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Film Basics • 3 Things to Know About ISO/Film Speed

Updated: Sep 19, 2023

In this series of Film Basics posts, we'll be going over some foundational film photography info for beginners to get familiar with how it all works and demystify some of the different concepts and terms that you'll hear as you start your film journey.

Today we are going to be going over three useful things to know about ISO/Film Speed. (In most cases, the terms ISO, ASA, and Film Speed can be used interchangeably because they all refer to the rated sensitivity of the film stock that you are using in your camera.)

#1, The Higher the Number, The More Sensitive to Light

Films come in different sensitivities, the higher the number, the more sensitive to light a given film is. You can imagine that if you're shooting outdoors on a bright sunny day, you'll have more light available than if you are indoors at a dimly lit concert. To make sure that you're able to get good exposures on your film, you want to make sure you're using a film with a sensitivity that is appropriate for the situation. To find out how sensitive your film is, just look at the box or on the roll itself and you'll see a number that usually ranges from 50 to 3200. The most common sensitivities are 100-400 with the films at the lower (50) and higher (3200) extremes being more specialized.

A great place to start out is with a ISO 400 film because it is sensitive enough to give you good images in many different conditions and can even work at night if you use a flash! If you know that you'll be shooting mostly in brighter conditions, you can load a lower ISO film in your camera. If you'll be shooting in heavy shade, at night, or need to freeze action, grab some higher ISO film and you'll be good to go.

#2, Appearance of Grain Varies with Film Speed

Generally, as films get more sensitive to light, they have stronger grain. This means that if you're looking for smooth and crisp images, you probably want to try a low ISO film like Kodak Ektar 100 or Ilford Pan F 50. If you want your images to have more visible grain and you enjoy some texture in your images, higher ISO films like Kodak Portra 800 or Ilford Delta 3200 are the way to go. Many of the most popular mid-speed films like Fujifilm Superia 400 and Ilford HP5+ 400 have a great balance of grain and clarity. Remember that since the ISO of the film determines both the light sensitivity and the appearance of grain, you won't be able to get a high ISO film with very low grain or a low ISO film with extremely prominent grain.

#3, You Can't Change Film ISO Mid-Roll

Unlike digital cameras, which let you change your ISO between every photo you take, shooting with film means that the entire roll needs to be shot at one ISO. Since each film roll gets processed with the whole roll remaining in the developing chemicals for the same amount of time, the whole roll will always only be one ISO speed. This means that your choice of ISO starts with the film you choose.

If you put a roll of ISO 200 film into your camera in the morning and later that evening you realize that you want to use ISO 800, you'll have to finish off that ISO 200 roll, and change it out for a new roll. If you leave the ISO 200 film in your camera and just change the ISO on your camera, the actual sensitivity of your film will not change and you will get muddy, underexposed photos!

That said, there is a lot more to talk about when it comes to how to rate your film or push process your film to different ISO ratings so once you've gotten comfortable with shooting your rolls at the ISO it says on the box, you can explore those other techniques.

If you have any questions about which film to use or want to know more about any of the info above, let us know! We'll have more Film Basics for you and some more in-depth technique articles soon!

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