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I am new to film photography and I really don’t get it.

Say I am shooting a roll of film, the box says ISO 400, do I have to shoot the whole roll on that ISO?

I have shot a whole roll changing the ISO setting on my camera for every picture and it turned out alright. I don’t want to shoot the whole thing at the speed specified on the box only to have evening shots turn out black because they were shot at ISO 400.

When I google this there’s not a single answer. Some say that ISO setting should not be touched others say to change it depending on lighting. I don’t get it.

If it matters: I am shooting in program mode.

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    \$\begingroup\$ "I have shot a whole roll changing the iso for every picture and it turned out alright". This surprises me a bit, I would expect some over/underexposed frames. But perhaps your lab is correcting for that (automatically). This is something I would check with the photo service. \$\endgroup\$
    – Saaru Lindestøkke
    Jul 15, 2021 at 15:20
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    \$\begingroup\$ @SaaruLindestøkke Color negative film and chromogenic B&W (like XP2 Super) are famous for their latitude. Many of them can be overexposed by two or three stops and underexposed by one with no change in development and still produce acceptable prints. \$\endgroup\$
    – Zeiss Ikon
    Jul 15, 2021 at 15:58
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    \$\begingroup\$ Unless you cut up the roll after you shot it and develop each piece separately, the whole roll will go through the same chemical process to develop it. So there are limits to how much you can intentionally deviate from what the film manufacturer said the standard process was supposed to be and still get "good" results. Of course if you develop the film yourself you can do whatever you like with it, and you might want to ignore the standard process to get a particular artistic effect. \$\endgroup\$
    – alephzero
    Jul 16, 2021 at 0:16
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    \$\begingroup\$ Related: What is the relationship between film speed and the ISO setting on my film camera? \$\endgroup\$
    – Michael C
    Jul 16, 2021 at 6:05
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    \$\begingroup\$ Setting the ISO switch from, say, 400 to 200 is exactly the same as closing one aperture step, or halving the exposure time. You are nominally under-exposing. That can still result in a good picture -- typically the dynamic range of a scene, especially in sunlight, exceeds what a film can take; closing one aperture will simply shift the brightness range of the scene that does not result in either absolute white or absolute black to brighter spots. That's not "wrong", that's different, hence your experience. But typically you would use the "+/-" dial to adjust that for single pictures. \$\endgroup\$
    – Peter - Reinstate Monica
    Jul 16, 2021 at 9:47

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The ISO of a film roll indicates how sensitive that whole film roll is to light. That's a chemical property of the film roll, which you cannot change shot by shot.

The ISO "setting" on your camera does not actually set the ISO of your film, as that is physically impossible. It does tell (the light meter of) the camera what the sensitivity is of the film you're currently using. You are supposed to set it to the ISO value of your film.

In P-mode (Program mode) (and other "automatic" modes like Av and Tv), the camera uses the ISO information, to set the aperture and shutter speed for you.
It measures the light, reads the ISO you've set and then uses a combination of rules (these differ per camera brand) to choose a certain aperture and shutter speed.

In these earlier questions you can learn more about the relationship of ISO, aperture and shutter:

When you are ready to explore more advanced film techniques, you could look into push/pull processing where you use the ISO setting to "fool" the camera and then compensate for it during the processing of your film for creative effect.

Note that, if you use a digital camera, you can set the ISO for every shot as then it's not a chemical property of the sensor, but a digital value that tells the sensor how much it should amplify the signal it receives.

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    \$\begingroup\$ @Sascha I understand that, but given that the question asker was thinking that you could change the ISO on the analog camera, talking about pushing/pulling seemed beyond their current learning level. I did add a link to a question about pushing/pulling, feel free to add/improve the answers there. \$\endgroup\$
    – Saaru Lindestøkke
    Jul 16, 2021 at 13:14
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    \$\begingroup\$ @Sascha That's not quite correct. You can underexpose that film and still develop a photo from it, but you're not "pushing" the film to another ISO. The film's ISO is what it is and cannot be changed by anything the photographer does. What you're describing is simply underexposure. \$\endgroup\$
    – J...
    Jul 16, 2021 at 14:06
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    \$\begingroup\$ @J... what Sascha means is probably the possibility to overdevelop or underdevelop the film by different developer bath timing and composition. \$\endgroup\$
    – fraxinus
    Jul 16, 2021 at 15:01
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    \$\begingroup\$ @J... I think you did not understand what i said. See the other comment of the author of the answer. He did \$\endgroup\$
    – Sascha
    Jul 16, 2021 at 15:02
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    \$\begingroup\$ @fraxinus Yes, I understand. I'm saying that calling underexposure "pushing the film to another ISO" is technically an incorrect interpretation of what's going on. Nothing is changing the ISO of the film, it's simply being underexposed and then overdeveloped. \$\endgroup\$
    – J...
    Jul 16, 2021 at 15:32
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In addition to Saaru's good answer, I wanted to point out that the reason you might have been switching around the ISO setting and not really noticing much difference is because film has reasonably good exposure latitude, and images can be "recovered" with some success from underexposed and overexposed film. Probably the machines/operators at the lab where you are having your film processed are doing this automatically for you.

But as Saaru mentioned, you should set the ISO setting on the camera to the ISO indicated on the film, so that the camera knows what exposure is needed for your film. This doesn't at all mean that "evening shots just turn out black". In low light, you just need to use a wide aperture and/or long exposure duration (or, add artificial light). Beware of camera shake - use a tripod in low light, or rest the camera on a wall or something.

I recommend always trying to expose film properly. If you think you can get away with underexposing your film too much - just because you want to use a faster shutter speed - you will be sure to be disappointed with your results. You must work within the constraints of film photography.

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    \$\begingroup\$ Just to add (since the OP is a beginner) that the compensation would be done (automatically) when the pictures are printed, not when the film is developed. That compensation can't magically regenerate information that is not on the negative at all, if the original film exposure was completely wrong! \$\endgroup\$
    – alephzero
    Jul 16, 2021 at 0:20
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    \$\begingroup\$ And note that the compensation can go very wrong, also. I had a black cat when I was growing up--the compensation always got it wrong, turning her into a featureless black mass. Even the pro place I took such film would make the same error but their people would understand and reprint them. \$\endgroup\$
    – Loren Pechtel
    Jul 16, 2021 at 3:48
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If I have ISO 100 speed film in a camera and choose to expose for ISO 400 speed film, I'm underexposing by two stops. It is true that ISO 400 film is more sensitive (and thus brighter) than ISO 100 film. But when I change my camera's ISO setting to '400' I'm not actually changing the film's sensitivity to ISO 400 - I've still got ISO 100 film loaded! What I am doing is reducing the amount of light the camera's meter says my film needs to be 'properly' exposed. So my film winds up underexposed by two stops.

In other words, the ISO setting for film cameras doesn't affect the sensitivity or 'speed' of the film at all. Rather, it affects the calibration of the camera's meter by telling it how sensitive the film that is loaded is.

When that film is developed, it needs to be pushed two stops brighter to get more or less 'proper' exposure. But the result will be lower contrast (brighter shadows) and coarser grain than if the film had been metered and exposed for ISO 100 and normally developed.

If you overexpose your film, you would normally pull process it by developing it for less time than normal. This will tend to increase contrast in the mid-tones and dark areas and muddy the shadows, but highlights that have been totally blown will not show detail, they'll just appear to be a uniform bright gray when printed.

The problem with changing the ISO setting while shooting a roll of film (unless you are thoughtfully using it as an exposure compensation control) is that the entire roll will almost certainly be developed for the same amount of time. Unless you're willing to skip every other frame when shooting, then fumble around in the dark and count sprocket holes by touch (without getting fingerprints on the actual exposed frames) in order to clip different parts of the film apart before developing it, the entire roll gets the same amount of development.

When you do a bit of both over and underexposing on the same roll and don't tell the film processors any differently, the film will be developed at box speed. When prints are made from the negatives, the machine that scans the film will usually automatically try to adjust for under/over exposure. Since most film has a pretty wide exposure latitude, in skilled hands the printing lab can usually pull decent results out of slightly over or underexposed negatives.

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Others have explained why you should set the ISO on the camera properly to reflect the film used.

However, note that there is a different control you can use after that: exposure compensation, often with a symbol that looks like a ± or a black and white gradient.

On old mechanical systems you can see that they are actually on the same knob; the ISO setting just moves the scale on the 0-centered exposure control.

Use this control to adjust the automatic exposure or the metering display, not the ISO dial.

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    \$\begingroup\$ Honestly, I think a beginner should stay well away from exposure compensation. It's often misunderstood, even by experienced photographers. Exposure compensation is just a way to fool or override your meter when the scene demands it. But if the meter is not being fooled, don't expect exposure compensation to help. In this case, it's just a shortcut to bad exposure. \$\endgroup\$
    – osullic
    Jul 16, 2021 at 0:02
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    \$\begingroup\$ @osullic I think especially beginners should stay away from automatic exposure ;-). They don't understand it and cannot use it correctly. They should measure spots and decide aperture and time based on the measurements for the interesting points in the scene. Automatic exposure is almost never perfect for any specific scene because it is a compromise. (Yes, this is is a bit tongue-in-cheek; if a person is happy with staying ignorant and rarely shooting really good pictures that's perfectly ok.) \$\endgroup\$
    – Peter - Reinstate Monica
    Jul 16, 2021 at 9:55
  • \$\begingroup\$ Especially for outdoor photography, multi-zone metering, if available, is generally extremely reliable. I'm sorry, but this claim that you won't shoot any really good pictures relying on the camera's meter and automatic exposure system is nonsense. \$\endgroup\$
    – osullic
    Jul 16, 2021 at 10:37
  • \$\begingroup\$ In some scenes, it will be impossible to avoid either overexposing highlights, having dark areas lose all detail, or both. In order to know what exposure to use, a camera would have to know which parts of the picture should be considered interesting. \$\endgroup\$
    – supercat
    Jul 17, 2021 at 19:48
  • \$\begingroup\$ I can't imagine a camera which is pointed at a table lit by paper lanterns, no matter how brilliant it might be, being able to magically know whether the photographer is interested in the lanterns or the other objects on the table. \$\endgroup\$
    – supercat
    Jul 17, 2021 at 20:00
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On cameras without dedicated exposure compensation controls, changing the ISO setting is the way exposure compensation is done in automatic exposure modes. It sounds like that is how you are using it.

Since you are not shooting the whole roll with a single ISO setting it is obvious that you don’t have to use a single ISO for the roll.

Technically what you are doing is changing the Exposure Index (EI). Exposure Index is how you as a photographer choose to rate the film based on your photographic intent. For example when the subject is backlit or when the sky is a distraction.

ISO is a technical measure of a particular film’s sensitivity that rolls a lot of different characteristics into a single number. It ignores variations in sensitivity at different frequencies, contrast characteristics, base fog, etc.

Similarly a camera light meter is at best making an educated guess as to what you might want. Since it measures reflected rather than incident light, it can be very much wrong.

Since you are happy with the results you are getting from switching ISO (EI), there is not an obvious reason to change what you are doing. It is fine not to change something that works.

Most photographic rules are nonsense. Ansel Adam’s three volume series, The Camera, The Negative,, and The Print are full of advice and very short on rules.

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    \$\begingroup\$ A very confusing answer for someone new to film photography \$\endgroup\$
    – osullic
    Jul 16, 2021 at 0:03
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    \$\begingroup\$ Much improved answer, however... the OP may be getting acceptable results only because others are making up for the OP's bad decisions. That's a very bad rationale for just "not changing something that works". E.g. If the OP thinks that changing the ISO setting from 400 to 800 will allow photos to be taken in lower light scenarios (as someone who is used to digital might think), what the OP is doing is counter-productive! The camera is underexposing the film. The OP needs to learn how film ISO should be understood, before moving on to your (much more advanced) techniques. \$\endgroup\$
    – osullic
    Jul 16, 2021 at 17:30
  • \$\begingroup\$ @osullic: When shooting fast-moving objects at widely-varied distances from the camera in low light, having an underexposed picture which was shot with a fast shutter and small aperture may be better than having a properly exposed picture which was shot with a slower shutter or larger aperture. \$\endgroup\$
    – supercat
    Jul 17, 2021 at 19:50
  • \$\begingroup\$ "On cameras without dedicated exposure compensation controls, changing the ISO setting is the way exposure compensation is done in automatic exposure modes. It sounds like that is how you are using it." It sounds to me more like the OP thinks changing the ISO setting on the camera somehow magically changes the sensitivity of the film, similar to how changing ISO with digital cameras changes the analog amplification of the signal. \$\endgroup\$
    – Michael C
    Jul 18, 2021 at 2:54
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    \$\begingroup\$ @BobMacaroniMcStevens Neither do I, at least not until they continually demonstrate time after time that they are. But it is not assuming someone is stupid because they indicate they do no know something they haven't had a chance to learn yet. Instead it's understanding what they are actually asking from the place they are currently in instead of assuming they already know whatever you might know. \$\endgroup\$
    – Michael C
    Jul 18, 2021 at 4:04

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