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01-30-2018, 08:47 AM   #16
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QuoteOriginally posted by skierd Quote
Other than the ďitís nice to knowĒ part. if the exposure comes out the way you expect it why does this matter? Just curious. It doesnít particularly bother me if shutter speeds are really a nominal speed than actual speed so long as the steps between are repeatable and consistent so I know what Iím getting in to when I take photos.
For 99% of images, you are right. But there are at least a three scenarios where it does matter:

1. Interval shooting: Intervalometers (including the ones built into the K-1) work by "pressing the shutter button" on a schedule of timed intervals. If you set the intervalometer to 30 seconds, then every 30 seconds it will try to take a picture. Now if the shutter speed is set to "30" but the exposure time is really 32 seconds long, then the next intervalometer button press will happen during the previous exposure and do nothing. Instead of getting one shot every 30 seconds, you'll get a shot, then a 28 second delay, then another shot, then a delay , etc. and half the number of shots you expected. Successful use of interval shooting depends on knowing the true exposure time and this work shows that the true exposure time (which is mathematically calculated for accuracy) is different than the dial time.

2. Interactions with artificial lights: Many types of artificial lights (fluorescent, CFL, LED, etc.) flicker stroboscopically in time with the power line or some internal clock. Often this flicker frequency is exactly twice the line frequency (e.g., 2*60 Hz = 120 flickers per second in the US). If you set the shutter speed to "1/60" you'd expect to always get an exposure illuminated by exactly 2 flickers of light. But we now know that the camera is actually shooting at 1/64 so it's not getting two full flickers. Because the intensity of the light source and color of the light source varies during the flicker cycle, shooting at 1/64 in a place lit by a 120 flicker/second light source may have weird color and image brightness variations between the shots.

3. Measurement: In technical photography, you can estimate the velocities of objects from the length of the blur they create in the image. But to get an accurate velocity estimate, you need to know the true exposure time which might be up to 13% different than the dial time.

Besides, it's a interesting foray into science and engineering to try to figure out exactly what the camera is doing. It's fun exercise for the little grey cells to figure a way to accurately measure the true exposure time of the camera as AstroDave has done.

01-31-2018, 12:36 AM   #17
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Interesting, and for me point number two in particular. Iíve had some weird exposure inconsistencies indoors that this might explain.
09-29-2020, 02:31 PM   #18
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Camera Simulator, Exposure Calculator, Light Value -49 to 69 Ev.

QuoteOriginally posted by AstroDave Quote
As this article explains, except for certain values (integer powers of two, and reciprocal integer powers - such as 1, 2, 4, 8, 1/2, 1/4, 1/8 seconds), the shutter speeds which we are familiar with are not quite correct.

The author refers specifically to Nikon, noting for instance, that a 30 second exposure is really 32 seconds.

With a really simple experiment - taking a "30" second exposure with my K-1 and timing it with my wrist watch - I get a similar result: a so-called 30 second exposure really is quite close to 32 seconds.

My simple search of the forum does not pop up any discussion of this, specifically with respect to Pentax.

Does anybody have good evidence to support these claims - or evidence to the contrary (i.e. real data from Pentax/Ricoh or elsewhere) for our Pentax cameras?

I am in the process of designing a digital circuit which will allow me to measure my K-1 and K-3 shutter speeds to about 1% accuracy (I already can do this to 2%) and will report results here when I have them. My system counts optical flashes from LEDs, recorded during the exposure, not the sometimes-hyped acoustic schemes using your computer audio input.

These differences are something to keep in mind if doing interval shooting with intervals of 15 or 30 seconds - not only do you have to allow for camera overhead such as writing to the SD card before starting the next exposure, but you should also keep in mind that the actual exposure is a second or two longer than you think!
QuoteOriginally posted by AstroDave Quote
Imagine a row of, say, 110 LEDs that turn on one after the other at a rate of such that there should be 100 of them illuminated during the expected time that the shutter is open. When the count gets to the last LED, the sequence starts over again, so the row of lights cycles endlessly at this rate. You take a picture of the LED array. If your shutter speed is correct, you should see 100 of the LEDs lit up. It doesn't matter where they begin - just count the lights. If there is an extra light or two, then your shutter is open LONGER than you expected. A few less than 100, your speed is faster.

If the lights are indeed in a row (my current system has an 8x8 matrix of LEDs), across the frame, this works even for high shutter speeds where the aperture is a moving slit between the top and bottom curtains. Conversely, if you are far enough away from the matrix, you will still see all the relevant lights within the slit.

Conceptually simple, but getting all those LEDs to light up in sequence takes a rats nest of wires!! I've got the design ready, just need to wire it up.

Another somewhat easy way to do it is to hang a single flashing LED on a long enough rope and swing it across the field of view. Takes only one LED, but you need a longish rope. (Fortunately, I have a place inside my house where I can hang a 15' pendulum - maybe I'll try this way first. I did something similar along time ago and had forgotten about that technique.)

In this way, you really do measure how long the shutter exposes the CCD.

I'll post my results (and circuit(s)) when I'm done.
I don't know if it could help you, but I re-built my Camera Simulator, Exposure Calculator,
with Java-Script in spring 2019, just between the launch of this thread and now...
Useful to find Equivalent Exposures : Astrophotography purpose, etc...

the Camera Simulator, Exposure Calculator is here :
Camera Simulator, Exposure Calculator, Light Value -49 to 69 Ev.

and I hope all the explanations are here :
Camera Simulator, Exposure Calculator : SLV -25 to 30 Ev mini. -

Last edited by geo444; 09-30-2020 at 01:54 AM.

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