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Context:

I recently purchased an Olympus OM-1 film camera. Upon receiving the camera, I realized the battery for the light meter was dead. I'm a (very) amateur photographer so, naturally, this is a problem for me. However, I had an idea. (For context, I've been shooting on ISO 200 film).

I own a Samsung Galaxy S8. The camera in this phone features a 'Pro Mode' which allows the user to manually adjust shutter speed, aperture and ISO and see the results live on screen, much like a DSLR.

Question:

If I set the ISO on my phone to 200 and set the aperture to the same width as what I have on the Olympus, will the correct shutter speed on the phone be the same as (or near to) the correct shutter speed on the Olympus?

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In theory, this should work perfectly. The combination of (shutter speed, aperture, ISO) determines the amount of light which falls on the sensor (per unit area), so should be transferable between devices.

In practice, there are a couple of things which mean it might not quite work:

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    May be worth adding that If the field of view of the phone is significantly different to the camera, it could meter for light variations that may not be visible to the camera (i.e. the phone could meter for the sun being visible - which could be out of shot for the camera) and vice versa. – dav1dsm1th Mar 8 '18 at 10:32
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    @PhilipKendall: Unfortunately on digital cameras the variation in claimed ISO can be a lot more than a 'few percent' out as is mentioned in the post you linked to. – James Mar 8 '18 at 11:25
  • on the other hand, exposure for film isn't nearly as critical as for digital, particulary when you err on the side of overexposure. – ths Mar 8 '18 at 12:29
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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.

Compound that with 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.

enter image description here

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.

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My first camera didn't have a built-in meter. This was in the pre-smartphone era, but I did have a light meter, so I'll answer from my experience.

The main problem I had was not getting the correct exposure for the picture, but getting lazy and not checking when I took the next picture. So as the light changed my settings didn't, and resulted in overexposure/underexposure.

Changing the battery in the camera will be a lot less work than constantly getting out your smartphone to check the exposure.

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