I finally 'get' equivalence

Started May 22, 2012 | Discussions thread
Great Bustard Forum Pro • Posts: 42,575
Re: 'Total Exposure'?

gollywop wrote:

Great Bustard wrote:

gollywop wrote:

The f-stop is only one part of the exposure; you're forgetting shutter speed and ISO.

ISO is not a part of exposure. The exposure is the density of light falling on the sensor, and is determined by three, and only three, factors:

  1. Scene Luminance

  2. t-stop (usually closely approximated by f-stop)

  3. Shutter Speed

I'm willing to go along with this, but only so far. Do you think the entire argument would make any sense whatsoever if the two shots were taken using entirely different ISOs? Clearly ISO makes a difference. That's the reason I equalized them in my post.

The role of the ISO control on cameras is primarily in that changing the ISO indirectly affects exposure by changing the f-ratio, shutter speed, and/or flash power.

Other than that, the role of ISO is merely processing (that is, changing the brightness of the photo).

However, for cameras using non-ISOless sensors, the higher ISOs have less read noise. For example, 1/100 ISO 400 is less noisy than 1/100 ISO 100 pushed two stops. Here's an example of that:


For cameras using ISOless sensors, you are best served using base ISO and applying the appropriate tone curve to the RAW conversion, if max IQ is your goal:


Of course, the FF sensors tend also to have deeper wells than the FTs, so this objection is mainly theoretical. But it's more than just relative sensor size.

It's not the sensor size, per se, but the diameter of the aperture of the lens, and the efficiency of the sensor (QE, Read Noise, and saturation).

I believe I just said above your statement that "it's more than just relative sensor size." I appreciate your addition as to what that "more" might entail.

Yes -- I thought I'd flesh out the "more" and how the "more" doesn't favor FF, as a general rule.

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