5d Mark III low iso poor performance.

Started Jun 20, 2012 | Discussions thread
John Sheehy Forum Pro • Posts: 21,225
Re: Take it back

Alexandros Trichos wrote:

So here are the raws.


You can judge for yourselves!

I agree with you that it is ridiculous that this banding exists - it would be very easy for Canon to eliminate it by just making the sensor chip a little taller and dedicating rows of masked pixxels to detect the vertical stripes and remove them.

However, your exposure is not as standard as you suggest; these are a couple stops lower exposure than "standard" for the ISO, and about 3 stops lower than what would be optimum. As you are not protecting highlights here, there is no need for the weak exposures.

If you're going to use Canon DSLRs, and want to avoid such issues, you need to know a few things:

1) Highlight Tone Priority (HTP) is disastrous to shadows in the low range of ISOs because it brings the signal 1 stop closer to a very harsh post-analog-gain noise floor (it is totally harmless at high ISOs, with their more linear noise).

2) Automatic Lighting Optimization (ALO) can hide from you the fact that your exposure is weak, by automatically brightening the review/embedded JPEG.

3) Super-fast apertures like f/0.95 to f/2 cause the camera to do something very stupid; the camera under-exposes, not to protect highlights, but just so that your f-stop and shutter speed are "true" to the ISO exposure index, to cover up the fact that the camera really can't do those apertures, and loses a good deal of the extra light you are supposed to get opening those lenses all the way because the microlenses can't handle them. Rather than just under-expose, the camera mathematically scales the RAW data, and clips away any extra highlights that may have otherwise been retained.

4) On Canon DSLRs with 1/3-stop ISOs, other than the 1D cameras and the original 5D, ISOs like 160/320 are just 200/400 over-exposed, and ISOs like 125/250 are just 100/200 under-exposed, making this latter group show off the banding and high noise floor even more.

Combine #1, #3, and #4, such as f/1.2, ISO 250 with HTP, and you have a recipe for major shadow disaster.

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