Talk:Film speed

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[edit] Graininess

Hi, all,
This is too much to just change in the main entry. May I have some comments so I can fix the proposed text up? My own comments are in [bracketed italics] 2B removed later. (I know there is an  !-- Invisible Note to editors -- mechanism, but these comments should be seen on the TalkPage. I'll hassle getting them out later.)

PROPOSED TEXT

Individual silver halide crystals suspended in a film emulsion aggregate into clumps ("grains") of random size. [Grains are not as regular and orderly as the crystals which lie at their heart, and at the heart of the energy storage mechanism that makes it possible to store and later develop a latent image] Even when these are not individually resolved in an image, a texture ("graininess") remains. Higher film speed brings more graininess. [nice direct statement from original more or less] Fine-grain stock, such as film used for portraiture or copying, is "slow", meaning that the amount of light used to expose it must be high or the shutter must be open longer. Fast films, used for shooting in poor light or for shooting fast motion, produce a grainy image. [Easily grasped contrast of different practical situations, also from original, more or less]

Since each grain of silver halide develops in an all-or-nothing way, photographic images actually consist of a mosaic of developed and undeveloped areas. In this sense, film is a threshold detector rather than a linear detector. [This is a delightful insight to present, at a time when so many of us are making the historic transition from "analog" to "digital". However, it didn't have enough of a "compare and contrast" discussion to clarify differences between the two ways of conceptualizing film's function.] If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. Fine gradations of grayness are also quantized, so that graininess interferes with the perception of shape from shading as well as with object contours (edges). [As the brain imposes structure on sensation to achieve perception, "shape from shading" has emerged in recent decades as an important mechanism of shape encoding from surfaces, just as contours and their elaboration in the brain is an important mechanism of shape encoding from edges, and of figure/background segmentation. Also, it makes for beautiful b/w photos :-) ] Photographers sometimes exploit film's quantization with high-contrast derivatives ("orthos") that really are only black or white, [These are typically made with photolithography film like Kodalith, hence the name. I put this in to be sure readers understand that it's hard to get even grainy shots to lose their grey-ness, unless special steps are taken. Perhaps it can be dropped to keep everything brief. Opinions? ] but the issue of quantization is moot in most photos, because grains are random in size, overlap, and are not individually resolved in the image. Under such typical circumstances, slow films have higher contrast, and faster films have not only lower contrast, but a much longer gray scale, beloved of available light photographers in journalism and movie making. [There was a problem in the original on this point. Maybe you can say it better, but we need some kind of clarification. An ASA 400 b/w film has what they call a "longer grey scale" than an ASA 25 film. Subjectively, I've always thought the grays of fast film "looked nicer", but technically it means that the negative film goes deeper into highlights before blocking/saturating, and it can throw a few grains into shadows before falling to baseline density at low image intensity.]

[Original: "Fast films are also relatively contrasty, for the same reason. That is, an area of the image will consist of bright areas and dark ones with few transitional areas of midtones." This statement is misleading at best. Slow films have a higher gamma. ]

END proposed text.

Please don't clobber me. If there are better ways to handle a suggested revision, just say. As for the revision itself, entries on technology are of necessity always a work in progress, so this won't be the last change, and not every change has to be made.
Thanks!
Jerry-va 17:09, 27 February 2006 (UTC)

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Does anyone know how films speed and the ISO ratings relate to digitalo cameras? I think this would be useful to add.

[edit] Delta 3200

A while ago, I removed Delta 3200 from the chart, and today an anonymous user, in what I assume to be a well-meaning edit, added it back (at 3200). The reason that I removed it is that Delta 3200 does not have a true ISO speed of 3200; according to the data sheet, the true ISO speed is 1000. Shooting it at the recommended speed of 3200 is actually "pushing" the film, and will result in higher contrast and less shadow detail than if it were shot at the ISO speed. For this reason, I don't think it's appropriate to list it on the chart. Listing it under 3200 is inaccurate, and listing it under 1000 would be confusing without a lengthy explanation. Similar arguments apply to Kodak's Tmax P3200. -- Coneslayer 18:00, 2005 Apr 17 (UTC)

That depends on what we mean by nominal film speed. The nominal speed of a number of popular films is different from the true ISO speed (which already contains a safety factor). Kodak TMZ and Ilford Delta 3200 are marketed and labeled as ISO 3200 films and can be shot as such. For that reason, it makes sense to list them here and provide more detailed explanation in separate articles about these two films. Your reasoning is not wrong, but by the same token you could challenge the inclusion of Fuji RVP 50, which has given rise to endless debate about whether it should be rated differently than its labeled speed, with recommendations ranging from ISO 40 to ISO 80. I'd say it's simplest if we stick with what's on the box (yes, that may be marketing hype) and explain what's really going on in separate articles for each film stock. --MarkSweep 20:25, 17 Apr 2005 (UTC)

Mark, to put it simply, I would say you're using the word ISO where it doesn't belong. The ISO specifies a method of testing film speed. The 3200 speed does not come from this testing methodology, and you won't see the word ISO near that speed on the packaging or datasheet. The ISO speed of Delta 3200 is 1000, period, no matter what you set the dial at, or how long you develop it. In contrast, RVP 50 is an ISO 50 film. You, and other photographers, might like the results better when it's shot at 40 or 80, but that doesn't mean the ISO speed is wrong, or that the speeds you like better are the true ISO speed. It doesn't make sense to say I'm shooting an ISO 50 film at ISO 40--that's a misuse of ISO, because I'm not using the ISO methodologies. The correct term would be EI 40 (exposure index). If you want to list Delta 3200 under ISO 1000 on this page, that's fine with me, but I would strongly object to listing any film under a speed other than the ISO speed. -- Coneslayer 21:23, 2005 Apr 17 (UTC)

Good point. I don't disagree with your assessment of the facts. My only concern was that it would be better to explain this in the article, since these issues are generally relevant here and likely to come up again. So how about listing Delta 3200 under ISO 1000 in the table and adding a note explaining what's going on? --MarkSweep 23:33, 17 Apr 2005 (UTC)

I have added a paragraph on this topic to the article. -- Coneslayer 21:35, 2005 Apr 17 (UTC)

[edit] f-number and aperture diameter

I have edited the article slightly to remove the common misconception that the f-number is equal to the ratio of the focal length to the aperture diameter. It is not. The f-number is equal to the ratio of the focal length to the diameter of the entrance pupil of the lens. The latter is proportional to the diameter of the aperture. It is true that doubling the f-number halves the diameter of the aperture but it is not true that the aperture diameter is equal to the focal length divided by the f-number.--Srleffler 04:29, 14 February 2006 (UTC)