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EV to F-Stop conversion


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#1 Gabrielle Demeestere

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Posted 08 November 2008 - 02:33 PM

Hello,

I shot a 16mm short film yesterday on Double X negative film (ISO 250) and my AC used the EV mode on the light meter by mistake. We were getting an 11 most of the day (which was cloudy and even), which seems to correspond to a 5.6 F stop on our light meter (at a constant shutter speed).

Do you think I could save the footage by pushing the film 2 stops or would it be best to sit in on the telecine?

thank you!

Gabrielle
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#2 Simon Wyss

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Posted 08 November 2008 - 02:53 PM

To be able to correct for an error one needs to know the shutter angle (camera brand, type and eventual modification) as well as the frame rate. Would you mind stating the values? Also there could be a bellows factor with macro shots.
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#3 Chris Keth

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Posted 08 November 2008 - 06:09 PM

Depending on your scenes, perhaps you can push 1 stop and deal with the rest in transfer. That should get you a very useable image without the huge grain and very high contrast that would come with a 2 stop push.
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#4 Adrian Sierkowski

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Posted 08 November 2008 - 06:11 PM

I second the 1 stop push and further correction in telecine. Expect a grain increase as well; though.
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#5 K Borowski

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Posted 08 November 2008 - 06:15 PM

DO NOT PUSH PROCESS THIS FILM!!!

Not to cut you off Simon, but, according to my flash meter here, switching back and forth between ExIn and F/stop, no no no, you went the WRONG WAY!

Assuming that you have a standard shutter angle, let's say it's 1/45 sec. for simplicity's sake, you would've needed to stop down to **F/45** to get proper exposure.

So you are probably OVEREXPOSED 4 stops, not underexposed two.

Please don't take this as an insult, but what on Earth were you thinking not taking into account the sunny 16 rule? With modern meters in particular, you always need to double check with what you think a scene should be in your head.

The way the Sunny 16 rule works is this: At F/16, your shutter speed should be approximately the film speed on a sunny day. So, that'd be 1/250 sec. at F/16. Since you were, I assume, shooting at around 1/45 sec., that'd be 2 1/2 stops smaller, so F/32 1/2, ,which is pretty darn close to 45. Let's say your cloud cover was a little lighter than usual.

I did all of this in my head 2,000 miles away. So memorize this rule, and it will save you from headaches like this in the future.

Now, as to your film, you'll probably have halation, even if you pull a stop (don't pull more than a stop), but it should be usable. You may need to get a special scan done too as the density will be way way way up. if your scanner operator doesn't know this film is way overexposed, it's very possible to get a scan with totally blown-out highlights, like the HD look you are, I assume, desperately trying to avoid by shooting film to begin with.

Looks like you really really really lucked out because film can be overexposed probably over 6 stops and still yield usable results. I once did five or six, but I was in a hurry, and that was with still photography. Unless you are shooting 10 o'clock news on 16mm, or a documentary, there's no reason why you shouldn't have caught this.

Had you UNDEREXPOSED four stops, you could have pushed probably six or eight stops and still have not gotten a usable image, contrast wise, disregarding what that much of a push would have done to the grain structure of a 16mm image.


So remember, shutters are fallible, meters are fallible, so you have to TEST everything beforehand, and know rules like "Sunny 16" so that when things do go wrong on the set you won't waste a whole day and have to reshoot everything, which would've happened here had you not lucked out and extremely overexposed instead of extremely underexposing.

And one final thing, how does ExIn 11 correspond to F/5.6 on your meter? I've checked every conceivable combination, even going from ExIn 5.6 to F/Number, and I can't get F/5.6 to go to 11, so I would NOT use that meter until you have it recalibrated.
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#6 Adrian Sierkowski

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Posted 08 November 2008 - 06:27 PM

Googling, I found this formula:

EV=log2(f^2)-log2(T)

where log2=logarithm base 2, f=f-number, T=shutter speed in seconds.
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#7 Chris Keth

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Posted 08 November 2008 - 06:31 PM

EV 11 corresponds to a 5.6 and 1/60th shutter speed on an EV table. If the ISO was set correctly on the meter and it told you EV 11 and you set the lens at f11, then you underexposed 2 stops like I said.

That is a plausible exposure if the cloudcover was heavy.
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#8 Adrian Sierkowski

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Posted 08 November 2008 - 06:35 PM

Also found this Chart via google. .xls format

http://www.fineart-p.../PhotoTools.xls
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#9 Chris Keth

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Posted 08 November 2008 - 06:36 PM

Googling, I found this formula:

EV=log2(f^2)-log2(T)

where log2=logarithm base 2, f=f-number, T=shutter speed in seconds.


an EV of 0 (zero) is the same as f1.0 and 1 second. Extend in any direction or use a table like the one here.

As for how an Ev of 11 corresponds to a 5.6, it's simple. An Ev of 11 ALWAYS corresponds to a 5.6 at 1/60th of a second. Just the same, it always corresponds to an f8 and 1/30, f4 at 1/125th etc.
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#10 Chris Keth

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Posted 08 November 2008 - 06:49 PM

I couldn't edit, so:

Gabrielle, bottom line is this: none of us were there so we can only speculate.

If it was very cloudy and quite dark for daytime, your measured exposure of EV 11 on 250 ISO film is plausible. If that is the case and you exposed at f11, you underexposed by about 2 stops. I would push 1 and fix the rest in transfer.

If what I described were not your shooting conditions, reply back into this thread and we can keep trying to help.
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#11 K Borowski

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Posted 08 November 2008 - 06:57 PM

Chris and Adrian are both right as far as the mathematical equivalent, but that isn't a guarantee that that is how your meter calculates it.

However, I was using my meter for the conversion, a Minolta IV meter more designed for still photography, but essentially the same calculations.

Apparently, what it is measuring is *relative E.V./Exposure Index* so it is quite possible that your meter does the same thing that mine does.

So it could be that you've underexposed two stops OR that you've overexposed four.

If you were shooting out of the shade with just the cloud cover, I'm probably right and you're four stops over.

If you were shooting in the shade on top of it being cloudy, then the conversion approach you, Chris, and Adrian are using is correct and you're two stops under.


So, we need to know your shutter angle, shooting conditions, E.I., and camera, or this will be a total "crap-shoot".
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#12 Simon Wyss

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Posted 09 November 2008 - 06:47 AM

I always look for the base. We have ISO 250 and your meter reading. That is not enough. First you must know exposure time, for which the shutter opening angle is crucial, and the frame rate. I assume 24 frames per second and 180 degrees with a reflex viewfinder camera, am I right ?

That makes 1/48th second. Cloudy overcast, no filters ( ? ), a medium-range zoom lens (am I right ?), focal distance not less than 3 feet, EV 11 is not far from f 5.6 (square root 32).

I'd also favor Karl's view. The correct iris setting should have been f 32 to 45. Do try to get the film(s) underdeveloped by two to three stops.
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#13 Tim Terner

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Posted 09 November 2008 - 01:46 PM

I always look for the base. We have ISO 250 and your meter reading. That is not enough. First you must know exposure time, for which the shutter opening angle is crucial, and the frame rate. I assume 24 frames per second and 180 degrees with a reflex viewfinder camera, am I right ?

That makes 1/48th second. Cloudy overcast, no filters ( ? ), a medium-range zoom lens (am I right ?), focal distance not less than 3 feet, EV 11 is not far from f 5.6 (square root 32).

I'd also favor Karl's view. The correct iris setting should have been f 32 to 45. Do try to get the film(s) underdeveloped by two to three stops.


Assuming he set the lightmeter up beforehand with the correct speed of the film he was using (in this case 250), getting a reading of EV 11 would indicate he needed an exposure setting of 1/60 at 5.6 - If shooting with a 180 degree shutter angle and at 24 FPS, he set the aperture to 11, he would be just under 2 and a third stops underexposed
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#14 K Borowski

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Posted 09 November 2008 - 02:06 PM

Assuming he set the lightmeter up beforehand with the correct speed of the film he was using (in this case 250), getting a reading of EV 11 would indicate he needed an exposure setting of 1/60 at 5.6 - If shooting with a 180 degree shutter angle and at 24 FPS, he set the aperture to 11, he would be just under 2 and a third stops underexposed


Well, Tim, as I found out the other day, my light meter reads Ex. In. (EV) as a *relative* function, so, without knowing what meter, or the shooting conditions, this really could be incorrect exposure in either direction.

I was outside shooting yesterday at an E.I. of 80, very overcast, and out of the shade, with the sun to my back, it was 1/125 @F/8, so that would've been about F/22 were I shooting at E.I. 250 under the same conditions, indicating a two-stop overerexposure.

It is bad to recommend one way or the other without knowing the specific meter, and shooting conditions (i.e. shade, sun position, supplemental lighting or diffusion or flagging, etc.)
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#15 Adrian Sierkowski

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Posted 09 November 2008 - 02:17 PM

And the moral of the story is always double check your meter settings/readings. ON the plus side, one you mess up once with this (as I did once too) you rarely ever make the same mistake.
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#16 Tim Terner

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Posted 09 November 2008 - 02:24 PM

It is bad to recommend one way or the other without knowing the specific meter, and shooting conditions (i.e. shade, sun position, supplemental lighting or diffusion or flagging, etc.)


Agreed Karl that the meter reading is only a guide to help you place the exposure where you need it

I was outside shooting yesterday at an E.I. of 80, very overcast, and out of the shade, with the sun to my back, it was 1/125 @F/8, so that would've been about F/22 were I shooting at E.I. 250 under the same conditions, indicating a two-stop overerexposure


an EV reading is only relevent to the ISO that you have fed into the meter beforehand for the speed of the film you are using. You'll get a different EV reading with an 80 set in the ISO than you will with 250 ISO set into the meter.
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#17 Tim Terner

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Posted 09 November 2008 - 02:29 PM

And the moral of the story is always double check your meter settings/readings. ON the plus side, one you mess up once with this (as I did once too) you rarely ever make the same mistake.


Thats spot on Adrian. I think we've all made the same mistakes by not setting the meter properly :-( at some time or another
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#18 Stephen Williams

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Posted 09 November 2008 - 02:44 PM

Thats spot on Adrian. I think we've all made the same mistakes by not setting the meter properly :-( at some time or another


Hi,

I generally know what the reading should be in any case, I turn the meter until it reads what I want
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#19 Dominic Case

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Posted 09 November 2008 - 07:51 PM

EV units are an unnecessary distraction. Thre are enough ways of measuring exposure without them, and so they usually lead to confusion.

I use two rules of thumb to understand EVs. The first is the definition of EV: Roughly, 0EV corresponds to 1 lux, which requires an exposure of 1 second at f/1 on 100 EI film. A change of 1 EV is one stop. (1 lux is about 10 fc).

However, as most of those 1s are off any real-world scale, my second version, though less memorable, is as follows: 1,000 lux is an EV of 10, which requires f/2.8 at 1/50th sec on 100 EI film. (These are to the nearest stop.)

If you read EV11, then you had one stop more light than that. So you should shut down to f/4 on 100EI film, or f/5.6 to f/8 on 250EI film.

In other words, you would be underexposed by 1 1/2 to 2 stops. Push one stop at the most: any more will blow the grain.

Nevertheless, that does seem quite a dark day.
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#20 Adrian Sierkowski

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Posted 09 November 2008 - 08:50 PM

Ha Stephen, you must be joking ;) I just shoot everything at F4 and tell the client I was representing my Pathos!


(btw, if you weren't joking, then that's pretty fantastic. I've guessed F-stops before, and been spot on on occasion; though never enough to totally disregard my meter. some mornings I can be a bit bleary eyed so I need a double check)
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