Perfect Exposures with Large Format Cameras
© 2006 KenRockwell.com

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INTRODUCTION

Correct exposure with large format cameras involves more issues than other film and digital cameras. Don't worry, Ansel Adams and everyone has dealt with these for over 100 years. They are:

1.) No through-the-lens (TTL) metering. You have to compensate manually for anything, like filters, which might alter the light coming through the lens.

2.) No Matrix or Evaluative metering. You have to compensate for light or dark subjects manually.

3.) Mechanical shutters, not electronic. Mechanical shutters work like wind-up mechanical watches. They often vary from their marked speeds.

4.) Large format cameras use bigger film. They are working at larger magnifications for any given composition. They may need manual exposure correction at close distances.

1.) NO TTL METERING

Since you're not metering through the lens, you must compensate manually for any filters.

Use the manufacturer's suggestions for most colored filters. Polarizers can vary a half stop from the specifications, so measure your own filter factors for best results.

This is easy. Make test shots at different exposures and keep notes. The correct filter factor is the factor which, added to the exposure when shooting through that filter, gives the same exposure as with no filter.

Make some shots at different exposures with the filter and one shot without. Use the factor correlating to the two shots that match. Easy!

2.) NO MATRIX METERING

Matrix metering has made exposure metering simple since the Nikon FA of the 1980s. Since we don't have matrix meters in large format, we use external meters and correct them if the subject is dark or light.

A regular meter reads to make whatever you point it at look medium gray. If you have an even mix of light and dark, no problem. If the shot is mostly light or dark, use the zone system.

A meter gives you medium gray regardless of where you point it. You have to add exposure to get white to look white, or subtract to get darks to look dark. A spot meter is the most precise.

Easy!

You can point a digital camera at the subject and copy the reading. See Using a Digital Camera as a Light Meter.

You can use an incident meter (one with a white dome held in the same light as the subject and pointed back at the light) and not worry about subject lightness.

See also Exposure.

3.) MECHANICAL SHUTTERS

Mechanical shutters often vary from their marked speeds.

I bought a Calumet Shutter Tester 15 years ago. It cost $69.95. Today the same thing sells for $109.99. I test each shutter and tape a correction table to each lens board.

if you're too cheap to buy a tester, bribe a camera repair shop.

If you're too cheap to bribe a repair shop, use these values. Most lenses do this:

1 sec. : fast 1/6 to 1/3 stop

1/2 sec.: fast 1/6 to 1/2 stop

1/4 - 1/125: usually OK.

1/250: add 1/3 stop

1/500: add 2/3 to 1 full stop.

Hint: You're probably doing something wrong if you're shooting a 4x5 camera at any faster than 1/30 of a second. Even in full sun I usually shoot at 1/15!

Hint: My correction table doesn't talk in stops. That would drive you crazy with mental calculations. Instead I make mark table with the actual shutter speeds. This makes it trivial to calculate the f/stop on the exposure calculation scale on my Pentax spot meter. I avoid meters with LCD read outs. You'll probably never figure out how to use them to add and subtract all the factors. If my table says the actual speed is 1/25 of a second, I look to see what f/stop lies across from 1/25.

I work in 1/3 stops, which is how my Pentax and most meters are calibrated.

Here's an example of a correction table. I round the speeds to the nearest 1/3 stop.

4.) BELLOWS EXTENSION FACTORS

As you rack out a lens to focus closer, the light has to travel further from lens to film. F/stop is the focal length divided by the opening of the lens. The opening of a 150mm lens at f/16 is 9.4mm. (150 / 9.4 = f/16.) If you rack the lens out to focus more closely, the effective f/number becomes greater. If you rack it out 20mm more than infinity focus, you're shooting at f/18. (150 + 20) / 9.4 = 170 / 9.4 = 18.

This is almost never an issue with ordinary cameras, because their little lenses extend very little when focusing closely. Any bellows factor is corrected automatically because their meters read through the lens.

Large format cameras use longer lenses, and have to extend a much larger percentage of their focal lengths when focusing at closer distances.

For example, a 50mm lens focused at six feet on a digital SLR needs an insignificant 0.073 stop correction at this 1:40 magnification ratio. This is corrected automatically by the meter reading through the lens anyway.

A 4x5" camera needs a 300mm lens to get the same composition on its bigger film! At six feet from the lens the 300mm lens needs to be extended two extra inches to focus, for a magnification ratio of 1:5.7. This requires an extra half-stop exposure, which your hand-held meter won't see. You have to add this half stop, or have underexposed images.

Buy a calculator from Calumet. You lay a target on the subject and read the correction from a ruler on your ground glass. It's easy! Calumet sells these for $10.99.

I'm cheap and good with math, so I calculated and drew my own scales. You can find the formulae in Ansel's Book "The Negative." You read from a different scale for each lens. I draw these on a ruler I lay on my focusing bed, and read the factors from my custom scales.

5.) SWINGS AND TILTS

You can ignore this.

Tilting the film spreads the light over more area. In theory you might want to add more exposure as the cosine of the angle of incidence, but I've never found it to be an issue.

We only use a few degrees of tilt in real photography. We never use the crazy tilts of which our cameras are capable. Even a crazy 10 degrees of tilt requires only 0.02 stops more exposure.

At an unfathomable 60 degrees of tilt the light is spread over twice the area, requiring an extra stop.

Since I have my calculator out, here's a table:

Here's an example of calculating for 60 degrees:

Compensation, stops = (Log [base 2] (Cosine of tilt angle))

Compensation, stops = (Log [base 2] (Cosine of 60 degrees))

Compensation, stops = (Log [base 2] (0.5))

Compensation, stops = -1

Hint: If using swings and tilts together, calculate factors based on degrees and add the stops of compensation. For example, with 10 degrees tilt and 20 degrees swing, 0.02 + 0.09 = 0.11 stops, which is too small to worry about.

Hint: Front movement matters less than rear movement if the lens is compensated for light falloff.

Hint: logs to base 2 are calculated on a pocket calculator by hitting Log [10 or e] and dividing that result by Log [10 or e] of 2. Use the same base, 10 or e, each time.

Bigger hint: forget this section. I never come close to ten degrees of tilt in real photography.

SUMMARY

It's easy to do all this. Do this and you'll be in complete control of your exposure and nail every shot the first time.

This was critical to Ansel Adams. Ansel wrote that bracketing was for wimps, and that what he's shooting doesn't always allow time for guessing. His "Moonrise, Hernandez, New Mexico" had light changing so fast that he had only one chance. He didn't even have time for a meter reading! He used the sunny f/16 rule based on the brightness of the moon and made his exposure. (Actually Ansel worked in footcandles and placed the disc of the moon on the zone he wanted, but it accomplishes the same thing today as the sunny f/16 rule. The luminance of the moon's disc is a constant.)

PLUG

If you find this as helpful as a book you might have had to buy or a workshop you may have had to take, feel free to help me write more with a donation. Thanks! Ken.