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Understanding Under-/Overexposure and Push-/Pull-Processes


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#1 Alexander Boyd

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Posted 22 November 2014 - 06:06 PM

Hello everyone,

 

a few questions of a beginner in terms of analog filmmaking that I haven't been able to answer by myself ...

 

 

1) what's the most common way to under- or overexpose? I guess it's by setting the ISO/EI to a higher/lower speed since you'd still want to have maximum control of DOF through the aperture?

 

2) what would happen if you under- or overexpose without pushing or pulling in development?

 

 

Thanks in advance for your help, I highly appreciate you taking the time to read this.


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#2 Stuart Brereton

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Posted 22 November 2014 - 07:50 PM

Setting ISO higher or lower is something that's only possible with Digital cameras. With Film, if you want to under or over-expose you would achieve this with the aperture or with ND filters for under-exposure.

 

If you under or over-expose film without compensating in development, the negative looks either under or over-exposed.

 

I think you may need to reframe your question to gain the desired answers.


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#3 Dennis Couzin

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Posted 22 November 2014 - 07:55 PM

@ Alexander: That's not how film exposure works.  There is no "setting the ISO" speed.  The film has an ISO speed.  You can adjust it by a stop or so by pushing or pulling its development, however not perfectly because the gamma then changes. 

 

You adjust exposure by changing the product of time × light intensity onto the frame.  So you change the time or the light intensity or both. 

Change the time.  You can make the time smaller by reducing the shutter angle.  You can make the time larger by reducing the frames per seond.  Both have undesirable side effects for capturing motion.

Change the light intensity.  If the illumination is under your control you can adjust that.  Otherwise you have just the lens aperture (which unfortunately adjusts DOF too) and ND filters.  ND filters are light intensity reducers only, but when shallow DOF is your goal they should be used instead of stopping down the lens.

 

Your question is about how to accomplish under- and overexposure, not what under- and overexposure do to film images.  That's a longer story.


Edited by Dennis Couzin, 22 November 2014 - 07:58 PM.

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#4 David Mullen ASC

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Posted 22 November 2014 - 08:48 PM

"Underexposure" is a pretty broad term.  In a "normally exposed" scene there are often very underexposed areas, so you have to be more specific when asking about deliberate underexposure.  For example, I could have an extremely hot backlight on a person's head in a close-up and decide that the face should be two-stops underexposed to look the way I want it to, but does that mean that the shot is underexposed? (No, unless I had to brighten the face back up in post because I overdid it...)

 

So when you are asking about the "common" way of under or over-exposing, do you mean a consistent, overall under or over-exposure that is corrected or compensated for in some way in post to get the images back to a "normal" or intended brightness level, or you do mean how one decides what parts of the frame should be hot or dark and by how much?

 

If you mean what is the most common way to rate a 500 ISO film stock at, let's say, 250 ISO or 1000 ISO, instead of the rating recommended by the manufacturer, then the most common method is to change the rating on the light meter and just treat the 500 ISO stock as if it were 250 ISO or 1000 ISO.  But some people just put a sticker or note on their light meter telling them to compensate their readings by whatever amount.

 

But after you decide that you are going to consistently under or overexpose everything by one-stop, then you have to decide if you will be correcting this back to a "normal" level of brightness and how, in development (by push or pull processing) or in printing (by using higher or lower printing light values) or in transfer to video and electronic color-correction.  And you can combine techniques -- you could, let's say, underexpose by two-thirds of a stop but push-process by one-stop to compensate, ending up on average with a 1/3-stop of extra density, which you then would probably correct back down to normal brightness in timing or transfer.  I'm talking about film here.

 

if you underexposed by one-stop and then push-processed by one stop, the negative would end up at normal density more or less, as if you had rated and exposed it normally.  The printer light values to get a print of "normal" brightness would be similar to if you had just shot it normally. But there would be an increase in graininess (from the underexposure, because fewer of the smaller, slower grains got exposed) and contrast (from the push-processing).  The blacks may also not be as pure and might pick up a faint green color-cast, though that is more of a problem with greater amounts of push-processing.

 

If you underexposed by one-stop and processed normally, the negative would be one-stop "thinner", less dense.  If you printed that at standard printing light values, the image would look one-stop darker.  If you printed that at lower printer light values to correct for the underexposure, you'd restore the brightness to normal but you'd see an increase in graininess and some milkiness to the blacks.


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#5 Dirk DeJonghe

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Posted 23 November 2014 - 01:06 AM

May I suggest that you read 'The Negative' by Ansel Adams. All your questions will be answered and much more.
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#6 Dennis Couzin

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Posted 23 November 2014 - 07:52 PM

Ansel Adams' tonal aesthetic should not be trusted for cinema.  Cinema (traditionally) is viewed in darkness.  The projected cine image contains a much greater range of luminances than a photographic print.  The projected cine image has no external white reference.  Cine and still involve different modes of seeing, requiring different tonal aesthetics.  (Also most cinematographers, unlike most still photographers, are limited to standard laboratory processing of what they shoot.)

 

My advice to cinematography students is to learn the theory of tone reproduction from a neutral, scientific source like Chapter 22 of Mees's "The Theory of the Photographic Process" (available online), rather than from artists or craftsmen.  Then they can specialize their knowledge with rules and hints from cinematographers, such as David Mullen gives in post #4 above.


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#7 Alexander Boyd

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Posted 23 January 2015 - 04:58 PM

Thanks for the advice everyone!

 

One last question: let's say the DP wants to underexpose by 2 stops (by either rating the stock or using the aperture). Would he turn up the intensity of the lights in order to "preserve" them and make sure that they won't be underexposed as well? 


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#8 David Mullen ASC

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Posted 23 January 2015 - 05:06 PM

???  

 

Doesn't make much sense, if he brightens the lighting after he meters it, then he wouldn't be underexposing, would he?  Besides, that's not how people light scenes, brightening the lighting once you are finished lighting and metering really is like re-lighting sort of, and then you'd be re-metering to figure out how much you brightened them.  Metering comes after lighting.

 

So you'd meter your lighting, decide what the correct exposure would be for the look you want, but then if you wanted to overall underexpose on top of that, you could do that, or just set your meter's ISO setting so you are basically tricked into underexposing consistently based on your readings.

 

Keep in mind that there is a difference between overall underexposure and lighting a scene with some areas that are darker than others.  Just because your fill light reads on the meter as three-stops darker than your key, doesn't mean you are underexposing the shot.


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#9 Mathew Collins

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Posted 27 December 2015 - 11:53 AM

 

if you underexposed by one-stop and then push-processed by one stop, the negative would end up at normal density more or less, as if you had rated and exposed it normally.  The printer light values to get a print of "normal" brightness would be similar to if you had just shot it normally. But there would be an increase in graininess (from the underexposure, because fewer of the smaller, slower grains got exposed) and contrast (from the push-processing).  The blacks may also not be as pure and might pick up a faint green color-cast, though that is more of a problem with greater amounts of push-processing.

 

David,

Could you explain (from the underexposure, because fewer of the smaller, slower grains got exposed)?

 

My understanding was, during under exposure film is getting 'less light' and slower grains are not getting enough exposure.


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#10 Satsuki Murashige

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Posted 27 December 2015 - 08:21 PM

The perception of 'graininess' comes from there being gaps between exposed grains. When both large 'fast' grains and small 'slow' grains are fully exposed, the overall grain structure of the image is less obvious because there is less micro-contrast between grain particles. When only the large grains are exposed, there are black gaps where the smaller grains are. Think of it like an LCD panel displaying a full field of white. Now imagine a bunch of dead black pixels on the panel. The overall pixel grid now looks more obvious, right? That's all it is.
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#11 David Mullen ASC

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Posted 27 December 2015 - 08:31 PM

Yes, to elaborate further, the smallest grains are the least efficient collectors of photons so are certain amount are not exposed enough to create a latent image and get washed away in processing, which is why black areas in the image are the clear areas on the negative.

 

So if you underexpose, after processing, you don't have as many small grains filling in the gaps between the large grains.

 

It's easier to look at how b&w negative works since it doesn't trade silver grains for color dye clouds at some step.  Silver halide grains, if they collect enough photon energy, can be developed into silver -- then the fixer and wash steps remove silver halide that didn't develop into silver, leaving only the silver.  With color film and processing, an equal amount of color dye forms with the silver, and then a bleach step converts that silver back into silver halide, which gets removed in the fixer and wash steps, leaving only color dye.

 

Bigger grains are better collectors of photons and therefore are faster than the smaller grains.


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#12 Mathew Collins

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Posted 28 December 2015 - 03:14 AM

The perception of 'graininess' comes from there being gaps between exposed grains. When both large 'fast' grains and small 'slow' grains are fully exposed, the overall grain structure of the image is less obvious because there is less micro-contrast between grain particles. When only the large grains are exposed, there are black gaps where the smaller grains are. Think of it like an LCD panel displaying a full field of white. Now imagine a bunch of dead black pixels on the panel. The overall pixel grid now looks more obvious, right? That's all it is.

 

Thank you Satsuki.

 
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#13 Mathew Collins

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Posted 28 December 2015 - 03:55 AM

Yes, to elaborate further, the smallest grains are the least efficient collectors of photons so are certain amount are not exposed enough to create a latent image and get washed away in processing, which is why black areas in the image are the clear areas on the negative.

 

So if you underexpose, after processing, you don't have as many small grains filling in the gaps between the large grains.

 

It's easier to look at how b&w negative works since it doesn't trade silver grains for color dye clouds at some step.  Silver halide grains, if they collect enough photon energy, can be developed into silver -- then the fixer and wash steps remove silver halide that didn't develop into silver, leaving only the silver.  With color film and processing, an equal amount of color dye forms with the silver, and then a bleach step converts that silver back into silver halide, which gets removed in the fixer and wash steps, leaving only color dye.

 

Bigger grains are better collectors of photons and therefore are faster than the smaller grains.

 

Thank you David.


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