Canon 5D Mark II: Banding... c'mon, still?

Rishi Sanyal , Dec 08, 2008; 10:11 p.m.

I have previously brought up the issue of banding in shadows for Canon digital SLRs (http://photo.net/canon-eos-digital-camera-forum/00RBTe). Very evident in Rebels, less evident in higher-end cameras like the 5D. But, nonetheless, there.

Disappointingly, it's still there in the 5D Mark II... you'll see some examples below.

These are more easily seen by lightening shadows using 'Fill Light' or whatever algorithm you choose. But sometimes they're visible on their own.

First question: Nikon CCD camera based users: do you see this?

Now, am I to understand that this banding results from variations in the amplifiers for each row OR from temporal variation in read-outs? And do these banding effects essentially result when these variations in gain or readout exceed the signal:noise ratio (which is much more likely to occur at low signal, i.e. shadows... OR, at high ISO where the gain for each amplifier is increased)?

Here are some examples from the new Canon 5D Mark II (sadly, yes):

Here's an example of some horizontal banding (note, the image was significantly brightened in DPP):
Horizontal banding in shadows_ISO100

Here's a comparison of the same area of a shot, at ISO 50 & at ISO 100, on the 5D Mark II, with Fill Light set to +100 to exaggerate the vertical banding in shadows:

Here's the link to the full-size image.

And here's the banding seen in a high ISO image (ISO 25600):

One thing that confuses me though -- sometimes it seems like the direction of the banding changes from image to image, by 90 degrees (and no, not because the image was portrait vs. landscape). I'm confused...

Anyway, the purpose of this post, to really figure out why it's there. Berg previously suggested that it's temporal variation, and if I understand this concept correctly, I don't understand why this temporal variation has to exist if scanners like the Nikon LS-9000, which scans 1 (or 3?) lines at a time, doesn't exhibit this sort of banding in extremely dense areas of Velvia 50 slide scans, as is evidenced below:
LS-9000: See? No banding!
Link to Full-Size Image

And that scan was significantly lightened with the Shadows/Highlights tool in Adobe PS CS3 (Shadows: +100, tonal width=14, black clipping = 0.01%).

Thanks in advance for your help,
Rishi

Responses

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G Dan Mitchell , Dec 08, 2008; 10:26 p.m.

Right. If you shoot at high ISO, underexpose, and use raise the fill setting... you will get banding, and noise, and a host of problems.

There is nothing wrong with the camera in such cases, and if you resolve those exposure and post-processing problems that you have created you won't see these "problems."

Scott Ferris , Dec 08, 2008; 10:56 p.m.

Rishi, cross posting is against forum rules. If you don't like the flak you take in one forum then you have to at least reword the question to post it again!

Rishi Sanyal , Dec 08, 2008; 11:22 p.m.

Oops. I just felt I'd posted it in the wrong section since that that thread also included point & shoots and this is specifically for Canon EOS.

That's why I reposted.

Hmm... I dunno what to do about it now.
Sorry,
Rishi

Bob Atkins , Dec 08, 2008; 11:26 p.m.

I'd assume all cameras will show banding if you look hard enough. It's almost certainly due to variations in readout noise as the array is read. I don't know how Canon readout their sensor arrays, but it's probably done on a line by line basis. A typical scheme is a parallel vertical shift register and a serial horizontal shift register, so each horizontal array of pixels is loaded into the horizontal shift register in sequence and read out serially. Tiny temporal differences in gain ("noise") can show up as banding.

For the Canon CMOS array I think each horizontal row is read out seperately via a multiplexing scheme (i.e. they don't have to be loaded into a shift register), so again there willo likely be very slight variation between the rows, resulting in "banding" if you look very closely at the array.

The only way around banding would be to read out the pixels one by one in a random fashion, which would presumably be much slower and more difficult to do. That might result in random noise rather than pattern noise.

Bob Atkins , Dec 08, 2008; 11:31 p.m.

BTW scanners only have one line, so they can be more uniform than sensors which have 4000 or more "lines" of pixels. Expecting every one to be identical is expecting too much. You might still see temporal changes with a scanner, so it appears the temporal component must be small.

Scott Ferris , Dec 08, 2008; 11:32 p.m.

Don't sweat it, normaly the moderators delete one. This is probably the better spot for it but the other one got more interest and is older, so who knows.

To put your mind at rest, or start a war, I would ask the question mentioning all the IQ issues you think are there. Something like, Are the real world images comming out of the 5D MkII really affected by the fringing, black spot and banding issues, or are they just pixel peeping non isssues that only show up in excessively post processed badly exposed shots? Please only answer if you own a 5D MkII and have printed your images at (whatever size you are going to print).

Take care, Scott.

Rishi Sanyal , Dec 08, 2008; 11:42 p.m.

Bob, thanks. Someone else had mentioned it's probably more a temporal component, but I wonder how much more it's due to readout noise as you mention or just differential gain between amplifiers, since each column shares one amplifier (I believe, from this: CCDvsCMOS ). The article states that gain (for each column amplifier) was sacrificed to increase uniformity... but that can only help so much (IMHO).

Is it possible that since on a CCD each pixel charge is read sequentially, that any non-uniformity would be less visible (and more randomly dispersed)?
Rishi

Bob Atkins , Dec 08, 2008; 11:55 p.m.

You'd have to ask Canon. I don't know the details of their sensor readout scheme for the EOS 5D MkII

I suspect this is mostly a non-issue except for those who really like to shoot at high ISO settings and post-process the &#@^ out the the resulting images.

If you look closely enough at pretty much anything, you'll find defects. Canon never claim perfection and you shouldn't assume it. It's a camera, not a precision photometer with every pixel calibrated (and corrected) to NIST standards. Maybe if they cooled it with liquid nitrogen they could do better.

If you want to do some experimental science (rather than photography), take multiple images and see if the patterning is consistent. If it isn't, it's probably mostly temporal. If it is, it's probably associated with some sort of difference between rows, either due to amplifier gains or some other chip level differences in layout or connection.

Rishi Sanyal , Dec 09, 2008; 12:09 a.m.

You're right in that perhaps I just shouldn't worry about it.

It's just that, to me, it's hard to stomach given that that LS-9000 scan of Velvia 50 received a +100 in the Shadows/Highlights tool (Tonal Range to which it was applied: 14 on a scale of 100), and still showed no dark non-uniformity.

The Imacon 848 I use, though, shows lots of dark non-uniformity, kind of like on par with what I'm seeing from certain Canon sensors... ok maybe that's a bit unfair. The Imacon (this one anyway) is probably considerably worse. Here you go:

Link to Full-Size Image

Again, same Shadows/Highlights adjustment to the Imacon scan. And apparently the Imacon's CCD is peltier-element actively cooled.

Some really cool results of cooling a Canon DSLR sensor here: http://www.pbase.com/terrylovejoy/image/65814409

Still don't know if it'd get rid of non-uniform noise in the form of banding due to differential gain of amps though. Not that active cooling is practical (or necessary?) in DSLRs for practical use anyway...

Cheers,
Rishi

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