[ccallana]

Anybody tried sending a message off to Pentax on this? Maybe NedB @ dpr? maybe there is a white paper or something they'd be willing to share with us.

[Lowell Goudge]

As a follow up, I have tried with my K10D experimenting with auto ISO.

I took 2 - 75 watt flood lamps, and a teddy bear in a chair as a subject, and camera on a tripod, to insure that I always took the exact same photo.

results are as follows:

In Tv Mode, lens was kept at maximum apature and ISO reduced as shutter speed was reduced until minimum ISO was reached, and then lens stopped down to control exposure

in Av Mode ISO remained at maximum until shutter speed reached 1/100 and then ISO began to reduced.

I need to retest with a different lens, and focal length to see if the 1/100 is a question of the 70 mm focal length I had used (sigma 70-210 F2.8) and a reasonable shutter speed for hand holding.

Overall it seems to work reasonably as I expected

[mattdm]

Interesting. In Nikon's implementation, you can set the shutter speed threshold where you want autoiso to kick in. Basically, this gives you the opportunity to tell the camera to do automatically exactly whatever you'd normally do manually. It'd be nice if Pentax would follow this lead.

[WMBP]

Interesting. In Nikon's implementation, you can set the shutter speed threshold where you want autoiso to kick in. Basically, this gives you the opportunity to tell the camera to do automatically exactly whatever you'd normally do manually. It'd be nice if Pentax would follow this lead.

Yes, precisely what's needed. Without this - and of course especially without ANY sort of clarity on this subject - then the AUTO ISO feature is not so useful.

Will

[Edvinas]

With Auto ISO everything is much more simple than it looks.

Camera allways try to set as low ISO as possible.

In Tv mode user sets shutter speed, camera sets lowest ISO (200 in case of K100D) and adjusts aperture to get correct exposure. When you make shutter speed fater, camera keeps ISO200 and opens aperture. When aperture of the lens is completely opened, camera starts to increase ISO.

In Av mode it is a little bit different. User set aperture, camera sets lowest ISO and then correct shutter speed. When user steps down aperture, camera decreases shutter speed up to the handheldable speed and then starts to increase ISO. When maximum ISO is reached, camera starts to decrease shutter speed again.

Handheldable speed is determined based on lens focal length. When you have 50mm lens on your Pentax DSLR in Av mode ISO will start to increase when shutter speed reaches 1/60 s. When focal length is 18mm, critical shutter speed is 1/30 s. With longer lenses it seems that limit is 1/125s. At least with my 80-320mm set to 320mm, K10D decreases shutter speed to 1/125 and just then starts to increase ISO.

[ccallana]

FYI - I sent a mail to Pentax USA thru the pentaximaging.com website.. This is the response I got:

Based on what information Japan has provided and experimenting on our own, it is our understanding that as light quantity decreases the camera will first lower the aperture to the minimum number (largest opening), followed by the shutter speed to a point where the flash indicator light would come on (usually 1/60th sec). It would be at this point that the camera would choose to increase the ISO. It is not predictable unless you can measure light in EV visually (I can’t) nor is it documented to my knowledge. Assuming you are aware of the properties of exposure, our advice would simply be to manually set the ISO desired rather than to rely on the automatic.

[WMBP]

FYI - I sent a mail to Pentax USA thru the pentaximaging.com website.. This is the response I got:

Heh. THANK YOU for posting this. Maybe it's NOT simpler than it looks, after all. :-)

Very often, reading the manual, I wish that they provided an explanation of WHY you'd want to change this or that setting, in addition to the very basic description of what the setting affects.

Will