How does ISO setting work?

John Sheehy wrote:
Paul Furman wrote in -
september.org:

Closest Nikon has is to underexpose then lift the shadows.

Well, Nikon has a number of cameras with very "liftable" low-ISO shadows.
Canon is plagued with excessive shadow banding at low ISOs. The 7D has a
lot of banding at low ISOs, but much of it is fixed pattern (and some
apparently caused by poor firmware correction), and could have conceivable
be calibrated out. I can get a decent small web-image from my 7D at ISO
100 under-exposed 8 stops, after subtracting a stack of blackframes.

Be interested to see an example of shadow banding at low ISO on the 7D -
a fragment at full resolution PNG lossless compressed would do.

Regards,
Martin Brown

Martin Brown wrote in
:

Be interested to see an example of shadow banding at low ISO on the 7D
- a fragment at full resolution PNG lossless compressed would do.

I'll soon be making examples for a thread on DPReview; I'll try to link to
the images here.

I've actually seen the banding in OOC JPEGs in deep shadow areas, without
any pulling up of the shadows (ALO off).

There are basically two components to 7D low ISO banding; one is the
typical recent-Canon non-fixed banding with lots of low-frequency content.
The other is unique to the 7D, at least as a general problem. It is
potential deep modulation of every 8th column of pixels, and different
blackpoints for odd and even pixel rows. Some people are lucky, in that
none of the 8 vertical lines in the 8-line period are deep, but most 7Ds
have at least one period member that's pretty deep.

Canon does a very ****-poor calibration of blackpoint-by-line in their
cameras; Canon does not guard against the pitfalls of complex readout, and
hence, their Rebels have better shadow IQ than their semi-pro siblings with
similar sensors.

John Sheehy wrote:
Martin Brown wrote in
:

Be interested to see an example of shadow banding at low ISO on the 7D
- a fragment at full resolution PNG lossless compressed would do.

I'll soon be making examples for a thread on DPReview; I'll try to link to
the images here.

I've actually seen the banding in OOC JPEGs in deep shadow areas, without
any pulling up of the shadows (ALO off).

If you can find one of these OOC JPEGs I would be interested to take a
look at it. My curiosity is peeked. The most obvious cause would be some
kind of rounding error in the transfer function from RGB to YCC.

Please send one to me privately at the odd looking reply to address.
(no alterations needed it is valid when unmodified)

Certain other well known leading packages have very similar faults
although the damage is usually most annoying in highlights where clear
chroma to luminance bands can appear in highlights.

A green to magenta diagonal gradient put through the JPEG encode/decode
cycle a few times is a very good test of rounding errors in the codec.
any rounding errors and it will break up into chunky bars of colour.

There are basically two components to 7D low ISO banding; one is the
typical recent-Canon non-fixed banding with lots of low-frequency content.
The other is unique to the 7D, at least as a general problem. It is
potential deep modulation of every 8th column of pixels, and different
blackpoints for odd and even pixel rows. Some people are lucky, in that
none of the 8 vertical lines in the 8-line period are deep, but most 7Ds
have at least one period member that's pretty deep.

Unfortunate given that JPEG relies on 8x8 periodicity.

Canon does a very ****-poor calibration of blackpoint-by-line in their
cameras; Canon does not guard against the pitfalls of complex readout, and
hence, their Rebels have better shadow IQ than their semi-pro siblings with
similar sensors.

I only have Canon Ixus P&S but I can't say I have ever had problems with
their black point calibration. Its main attraction is always being with me.

Regards,
Martin Brown

Martin Brown wrote in
:

If you can find one of these OOC JPEGs I would be interested to take a
look at it. My curiosity is peeked. The most obvious cause would be
some kind of rounding error in the transfer function from RGB to YCC.

I didn't say anything about the problem *being* a JPEG issue. This is
purely a RAW thing. What I did say is that it could be seen in the JPEG
review image, meaning, with no pushing of the shadows. This is purely a
RAW shadows issue.