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07-22-2015, 02:46 PM   #16
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QuoteOriginally posted by rawr Quote
Maybe I got that partly wrong. You are right if everything else stays the same. Maybe it depends in part on SR being turned on or off? SR and the program line will likely directly interact with each other as the lens focal length varies.

The chart below for the K10D shows the program line at work with the 18-55 kit lens and the FA 50 1.4. It kind of adds to the confusion a little bit about focal length and the program line though.


A an aside, it's interesting to see how much 'neater' a Nikon normal program line looks, especially when you compare the two 50mm f1.4 program lines:
This is fascinating. Thanks for sharing. I wonder how much of the difference between the two program lines for the 18-55 kit is due to the change in maximum aperture through the zoom range? The lens transmits that information in real time as it is zoomed*. Still though, these diagrams help illustrate some of the complexity of programmed exposure.

In order to support programmed exposure, body must know both the maximum aperture and aperture range for the mounted lens. That information along with the shutter speed range allow for appropriate use of the program line as well as providing the option for a range of program lines tuned for different speed lenses.


Steve

* This is not uniformly true, but generally holds for the DA-series Pentax glass.

07-22-2015, 06:55 PM   #17
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I'm starting to believe that the concept of a X-Y chart 'program line' doesn't quite capture the matrix of inputs, settings and events that may go into shaping a DSLR's decisions about shutter/aperture/ISO.

It's not just camera reading of light levels, then exposure triangle adjustments of aperture, shutter and ISO. There are probably a multitude of 'if-then' tests which shape the response curve to take into account focal length, shake reduction status, flash settings etc.

I'm not even totally sure now whether the P-mode settings for program line [Auto/Normal/High-speed/DOF/MTF priority] are entirely sand-boxed just for P-mode use [and green-button], or may indeed spread their influence over Tv, Av, TAv and Sv mode too.
07-23-2015, 04:20 AM   #18
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QuoteOriginally posted by stevebrot Quote
I just tried this, but got the same P mode shutter/aperture at 17mm, 50mm, 70mm and 200mm.


Steve
Steve, I think the lens FL determines the minimum shutter speed only. If the lighting dictates a shutter speed of 1/500 seconds for example, it wouldn't matter which lens from your list is attached.

Fujifilm has an interesting approach to this function. In Auto ISO, the user inputs the minimum shutter speed for the camera. The camera won't go below this speed until the ISO is maxed out. There are pluses and minuses to this concept.
07-23-2015, 09:56 AM   #19
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QuoteOriginally posted by IchabodCrane Quote
Steve, I think the lens FL determines the minimum shutter speed only. If the lighting dictates a shutter speed of 1/500 seconds for example, it wouldn't matter which lens from your list is attached.

Fujifilm has an interesting approach to this function. In Auto ISO, the user inputs the minimum shutter speed for the camera. The camera won't go below this speed until the ISO is maxed out. There are pluses and minuses to this concept.
To be honest, I think there is plenty of opportunity to overthink these features. We like to think that the camera is more sophisticated than it is just because we see a behavior that we don't understand. This is particularly true for border or corner cases.

That being said...

I took a mental step back for a broader view on this yesterday and it occurred to me that the job of auto ISO may simply be to preserve as much of the EV range on the program line as possible. After all, ISO is not really an exposure (light to sensor) setting. It is simply response amplification. As a result, one of the side effects is that it defines the range of available exposure settings (if any) for a given EV.* Set it too high and the program curve (out of necessity) flattens.

Steve

* The K-3 EV range for body-controlled exposure is -4.5 to 22 with the Pentax-A 50/1.2. The range will vary according to the available maximum/minimum aperture. Set the ISO too high and the range will contract at the high end as will the number of available setting combinations.

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