K10d and automatic ISO change criterion

I've been searching this forum but in vain, at least so far. My question (no.1) is: how does k10d decide to change the ISO when set to AUTO let's say 100-400? In other words, can I set the minimum speed or is there a criterion at which k10d will switch to higher ISO? I've noticed that though in AUTO ISO 100-400 mode, it didn't switch to 200 even at speeds of 1/45 or so. Other conditions: Av mode, AF and NR on.
And just curious question: does anybody here have any experience with Tokina 28-70/2.8 ATX-pro and k10d lens? To me it seems surprisingly less sharp than the 18-55. Perhaps because it is recognized as 28-70/3.5-4.5? Does this affect something?

1) If you want a minimum speed use Tv mode. In that mode you set the shutterspeed.

2)In general you can shots iso through the fn menu. Push the fn button to enter it.

3) In the custom settings menu you can change the two handle in each to suit you. Eg I have Iso on the back and apperture on the front wheel in Av mode. Others have it the other way around....and so on.

I hope that helps.

I don't know the tokina lens. Try ask in the lens section of this site. The 3.5 - 4.5 is tha maximum aperture. The lower the number the better the lens is. A "high" (good) apperture is around 2.8 and lover.

I haven't noticed or worked out exactly what it is, but the K10D's criterion is based on the shutter speed and focal length. It tries to prevent you from going below a certain speed for a certain length. You might find it is a bit optimistic, though.

TAv is also useful when you want to make sure the shutter speed doesn't get too low (or high). Or just use any mode you like with manual ISO control.

I've tried Auto Iso a little but also found it a bit unpredictable so don't use it anymore....

Don't forget that manual Iso can be changed extremely quickly/easily in AV/TV modes by simply pressing the "OK" button & using the front control wheel - I always use this instead of Auto

simon

I use Av mode with Auto ISO.
However, I have programmed the front wheel to be ISO - rotating the finger wheel will fix the ISO, and pressing the green will set it back to Auto.
In that mode the viewfinder shows Av, Tv, and ISO

Takes a little getting used to (reading three values at once) but it works for me!
Matt

Which mode are you using?

Isn't this controlled by the Program Line setting?

iceblinker: your idea seems to be the right one. I'll try to verify that.

MattMatic: I'm using exactly the same config as you, it works great. The only problem is getting used to these three numbers. But the advantage of getting more ISO than just one is worth it! And I like the front wheel changing ISO, as well as the green button. K10d is great with the wheel configurations, I like it a lot!

Nimitz: I know about Tv, TAv and the rest... my question was only about the criterion of ISO increase. The same about the lens, my Tokina is 2.8 all length. 3.5-4.5 is the minimum (maximum?) aperture at various focal distances.

Ericp: hmmm, frankly I don't know. Let me check it tonight but I doubt it.

F-values is quite confusing. You call it maximum apperture eventhoug it is the lowest number.

If it is F2.8 all lenght it is f2.8 all lenght. I don't get what you mean by recognized to be 3.5-4.5. Normally 3.5-4.5 mens that maximum apperture at the wide end is F3.5 and maximum apperture at the long end is F4.5. So yes it changes as you zoom.

A lens with F2.8 in the entire zoomrange is usually best stopped down one or two stops - eg. best at F3.5-4.

When I look at the EXIF data of an image taken with the Tokina it says 28-70/3.5-4.5 not 28-70/2.8. This is what I meant by 'recognized as 28-70/3.5-4.5 lens'.
Well according to photozone the Tokina ain't such a great lens but once I have I'll use it.

Thanks for pointing that out Matt. Should be very handy.
cheers
bb2

beachboy: yes indeed it is very handy, it was one of the first things that I set up on my k10.

Some experiments from yesterday: the criterion for ISO change depends on the focal length - as nimitz suggested. I tried this with the kit 18-55 lens: k10 changes ISO at 1/30 for 18mm and at 1/60 for 55mm. I will try more tonight with longer focal lengths.

I would expect that the criteria for selecting sensor sensitivity would include some or all of the following:

1 The focal length. A longer length demands a higher shutter speed, so either sensitivity or aperture must be changed to compensate.

2 Lens MTF. This information is included in recent Pentax lenses, and tells the camera the "best" aperture to use (in other words, the aperture that gives the best results for the given lens).

3 Depth of field. There might be an attempt to avoid very wide apertures in order to maintain DOF.

There are no doubt others I haven't thought of.

Number 2 provides one reason why a Pentax lens might be treated differently from a third party lens, or indeed from an A series lens.

As for comparative lens sharpness, the clue might lie in the names on the lenses. There is a reason for the Riley Rule.

G

I agree with 1), 2) - not using recent Pentax lenses therefore I can't verify. 3) I was working in Av hence the aperture was set by me.
Tokina 28-70/2.8: Just found that EXIF data from Photome show Tokina ATX 28-70/2.6-2.8 but Pentax Photo Browser shows 28-80/3.5-4.5... a Pentax lens with closest parameters to Tokina perhaps?
Btw as from very, very rough comparison from yesterday Pentax 18-55/3.5-5.6 is not better than the Tokina.

Not better than the Tokina 28-80mm do you mean? If so, these are two very different sorts of lenses and not really comparable directly.

Lens testing is a very precise art that takes a considerable amount of effort to produce meaningful results. Apart obviously from being able to spot reals "duds" straight away!

Well I said very very rough comparison. I had some doubts from several photos taken with the Tokina lens but I agree that lens testing is long process and it requires certain degree of experience especially in optics. I won't adventure into this, at least not now. I was just trying to find out whether there are some 'duds' as you say but found none so far.