Hi Veriwide,
Thank you for getting back to us.
Let's think out loud together here. (For just my conclusion, please go to my next post)
I don't think too many people have done this before, trying to figure out the basics of how shutter speeds, apertures AND Auto ISOs under various modes on digital cameras were or should be designed to work together. Very few digital cameras even have really usable or working Auto ISO. F10/11s are among the very few. Future F10/11 successors can or may have even better Auto ISOs. F10 doesn't have S or A mode. F11 has S and A modes but they don't work with Auto ISO.
Here, we want to see if there is something we can learn from Veriwide's Pentax DSLR that has an Auto ISO that works in P, S and A modes.
Hi and thanks for waiting!
It seems that the Pentax is actually working as I described
previously!
It tries to set speed, aperture and iso to maximize image quality:
That is exactly the correct design philosophy.
Others would say of course they knew that.
But what is new here that they may not notice is that ISO now gets into the consideration on each and every picture from picture to picture. It is only possible with digital cameras. A good Auto ISO scheme can simplify the picture taking process without sacrificing image quality.
Examples:
In a well lit room (3 bulbs):
With the 17-35 f2.8 @ 17mm, point to the center of the room:
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In P mode (everything AUTO):
(center of the room) 1/15s, f3.2 @ ISO 400;
Why would it be F3.2 and not go all the way to F2.8?
It probably wouldn't have helped to speed up the shutter speed that much (perhaps to 1/18s or so only).
The ISO probably would have to go to 400 anyway due to the dim lighting condition in the center of the room.
What it does show is the constraint of not wanting to go even higher to ISO 800 in favor of a faster shutter speed like 1/30s.
They probably figure that serious DSLR users would know how to handle 1/15s better than compact digicam users can.
Point to one of the light bulbs: 1/60s ,f4.5 @ ISO 200 (the min
with the Pentax).
So when there is more light to share (already at Pantax's lowest ISO of 200, which is 1 stop lower than ISO 400), they give some for faster shutter speed (from 1/30s to 1/60s - about 1 Stop) and some for smaller Aperture (from F3.2 to F4.5 - also about 1 EV)
Looks like your Pentax sees your light bulbs as 3 EV or 3 stop total brighter than it sees the center of the room: 1 Stop from ISOs, I Stop from Shutter Speeds and I Stop from Apertures.
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In S mode (shutter to 1/30s) :
Center of the room: f3.2 @ ISO 800;
Again, I don't understand why it wouldn't go to F2.8. And again, it probably would have to go to ISO 800 even at F2.8.
Now I got it!
It probably had more light to spare to be at ISO 800 and to be at F2.8 (but definitely not enough light to be at ISO 400). That answers my earlier questions about the P mode too.
In order to avoid slight overexposure, the Aperture went to F3.2 (You fixed the shutter speed at 1/30s here).
Other than that, it was simply the P mode choosing 1/15s and ISO 400, while with the S mode that you picked 1/30s which is one Stop faster than P mode's 1/15s, and the Pentax's Auto ISO simply picked an ISO one Stop higher (from 400 to 800) to offset.
Light bulb: f5.6 @ ISO 200;
Compared with P mode pointing at light bulb: P mode picked 1/60s; and with S mode here you picked 1/30s. There is one Stop difference. But the S mode's F5.6 is not quite one stop narrower than P mode's F4.5 (Should also be one stop to offset). Perhaps there was an experimental error. You didn't point quite at the same spot which wasn't easy to do.
Again, when pointing at the light bulb, there was more than just enough light at ISO 200 (the lowest for Pentax already). Since it was the S mode and you picked 1/30s and that wouldn't change, the excess light only caused the Aperture to narrow.
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If I use the 35mm end of the zoom, the same exposure but with a
speed never less than 1/30s in P mode.
because your 17 - 35mm Zoom lens has a constant Widest Aperture of F2.8 across the Zoom range. (F10/11 has smaller max Apertures with more Zoom)
The faster Shutter Speed (from 1/15s to 1/30s) is to avoid the Zoom multiplication effect of camera-shake.
I think it’s actually a clever way to work even if it prefers to
bump to ISO 800 in the room shot in S mode instead of opening to
f2.8 because the level of noise at ISO 800 is really excellent on
the Pentax (plus opening to f2.8 only wouldn’t be enough…)
What do you think?
You have given me excellent examples and numbers (except I am still curious what Auto ISO at A mode with F2.8 will do, but I can almost guess now) !
