Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Jack Hogan Veteran Member • Posts: 6,951
Re: Exposure basics, lesson two point one (& ISO)

Ulric wrote:

Right, I'm willing to accept that increasing the iso setting on the g3 from 800 to 1600 will reduce noise.

I am not sure I understand.

What I wondered about was this exchange:


Case A

Jack Hogan wrote: It appears that if one wanted to capture the very same tonal range from a scene with the EM5 and the G3, same exposure and all, one would have to set the EM5 at in-camera ISO 3200 while the G3 at in-camera ISO 1600, with roughly similar resulting SNR (EM5 slightly better as per DxO).

Case B

On the other hand, if one set both at 3200 maintaining the same exposure they would no longer be directly comparable, but the G3 would appear noisier to a naive observer.


Perhaps a different tack would help: the speedometers of two different cars have been set by their respective manufacturers to read about 5% and 100% more than the actual speed the cars are travelling, so when the first car is showing 100km/hr on its speedometer it is in fact travelling at about 95 and when the second car is showing 100 it is in fact travelling at 50km/hr. They both have the same performance and fuel consumption at a real speed of 100 Km/hr .

A gentleman who doesn't know this but would like to decide which car to buy on the basis of fuel consumption performance decides to take each on a 100km highway test drive and measure how much fuel each consumes during the trip. Day one he takes the first car and drives it for the duration of the trip at a speedometer-indicated 100km/hr, taking just over an hour to complete it. He consumes about 10 liters of gasoline. The next day he takes the second car on the same trip. Because it's a nice day for a drive he casually notices that it has taken him longer to travel the 100km, but he is pleasantly surprised that he has cosumed only 7 liters of gasoline to cover the same distance at the same speedometer indicated 100km/hr. Convinced by the second car's fuel performance evidence, he naively buys the second car without hesitation and drives away happily. This is case B.

The next week, an expert friend of the first gentleman goes through the same buying process. However he understands that speedometers in this country are not labelled consistently because the relative ISO standard allows manufacturers a lot of latitude in speedometer display correlation to actual speed, so he brings his own GPS. He makes the two trips on the same two cars but this time following the real speed indication of the GPS. It takes him exactly the same time to complete the journeys on both cars and his fuel consumption is exactly the same, even though the in-car ISO speedometers showed 105km/hr in one case and 200km/hr in the other. Happy that both cars perform about the same, he makes his buying decision based on price and other features. This is case A.

Case A: you toss ISO out of the window because you know it is unreliable and simply set up the two cars to drive at the same Speed by another mean (ETTR in my previus post is one of them) - who cares what in-car ISO speedometers show, we know they are not correct.  The important thing for a fair fuel consumption (SNR) comparison is that they are driven at the same Speed (S) in the same conditions (Exposure).

Case B: you take in-car ISO speedometers at face value, but you end up comapring the fuel performance (SNR) of a car, in the same conditions (Exposure) that is going at a real Speed (S) of 95 to one that is going at a real Speed (S) of 50km/hr, while naively thinking that you are actually going at the same speed because that's what the ISO speedometer shows


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