VX2100 > F2.4 +3db > how is this possible? at DVinfo.net

John Jay · January 9th, 2005, 11:53 AM

Filming with the VX2100 last night, and to my surprise the display read F2.4 +3db when I turned it on.

thumbing the wheel for smaller iris kept the +3db however when I opened up the iris to 'open' it reset to 0db and then everything was as usual.

question is how did that happen? I would like to know because that could be useful, perhaps there is some button sequence that does it which I stumbled across?

Bob Harotunian · January 10th, 2005, 01:00 PM

If the vx 2100 is like the PD 170, I'm thinking that you had gain and speed in auto. When you opened the iris, the camera compensated with less gain.

John Jay · January 10th, 2005, 03:21 PM

yes

the 170 has independent gain control, but the VX does not and the only way to see a gain increase in the LCD with the VX is after passing through 'open'

AEA exposure mode does not show the gain/speed being applied for a fixed iris setting

Its almost like theres a PD170 under the hood of my VX which is bursting to get out !!

Maybe it will evolve into a PD170 through natural selection

Perhaps I should kept it in a dark cupboard when I am not using it and who knows :)

Boyd Ostroff · January 10th, 2005, 07:09 PM

John: I hate to burst your bubble, but your VX-2100 has not morphed into a PD-170 ;-) I just tried on my VX-2000 and it can also do this (or add any other amount of gain desired at f2.4).

How? Simple: the maximum f stop varies from f1.6 to f2.4 as you zoom from wide to telephoto. That's why the finder simply displays "OPEN" when you reach the max. Zoom your camera all the way to telephoto then turn the exposure wheel. As you open the iris it will eventually get to f2.8, then to OPEN. At full telephoto OPEN = F2.4 (look at the specs in the back of your manual). Therefore as you turn the exposure wheel you will start adding gain.

Now here's where it gets interesting. Zoom out slowly - as you get about halfway out the display changes from OPEN 3dB to F2.4 3dB. Now zoom all the way wide and turn the exposure wheel - it changes to F2.4 0dB, turn it again and you get F2.0 0dB, etc.

For another experiment, zoom in and turn the gain way up, something like OPEN 12dB for example. Zoom full wide and you get F2.4 12dB. Turn the wheel one click and it changes to OPEN 9dB. This is interesting... it jumps to OPEN 9dB, which is the equivalent of F1.6 9dB!

John Jay · January 11th, 2005, 10:41 AM

Hey Boyd,

Thats it! Excellent detective work!!

Turning on the cam I remember it was part zoomed so your explanation is right on target, it just threw me to see the cam display that after power up

Tom Hardwick · January 12th, 2005, 03:24 AM

Yes, that's just the way my 2000 works as well. It's not been talked out here though, as most people seem happily unaware that the lens loses a stop as you zoom from one end to the other.

Dave Largent · January 29th, 2005, 01:55 AM

So in low light, if you use a tele lens, you
can avoid having the cam go to smaller f-stops,
because you're not zooming as much?

Lamar Lamb · January 29th, 2005, 07:00 AM

It's simple math. The f-stop is simply a measure of the focal length divided by the lens opening.

F-stop = focal length / lens opening

So when you zoom in your f-stop will increase and when you zoom out it will decrease. The diameter of the aperture is remaining constant.

If you are familliar with the inverse square law you will understand how this can happen. Light intensity diminishes as the square of the distance from the source. I don't know if this law applies exactly to the focused light from the lens but it helps to understand what is going on. When you zoom in on a subject with a constant aperture setting the amount of light hitting a given surface area (ccd) at a given distance from the lens is reduced. Your lens is allowing a given amount of light "i" through at all focal lengths based on a given aperture setting. Lets say at full wide all of this light, the entire image from the lens, is focused on the ccd's. However, when you zoom in the image that is cast at the ccd gets larger but the ccd is still the same size and can only see the middle portion of that image. The same amount of light "i" is getting through the lens to make up the entire image but because the entire image is much larger than the ccd you are only getting a fraction of that light "i" that is hitting the ccd. What the ccd sees is a smaller portion of the same image seen at wide. This is what makes the objects in the image look bigger and darker for a given aperture. This is my analysis of what is happening anyway.

Edit:
I did a Google search and found a much more thorough explanation for anyone who may be interested.
http://www.faqs.org/faqs/rec-photo/lenses/tutorial/

Tom Hardwick · January 29th, 2005, 07:23 AM

Look at it this way Dave. If you're filming in a dimly lit room using maximum wide-angle and maximum aperture (f 1.6) then as soon as you start to zoom the camera will start to apply gain-up. By half way (36 mm focal length) the gain-up will be +3 dB and by maximum telephoto (72 mm) the camera will be at +6 dB.

If you wanted to film at zero gain up, you'd have to double the amount of light in the room to film at full telephoto.

Why? because the Sony lens designers wanted to keep the size, weight and cost of the zoom down, and they do this by making the maximum aperture at full tele one whole stop smaller than at wide-angle. It doesn't have to be so, but this is what the Sony design team settled on. Some cameras (the Canon MVX3i for instance) lose less than half a stop, whereas others lose nearly two stops.

tom.

Lamar Lamb · January 29th, 2005, 07:34 AM

Sorry Tom. I didn't mean to throw that edit above in on top of you you.

January 9th, 2005, 11:53 AM	#1
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	VX2100 > F2.4 +3db > how is this possible? Filming with the VX2100 last night, and to my surprise the display read F2.4 +3db when I turned it on. thumbing the wheel for smaller iris kept the +3db however when I opened up the iris to 'open' it reset to 0db and then everything was as usual. question is how did that happen? I would like to know because that could be useful, perhaps there is some button sequence that does it which I stumbled across? __________________ John Jay Beware *PLUGGER-BYTES*

January 10th, 2005, 03:21 PM	#3
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	yes the 170 has independent gain control, but the VX does not and the only way to see a gain increase in the LCD with the VX is after passing through 'open' AEA exposure mode does not show the gain/speed being applied for a fixed iris setting Its almost like theres a PD170 under the hood of my VX which is bursting to get out !! Maybe it will evolve into a PD170 through natural selection Perhaps I should kept it in a dark cupboard when I am not using it and who knows :) __________________ John Jay Beware *PLUGGER-BYTES*

January 11th, 2005, 10:41 AM	#5
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	Hey Boyd, Thats it! Excellent detective work!! Turning on the cam I remember it was part zoomed so your explanation is right on target, it just threw me to see the cam display that after power up __________________ John Jay Beware *PLUGGER-BYTES*

January 29th, 2005, 07:00 AM	#8
Lamar Lamb Regular Crew Join Date: Jul 2004 Location: Baxley, Georgia Posts: 103	Just Math It's simple math. The f-stop is simply a measure of the focal length divided by the lens opening. F-stop = focal length / lens opening So when you zoom in your f-stop will increase and when you zoom out it will decrease. The diameter of the aperture is remaining constant. If you are familliar with the inverse square law you will understand how this can happen. Light intensity diminishes as the square of the distance from the source. I don't know if this law applies exactly to the focused light from the lens but it helps to understand what is going on. When you zoom in on a subject with a constant aperture setting the amount of light hitting a given surface area (ccd) at a given distance from the lens is reduced. Your lens is allowing a given amount of light "i" through at all focal lengths based on a given aperture setting. Lets say at full wide all of this light, the entire image from the lens, is focused on the ccd's. However, when you zoom in the image that is cast at the ccd gets larger but the ccd is still the same size and can only see the middle portion of that image. The same amount of light "i" is getting through the lens to make up the entire image but because the entire image is much larger than the ccd you are only getting a fraction of that light "i" that is hitting the ccd. What the ccd sees is a smaller portion of the same image seen at wide. This is what makes the objects in the image look bigger and darker for a given aperture. This is my analysis of what is happening anyway. Edit: I did a Google search and found a much more thorough explanation for anyone who may be interested. http://www.faqs.org/faqs/rec-photo/lenses/tutorial/ __________________ Lamar

January 29th, 2005, 07:34 AM	#10
Lamar Lamb Regular Crew Join Date: Jul 2004 Location: Baxley, Georgia Posts: 103	Sorry Tom. I didn't mean to throw that edit above in on top of you you. __________________ Lamar

January 10th, 2005, 01:00 PM	#2
Bob Harotunian Major Player Join Date: Mar 2003 Location: Somerville, MA Posts: 360	If the vx 2100 is like the PD 170, I'm thinking that you had gain and speed in auto. When you opened the iris, the camera compensated with less gain.

January 10th, 2005, 07:09 PM	#4
Boyd Ostroff Wrangler Join Date: Dec 2002 Location: Mays Landing, NJ Posts: 11,787	John: I hate to burst your bubble, but your VX-2100 has not morphed into a PD-170 ;-) I just tried on my VX-2000 and it can also do this (or add any other amount of gain desired at f2.4). How? Simple: the maximum f stop varies from f1.6 to f2.4 as you zoom from wide to telephoto. That's why the finder simply displays "OPEN" when you reach the max. Zoom your camera all the way to telephoto then turn the exposure wheel. As you open the iris it will eventually get to f2.8, then to OPEN. At full telephoto OPEN = F2.4 (look at the specs in the back of your manual). Therefore as you turn the exposure wheel you will start adding gain. Now here's where it gets interesting. Zoom out slowly - as you get about halfway out the display changes from OPEN 3dB to F2.4 3dB. Now zoom all the way wide and turn the exposure wheel - it changes to F2.4 0dB, turn it again and you get F2.0 0dB, etc. For another experiment, zoom in and turn the gain way up, something like OPEN 12dB for example. Zoom full wide and you get F2.4 12dB. Turn the wheel one click and it changes to OPEN 9dB. This is interesting... it jumps to OPEN 9dB, which is the equivalent of F1.6 9dB!

January 12th, 2005, 03:24 AM	#6
Tom Hardwick Inner Circle Join Date: Mar 2003 Location: Billericay, England UK Posts: 4,711	Yes, that's just the way my 2000 works as well. It's not been talked out here though, as most people seem happily unaware that the lens loses a stop as you zoom from one end to the other.

January 29th, 2005, 01:55 AM	#7
Dave Largent Trustee Join Date: Aug 2003 Posts: 1,483	So in low light, if you use a tele lens, you can avoid having the cam go to smaller f-stops, because you're not zooming as much?

January 29th, 2005, 07:23 AM	#9
Tom Hardwick Inner Circle Join Date: Mar 2003 Location: Billericay, England UK Posts: 4,711	Look at it this way Dave. If you're filming in a dimly lit room using maximum wide-angle and maximum aperture (f 1.6) then as soon as you start to zoom the camera will start to apply gain-up. By half way (36 mm focal length) the gain-up will be +3 dB and by maximum telephoto (72 mm) the camera will be at +6 dB. If you wanted to film at zero gain up, you'd have to double the amount of light in the room to film at full telephoto. Why? because the Sony lens designers wanted to keep the size, weight and cost of the zoom down, and they do this by making the maximum aperture at full tele one whole stop smaller than at wide-angle. It doesn't have to be so, but this is what the Sony design team settled on. Some cameras (the Canon MVX3i for instance) lose less than half a stop, whereas others lose nearly two stops. tom.