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Lens Ratios and Pushing Film


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#1 Jase Ryan

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Posted 04 January 2009 - 10:39 PM

I am still not 100% clear about these two things.

First, Lens Ratios.

When I read about a lens that say has a ratio of 11:1 or 4:1 or whatever, what exactly is this ratio telling me about the lens? Am I right to assume it has nothing to do with focal length? I ask this because I'm sure I've seen different length zooms with the same ratio. Any explanation on this would be great.

Second question is, Pushing Film.

From my understanding, if I'm shooting a scene that is underexposed by lets say two stops, I would then tell the lab to push it two stops to compensate, correct? Is this basically developing the film a bit longer to get more of the image?

So if I'm on set and I'm shooting 500T film, yet I'm still two stops under, would I then set the ISO on my meter to 2000 ISO and tell the lab I rated this film at this speed?

Thanks in advance for helping clear this up!
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#2 Hal Smith

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Posted 04 January 2009 - 11:37 PM

I am still not 100% clear about these two things.
First, Lens Ratios.
When I read about a lens that say has a ratio of 11:1 or 4:1 or whatever, what exactly is this ratio telling me about the lens?


Lens Ratio is the ratio of a zoom lens' longest focal length to its shortest focal length.

For instance: An Angenieux 12 to 120mm lens has a 10:1 ratio and my Angenieux 35 to 140mm lens has a ratio of 4:1. It's a convenient way of expressing the extremes of focal length of a given lens.
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#3 David Mullen ASC

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Posted 05 January 2009 - 01:11 AM

As negative film gets more exposure, more grains become "developable" from silver halide to silver (in color film, an equal amount of color dye is formed during processing relative to the amount of silver formed - then the silver and the unexposed silver halide are all removed in later steps, leaving only color dye.) On a negative, the brightest areas in the real-world subject (the ones providing the most exposure) create the most silver and therefore are the most dense / darkest areas; the dark areas of the subject form the least amount of silver and therefore are the most thin areas.

So more exposure will create more density on the negative.

So will extended development (push-processing).

But pushing can only increase the density of information that originally got captured by the negative, which is determined by its sensitivity -- pushing doesn't increase actual sensitivity. Hence why there is an increase in graininess and contrast when you push process -- information has only been captured by the larger grains (which are the most sensitive) and you're taking less information and stretching it out to increase the brightness of the highlights.

There is also an increase in base fog level when push-processing. So there is both more contrast and some loss in blacks.

Anyway, it's always safer to underexpose less than the amount you are asking to push-process, just to make sure you are capturing more information to work with. So if you are going to push a 500 ASA film stock by two-stops, rate it at 1250 or 1600 ASA rather than 2000 ASA, just to be safe. Plus pushing is not always exact in that you don't get exactly two more stops of density, it can vary by a 1/3 of a stop sometimes.
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#4 K Borowski

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Posted 05 January 2009 - 01:25 AM

First, Lens Ratios.

When I read about a lens that say has a ratio of 11:1 or 4:1 or whatever, what exactly is this ratio telling me about the lens?


Be careful though. Sometimes, especially in non-English countries, there is a tendency to use the colon instead of a "/" to refer to F/stops, so it is possible that these could refer to F/11 or F/4.

I think the ratio is the other way around, but just want you to be aware that there aren't international conventions for describing lens characteristics. . .
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#5 David Auner aac

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Posted 05 January 2009 - 03:23 AM

Be careful though. Sometimes, especially in non-English countries, there is a tendency to use the colon instead of a "/" to refer to F/stops, so it is possible that these could refer to F/11 or F/4.


Here is Austria/Germany etc. you would describe a lens's speed thus: 50mm, F 1:1.4. AFAIK this originally comes from much simpler optical devices such as Newtonian (reflecting) telescopes. These usually are F 1:6.3 or F 1:10, meaning they are 6.3 or 10 diameters long. It also works when talking about a simple refracting telescope where the focal length is divided by the diameter of the front lens to calculate the ratio. The same as with lenses, this describes the speed of the telescope, which, in German, is called "light strength".

Cheers, Dave
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#6 Daniel Porto

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Posted 05 January 2009 - 04:10 AM

So if you are going to push a 500 ASA film stock by two-stops, rate it at 1250 or 1600 ASA rather than 2000 ASA, just to be safe.


Does that mean you should then tell the lab that you rated it at 2000ASA (even though you did it at 1600.... considering the fact that there is a sometimes a 1/3 stop variance)????

Thanks DAVID!
DANIEL PORTO
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#7 Satsuki Murashige

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Posted 05 January 2009 - 05:22 AM

Does that mean you should then tell the lab that you rated it at 2000ASA (even though you did it at 1600.... considering the fact that there is a sometimes a 1/3 stop variance)????

I would avoid giving the lab anything more than simple instructions when it comes processing your film. Too much info can lead to confusion on their part. The lab only needs to know if you want your film processed normally, or if you want it pushed or pulled and then by how much (1 stop, 2 stops). How you rated the film is entirely your business, they don't really care anyway.

If you're going to make a print of the film, I would recommend shooting a grey card or grey scale at the head of every roll of film, exposed at the same rated ASA as the rest of the roll. So if you rated 5218 500T at 320T, then you'd shoot the grey card at 320T also. This tells the lab that even though the film is overexposed by 2/3 of a stop, you want them to make it darker so that it looks normal (ie. the 18% grey card should look 18% grey). If you get your print back and it looks darker or brighter than what you intended, first check the the grey card. If it also looks too bright or too dark, then you know that the lab made the mistake and not you. If the grey card looks ok, then you probably over or underexposed the film by mistake. Either way though, shooting the grey card is essentially the same thing telling the color timer how you intended the image to look (ie. how you rated the film, how you exposed the film, how you color balanced the film, etc.), but in a language that they understand (even a grey card still seems to confuse some of them though!!). If you go with the grey card, definitely include a note when you send in the film to "print to the grey card."

BTW, all this applies to telecine as well, but since a colorist can make much greater changes to the look of an image in a telecine suite than a color timer in the lab, the opportunity for screw-ups in the transfer process increases.
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#8 Daniel Porto

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Posted 05 January 2009 - 06:29 AM

I would avoid giving the lab anything more than simple instructions when it comes processing your film. Too much info can lead to confusion on their part. The lab only needs to know if you want your film processed normally, or if you want it pushed or pulled and then by how much (1 stop, 2 stops). How you rated the film is entirely your business, they don't really care anyway.

If you're going to make a print of the film, I would recommend shooting a grey card or grey scale at the head of every roll of film, exposed at the same rated ASA as the rest of the roll. So if you rated 5218 500T at 320T, then you'd shoot the grey card at 320T also. This tells the lab that even though the film is overexposed by 2/3 of a stop, you want them to make it darker so that it looks normal (ie. the 18% grey card should look 18% grey). If you get your print back and it looks darker or brighter than what you intended, first check the the grey card. If it also looks too bright or too dark, then you know that the lab made the mistake and not you. If the grey card looks ok, then you probably over or underexposed the film by mistake. Either way though, shooting the grey card is essentially the same thing telling the color timer how you intended the image to look (ie. how you rated the film, how you exposed the film, how you color balanced the film, etc.), but in a language that they understand (even a grey card still seems to confuse some of them though!!). If you go with the grey card, definitely include a note when you send in the film to "print to the grey card."

BTW, all this applies to telecine as well, but since a colorist can make much greater changes to the look of an image in a telecine suite than a color timer in the lab, the opportunity for screw-ups in the transfer process increases.


Ok. Lets just say that I over-exposed the film by 2 stops, should I then tell the lab that I over-exposed by 1 and 3/4 stops to account for what David was saying about the variance and just to be safe. (So basically if I didn't want to get the right exposure by using a grey card)

THANKS AGAIN
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#9 Alfeo Dixon

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Posted 05 January 2009 - 09:02 AM

Ok. Lets just say that I over-exposed the film by 2 stops, should I then tell the lab that I over-exposed by 1 and 3/4 stops to account for what David was saying about the variance and just to be safe. (So basically if I didn't want to get the right exposure by using a grey card)

THANKS AGAIN

Just the opposite. What David is advising is you set your meter (rating it) at 1250 or 1600 and the notes to the lab are to PUSH 2 STOPS and thats it... (500iso -> 2000iso)

By rating it at 1250 or 1600, you are actually over exposing the the 2000 iso lab push by approximately 1/3 stop to give your exposure just a little bit more bump.

Edited by Alfeo Dixon, 05 January 2009 - 09:04 AM.

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#10 Daniel Porto

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Posted 05 January 2009 - 10:28 AM

Just the opposite. What David is advising is you set your meter (rating it) at 1250 or 1600 and the notes to the lab are to PUSH 2 STOPS and thats it... (500iso -> 2000iso)

By rating it at 1250 or 1600, you are actually over exposing the the 2000 iso lab push by approximately 1/3 stop to give your exposure just a little bit more bump.


I got mixed up with my wording but you answered my question Alfeo!

THANKS!
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#11 Dan Salzmann

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Posted 05 January 2009 - 10:38 AM

I would like to second what Satsuki said about not telling them what you did but telling them what THEY should do to the film.
Sometimes film can be under or over-exposed on purpose in certain situations.
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#12 David Mullen ASC

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Posted 05 January 2009 - 12:16 PM

The lab person doing the negative development doesn't care how you exposed a shot, they just need to know how to process the footage. They don't even care if it is a 100T or 250D or 500T negative stock, or even Fuji or Kodak -- it all goes into the same ECN2 bath anyway and no one can see an image until after the processing is done, so know one is going to know how you exposed the roll actually until it's too late.

Just mark the rolls, camera reports, and work orders with PROCESS NORMAL, or PUSH ONE STOP, etc. whatever you need them to do.

Shoot a grey scale at the head of the roll at the actual rating you are using so that the colorist doing the transfer or the timer making a print can see what your intentions are in terms of what a "normal" exposure is.
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#13 K Borowski

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Posted 06 January 2009 - 07:13 AM

Just mark the rolls, camera reports, and work orders with PROCESS NORMAL, or PUSH ONE STOP, etc. whatever you need them to do.


One other recommendation I have: if and when you push film two stops, always spell out the number "TWO" and underline it so that it reads: PUSH TWO STOPS so that there is no confusion and the "2" isn't overlooked.
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