Units of dynamic range

D Cox said:

Iliah Borg said:

TN Args said:

Hi everyone, I would like to get some clarification please.

I have seen dynamic range described using various units, as a number of EV, stops, dB, bits.

Firstly, are all those units legitimate units of dynamic range?

Also, are there any conversion factors?

• 12 EV of dynamic range
• 12 stops of dynamic range
• 12 dB of dynamic range
• 12 bits of dynamic range
• 12=12=12=12?

Finally, does the appropriate unit vary with what is being measured?

• the live scene in front of me has 12 (unit) of dynamic range
• the sensor has 12 (unit) of dynamic range
• the image has 12 (unit) of dynamic range
• the PC monitor has 12 (unit) of dynamic range

As an aside, some PC monitors or video projectors seem to use a contrast ratio (like 5000:1) when they are describing what I would call the dynamic range. Can one convert from contrast ratio to EV / stops / dB / bits?

Thanks for any responses.

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1 EV = 1 stop

I'm not fond of measuring DR in bits, but theoretically 12 bits = 12 EV

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No, it doesn't. You can map a 12 EV range onto any number of steps.

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alanr0 said:

JimKasson said:

TN Args said:

Hi everyone, I would like to get some clarification please.

I have seen dynamic range described using various units, as a number of EV, stops, dB, bits.

Firstly, are all those units legitimate units of dynamic range?

Also, are there any conversion factors?

• 12 EV of dynamic range
• 12 stops of dynamic range
• 12 dB of dynamic range
• 12 bits of dynamic range
• 12=12=12=12?

Finally, does the appropriate unit vary with what is being measured?

• the live scene in front of me has 12 (unit) of dynamic range
• the sensor has 12 (unit) of dynamic range
• the image has 12 (unit) of dynamic range
• the PC monitor has 12 (unit) of dynamic range

As an aside, some PC monitors or video projectors seem to use a contrast ratio (like 5000:1) when they are describing what I would call the dynamic range. Can one convert from contrast ratio to EV / stops / dB / bits?

Thanks for any responses.

Click to expand...

I see what you're getting at. Dynamic range has no dimensions. As others have said here, it is the ratio of two quantities. In photography, those quantities have the same dimensions (like volts), so the dimensions cancel when the ratio is created, and you are left with a dimensionless number. That number can be expressed in many ways. Say it's 5000, which is the example you used in your original post.

log2(5000) = 12.29. Since each stop is a doubling of light, that's 12.29 stops, or that many EV. Since one bit position is also a doubling, you could call that 12.29 bits.

If we're talking voltage, 20*log10(5000) = 74 decibels (dB). See how the dB number is different? That's because dB is based on log base 10, not log base 2.

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This is certainly how photographic sensors are typically specified, but it is worth bearing in mind that this convention is not adopted in other fields.

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alanr0 said:

Safer to stick to EV or a linear intensity ratio unless one is confident that one's audience is familiar with your choice of dB convention.

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tlwalton said:

DR is dimensionless; it's a ratio of two quantities with the same units.

All this measure-theory/Borel set/sigma algebra/Lebesgue measure claptrap is so much jawboning. It only impresses people who have never studied it. Fractional bits are a useful engineering shorthand for log(2) of a dimensionless ratio. It's an exponent, presumably real in an engineering context. Put 'em back in your pants, boys.

-Tom

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tlwalton said:

DR is dimensionless; it's a ratio of two quantities with the same units.

All this measure-theory/Borel set/sigma algebra/Lebesgue measure claptrap is so much jawboning. It only impresses people who have never studied it.

Click to expand...

tlwalton said:

Fractional bits are a useful engineering shorthand for log(2) of a dimensionless ratio. It's an exponent, presumably real in an engineering context. Put 'em back in your pants, boys.

-Tom

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bobn2 said:

A 'bit' is strictly a unit of information, and white noise contains no information. If you take 'bit' as 'binary digit', then of course you can encode your 12 EV range into any number of 'bits' that you like, but only 12 will encode information (the rest will be encoding noise, which is not information).

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