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09-07-2009, 06:13 PM   #1
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Lens focal length used in computing exposure time?

Pardon the possibly stooopid question.

I've noticed that my K200D appears to pay no heed to the current focal length of the lens when computing exposure and shutter speed under its P or 'Auto pict' modes.

Eg I'm taking a shot of a distant scene with my Tamron 18-200 mm at 200mm focal length for example in hazy daylight. Using basic P mode the camera might select f8 and 1/180 sec and 100 ISO for the shot. But if I do the same shot with my FA50 1.4, the camera will also select pretty much the same parameters: ie f8-1/180 -100ISO.

This will result in a correctly exposed image for the photos taken by both lenses, but for the telephoto shot the chances are you will see some blur.

Given that the camera usually knows about the lens focal length (for the SRS) shouldn't it also know to bump up the shutter speed a notch or two if you are using a long focal length lens in order to improve the chances of getting less blur? That doesn't seem to happen in the algorithm used for P and AUTO PICT.

It's usually not a real issue for me since when using telephoto I usually go to shutter priority to put the shutter speed well above the lens focal length, but it is just something I was curious about.

Am I mistaken in assuming that the standard K200D exposure modes pay no heed to lens focal length?

09-07-2009, 06:42 PM   #2
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There may be a little more to it.
Have you checked your P mode settings in the menu with regards to how it selects Av, Tv and ISO?
Was your SR on?
It may explain why it's chosen f/8 rather than f/5.6 despite the slower shutter speed. With SR on, it may compute that it would be sharper going f/8 and 1/180sec rather than f/5.6 and 1/320sec (or whatever).

As for AutoPict modes, I'm not sure - the camera just takes over, but it's possible that SR has something to do with the camera believing it's OK to exceed the 1/(focal length) barrier.
09-07-2009, 06:47 PM   #3
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Are you saying that you expect the camera to automatically use a different program curve for longer focal length lenses?

Fascinating idea...though I don't think that is an advertised feature of your camera. You can set the program line as a setup setting however.

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09-07-2009, 07:29 PM   #4
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QuoteOriginally posted by stevebrot Quote
Are you saying that you expect the camera to automatically use a different program curve for longer focal length lenses?

Fascinating idea...though I don't think that is an advertised feature of your camera. You can set the program line as a setup setting however.

Steve
That is an intriguing idea. This may require some testing.

I have a feeling that the focal length is only used for SR calculations, and not for the program line.

09-07-2009, 07:35 PM   #5
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QuoteOriginally posted by stevebrot Quote
Are you saying that you expect the camera to automatically use a different program curve for longer focal length lenses?
Yeah, that's it exactly.

If it did that, that would be really cool.

It would see the camera try to better balance both achieving a technically correct exposure with delivering a 'keeper' image to the user, depending on the lens focal length used.

Last edited by rawr; 09-07-2009 at 07:45 PM.
09-07-2009, 09:25 PM   #6
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I thought it did? That is would try to avoid shutter speeds less than 1/FL in P mode? Or maybe that's only when using flash? Needless to say, I don't use that mode much, or I wouldn't be guessing...
09-08-2009, 12:58 AM   #7
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QuoteOriginally posted by rawr Quote

If it did that, that would be really cool.
assuming it knows its not on a tripod
09-08-2009, 02:03 AM   #8
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Even on a tripod telephoto blur can still be an issue.

But I guess you could always have the option to turn off this mode (as you can SRS) if you are on a nice solid tripod or other support, or select another exposure mode (like Tv) anyhow.

Having a program mode sensitive to long lens focal lengths, in combination with SRS, would be pretty powerful though.

09-08-2009, 03:18 AM   #9
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Pentax used to publish the Program line graphs for its film cameras. I guess with Auto ISO thrown in as another variable for dSLRs, and the proliferation of lenses, its become too complicated and possibly confusing to print such a graph.
For what its worth, for a given fixed ISO and focal length, I can see Pentax hasn't changed its Program line that much even from the film days.

If I refer back to the ZX10 manual, the Program line stays at maximum aperture/maximum possible shutter speed (for correct exposure) for a wide range of brightness. It only starts to reduce the aperture when the light is pretty bright. That remains the case even with the latest K-7 for the standard program, unless you are on MTF mode, which tries to hit the "sharpest" aperture for the lens.

If you get a metering of f/8 and 1/180 secs, that's pretty decent light. With SR, you should not get any blur from camera shake, even at 200mm focal length. I've easily got sharp photos at 1/40 secs on a 200mm zoom with SR.
Motion blur is a totally different matter. For some applications, even 1/1000 secs is not fast enough.

Yes, the Program line changes with Focal length. The maximum "target" shutter speed before the Program line reduces the aperture increases with focal length.

Last edited by kittykat46; 09-08-2009 at 03:27 AM.
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