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Lowest aperture on highest zoom?

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Offline TMHKR

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Lowest aperture on highest zoom?
« on: 20 / March / 2015, 21:06:26 »
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Is there any way to force CHDK to use the lowest aperture on my SX170 (which is f/3.5) while fully zoomed in? Canon factory limit is f/5.9 at full zoom. I tried overriding aperture to f/3.5 and zoomed in, but resulting aperture is f/5.9 each time.

I don't understand - if the hardware limits of the SX170 aperture are min. f/3.5 and max. f/8.0 (CHDK extends max. to f/16), why can't the aperture simply stay opened at f/3.5 while I zoom in? I don't see how this could be a hardware limitation, because zoom and aperture systems are controlled separately inside the lens.

Thanks in advance.
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Offline waterwingz

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Re: Lowest aperture on highest zoom?
« Reply #1 on: 20 / March / 2015, 22:31:58 »
So I understand your confusion.   There are several things happening here that make this hard to understand and the "marketing" data that passes as a user manual these days does not make this easier.

First of all,  the available f-stops on your camera change as you change the zoom position.   At the wide angle zoom position, you can adjust the f-stop to the following positions : f3.5, f4.0, f4.5, f5.0, f5.6, f5.9,f6.4, f7.1, f8.0

As you zoom in, the minimum f-stop of your lens changes and you lose some of those values.   This is driven by simple lens physics - you can't changes that.  So when you get to maximum zoom,  your only f-stop options are f5.9. f6.4, f7.1, f8.0.

What is confusing when you say this "I don't see how this could be a hardware limitation, because zoom and aperture systems are controlled separately inside the lens" is that you don't realize that changing the zoom changes the largest possoible lens opening ( lowest f-stop number).  Longer zoom = smaller internal aperture.  Simple optics.

Nothing CHDK can do will change that - it's defined by the physical glass components of the lens.

As far as CHDK extending the max (smallest) f-stop to f16.  well,  that's a fantasy too.  CHDK will TRY to set that value but nothing says the camera can actually do that.  Sorry - sort of like finding out there is no Santa Claus, right ?    On some cameras it has been reported that CHDK can achieve one additional f-stop beyond the Canon range - but it is also reported that the resulting images look terrible.

reference :  http://gdlp01.c-wss.com/gds/6/0300012156/01/pssx170is-cu-en.pdf



 
« Last Edit: 20 / March / 2015, 22:34:57 by waterwingz »
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Offline TMHKR

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Re: Lowest aperture on highest zoom?
« Reply #2 on: 21 / March / 2015, 06:33:39 »
Actually it's possible to use f/16 - I tested it before at full zoom, and images do appear a bit darker. But yes, it's not very useful.
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Offline waterwingz

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Re: Lowest aperture on highest zoom?
« Reply #3 on: 21 / March / 2015, 08:39:02 »
Quote from: TMHKR on 21 / March / 2015, 06:33:39
Actually it's possible to use f/16 - I tested it before at full zoom, and images do appear a bit darker. But yes, it's not very useful.
Well, it can "sort of" give you f16.  Or at least the equivalent exposure to f16.

Your camera (like most Canon P&S cameras) includes a built-in switchable Neutral Density filter. When the filter moves into the light path, it reduces the amount of light reaching the camera's image sensor by the equivalent of approximately three f-stops.  So in bright lighting situations where you want to use a slow shutter speed (adding motion blur for example) then the camera will automatically activate the filter to help reduce the amount of light hitting the sensor.

So when the camera reports f16,  it's really giving you a lens opening of f5.6 or f8.0 plus the equivalent of an additional three stops from the ND filter (see http://en.wikipedia.org/wiki/F-number#Standard_full-stop_f-number_scale).

Why does this matter?  Well, in the days of 35mm film photography, (and today with full frame DSLR;s),  a standard 50mm lens would have a huge difference in depth of field at f5.6 vs f16.   So a "fake" f16 would not have nearly the depth of field of a "real" f16 lens opening.    However, with P&S cameras and their small lenses and sensors,  the depth of field at almost every f stop setting is already pretty large so you will not see the kind of difference there that you would with a full size camera.

Note the you can override the ND filter from the CHDK enhanced photo operations menu if you want to test this out.  The Canon firmware won't know you have done that,  so if you override to the opposite state that the camera was expecting, the exposure will be wrong.  But it gives you a chance to try this for yourself and see how it works out.  CHDK User Manual : ND filter state



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Offline srsa_4c

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Re: Lowest aperture on highest zoom?
« Reply #4 on: 21 / March / 2015, 09:17:03 »
Quote from: waterwingz on 21 / March / 2015, 08:39:02
Your camera (like most Canon P&S cameras) includes a built-in switchable Neutral Density filter.
I'd like to correct this one: the sx170is is the last 'cheap' superzoom PowerShot with manual modes and a real adjustable iris. It does not have an ND filter.
The followup models (sx400 and sx600 series) no longer have manual modes and adjustable iris.
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Offline waterwingz

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Re: Lowest aperture on highest zoom?
« Reply #5 on: 21 / March / 2015, 09:27:58 »
Quote from: srsa_4c on 21 / March / 2015, 09:17:03
I'd like to correct this one: the sx170is is the last 'cheap' superzoom PowerShot with manual modes and a real adjustable iris.
I don't think I said that it does not have an adjustable iris.  In fact I quoted the specs from the manual :
Quote from: waterwingz on 20 / March / 2015, 22:31:58
At the wide angle zoom position, you can adjust the f-stop to the following positions : f3.5, f4.0, f4.5, f5.0, f5.6, f5.9,f6.4, f7.1, f8.0

The OP stated he was observing an f16 reported setting.  That's either an ND filter at work or he was setting that in CHDK and assuming it worked when it really didn't?

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It does not have an ND filter.
Hmmm .. probably need to correct the platform_camera.h file for the sx170is then.

Edit :  also,  the original question here was why the largest aperture (smallest f-stop) decreases as the zoom position increases.  That's not a Canon "take away your freedom" thing - it's simple optics/physics.
« Last Edit: 21 / March / 2015, 10:01:54 by waterwingz »
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Offline srsa_4c

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Re: Lowest aperture on highest zoom?
« Reply #6 on: 21 / March / 2015, 10:04:25 »
Quote from: waterwingz on 21 / March / 2015, 09:27:58
I don't think I said that it does not have an adjustable iris.
Oops. You're right. I only read the first two paragraph of your post.
I'm not sure how CAM_HAS_ND_FILTER ended up in platform_camera.h. As far as I know, an additional (undocumented) ND filter was only included in the sx2xx series cams. Perhaps TMHKR or nafraf could clarify the situation by trying a depth-of-field or diffraction based test. Or, by looking into the lens while pointing it to a strong light source.
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Offline waterwingz

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Re: Lowest aperture on highest zoom?
« Reply #7 on: 21 / March / 2015, 10:14:21 »
Quote from: srsa_4c on 21 / March / 2015, 10:04:25
Perhaps TMHKR or nafraf could clarify the situation by trying a depth-of-field or diffraction based test. Or, by looking into the lens while pointing it to a strong light source.
ND filter control is enabled in the CHDK menu.  A couple of quick shots would tell you if that has any effect.  Or a simple script would tell you too.  Assuming the CHDK code is correct.
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Offline TMHKR

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Re: Lowest aperture on highest zoom?
« Reply #8 on: 21 / March / 2015, 14:58:19 »
Quote from: srsa_4c on 21 / March / 2015, 09:17:03
Quote from: waterwingz on 21 / March / 2015, 08:39:02
Your camera (like most Canon P&S cameras) includes a built-in switchable Neutral Density filter.
I'd like to correct this one: the sx170is is the last 'cheap' superzoom PowerShot with manual modes and a real adjustable iris. It does not have an ND filter.
The followup models (sx400 and sx600 series) no longer have manual modes and adjustable iris.

I believe it DOES have an ND filter - when I point the lens to a bright scene, I can hear some clicking inside the camera. Actually, I hear exactly TWO clicks - the first one when "approaching" the bright scene, and the second when the scene fills the screen. I also tested the CHDK ND override, and it does make a difference in brightness. At each click the brightness decreases. Could that mean that SX170 has double ND filters? Would you like me to perform some testing?

EDIT: Tested with the provided ND filter test script, default script settings. Results:

*** STARTED ***
ND Test : avmode= 2
 ND out tv=96 av=352 sv=411 bv=-139 exp=16
 ND in tv=-192 av=352 sv=411 bv=-138 exp=9
...done
*** FINISHED ***

EDIT 2: I looked into the lens while pointing it towards the sun - there's a tiny piece of something that moves inside the lens center.
« Last Edit: 21 / March / 2015, 15:24:23 by TMHKR »
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Offline waterwingz

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Re: Lowest aperture on highest zoom?
« Reply #9 on: 21 / March / 2015, 15:24:59 »
Quote from: TMHKR on 21 / March / 2015, 14:58:19
I believe it DOES have an ND filter - when I point the lens to a bright scene, I can hear some clicking inside the camera. Actually, I hear exactly TWO clicks - the first one when "approaching" the bright scene, and the second when the scene fills the screen.
One click could be the aperture adjusting to the minimum opening position and the other click could be the ND filter inserting.  Or possibly the ND filter inserts silently but the aperture adusts back open a bit with it does and that's the second click you hear.   Kind of hard to diagnose this without having the camera  :P

Quote
Could that mean that SX170 has double ND filters?
That seems very unlikely and would certainly be unique in the P&S camera world if it was true.


Quote
Tested with the provided ND filter test script, default script settings. Results:
*** STARTED ***
ND Test : avmode= 2
 ND out tv=96 av=352 sv=411 bv=-139 exp=16
 ND in tv=-192 av=352 sv=411 bv=-138 exp=9
...done
*** FINISHED ***
This says that there is an ND filter and it adds the equivalent of 3 stops ( 96- (-192) = 288 which in APEX96 units = 3 f-stops)
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