Best low light FF Canon DSLR

Started 3 months ago | Discussions thread
Dr_Jon Veteran Member • Posts: 6,085
Re: Light
1

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. Oh and I should have said:
"It only helps if you can reduce the ISO, which changes how much light you can collect."

There are two things that screw you over:

(1) Collecting more light than a pixel can hold - get white.

(2) Collecting less light than the pixel can hold but setting the ISO amplifier so it amplifies it off the top of the range of the ADC that digitises the signal - get white.

Neither gets you a useful image.

(Somewhat simplified) example of case #2...
Let's say the pixel can collect enough electrons we can make 3V if it's full. We have a digitiser that has an input range of 0-3V and divides that into 16k steps. If at ISO 100 the amplifier just passes the signal through we only get the top reading if the pixel is full. At ISO 200 it doubles the voltage before the digitiser, so anything over a 1/2 full pixel will give you pure white (you collected more light, but the bright bits were off the top of the range after amplification).

So, another (also simplified) example:

* If at ISO 100, and f/2 the camera selected 1/500th as the correct shutter speed it would work that out to put what it saw as the brightest parts of the scene at 60% (made-up number, varies a lot with manufacturer, but they have a number) of the sensor's capacity (to allow highlight headroom).

* If you then shot at f/1.4 in an automatic mode (with the same ISO) the camera would halve the shutter open time so you still get it's 60% full pixel for white. You get the same amount of light collected.

* If you shot at f /1.4 in manual and kept everything the same you'd get 2x the light and overflow the pixel in the brighter areas (like pouring two 60% full buckets into another same-size bucket), so they'd be all white.

ISO example:

* If you shot at ISO 200 f/2 and the camera (or your light meter) said the scene required 1/1000th that would fill the sensor 30% for white (using my 60% made-up number). That's because the ISO amplifier will double the signal before the digitiser, giving 60% of its range (which is what the camera wants).

* Changing to a f/1.4 lens (keeping ISO and shutter speed) will fill the pixel to 60% and then the ISO amplifier doubles it to 120% and you lose the highlights as they are off the top of the digitiser's range (strictly they were off the top of the amplifier's range, so it would limit them).

* BUT (still at f/1.4, with the same shutter speed) changing the ISO to 100 fills the pixel to 60%, as previously, but doesn't amplify it and gives you 2x the light in your data.

* However if you're already at the lowest ISO you can't get more light from a faster lens, just a higher shutter speed and less DoF. (The camera will just do the 60% thing, or you will.)

Somewhat over-simplified, but anything more will lose everyone (if I'm not there already)....and it's basically what happens.

P.S. Note you can over-expose (e.g. ETTR) to reduce the highlight room the camera will use for a "correct exposure" (e.g. replacing 60% with 90%), although the image you see on the camera's screen and the JPEG won't be that useful (as the camera made them assuming 60% was the bright bits). However you get some more light and so less noise, but highlights can get lost (as small areas can be over what the camera took at the bright bits, which is why it went with 60%).

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