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10-16-2012, 08:10 AM   #1
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Exposure correction functioning (home testing results)

Dear Forum,

first a few words about me because it is my first post on this platform. My name is Florian, I am 24 study Neuro Cognitive Psychology in Munich. I focus mainly on Nature Photography. I profited from this community for quite a while now so I feel like it's time to give something back.

After a long and stupid discussion on a German platform about how the exposure correction setting changes actual exposure parameters I decided to analyse it myself. I feel like the results might be of interest of even helpful to some of you. So here we go:


Camera: Pentax K-5
Light-Metering: 77 Segment TTL
Autofocus: Center Spot
Auto ISO settings: 80-1600 // SLOW (camera raises ISO very conservatively)
White Balance: auto
SR: 1
lens: da* 16-50 2.8 @ 16mm and 50mm

*****

Mode M: Exposre correction only changes the zero point of the light meter but NOT the actual exposure parameters(ISO, aperture, shuttertime). This is of practical relevance for the use of green auto exposure button.

Mode Av: Camera changes only the shuttertiming. If the desired correction exceeds the camera's possibilities it just tries to do the maximum possible.
Mode Av + Auto ISO: Camera tries to keep shutter time faster than 1/25s (1/50s@ 50mm), so it first changes the ISO and only goes for longer shutter times if the ISO is already at maximum.

Mode Tv: Camera changes only the Aperture. If the desired correction exceeds the camera's possibilities it just tries to do the maximum possible.
Mode Tv + Auto ISO: The Camera tries to avoid stopping down more than 10, so it first tries to adjust the ISO.

Mode Sv: Highlight -> The camera tries to avoid stopping down more than 10 ; Lowlight -> Camera tries to keep shutter time faster than 1/25s (1/50s@ 50mm)

Mode TAv: Camera only changes ISO. If the desired correction exceeds the camera's possibilities it just tries to do the maximum possible.

Mode P: Camera reacts analogue to mode Av respectively mode Tv

Now does the SR-unit influence these algorhitms?
Yes, with the SR-unit disabled the focal length specific shutter time tresholds mentioned above seem to be non existent.

*****

Of course this is not the full story. At the moment this only shows how the camera reacts under those specific conditions. There are many open questions. I wonder specifically:

Can the results be modulated by changing the Auto ISO sensitivity setting (SLOW/NORMAL/FAST)?
What shutter time treshhold is chosen when using a manual lens? Maybe it is dependent on the focal length you set up in the camera?
What happens when using flash(es)?
Which other parameters affect exposure correction? For example the camera might prefer shorter exposure times when in AF-C mode to avoid motion blur.

I am looking forward to your input. Sorry for my clumsy verbalization.

Best,
Florian

http://www.flickr.com/photos/floriko/
http://floriko.wordpress.com/

10-17-2012, 04:00 AM   #2
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Location: Murcia, Spain
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Posts: 72
QuoteOriginally posted by floriko Quote
Mode M: Exposre correction only changes the zero point of the light meter but NOT the actual exposure parameters(ISO, aperture, shuttertime). This is of practical relevance for the use of green auto exposure button.
... and for P-TTL flash exposure (very practical, indeed; I always use M-Mode for flash; the degree of underexposure determines the mix of flash to ambient light, the exposure correction setting influences absolute exposure level)

Cheers - Klaus
10-17-2012, 08:29 AM   #3
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Ive added a few comments against specific points, but the key issue here, is that this is only for the K5D, other cameras do not behave necessarily the same, which is a pity
QuoteOriginally posted by floriko Quote
Dear Forum,

first a few words about me because it is my first post on this platform. My name is Florian, I am 24 study Neuro Cognitive Psychology in Munich. I focus mainly on Nature Photography. I profited from this community for quite a while now so I feel like it's time to give something back.

After a long and stupid discussion on a German platform about how the exposure correction setting changes actual exposure parameters I decided to analyse it myself. I feel like the results might be of interest of even helpful to some of you. So here we go:


Camera: Pentax K-5
Light-Metering: 77 Segment TTL
Autofocus: Center Spot
Auto ISO settings: 80-1600 // SLOW (camera raises ISO very conservatively)
White Balance: auto
SR: 1
lens: da* 16-50 2.8 @ 16mm and 50mm
not sure what the above has to do with things,other than they represent your settings
QuoteQuote:

*****

Mode M: Exposre correction only changes the zero point of the light meter but NOT the actual exposure parameters(ISO, aperture, shuttertime). This is of practical relevance for the use of green auto exposure button.
this changes camera model to camera model, K10D for example has no Ev compensation in manual at all, and it can also impact P-TTL flash
QuoteQuote:

Mode Av: Camera changes only the shuttertiming. If the desired correction exceeds the camera's possibilities it just tries to do the maximum possible.
Mode Av + Auto ISO: Camera tries to keep shutter time faster than 1/25s (1/50s@ 50mm), so it first changes the ISO and only goes for longer shutter times if the ISO is already at maximum.
actually the shutter speed is something close to the hand held rule of thumb for the sensors, i.e. 1/(FL*1.5) . In fact the process is actually that it will retain the shutter speed at 1/(flx1.5) until the ISO can be reduced to ISO minimum at which point it raises shutter speed, the camera always adjusts ISO before other settings
QuoteQuote:

Mode Tv: Camera changes only the Aperture. If the desired correction exceeds the camera's possibilities it just tries to do the maximum possible.
Mode Tv + Auto ISO: The Camera tries to avoid stopping down more than 10, so it first tries to adjust the ISO.
need to check this, I suspect that may be a function of lens or more specifically whether the camera in program mode is to follow maximum shutter speed line, maximum DOF line or the MTF program line.
QuoteQuote:

Mode Sv: Highlight -> The camera tries to avoid stopping down more than 10 ; Lowlight -> Camera tries to keep shutter time faster than 1/25s (1/50s@ 50mm)

Mode TAv: Camera only changes ISO. If the desired correction exceeds the camera's possibilities it just tries to do the maximum possible.

Mode P: Camera reacts analogue to mode Av respectively mode Tv

Now does the SR-unit influence these algorhitms?
Yes, with the SR-unit disabled the focal length specific shutter time tresholds mentioned above seem to be non existent.
be careful on this point, they seem to follow still 1/(FL x 1.5)
QuoteQuote:

*****

Of course this is not the full story. At the moment this only shows how the camera reacts under those specific conditions. There are many open questions. I wonder specifically:

Can the results be modulated by changing the Auto ISO sensitivity setting (SLOW/NORMAL/FAST)?
What shutter time treshhold is chosen when using a manual lens? Maybe it is dependent on the focal length you set up in the camera?
not sure about normal modes but for flash, it is set to 1/180
QuoteQuote:
What happens when using flash(es)?
the camera adjusts in any auto mode to the extent possible to achieve exposure, and then uses the flash for the remainder. i.e. it tries to get a natural light solution first. auto everything in flash mode is really dangerous (to image quality) because it will bump ISO first to get a solution
QuoteQuote:
Which other parameters affect exposure correction? For example the camera might prefer shorter exposure times when in AF-C mode to avoid motion blur.
the biggest thing will be the program line you pick and whether the camera is set to flooow the MTF curve of the lens or not.
QuoteQuote:

I am looking forward to your input. Sorry for my clumsy verbalization.

Best,
Florian

Flickr: floriko's Photostream
floriko photo – all rights reserved
10-18-2012, 03:03 AM   #4
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Posts: 12
Original Poster
Heya, thanks for the input. Of course these findings are only valid for the pentax K-5 but ofc it maybe possible that they also apply to other camera models.

I provided my settings so others can retrace and maybe even replicate those results. Usually I work in science, maybe it's just one of those odd profession habbits. The parameters listed might critically affect the functioning of exposure correction.

"actually the shutter speed is something close to the hand held rule of thumb for the sensors, i.e. 1/(FL*1.5) . In fact the process is actually that it will retain the shutter speed at 1/(flx1.5) until the ISO can be reduced to ISO minimum at which point it raises shutter speed, the camera always adjusts ISO before other settings"

That's what I suspected first, too. But how would you explain the 1/50s @ 50mm then? According to that algorhithm it should be 1/75s.

Do you have any evidence for this maximizing MTF behavior of the pentax K5?

10-24-2012, 05:15 PM   #5
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Join Date: Sep 2012
Location: Tokyo
Posts: 64
QuoteOriginally posted by floriko Quote
Do you have any evidence for this maximizing MTF behavior of the pentax K5?
As mentioned, in P you can choose how the camera biases the aperture/shutter settings for the same exposure. It's a menu setting. You can, for example, tell it to bias towards the MTF for the lens, or bias towards larger aperture, or smaller aperture ... take a look at your K5 menu.

Paul
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