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 COLM CASSERLY |
Film ASA/ISO?? Hi, this might sound silly but I dont really understand ASA and film speeds. I thought it was simply a case of matching the number on the kodak film box (ie 400) with the setting on my camera at 400. but I read people on this forum suggesting changing the ASA on the camera to achieve different effects? such as for waterfalls to gain a veli-like effect. what does all this mean?? also on this matter - example: if I want to take a photo of someone but the meter on my camera says to use no higher than 1/4 shutter speed at highest appurture (f3.5)(which would mean camera shake without a tripod), can I compensate this and get correct exposure by changing the ASA setting? or maybe by using the +/- exposure compensation dial. im not sure what this does exactly. the nikon manual book says its for things like shooting a black cat in the snow?!?! ps I have a nikon fe2 manual slr. its tricky to use but I really want to learn to use it to its fullest. I appreciate any help guys, thanks a million
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Christopher A. Vedros
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Film speed (ASA or ISO) is a standardized way of measuring the light sensitivity of a certain film. ISO 400 speed film is twice as sensitive to light as ISO 200 speed. Therefore, if your camera meter indicates 1/4 second shutter speed at f3.5 with ISO 200 film, it would indicate 1/8 second shutter speed at f3.5 for the same scene with ISO 400 film. For the most part, it is simply a case of matching the film speed with the setting on your camera. In almost all cases, that is what you do. There are some cases where you can shoot a roll of film at a higher or lower ISO setting than what is on the box. This is called pushing or pulling the film. This can only be done if you shoot the whole roll that way, and then you need to give instructions to the developer, so that they can adjust the development to account for it. Some photographers will push film to a higher ISO to shoot an indoor wedding without a flash, for example. Some films respond better to this than others. The thing to remember is that you can only do this if the whole roll is shot that way and developed accordingly. In your example, when your meter indicates 1/4 second shutter speed at f3.5, changing the ISO setting will only help if you can shoot the whole roll that way. Besides, since each jump in film speed will double your shutter speed, you would have to push the film 3 or 4 stops to get up to a safe hand held speed, and film is usually only pushed 1 or 2 stops. The Exposure Compensation dial is used in situations where the amount of light reflecting off a subject could fool the camera meter into recommending an over or under exposure. A camera's meter is calibrated to measure light as if it is reflecting off of a medium gray object. To the camera's meter, a black cat will look like it is not reflecting enough light, so it will indicate settings that will end up overexposing the cat, making it look more gray than black. Snow, on the other hand, reflects more light, so the camera will try to underexpose it to make it look more gray than white. As you learn to recognize these scenes, where you know that your camera is metering something that is very dark colored or very light colored, you can dial in an intentional underexposure or overexposure in order to properly expose the subjects. These are just the basics. A book like Bryan Peterson's "Understanding Exposure" would be a great learning tool for you. I've read it several times. Good luck.
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| Vince Broesch |
Film is given an ASA by the engineers who design it. It is therefore, the manufacturers recommended amount of light that will result in the best image. But there is also a tolerance, you could be under-exposed or over-exposed, and the film’s exposure latitude will still give you an acceptable picture. The labs automatic printer will correct for color negative film which is under-exposed by about 2 stops to over-exposed by about 3 stops, and will still yield an acceptable print. You’ll hear a lot about “pushing” and “pulling” which means intentionally under-exposing or over-exposing the film, and then compensating for that error by changing the processing of the film. With color negative film, they are just taking advantage of that built-in latitude which is there anyway. Color negative film really can’t be pushed or pulled, the attempt will change contrast (HD-LD) but not speed (LD). LD is the key issue. I have worked in photo labs for 30 years and I have a recent degree in digital imaging technology. I don’t claim to know it all, but my recommendation has been the same for 30 years... shoot film at the manufacturers recommended ASA, the engineers who designed that film where not bone-heads. B&W or E-6 process films can be pushed or pulled with acceptable results. Vince
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Gregory LaGrange
gregorylagrange.org |
There's ASA, and then there's NASA.
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Kevin Ekstrom
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ASA = American Standard Association. ISO = International Standards Organization. Everyone has pretty much covered the bases here. Just keep in mind that ISO numbers are in direct relationship, just like shutter speeds and f-stops. So 100 is twice as fast as 50 and so on.
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Jon Close
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Re - >>"...but I read people on this forum suggesting changing the ASA on the camera to achieve different effects? such as for waterfalls to gain a veil-like effect."<< That particular question came from a user with a digital camera. With a digital one is free to change the ISO from shot to shot. Changing the ISO setting on a digital not only adjusts the meter's calibration, it also changes the digital sensor's light sensitivity. With a film camera, changing the ISO setting changes the meter calibration only. The light sensitivity of the film in the camera does not change. While one can push- or pull-process the film to adjust for different ISO settings, such processing (as Chris V. noted) can only be applied to the entire roll. It is not possible to select individual frames on the roll to push, pull, or develop normally.
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| COLM CASSERLY |
Thanks a million guys, cleared it up perfectly!
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John A. Lind
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The ASA and ISO film speed standards are different. When ISO adopted its standard for film speed, it started with the ASA standard and made some changes in the specific lab testing procedures to arrive at a film's speed rating. I freely admit it's inconsequential in practice by photographers using the film. An ASA speed rating number can be considered the same as an ISO speed rating number. The two different methods arrive at essentially the same result (same number), which was one of ISO's goals. It is refreshing to see it referred to as "ASA" periodically . . . I occasionally slip and use "ASA" when talking to someone about film speed, and if they're young enough it results in a rather puzzled look. Gregory, -- John Lind
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