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I want to do a photo shoot using film, but due to not being able to see the shots I've taken I would first like to take a digital photo, then recreate it using film.
If I used the same focal length, ISO, aperture size and shutter speed, would the photo was taken digitally be exposed the same as the film photo? Maybe crop vs non crop sensor may affect things?
If not, what can I do to achieve similar levels of exposure (I don't just want to rely on the film cameras light meter all the time)?
Even in theory there are differences in the way digital sensors and films record light that makes ISO values only approximate. But these differences are usually fairly subtle and theoretically exposure should be more or less equal if you use the same ISO, aperture, and shutter time. For more about this, please see: Why are these film photos brighter than digital photos taken at the same time with the same settings?
In practice there are even greater differences that may affect each of these basic components of exposure.
ISO: Since digital sensors have a linear response to varying brightness levels of light and film has a more logarithmic response, comparing an ISO value for a particular digital sensor and an ISO value of a particular film is only approximate. This value is usually closest in the mid-tones but will vary more in the highlights and shadows.
Keep in mind that not all films with a specific ISO/ASA rating have the exact same response curves. Some may have deeper blacks and brighter highlights for higher contrast while others may have lighter shadows and more restrained highlights for less overall contrast. These curves can also be manipulated by modifying exposure times and then compensating by altering the developing time. That's basically what Ansel Adams' 'Zone System' was about.
Compound that with digital cameras that actually use different ISO values internally than they are labeled in the settings. They usually do this specifically to preserve highlight detail in the raw image data collected.
So digital cameras tend to have their actual ISO sensitivity for a particular setting rounded up. On the other hand, film manufacturers tend to round the sensitivity of their films down to the next nearest "standard" value.
With exposures for film longer than about 1 second the Schwarzschild effect, sometimes referred to as reciprocity failure, must be taken into account. The sensitivity of films at longer exposure times is not linear. This must usually be taken into account when exposing film for longer than one second. This can very significantly impact exposure times, and it varies by the specific film in question. The manufacturer of your film should be able to provide information regarding how much compensation is needed for longer exposures.
Aperture (Av): Different lenses labeled with the same aperture value may not be equally bright. This is partly due to differences in transmission loss through the various elements of each lens. But at maximum aperture it is also due to the values of each lens being rounded to the nearest or (usually) next wider standard f-number.
The differences due to transmission loss are carried across the entire range of aperture settings. The differences between stated and actual aperture when wide open tend to be reflected in successive apertures settings as well in order to preserve the differences in stops between the maximum aperture setting and the others. Sometimes the further one moves from the maximum aperture the more "honest" the actual f-number is with regard to the actual diameter of the entrance pupil relative to the lens' focal length. By the way, focal lengths are also approximated and rounded to the nearest "standard" number in the most favorable direction!
Here are the actual transmission measurements for three different Canon "L" lenses with an "f/4" maximum aperture. Even when using each of the respective lenses on the same camera, the exposure values would need to be adjusted slightly to give the same brightness of exposure.
The EF 24-70mm f/4 is essentially an "honest" f/4 lens throughout its zoom range. The EF 17-40mm f/4 is one-third stop slower at about f/4.4 and the EF 24-105mm f/4 is two-thirds stops slower at around f/5.1.
Shutter Time (Tv): Like the other two basic components of exposure, shutter times are only approximate. Even the numbers we assign to them are rounded to easy to use values.
Of ISO, Av, and Tv, the latter is usually most consistent across digital and film platforms if the camera has an electrically controlled physical shutter or a purely electronic shutter. If the film camera has a mechanically controlled focal plane shutter or iris shutter, all bets are off as to which exposure components (ISO, Av, Tv) will be most and least accurate.
In abstract theoretical terms, the exposure parameters — aperture, shutter speed, and ISO — are the same no matter the camera or lens. In an ideal sense, f/2.8 at ISO 100 and ¹⁄₁₀₀th of a second will give the same exposure in the result on any camera.
You specifically ask if the sensor size makes a difference. It doesn't, as covered at Do the same camera settings lead to the same exposure across different sensor sizes?. Exposure is per unit area, not across the whole frame.
In practical terms, cameras just aren't that precise, and any of the values can vary by a significant fraction of a stop. Digital ISO, in particular, is prone to a lot of variation from brand to brand and camera model to camera model. And, as Michael notes, there are differences in film and digital, especially regarding long exposures.
For your use of preview for film photography, though, this can work. I'd recommend shooting a roll with your preferred film in different situations, carefully recording both the digital camera's values and the settings you used on the film camera. Once you've done that, go through and compare the results; from that, you should get a general sense of whether you need to estimate up or down from the digital camera's metering — and whether there are situations which are special cases.
(See Am I wrong to judge my exposure using my smartphone? for more.)
There should be no problem trusting the exposure meter built into the camera. Film has exposure latitude that digital (still) does not. So even if exposure is not spot on, it can be adjusted in post. This is probably bad form, but because of film latitude, it's possible to meter once and use the same exposure setting throughout a shoot, as long as lighting doesn't change significantly. This is basically how disposable cameras operate as well.
There are scenes that film cameras are more likely to have difficulty metering than digital cameras. If the camera misjudges by only a few stops, film latitude should be sufficient. Otherwise, you can use exposure compensation or exposure bracketing.
Note: When I write, "same exposure setting", I do not mean that ISO, shutter speed, and aperture have to stay the same. Rather, the settings can be adjusted with respect to each other to keep exposure "constant" without referring back to the meter.
The idea is to think of balancing a math equation in terms of "stops". If one setting goes up a stop, another setting has to go down. For shutter speed and ISO, multiply and divide by 2. For aperture, the stops are at 1.4, 2.0, 2.8, 3.5, 4.0, 5.6, 8.0, 11, 16, 22. There are exceptions and further details, but this is good enough for the typical case.
Personally, I'd use aperture priority and trust the built-in meter. I have a bunch of nice pictures that were taken with a film camera that provided only a programmed auto exposure mode that could be kicked into aperture priority if desired.
A “good” exposure is defined as that that amount of light energy that produces an image with satisfactory tonalities and color characteristics. To allow repeated and duplicated results, the industry has standardized the ingredients of the exposure. 1. Scene brightness (Lux or Lumen level) as measured by photographic light meter system. 2. Sensitivity to light of film or digital sensor (ISO). 3. Shutter speed (accurate timing apparatus). 4. Capability of the lens to gather and transfer light (f-number).
Given the standardization there remain numerous pitfalls. The accuracy of brightness measurements are a variable based on the truthfulness of the instrument and the procedure performed by the user. The sensitivity of film is well regulated by the International Standards Organization, digital sensors only loosely follow these guidelines. Modern shutters are quite accurate, nevertheless there will be errors. The f-numbers are a ratio that equalizes all lenses, regardless of their dimensions. However this system is slightly flawed because the f-number does not take into account light absorption with the system.
Nevertheless, you can count on a reasonable like rendering of the subject provided the cameras are set to the same equivalent exposure. The differences encountered are facts of life. No two pictures taken with different cameras will be exactly alike. The difference is likely minuscule however when we compare film and digital there will are dissimilarities inherent to between the various medias.
