Confusion on F stops & ND Filters?

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ericcosh

Well-known member
I know this is going to sound like a crazy question, but I'm confused about 2 things with regard to the DVX100 & 100A.

F stops. As most of you are aware, the electronic F stops on the 100 are as follows:

1.7 2 2.4 2.8 3.4 4 4.8 5.6 6.8 8 9.6 11 14 & F16.

In photography, typical F stops could be below F1 (very expensive) and then would be something like:

1 1.4 2 2.8 4 5.6 8 11 16 22 32 45.

When dealing with F stops, the rule of thumb has always been that with each F number, it either lets in twice as much light or takes away twice as much light.

It seems to me that the 100 is dealing with 1/2 numbers, i.e , 2 to 2.4. to 2.8. to 3.4 to 4 to 4.8 etc...

Are we to assume that we in fact only let in one times the amount on each number?


Second Question:

I'm very confused about the ND filter numbers.

The manual says 1/8: The light quality is reduced to about one eighth.

1/64th: The light quality is reduced to about one-sixty-forth.

Are they saying that starting at zero we go 1/2, 1/4, 1/8 or 3 F stops? and that 1/64th would be an additional 3 F stops? (i.e. 1/16th, 1/32nd, 1/64?) or a total of 6 F stops?

Any help on this issue would be greatly appreciated.

thanks,

eric
 
Re: Confusion on F stops & ND Filters?

[quote author=ericcosh link=board=Events;num=1091597258;start=0#0 date=08/03/04 at 22:27:38]
It seems to me that the 100 is dealing with 1/2 numbers, i.e , 2  to 2.4. to 2.8. to 3.4 to 4 to 4.8 etc...

Are we to assume that we in fact only let in one times the amount on each number?
[/quote]
The DVX iris operates in 1/6 of a stop increments. The numbers in the viewfinder are rounded off to 1/2-stop increments. So f/6.8 is the same as stopping down half a stop from f/5.6, or opening up half a stop from f/8.

Each tick of the iris lets in (or blocks out) 1/6 of a stop of light, so if you are at f/4 and you stop down three ticks, the viewfinder will now show f/4.8 and you will be letting in 3/4 as much light as you used to be.

Are they saying that starting at zero we go 1/2, 1/4, 1/8 or 3 F stops? and that 1/64th would  be an additional 3 F stops? (i.e. 1/16th, 1/32nd, 1/64?) or a total of 6 F stops?
Click to expand...
Yes. Sort of.

You are correct, but in my simple testing I found that ND1 is about 3.5 f-stops, and ND2 is about 4.5 f-stops, for a total of about 8 f-stops.
 
Re: Confusion on F stops & ND Filters?

Each tick of the iris lets in (or blocks out) 1/6 of a stop of light, so if you are at f/4 and you stop down three ticks, the viewfinder will now show f/4.8 and you will be letting in 3/4 as much light as you used to be.

Hi Barry: Thanks for your very quick reply to my question. I think I'm still confused though on your answer above. You say that if you stop down from F4 3 ticks you will now be at 4.8? Did you mean 6.8? or am I still missing something?

thanks,

eric
 
Re: Confusion on F stops & ND Filters?

Hi Barry:

I think I get it now. When you said "ticks", I was thinking of actual "clicks or numbers". I think what you were really saying was that if you move the iris wheel ever so slightly there are approximately 3 slight movements of the wheel before the actual numbers change from say F:4 to F 4:8 which is 3X1/6 or 3/4's as much light.

Am I now on the right track?

thanks again Barry,

eric
 
Re: Confusion on F stops & ND Filters?

You are indeed on the right track. The iris in the DVX is calibrated to open/close in 1/6-stop increments, so if you watch a monitor, as you slowly close the iris wheel from f/4 down to f/5.6 you'll see six distinct brightness level changes. However, the display only updates at the halfway point.

So the iris goes f/4, f/4+1/6, f/4+2/6, f/4.8, f/4.8+1/6, f/4.8+2/6, f/5.6...
 
Re: Confusion on F stops & ND Filters?

I found that the neutral density filter blocked out a whole lot of light, more than any of you said. Since Barry's a genius I figured maybe he could help me out. I was doing DOF tests and with ND2 on (1/64th) the iris was OPEN and the image was dark enough that I had to use +6db of gain to get the same brightness as at F16 (or was it F14? it was around there, pretty sure F16.)

Here are the pics, btw. You can guess from the DOF (what a difference!) which is which.

f0.jpg

f16.jpg
 
Re: Confusion on F stops & ND Filters?

Not sure where the question is. Are you asking how much light the ND blocks out? At 8 stops for ND2, it's allowing in 1/256 as much light as with the ND off. Yes, it blocks a LOT of light. More than enough to cover the entire f-stop scale (so yes, you could be at f/16 with ND off, and WIDE OPEN with ND2 on).
 
Re: Confusion on F stops & ND Filters?

That's basically what I was wondering....why the ND filter seemed to block more light than f/16. I think I just read what your wrote previously wrong.

Plus, I had just taken those screen grabs and wanted somewhere to put them!
 
Re: Confusion on F stops & ND Filters?

[quote author=Policar link=board=Events;num=1091597258;start=0#7 date=08/05/04 at 07:57:30]why the ND filter seemed to block more light than f/16.[/quote]
Because it does. Going from f/2 (basically wide-open) to f/16 involves six stops. Using the ND2 switch is approximately 8 stops (even though it's labeled 1/64, it's actually more like 1/256).
 
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