Monday, August 30, 2004

What is Push and Pull


Mark M. Hancock / © The Dallas Morning News

Brass players of the Carroll High School marching band rehearse for the homecoming performance at Dragon Stadium in Southlake on Wednesday, September 17, 2003.

This post is about film not light. OK. It's about light, but not metering. OK. It's about metering, but doing so incorrectly. OK. It's about doing so incorrectly but with knowledge of expected results. OK. It's about light metering again.

I know I should break this down into more reasonable entries, but frankly, I'm tired of this subject and want to move on to something else. When your brain explodes, stop reading, clean the walls and come back to this another day.

Use caution before pushing or pulling film
New PJs often hear experienced PJs talk about "pushing" or "pulling" film to make it do something it shouldn't otherwise do. Most frequently, the term "push" arises in sports photography (particularly nighttime football and soccer).

Before anyone gets a brilliant idea to push film like the old-timers, they might want to know how the photographic process actually works and what the old-timers are doing to accomplish this magical feat.

Expose for shadows, develop for highlights
The rule of classical fine art photography is: "expose for the shadows and develop for the highlights." This works fine on hand-processed, B&W sheet film and relatively well on 35mm B&W film. It gets a bit trickier on color film. It gets really crazy on high-speed film, and it leans toward impossible with digital cameras.

However, it still teaches PJs some important lessons about how the photographic process works. With film, the exposure is critical because this sets the shadow detail. There’s absolutely no way to add shadow detail if the image is underexposed.

There is, however, a way to hold down the highlights or “flatten” the image while chemically processing. This is when the PJ “pulls” the development time (and/or temperature with B&W) to reduce the amount of oxidization on the film (ie. density). The end result is a photograph with both shadow and highlight density and detail.

How film exposures work
Exposures are cumulative like arsenic. As light is directed onto the film by the lens, it builds from emptiness to full. Sections of the scene which don’t get much light are represented as black on the final product (print, slide or digital file). Sections of the scene which received a maximum threshold of light (on a properly exposed image) are represented as white on the final product.

Without getting too much into negative film composition and density, those areas which have the highest concentrations of light (and silver) are more opaque. Those areas which have the lowest concentrations of light (and silver) are more transparent. It is the opposite or negative of the same info above.

If a camera is set below the appropriate exposure (in all cases), the shadow detail will be lost in blackness. If the camera is set above the proper exposure, highlight details will blow out to white.

How film processes
Exposed film holds a latent image. This image must be realized through a chemical oxidization process. I'll skip the chemistry lesson and concentrate on the process.

As the latent image is processed, density grows in the areas of greatest exposure. The development is equal across the film surface until the shadow detail development is complete. Then, the shadows stop adding density while the midtones and highlights continue to add density.

As film remains in the "soup" (developer), more density is added to the midtones and highlights. As the midtones near proper density, the highlights continue to develop at a high rate while the midtones develop at a lower rate. Finally the highlights reach maximum density (white) and the film is then moved to a different chemical to stop development and then to "fix" the image onto the film substrate. These latter two chemicals are commonly called the "stop bath" and "fix." Again, I'll spare you the blow-by-blow of the whole process.

After a standard development of properly exposed film, the PJ is left with complete shadow details and spectral highlights with an even tonal range across the image.

How film reacts when pushed
When film is pushed too far, it whips out an Uzi and pops some caps. I can’t be serious for too long. Sorry. I’m better now.

When film is pushed, the PJ has chosen to allow the highlights to develop beyond the suggested time and build additional density. Spectral highlights can be absorbed by surrounding areas, and other areas, which would be white with detail, become solid white.

Since shadow detail is already set, this only increases the overall contrast of the final image. Again, it creates NO useful change in the shadow detail.

Pushing film has specific uses for the PJ. It can increase the contrast of flat light or an even tonal scene. However, it's most often used as listed below, BUT with proper knowledge of the outcome.

How PJs push film
Sometimes the light at a venue simply sucks. If the coach is using a cigarette lighter to read the playbook, the PJ is in for a fun night. Nonetheless, images are due in the system at 10 p.m. sharp.

So, the PJ meters the light, does EV calculations and finds the readings equal a cave inadequate. S/he attaches the dedicated flash and hooks up the turbo to keep it firing at a decent rate for the evening. Note: while the flash adds to overall film exposure, it really kicks up the highlights due to water/sweat’s reflective properties. This will be important in a moment.

Consequently, the PJ must decide to push the film or not. If both teams are known for their defenses and wishbone offenses, there's no other option (because they sit in the middle of the field and pound on each other all night).

Next, the PJ must determine how much to push the film. If this is a soccer game in Norway (typically highly reflective skin tones), go for three stops. If this is a soccer game in the Democratic Republic of the Congo (typically non-reflective skin tones), one stop is probably too much. Either way, all the frames will get the same amount of development, so the decision must be standard for the entire roll of film.

My suggestion is to bump up each roll as the light fades, but shoot like a maniac while there is still some ambient light at the beginning of the game. Clearly mark how many stops each roll was pushed to avoid permanent errors, which can’t be corrected.

Development
Develop as appropriate. Get the correct compensation numbers from two sources on the Internet or film charts before attempting a push. Shoot and push a test roll before leaving it to chance during deadline.

On high-end machines, use the rpms instead of the standard gauge for compensation. On low-end machines, turn off the machine while the film is still in the developer. For manual development, compensate exactly as is directed on film and/or chemistry charts.

I’m smiling because I remember the ancient days (a few years ago) waiting for the processor to complete someone’s +3 film so I could do my +1 film and then next person could do their +2 film while someone else was finishing their +0 film on the other machine, but we all (15+ PJs) had to turn out the images by 10 p.m. Maybe digital isn’t so bad after all.

The end result
When the film finishes drying, the PJ finds out how good or bad her/his decisions were. The end result will be an image with extremely hot highlights and few shadow details. If the action captured is compelling enough as uniformed, faceless beings (mid-air collisions), it was probably a good decision. If the game was a ground war with only streaks of reflected sweat inside glowing-white helmets, it was probably a bad decision.

Enough for now,

4 comments:

Flamenca said...

which film do you recommend for pushing? Im doing work in tropicsl climate, and I want to try a film that is dramatic-so far I havent foun any-Ive been using portra from kodak and some fuji, should I try a specific pro film..
thanks

Mark M. Hancock said...

Any film can be pushed. The result is the same: increased contrast and lost detail. Instead, use appropriate film speeds for light levels. If you want extra contrast, it can be added while printing or using Photo Shop.

Eirik said...

Thanks for the article Mark.

if I just want my photos to have low contrast and look slightly overexposed, should I:

- shoot ISO800 at ISO800 in camera, then process it at ISO400 (is this pulling 1 stop?); or
- shoot ISO800 at ISO400 in camera, then process it at ISO800

I hope I make sense.

Mark M. Hancock said...

Hi Eirik,
It's always best to shoot ideal frames if possible. This makes them useful later as well. When you push or pull images, you're losing something.
Shoot 400 or 200 iso on a tripod. To achieve low contrast, shoot in completely shaded places (open shade) or on cloudy days.
If you're shooting film, pulling film is the better option. To pull film (for reduced contrast), you would overexpose and under develop by controlling either the RPMs in a commercial processor or by reducing the developing time in a tank. It can also be done by cooling the chemicals more than recommended, but this can create color shifts so you will need to run some tests before an important shoot.
Another option is to flatten the contrast ratio in Photoshop by using the highlights and shadows control. Since this isn't manipulating the pixels, it's an acceptable PJ toning technique.
To make the images appear overexposed, you can adjust the curves in Photoshop and increase the highlight input while reducing the shadow input (to keep your blacks from washing out).