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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 15 Dec 2009 (Tuesday) 12:53
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How do you think the 7D chooses the ISO "program line?"

 
ASA_5
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Dec 15, 2009 12:53 |  #1

Since the 7D has auto iso (which I understand is a relatively new feature for Canon?) I was wondering what you think it does when calculating the iso. So if you are in Tv mode, does it choose aperture, along with the lowest possible iso? Vice versa? This thread got me thinking about this:
https://photography-on-the.net/forum/showthre​ad.php?t=796282

For example, on Pentax cameras, there is something that is called the "Program line" that governs how ISO, shutter speed, and aperture are controlled depending on what mode you are in (P, Tv, Av, Sv, TAv, etc.) You can control how you want the auto iso in combination with the other parameters to behave. You can set it several different ways. For example, you can set it to "DOF Priority." If you then are in Tv mode, and the camera selects the aperture and ISO, it will choose a higher iso and smaller aperture such that DOF is larger. etc. etc.

I have looked, and I don't believe there is any setting to adjust the type of priority the 7D uses when auto-selecting iso.

Now in order to head-off the inevitable "i don't use auto iso, I just select an iso" response...I know that I can do that, and often do. I am just interested in others' experience with the auto iso.:p


Canon 7D | EF-S 17-55 f/2.8 IS | EF 70-200 f/4L IS | EF 100-400 f/4.5-5.6L IS

  
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hpulley
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Dec 15, 2009 12:59 |  #2

My old EOS 650's manual includes the program line but I don't seem to recall seeing the program graph for my T1i. Does the 7D's manual graph the program? It is probably more complicated than it was in 1987 though the 620 did have program shift (the 650 did not).


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ASA_5
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Dec 15, 2009 13:11 as a reply to  @ hpulley's post |  #3

Interesting. Nowhere in the manual am I able to find the program line. The closest they get is discussion of the auto and creative auto. (Two modes I will never use).


Canon 7D | EF-S 17-55 f/2.8 IS | EF 70-200 f/4L IS | EF 100-400 f/4.5-5.6L IS

  
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hpulley
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Dec 15, 2009 14:13 |  #4

I'll create an artistically derivative work which contains the diagrams later tonight in case you're interested. Knowing Canon the program line may not have changed much in 20 years...


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ASA_5
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Dec 15, 2009 15:16 |  #5

hpulley wrote in post #9203471 (external link)
I'll create an artistically derivative work which contains the diagrams later tonight in case you're interested. Knowing Canon the program line may not have changed much in 20 years...

Sure!


Canon 7D | EF-S 17-55 f/2.8 IS | EF 70-200 f/4L IS | EF 100-400 f/4.5-5.6L IS

  
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msowsun
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Dec 16, 2009 15:06 |  #6

All Canon cameras seem to use the same Program Line. I don't see why the 7D would be any different. (these are from four different cameras)

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hpulley
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Dec 16, 2009 15:14 |  #7

Thanks. I forgot to post mine.


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ASA_5
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Dec 16, 2009 15:47 as a reply to  @ hpulley's post |  #8

Awesome Mike! You are a wealth of information. I see now how shutter speed and aperture are related, but how does the camera decide what ISO to use? I must be missing it on the graphs.


Canon 7D | EF-S 17-55 f/2.8 IS | EF 70-200 f/4L IS | EF 100-400 f/4.5-5.6L IS

  
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msowsun
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Dec 16, 2009 15:51 |  #9

These graphs are for cameras that don't have Auto ISO. (5D classic, 10D, 20D, 30D)

Maybe that is why they don't include the charts in the manuals for the newer cameras with Auto ISO.


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ASA_5
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Dec 16, 2009 16:02 |  #10

msowsun wrote in post #9211016 (external link)
These graphs are for cameras that don't have Auto ISO. (5D classic, 10D, 20D, 30D)

Maybe that is why they don't include the charts in the manuals for the newer cameras with Auto ISO.

Yes it seems like a much more complicated decision since you have three variables an no "unique solution." That is why the the variable priority system I described above made sense to me. Canon has a fixed priority system but I can't quite figure it out. It looks like it prioritizes low iso and larger apertures.


Canon 7D | EF-S 17-55 f/2.8 IS | EF 70-200 f/4L IS | EF 100-400 f/4.5-5.6L IS

  
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apersson850
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Dec 16, 2009 16:19 as a reply to  @ ASA_5's post |  #11

The experience I have says that when it gets dark, they do follow these program lines, at ISO 100, until they reach the max aperture or the 1/focal length shutter speed. Once they do that, they hold the aperture there (for obvious reasons), or the shutter speed, because they want to, and start increasing ISO until it reaches 3200.

When that happens, underexposure will be indicated in Tv mode. In Av mode, the shutter speed will start becoming slower again.
If safety shift is enabled, the exposure time will be longer in Tv mode as well.


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Dec 16, 2009 16:52 as a reply to  @ apersson850's post |  #12

Anders, so you are saying that as light is diminished, the program "rolls" down the curve untill the knee in the graph, at which time it jumps iso one stop and rolls down till the new knee etc?

If I had a few minutes to test, I guess I could put the camera in a bright room and slowly cover the lens, and track exposure settings, and come up with the actual curve.


Canon 7D | EF-S 17-55 f/2.8 IS | EF 70-200 f/4L IS | EF 100-400 f/4.5-5.6L IS

  
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apersson850
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Dec 17, 2009 08:16 as a reply to  @ ASA_5's post |  #13

I haven't tried it much in P mode. What I described is mainly valid for Av and Tv.

Without testing in P mode, I think it rolls downhill to the knee, at ISO 100, then remains at the knee by raising ISO to 3200, and only then continues to the left, along the horizontal part of the curve.


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Tom ­ W
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Dec 17, 2009 09:22 |  #14

apersson850 wrote in post #9211198 (external link)
The experience I have says that when it gets dark, they do follow these program lines, at ISO 100, until they reach the max aperture or the 1/focal length shutter speed. Once they do that, they hold the aperture there (for obvious reasons), or the shutter speed, because they want to, and start increasing ISO until it reaches 3200.

When that happens, underexposure will be indicated in Tv mode. In Av mode, the shutter speed will start becoming slower again.
If safety shift is enabled, the exposure time will be longer in Tv mode as well.

I think you're right - aperture is limited by the max of the lens, and shutter speed is limited by 1/focal length (at least until ISO 3200 is reached). The camera will raise ISO when both of those (or either in the case of AV or TV mode) are reached so that the shutter speed is always above 1/focal length and the aperture stays at maximum or smaller. That seems to be the pattern with the 5D2. I'm not sure if the APS-C bodies take into account the "crop factor" or not.


Tom
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apersson850
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Dec 18, 2009 02:04 as a reply to  @ Tom W's post |  #15

As far as I've seen, they don't. Someone at Canon wasn't payed enough to change the software just for that.


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How do you think the 7D chooses the ISO "program line?"
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