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What is ISO in Film Photography?

And how does it impact your exposure?

ISO is a standard system of measurement for how sensitive a photographic film is to light. It is one of the three main factors that determine the exposure of a photograph, along with aperture and shutter speed. The higher the ISO number, the more sensitive the film is, and the less light it needs to produce an image. The lower the ISO number, the less sensitive the film is, and the more light it needs to produce an image.


As we explored in The "Correct" Exposure of Film, film has a certain exposure latitude around which you can very easily achieve acceptable images. So if that's the case, what does this ISO rating actually mean, and do you have to rigorously adhere to it?


A box full of 35mm film canisters.
What does the number on your film canister actually mean? Photo by @brianhuynh/Brian Huynh on Unsplash

ISO stands for International Organization for Standardization, which is a body that sets various standards for different fields and industries. The ISO system for film speed was introduced in 1974, and replaced the older systems of ASA (American Standards Association) and DIN (Deutsches Institut für Normung), which were based on different scales and methods of measurement. The ISO system is also used to measure the sensitivity of digital camera sensors, although there are some differences between film and digital ISO.


The ISO rating of a film is usually indicated on the film box and the film canister, along with other information such as film type, format, expiry date, and number of exposures. The most common ISO ratings for film in the modern age are 100, 200, 400, and 800, although there are also films with lower ratings, such as 25 (e.g. Kodak Technical Pan), 32 (Kodak Panatomic-X) and 50; and there are films marketed as 1600, or even 3200 speed, though these are exposure index ratings by the manufacturers and involve essentially push-processing as the normal process.


Generally speaking, films with lower ISO ratings are finer grained, sharper, and more saturated, while films with higher ISO ratings are coarser grained, softer, and less saturated.


 

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How can I determine my own personal Exposure Index for a film?

A set of black and white negatives being examined on a light table.
Check your negative density by shooting brackets of the same image at different effective speeds. Photo by Annushka Ahuja on Pexels

The ISO rating of a film is not an absolute value, but rather a guideline based on standardised testing conditions and methods. Different photographers may have different preferences and expectations for how they want their images to look (similarly to how they may not adhere to the rules of composition and style), and may also use different cameras, lenses, filters, light meters, developers, and printing methods. Therefore, it may be useful to test for your own personal ISO rating for a film, which is also known as your personal exposure index (EI).


To test for your own personal EI for a film, you will need to shoot a series of test shots using different exposure settings on your camera. You will also need to use a consistent light source and subject matter for your test shots. For example, you can use a gray card or a colour chart as your subject, and shoot it under daylight or artificial light. You will need to shoot at least one frame at the box speed of the film (the ISO rating indicated on the film box), and then shoot additional frames at different exposure values (EV) above or below the box speed. For example, if you are testing a film with a box speed of ISO 100, you can shoot one frame at EV 0 (ISO 100), one frame at EV +1 (ISO 50), one frame at EV -1 (ISO 200), one frame at EV +2 (ISO 25), one frame at EV -2 (ISO 400), and so on. You can use an exposure calculator or an app to help you determine the correct aperture and shutter speed combinations for each EV.


After shooting your test shots, you will need to develop them using your preferred developer and method. You will also need to make sure that you use the same development time and temperature for all your test shots. After developing your test shots, you will need to examine them using a loupe or a scanner to see which frame has the best exposure for your liking. You will need to look at factors such as contrast, tonality, detail, grain, sharpness, and colour rendition. The frame that has the best exposure for your liking will indicate your personal EI for that film. For example, if you find that the frame shot at EV -1 (ISO 200) has the best exposure for your liking, then your personal EI for that film is ISO 200.

 

Pushing and pulling film are techniques that involve deliberately exposing a film at a different ISO rating than its box speed, and then compensating for it during development. Pushing film means exposing a film at a higher ISO rating than its box speed, and then developing it for longer than normal. Pulling film means exposing a film at a lower ISO rating than its box speed, and then developing it for shorter than normal.


The purpose of pushing and pulling film is to achieve different creative effects or to cope with different lighting conditions. For example, pushing film can be used to create images with more contrast and grain, or to shoot in low-light situations without using a flash or a tripod. Pulling film can be used to create images with less contrast and grain, or to shoot in bright-light situations without using filters or stopping down the aperture.


Pushing and pulling film can also be done partially or selectively. For example, you can push a film by one stop during exposure, and then pull it by half a stop during development. This way, you can achieve a compromise between increasing the effective film speed and maintaining some tonal range.


Pushing and pulling film are techniques that require some trial and error and careful control over the development process. They also affect the final image quality in terms of contrast, tonality, detail, grain, sharpness, and colour rendition. Therefore, pushing and pulling film are techniques that require some trial and error and careful control over the development process. They also affect the final image quality in terms of contrast, tonality, detail, grain, sharpness, and colour rendition. Therefore, they should be used with caution and discretion, and only when necessary or desired.

 

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Does black and white developer choice impact film speed?

A safelit photographic darkroom with measuring jug, squeegee and developing tank set out on developing trays.
Developer choice has an impact on film speed. Photo by Tima Miroshnichenko on Pexels

One of the factors that can affect the film speed - in black and white film photography certainly - and thus the actual ISO rating achievable is the type of developer used to process the film. Different developers have different characteristics and effects on the film emulsion. Some developers are more active or energetic than others, meaning that they can develop the latent image on the film faster or slower. Some developers are also more contrasty or compensating than others, meaning that they can affect the tonal range or gradation of the image.


Broadly speaking, where you use a more active or energetic developer, you will tend to increase the effective film speed or sensitivity of the film emulsion, and they can then produce acceptable images with less exposure or light than normal. However, with less active or energetic developers you will mostly decrease the effective film speed, and so you will require more exposure or light than normal to produce acceptable images.


In the energetic developer category, you have Rodinal, D-76, and HC-110 (at higher concentrations) for example. These developers can increase the effective film speed by one or two stops, depending on the dilution and development time used. So, if you use Rodinal at 1+25 dilution and develop for 6 minutes, you could theoretically increase the effective film speed of an ISO 100 film to ISO 200 or even 400 at a push. On the other hand, less active developers include Perceptol, Microphen, and Xtol, among others. With these developers, you may see a one - or even two - stop film speed loss, depending on the dilution and development time used. For example, if you use Perceptol at 1+3 dilution and develop for 12 minutes, you can decrease the effective film speed of an ISO 100 film to ISO 50 or 25.


On the other hand, more energetic developers will lead in general to an increase in the contrast of the image. This means that they can produce images with more punch or drama, but also with less tonal range or gradation. And similarly, more compensating (or less active) developers tend to decrease the contrast of the image, and so you can produce images with more tonal range or gradation, but also with less punch or drama.


 

Shooting Kodak Portra 400 at 200 or 100

Another example of how ISO rating is routinely ignored by film photographers is the practice of shooting Kodak Portra 400 at 200 or 100. Kodak Portra 400 is a popular colour negative film that has a box speed of ISO 400. However, many film photographers prefer to shoot it at ISO 200 or 100, which means overexposing it by one or two stops. The reason for this is to achieve a certain aesthetic or look that is characterised by pastel tones, soft colours, low contrast, and fine grain.


Shooting Kodak Portra 400 at 200 or 100 is possible because of the high latitude or exposure range of the film. Latitude is the ability of a film to produce acceptable images even when overexposed or underexposed by several stops. colour negative films generally have higher latitude than black and white films or colour slide films, which means that they can tolerate more exposure errors or variations. Kodak Portra 400 has one of the highest latitudes among colour negative films, which means that it can produce acceptable images even when overexposed by up to five stops or underexposed by up to three stops.


A cloud formation photographed on Portra 400 film with rebate and sprockets showing.
Kodak Portra 400 is a popular film stock among the new generation of film photographers. Photo by Chay García on Pexels

However, shooting Kodak Portra 400 at 200 or 100 is not without consequences. Overexposing the film by one or two stops means that the film receives more light than it needs to produce an image. This results in a loss of highlight detail, a reduction of contrast, a shift of colour balance, and an increase of grain. These effects may be desirable for some photographers who want a certain look or mood for their images, but they may not be suitable for others who want more accurate or realistic colours and tones.


Therefore, shooting Kodak Portra 400 at 200 or 100 is a matter of personal preference and artistic expression. It is not a rule or a requirement for using the film. It is also not a guarantee for achieving the best image quality possible with the film. It is simply one way of manipulating the exposure and development process to create a different outcome than what the film was originally designed for.

 

Is ISO a hard and fast rule, then?

As we have seen in the previous sections, there are many factors that can affect the film speed and thus the actual ISO rating achievable for a film. These factors include personal preferences, exposure settings, light sources, subject matter, developers, development times, development temperatures, printing methods, and so on. Therefore, ISO should only be considered a guide and not an actual hard and fast rule for determining the exposure of a photograph.


ISO is a useful tool for helping us to estimate how much light we need to expose a film properly. However, it is not a precise or accurate measure of how sensitive a film is to light in every situation. Different films may have different responses to light depending on their emulsion type, colour balance, grain structure, latitude, reciprocity failure, and so on. Different photographers may also have different tastes and expectations for how they want their images to look in terms of contrast, tonality, detail, grain, sharpness, colour rendition, and so on.


Therefore, it is important to experiment with different films and different exposure settings to find out what works best for us in different situations. It is also important to test for our own personal EI for each film we use, and adjust our exposure accordingly. By doing so, we can achieve more consistent and satisfying results with our film photography.



How does the age/expiration of your film affect film speed?

Another factor that can affect the film speed and thus the ISO rating achievable for a film is the age and expiration of the film. Film is a perishable product that has a limited shelf life. Over time, the chemical components of the film emulsion degrade and lose their sensitivity to light. This results in a decrease of effective film speed or sensitivity of the film emulsion.


The expiration date of a film is usually indicated on the film box and the film canister, along with other information such as film type, format, ISO rating, and number of exposures. The expiration date is an estimate of how long the film can be stored under normal conditions (usually at room temperature) before it starts to deteriorate significantly. However, the expiration date is not an absolute deadline for using the film. It is possible to use expired film and still get acceptable images, depending on how much the film has degraded and how it was stored.


The rate and extent of degradation of a film depends on several factors, such as original film speed, emulsion type, storage conditions, and so on. Generally speaking, faster films (higher ISO ratings) degrade faster than slower films (lower ISO ratings), because they have more silver halide crystals in their emulsion, which are more prone to oxidation. Colour films degrade faster than black and white films, because they have more layers and dyes in their emulsion, which are more sensitive to heat and humidity, and the ability to pull an acceptable image out of a colour negative from a film that has expired is complicated by the colour balance, since the different colour layers degrade at different speeds. Slide films degrade faster than negative films, because they have less exposure latitude and more complex colour balance.


Storage conditions also play a significant role in determining how fast and how much a film degrades. The main enemies of film are heat, humidity, light, and radiation. These factors can accelerate the chemical reactions that cause the film to lose its sensitivity to light. Therefore, the best way to preserve the quality and longevity of a film is to store it in a cool, dry, dark, and shielded place. For example, a refrigerator, a freezer, or a lead-lined container.


A common rule of thumb for determining how much the effective speed of a film decreases once it expires is to subtract one stop per decade from its box speed. For example, if you have an expired ISO 100 film that was manufactured in 2010 and expired in 2020, you can assume that its effective speed is now ISO 50. However, this rule is only an approximation and may not be accurate for every case. As mentioned earlier, different films may degrade differently depending on their characteristics and storage conditions. Therefore, it is advisable to test the actual speed of an expired film before using it for important or critical shots.


To test the actual speed of an expired film, you can use a similar method as testing for your own personal EI for a film, as described in a previous section. You will need to shoot a series of test shots using different exposure settings on your camera, and then develop them using your preferred developer and method. You will then need to examine them using a loupe or a scanner to see which frame has the best exposure for your liking. The frame that has the best exposure for your liking will indicate the actual speed of the expired film.

 

In conclusion, ISO is a standard system of measurement for how sensitive a photographic film is to light. It is one of the three main factors that determine the exposure of a photograph, along with aperture and shutter speed. However, ISO is not an absolute value, but rather a guideline based on standardised testing conditions and methods. Different factors can affect the film speed and thus the actual ISO rating achievable for a film, such as personal preferences, exposure settings, light sources, subject matter, developers, development times, development temperatures, printing methods, age, expiration, and so on. Therefore, ISO should only be considered a guide and not an actual hard and fast rule for determining the exposure of a photograph.


ISO is a useful tool for helping us to estimate how much light we need to expose a film properly. However, it is not a precise or accurate measure of how sensitive a film is to light in every situation. Different films may have different responses to light depending on their emulsion type, colour balance, grain structure, latitude, reciprocity failure, and so on. Different photographers may also have different tastes and expectations for how they want their images to look in terms of contrast, tonality, detail, grain, sharpness, colour rendition, and so on.


Therefore, it is important to experiment with different films and different exposure settings to find out what works best for us in different situations. It is also important to test for our own personal EI for each film we use, and adjust our exposure accordingly. By doing so, we can achieve more consistent and satisfying results with our film photography.


ISO is also not a fixed or immutable value, but rather a variable or flexible one that can be manipulated or modified by various techniques, such as pushing and pulling film, shooting at different speeds than the box speed, or using expired film. These techniques can be used to achieve different creative effects or to cope with different lighting conditions. However, these techniques also have consequences and trade-offs on the final image quality in terms of contrast, tonality, detail, grain, sharpness, and colour rendition. Therefore, they should be used with caution and discretion, and only when necessary or desired.


In summary, ISO is a great starting point and pretty much every beginner film photographer should shoot their film at box speed, and understand how their film performs at box speed. However, when more familiar, shooting at a different speed than the ISO given on the box can be an effective creative control in the film photography process. ISO is not a rule or a requirement, but rather a tool or an option that can help us to create better images with film.

 

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Rated 5 out of 5 stars.

Great article and very informative

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Michael Elliott
Michael Elliott
Oct 23, 2023
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Thank you so much for your comment, David! You can find more informative articles in the archive! Cheers 😊

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