BaselineExposure: which way is it?

[perb]

Advertisement (gone after registration)

Dear all,

 

I was just reading the latest essay on LL:

 

Optimizing Exposure

 

(Please note, my question is NOT about ETTR, so don't go there in this thread

 

 

The thing is, near the end Michael Reichmann is quoting a colleague:

 

"DNG has a tag named BaselineExposure that can be used for exactly this purpose. For example, if a camera captures an extra 1.5 stops above the auto-metered value, the camera should record a BaselineExposure tag value of -1.5. The raw processing software compensates by reducing the exposure of the recorded values by 1.5 stops from its default rendering of the image. Thus, the photographer reaps the benefit of ETTR and sees a sensible default rendering, without jumping through extra hoops."

 

I remembered reading about BaselineExposure here (including posts following):

 

http://www.l-camera-forum.com/leica-forum/leica-m9-forum/125227-m9-constantly-underexposes-4.html#post1327336

 

From the above thread my understanding was that if the numerical value of the DNG tag BaselineExposure is negative, it means that the camera has exposed less than it nominally would from metering. In the thread we saw that the value for the M9 is -0.5 (except for ISO80), and the message I took with me was that the M9 indeed exposes 0.5 stops *below* metering, presumably to protect highlights.

 

However, the quote above from LL suggests that it works in the other direction. So, a negative value means that the camera has actually exposed *more* than nominal, and the raw converter uses the tag value to determine that it should present the image to the user *as if* the exposure was lower, i.e. nominal.

 

I had a closer look at the image in post #72 by jaapv (a few posts below the one I link to), and though the text in the "yellow box" is difficult to read, this is what I see in the last paragraph:

 

"Because of these differences, a raw converter needs to vary the zero point of the exposure compensation control from model to model. BaselineExposure specifies by how much (in EV units) to move the zero point. Positive values result in brighter default results, while negative values result in darker default results."

 

I find that the LL quote matches the "yellow box" quote; in other words:

 

A positive BaselineExposure value means that the camera exposed to the left, and a negative value means that the camera exposed to the right.

 

So, if this last statement is correct, then the M9 actually exposes 0.5 EV to the right. However, the raw converter will present the image 0.5 EV darker than the raw data suggests.

 

Am i right?

 

This is opposite to how I originally read the referenced thread.

 

Regards

Per

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[sandymc]

This is what the DNG spec says:

 

Positive values result in brighter default results, while negative values result in darker default results.

 

However, don't think about it as the M9 exposing this way or that way. Eric Chan, part of the camera raw engineering team, said this on the LL forums a few years ago:

 

There is a fundamental tradeoff between (a) honoring the original distribution of raw values (between black point and white point) and ( b ) having a given exposure (e.g., f/8, 1/10th sec, ISO 400) be rendered the same way across different cameras. In order to have common controls such as Exposure compensation behave the same way across the different cameras, Camera Raw applies a baseline exposure compensation that varies from model to model in order to get them all to behave similarly when the exposure compensation control is set to its default value of zero.

 

So essentially, it's there to match to LR's sliders, nothing else.

 

There are a lot of threads on this on the LL forums. Look for Eric's posts; what he says is generally trustworthy. Some others on LL........not so much.

 

Sandy