensitivity -- the sensitivity where they have their best
dynamic range. ISO changes from that are made by either boosting or
damping the signal from the sensor. Both decrease dynamic range:
boosting amplifies noise and decreases DR in the shadows. Damping,
however, doesn't affect the rate at which the photosites saturate:
that is, the highlights will blow out once the "wells" fill up no
matter what's done to the signal afterwards.

Therefore, if you have a sensor whose nominal sensitivity is ISO100
and you damped it to ISO25, you would get blown highlights about as
often as if you'd overexposed by two stops. This is unacceptable
for most uses.

Click to expand...

Many thanks for the explanation, I guess there's going to be a need
for ND filters for some time to come,

Julie

Click to expand...

Julie Sinar said:

I'd love to have a DSLR with such a feature, if nothing else it
would be nice not having to carry around ND filters all the while.

Click to expand...

Yep; I agree. Kodak has full-frame dSLRs that go down to 6, which is an automatic four stops. Very useful in a few particular situations.

I've tried shooting the same river from a tripod in quick sucession and then stacking the images, but it's just not the same thing.

Following your logic, would you then say that generally a camera will have more dynamic range in ISO 100 if it allows ISO 50? Are you suggesting that just like Aperture range, there is a sweet spot in ISO?

EXTREMELY INTERESTED in your response.

Petteri Sulonen said:

Petteri Sulonen said:

I was wondering if there was any technical reason that DSLR's do
not offer lower ISO settings, or whether the manufacturers don't
bother with lower ISO's because they don't think photographers
want/need them.

I'd love to have a DSLR with such a feature, if nothing else it
would be nice not having to carry around ND filters all the while.

I'd love to hear other peoples comments on this,

Click to expand...

There is a technical reason.

Sensor have something called "nominal sensitivity." This is their
"natural" sensitivity -- the sensitivity where they have their best
dynamic range. ISO changes from that are made by either boosting or
damping the signal from the sensor. Both decrease dynamic range:
boosting amplifies noise and decreases DR in the shadows. Damping,
however, doesn't affect the rate at which the photosites saturate:
that is, the highlights will blow out once the "wells" fill up no
matter what's done to the signal afterwards.

Therefore, if you have a sensor whose nominal sensitivity is ISO100
and you damped it to ISO25, you would get blown highlights about as
often as if you'd overexposed by two stops. This is unacceptable
for most uses.

The EOS-1D at least does implement a "damped" ISO100 -- the nominal
sensitivity of its sensor is ISO200. This effect is quite clear --
significantly reduced dynamic range in the highlights.

BTW, if the scene doesn't have too much dynamic range, you can
always shoot in RAW and "expose to the right" -- that is,
overexpose as much as you can without losing the highlights. Then
pull down the tones. You will get better shadow tonality and lower
noise (as if that matters) -- IOW, all the advantages of a "damped"
sensor.

Petteri
--
[ http://www.prime-junta.tk ]

Click to expand...

--

If you can't do anything right, at least you can stand aside and let someone else do it.

AhLimPaS said:

Following your logic, would you then say that generally a camera
will have more dynamic range in ISO 100 if it allows ISO 50? Are
you suggesting that just like Aperture range, there is a sweet spot
in ISO?

Click to expand...

In general, the "sweet spot" is the lowest sensitivity. Some cameras do offer damped sensitivities, but these are usually well documented or even have to be accessed differently from the other ISO's (e.g. the 1D and 1D Mark II).

Some people do claim that the EOS-10D, for example, has slightly more dynamic range at ISO200 than at ISO100 -- which would mean that the nominal sensitivity of the sensor is greater than 100. I haven't noticed this in practice, though.

It might be worth investigating, though. Take a contrasty scene and shoot a series, see if the highlights blow sooner at the lowest ISO than the next-lowest. If that happens... well, you've learned something that may be useful.

But in general, it's a good rule of thumb to shoot at as low ISO as the lighting allows.

Petteri
--
[ http://www.prime-junta.tk ]