why do my f stops change in manual mode? at DVinfo.net

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Canon GL Series DV Camcorders
Canon GL2, GL1 and PAL versions XM2, XM1.

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Old June 28th, 2004, 10:29 PM   #1
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why do my f stops change in manual mode?


I have been frusturated by this occurence for a while now and have always forgotten about it when I am on the message boards...

I find that when I am shooting at a very wide (low) f stop in low light and i zoom into something that is even darker or if the setting becomes even more dark that my f stop will jump up half a stop or more and then i wont be able to go back and lower it.
For example when filming surfing the other day on an overcast day at f1.? i zoomed into the dark kelp near the waters edge and witnessed the scene get darker as the fstop jumped up to f2.? --- can anyone explain how or why this occurs and if anything can be done to avoid it?


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Old June 28th, 2004, 10:41 PM   #2
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Jerry, there are posts about this elsewhere (where, escapes me at present) but basically it's just a way the lenses work. You can't do anything about it. Why I'm not sure but it just can't maintain an open iris when zoomed in that far. I think on the GL/XM2 it will drop as low as 2.2 when fully zoomed.

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Old June 28th, 2004, 11:55 PM   #3
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Basic photo physics and a property of all zoom lenses. The closer you zoom-in on a subject, the less light passes through the lens. Consequently the maximum effective aperture of the lens must shrink.

But the GL2's lens is quite "fast" (bright) by photographic standards, even at 20x.
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Old June 28th, 2004, 11:58 PM   #4
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Basically most, if not all, consumer/pro-sumer cams use a variable aperture lens as it is cheaper to produce than a constant aperture lens. There are also other reasons like size and weight that come into play. Have you ever seen the size of a constant aperture zoom like a 70-200/2.8 compared to a similar range lens with variable aperture like 4-5.6. The constant aperture lens is much larger, heaver and costs 3x+ as much.

If you look on the barrel of you lens you will see a figure like f1.6-2.4. This is the max f-stop at wide open (f1.6) and full zoom (f2.4) and accounts for why you saw the lens stopping down as you zoomed.

Unfortunately with the GL there is nothing you can do about it other than change your technique. If you had an XL or other cam with interchangable lenses you could get a constant aperture lens like Canon's 16x manual lens.
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Old June 29th, 2004, 09:36 AM   #5
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thanks alot guys, im mostly relieved to know that it isnt a problem in my camera but something that will just have to be accepted - i can certainly deal with it.
it would be nice to not have a variable aperture like on the XL's interchangeable lenses but for the price of my GL2 im definately not complaining!

thanks for the quick response...
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Old June 29th, 2004, 10:00 AM   #6
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Just so none of us gl2 users feel inadequete, the only current xl1 lens with a constant aperture is the Manual 16x lens (which costs almost as much as the gl2 itself). The supplied auto lens is a F1.6-2.6, and the 3x is f1.6-2.2.

F-numbers are essentially fractions, or ratios of the focal length as compared to the aperture of the lens....so if the focal length of the zoom increases, and the aperture remains constant, the F-number goes up... which of course means less light.


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Old June 29th, 2004, 06:16 PM   #7
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hahah ya we might not be the best roommates to have;)
you should definately try to hook one up, they are so much fun! that site needs to be updated because ive got a ton of great holga shots from mexico and my drive across the country... let me know if you ever hook one up.

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Old June 29th, 2004, 07:10 PM   #8
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If you shoot Av (aperture-priority) mode, and set the f-stop at f1.6 (aka "wide open"). The camera will automatically maintain the largest aperture possible for whatever focal length you happen to be shooting. When you zoom in (longer focal length), the aperture will get smaller, but when you zoom out (shorter focal length), the aperture will open up again. Automagically ;)

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Old June 30th, 2004, 02:14 AM   #9
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Yeah, and if you set F2.8 at full wide (switch ND off, if needed), then you can zoom into full tele, without any aperture change :)
I think F1.6 is only for, if i us ND, and want a really small DOF
And if i'm right, the lens has a maximum sharpness (and minimum vignetting) at F3-4.

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