Question About EI
Hi, I am brand new to APUG and to large format photography. In fact, my Pacemaker Speed Graphic just arrived yesterday. There are some terms I am unfamiliar with and I was hoping someone on this forum could help. There was a gorgeous photo on Flickr I really liked (http://www.flickr.com/photos/ulvo/4291950562/) In the description it said:
Fomapan 100 @ EI 50 (over-exposed by one stop)
I know the film, but I'm unsure if EI means exposure index??? And how does one get EI 50 on a Speed Graphic? This isn't Aperture or Shutter speed, is it? I'm so sorry, but I really don't get what they mean and I've been seeing EI a lot!! I'd really appreciate it if anyone who would be kind enough to shed some light on what this really means and what they're doing. Even enough so I can continue my research......thanks!
You have it right. EI means "exposure index." Technically, any time you expose at other than the speed specified as ISO, you are not exposing at ISO. There is only one ISO because that's a standardized speed with strictly defined criteria. Anything else is an exposure index.
It refers to the film speed. You "get" EI 50 with any camera the same way - you take a meter reading, either with an internal meter or, in the case of a Speed Graphic with an external one, and set the film speed to that speed, or adjust from a different setting to get that effective setting. With a manual camera you transfer the shutter speed and aperture for that exposure to the camera lens and shutter.
Yes, EI is Exposure Index. It's a description of how much you exposed the film (bigger numbers are less exposure), so it is a function of the meter reading, the aperture and the shutter speed.
Simplest way to achieve a specific EI is to set your meter's ISO setting to that EI, take a light reading and put those settings (aperture and shutter) onto the camera.
ISO is a definition of how much exposure a film should get for normal exposure and development, EI is how much exposure you actually give it. Most people find that Fomapan is quite optimistic in its ISO rating, i.e. they get less shadow detail than they want, so they expose it at EI50 instead of EI100. 50 is twice as much exposure, i.e. one stop more, which gives more shadow detail and a denser negative.
Usually when adjusting EI, one would also adjust development. You don't just randomly pick an EI, you typically decide on how much contrast you need for a particular scene, which defines how much development you're going to give that sheet, which defines the appropriate EI. Have a read about Zone System and Beyond The Zone System (BTZS).
EI = exposure index, an abritrary "re-rating" of the film to suit the contrast values of a scene or your prior experience (ideally) with that particular type of scene's contrast.
Typically EI references a different film speed to provide over-or under-exposure. An example is Velvia (transparency film with a bad temper! :)) rated at EI40 is +0.3 (third-stop) over-exposure; EI32 is +0.6 (often too much!). It's never referred to as ISO40 or ISO32. Negative films have a lot more exposure than slide films, and so an EI is often given in the range of one or two stops above or below, then development adjusted to suit. EI has more use in LF where sheet film allows individual development to a very fine degree. With roll film (120, 35mm) you are simply giving more or less exposure to suit and possibly even then specifying push or pull processing (something else for you to learn ... later!)
The best way to learn about exposure index is to load some film up, re-rate it (e.g ISO 100 film to EI 200, maybe other indices), expose it normally but importantly, keep notes as you go along so you can reference them examining the negs/trannies.
EI = speed setting in analogy to ISO, BUT based on your specific process, requirements and expectations
Very simply ISO is the film speed that the manufacturer determines for a film. EI is the speed which produces the best results for your camera and processing method.
In Canada, it means "Employment Insurance" :).
As others have posted, it means Exposure Index when you are talking about film photography.
It is the light sensitivity rating for the film you use with your meter. Sometimes referred to as the "speed" of the film.
It reflects your practice, your exposure preferences and your equipment. It is usually close to or identical to the standard ISO ("box") light sensitivity rating for the film, but not always.
Originally Posted by MattKing
Welcome to APUG, Squirt! You have asked an important question, and you got good answers here. If you stick with this sort of photography for a while, you will have many more questions about EI, ISO, development times, CI, gradients, and much much more. It may help you to get along this wonderful journey to read up something that will explain it all in a gentle, concise and authoritative way. One of the best such reads is a set of three books by Ansel Adams, titled: The Camera, The Negative, and The Print. Get the more recent editions.
Enjoy, and keep asking questions.