Best low light FF Canon DSLR

Started 3 months ago | Discussions thread
Dr_Jon Veteran Member • Posts: 6,080
Re: Never
1

Mako2011 wrote:

Dr_Jon wrote:

Mako2011 wrote:

Dr_Jon wrote:

I said this below, but I'll point it out again in case it's helpful. A f/1.4 lens gives you the same light as a f/2 lens as the camera will just halve the time the shutter is open to give a correct exposure. It only helps if you can reduce the ISO, which changes how much light you collect to give a correct exposure.

Just so folks are clear. Changing ISO alone has no effect on how much light you can collect. Only a chance in shutter speed and/or aperture and/or scene luminance changes how much light you collect

I'd disagree about the point here - what ISO changes is the upper limit to how much light you can collect, as it amplifies the signal coming from the pixel and so will clip at a lower level.

It never changes the amount of light you can collect. As an example....blank wall lit by constant artificial light. Set camera on tripod at f2 and 1/60s in MANUAL EXPOSURE mode. Notice that regardless of what ISO setting you select...the sensor never captures more or less light. How the resulting image looks may change...but the amount of light captured by the sensor never ever does. The ISO setting has no effect on the Full Well Capacity nor the min charge an individual pixel can hold

I think my view is the more useful one here, as while ISO doesn't change the amount of light the sensor collects it does change whether you can use that for an image. Every time you crank the ISO up another part of the sensor's capacity becomes hidden and you can't measure it and can't use it to make an image. Hence while the pixel's Full Well Capacity remains the same every time you crank the ISO up a stop you can't use the top half of of the previous capacity, hence the Full Well Capacity as seen by the photographer halves.

So yes, if a f/2 lens fills the sensor to 30% a f/1.4 lens (everything else the same) will fill it to 60%, so you get twice as much light captured. But at ISO 200 (assuming ISO 100 = no amplification) they will be reported as 60% and 100% (as the 60% will be clipped by the amplifier). AT ISO 400 they will both be 100% and neither will provide image information.

So, with no ISO amplification you get to use 100% of the pixel's capacity to get image data.
At 2x (e.g. ISO 200) you can only use the bottom 50% of the capacity, anything above that will report as 100%.

ISO - Part of pixel capacity can use

100 - 100%

200 - 50% (anything over 50% will be clipped)

400 - 25%

800 - 12.5%

etc.

This is important as the camera will try to set a "correct exposure" to use a decent amount of the pixel's capacity (I chose 60% as an easy-to-work-with example). Every time you double the ISO that will be 60% of a smaller, usable, part of the ISO 100 capacity. Hence the shutter speed increases to cut the amount of light collected.

For example here are the measured Full Well Capacities for the 6D:

As you said the pixel's Full Well Capacity will always stay the same (probably the ISO 100 value), but my point is that doesn't matter as you can't use that at higher ISOs as every extra ISO stop increase amplifies the top half of it (approximately, they can play with the highlight headroom over increasing ISO) off the top of what can be measured.

P.S. the high read-noise (the noise due to the camera, which is usually swamped by the noise in the light) at lower ISOs is due to noise picked up on the trip between the ISO amplifier (on chip) and the digitiser (off-chip in the 6D). The 5D4 sensor has an on-chip digitiser (well, digitisers) and so its low ISO noise is much less and doesn't fall much as ISO increases.

P.P.S. (For completeness) The reason for the read (camera) noise falling at is at every stop of ISO increase the signal (and the noise) prior to the ISO amplifier is doubled by the amplifier (note that noise is of the order of a couple of electrons). You then add a big bunch of noise going from the ISO amplifier on it's trip off-chip (for a 6D). However that is fixed and with each doubling of the data it becomes a less and less significant part of it.

If you capture 10,000 electrons in a pixel then, at the input to the ISO amplifier, you will have, on average, 100 electrons of noise from the light and a bit less than 2 from the sensor + trip so far (note noise doesn't just add, so 121 and a bit total). If you don't amplify it you will still have 10,000 and add another 25 (-ish, on average) electrons worth of noise on the trip to the digitiser. At ISO 400 it gets amplified 4x to 40,000 electrons, but you still only add 25 of noise, so a much smaller part.

 Dr_Jon's gear list:Dr_Jon's gear list
Nikon Coolpix 950 Fujifilm FinePix Real 3D W3 Sony RX100 V Canon EOS 5DS R Panasonic GH5 +29 more
Post (hide subjects) Posted by
BAK
MAC
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
MOD Mako2011
BAK
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow