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01-18-2011, 06:55 PM   #1
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How does the camera determine ISO?

I did a title search and didn't come up with anything specific to my question.

I just bought my first DSLR (K20D) after using film for 30 years.

I spent quality time with my manual as well as hours reading everything I can absorb from the forum here.

One thing that I need to get my head around, is the variable ISO. When shooting film of course, that isn't a variable once you load your desired film.

Getting used to the new camera, I've set the ISO to AUTO with a range of 100-800. Now I know I can set a constant ISO, but am still experimenting.

My question is: If I am in aperture priority or shutter priority, and my ISO set to auto, how does the camera determine the ISO? Does is have certain parameters such as the available light, then set the ISO, then set the shutter speed or aperture?

I hope I am explaining my question properly - this is one of those things that I need to know how it works before I can move on.

01-18-2011, 07:17 PM   #2
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Per my observation: In P and Av mode it will start increasing the ISO when the shutter speed would otherwise drop below what Pentax considers 'safe' for hand held shooting. Basically the old 1/focal-length rule adjusted for the size of the sensor being smaller than 24x36mm.
01-18-2011, 07:43 PM   #3
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Ole said it well. Basically, when in Auto ISO, the camera is going to choose the lowest ISO that is compatable with the shutter speed. In P it will adjust both apature and shutter speed before bumping the ISO. In AV, since your apature is set, it will adjust the shutter speed down until it reaches the minimum speed for hand-holding before bumping the ISO.
It's actually a cool feature. Because you've been shooting so long, you probably remember the term "idiot camera." With modern digital, you can change that term to genius camera.
However, just as with film, everything is better at lower ISOs, so I keep mine set on 100, switching to auto only when the light is low.
01-18-2011, 08:23 PM   #4
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Thank you both for the explanation - makes perfect sense and really helps me with the learning process.

Actually Ron, as I learn this new dimension, I was going to keep the ISO set at 100 as you state to try to simulate what I am used to. I just needed to understand how the camera "thinks" in those situations.

And now knowing that it is making its settings assuming being hand held is very important as I do a lot of work on my tripod. Too bad the camera doesn't know when it is attached to a tripod and automatically set the ISO to 100 (just kidding!!)

I've learned something today - it is a good day!

01-18-2011, 10:50 PM   #5
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I use a tripod whenever possible, and shoot in AV most of the time, and all the time for scenic shots. To increase the DOF, I'll shoot from f-8 to f-14, depending upon the lens used, which means most of the time my shutter speed is to slow for hand-holding. The camera, of course doesn't know I have it on a tripod, but since I have the ISO set at 100, it doesn't matter.
When using a tripod, you also need to remember to turn off SR. For quite a while I either forgot to turn it off, or forgot to turn it back on afterwords for hand=held shooting. But a really neat feature of the K20D, is if you also use a delay or a remote, when using the camera on a tripod, it automatically disables SR, and you get mirror-up function for maximum stability and clarity. When you take the camera back off delay, it automatically turns SR back on.
01-18-2011, 11:08 PM   #6
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Great tip!

I just took a look at the manual - just need to clarify something.

I just happened to receive the "Remote F" today, so this is quite timely.

For the mirror up function, do I need to set the timer function in addition to using the remote? It mentions the setting for the 3 sec. delay for the remote, but no mention of the mirror up function working. The mirror up function is only mentioned using the 2 sec. timer in conjunction with the regular shutter release.
01-19-2011, 01:00 AM   #7
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I'm reasonably certain that you cannot select both functions for wireless remote. If you want to use the Mirror Up with remote, you'll need a wired remote which works as an extension of the shutter button.

01-19-2011, 08:55 AM   #8
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That makes sense, and is how I interpreted it from the manual also.

The reason I am looking at this feature - my wife has a huge flock of Yellow Finches at her feeders constantly. In the summer they just explode with color, and want to capture them. But, they are real flighty. I plan to set up on a tripod and have myself in range with the remote to get these images. Having the mirror up function using the remote for this would be great, but certainly not a deal breaker.

01-19-2011, 09:07 AM   #9
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This is similar to the way I shot hummingbirds.
To use the remote, you need to set it to that in your FN menu. The mirror goes up automatically, and a little light will blink on the back. It also blinks on the front, which can makes birds cautious for close shots. I covered the front light with a piece of electrical tape. You also need to prefocus in MF and capture the birds when they enter the zone.
When using the remote, you don't need, nor would you want, delay for birds.
01-19-2011, 09:33 AM   #10
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QuoteOriginally posted by Ron Kruger Quote
To use the remote, you need to set it to that in your FN menu. The mirror goes up automatically.
That's great to hear that the mirror goes up automatically when put in remote mode.

I am going to experiment with this over the weekend.

Now I know how to set/use my ISO settings and how to use my remote.
01-19-2011, 10:18 AM   #11
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Be aware however, that the Remote F / K20d is pretty much a direct line of sight operation. It doesn't do well at distances, or too much off to the side. That was my experience with the combo anyway. Practice is a wise exercise so you can see what will and will not work.

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