film exposure latitude in f-stops + metering

Discussion in 'Beyond the Basics' started by J_S, Feb 19, 2005.

  1. J_S

    J_S TPF Noob!

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    Can anyone tell me the exposure latitude (range) of camera films in f-stops?

    I've been researching this.. i've found suggestions that b&w negative film has a 7-stop latitude and colour slide (transparency) film has a 5-stop latitude, but am not sure if this is true or not.

    If this is true, if I use my (colour slide film loaded) camera to take a spot-meter reading of something which is 18% grey (the grey that all lightmeters are calibrated to), and I set a 4 stop underexposure using the camera's exposure compensation feature, then the resultant slide will show that area to be totally darkest black. And likewise, the other way, if I overexpose the slide by 1 stop, this area should be brilliant white. Is this true?! This makes the 5-stop range for this film seem a little narrow from my experience. I am assuming here that my camera's inbuilt lightmeter is calibrated to 18% grey, and this is located 18% of the way between white and black, and not located at the midpoint (50%).

    Ideally what i'd like to do, is to take a spot-meter reading of a tone, then shift the exposure using the exposure compenstion feature on my camera and be able to predict what the resultant tone will look like on the slide/negative.

    I look forward to hearing any suggestions about this topic
     
  2. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    The latitude they are talking about isn't between solid white and solid black, but between the brightest tone and the darkest tone where you still get 100% of the detail. The ranges you post are ones that I commonly see, but there are folks using certain developers and techniques claiming 9+ stops for some BW neg films. I recently read an article where the guy said he was getting 15 stops of full detail. An exact range probably differs from film to film.

    18% gray is smack dab in the middle, at least from the point of view of how almost everything relating to it and photography is thought of. Think of the tonal scale from black to white. Instead of an even gradient, break it into 10 parts or "zones", each a solid tone (some folks like to use 11 zones instead of 10). Zone 0 is solid black (no detail). Zone 9 is solid white (again, no detail). Zone 5 is 18% gray (with 100% of the detail). Just search for the term "zone system", and these sorts of tonal scales will be at almost every website you hit (and a lot of good info too).

    When they say a 5 stop latitude, they mean that if you expose for zone 5 ( =18% gray = middle gray ) you will still get 100% detail 2 stops below and above, zone 3 through 7. As you move past these detail is lost.

    The problem with shifting tones with exposure is that without development control if you make one tone darker, they all get darker. If you make one tone lighter, they all get lighter. E6 and C41 processes are pretty standardized, and even if you do it yourself, I don't think you'd have the sort of options as with traditional process BW neg developing. With BW negs you can contract and expand the tonal scale in places to adjust certain tones, while leaving others alone.

    If you underexpose a slide by 4 stops it's going to be very, very dark (middle gray would be almost black). If you over expose it by 1 stop, it would look a little washed out (middle gray would be a lighter gray).

    When I'm shooting slide film I try to stick with exposing for middle gray, and if I'm going to err some it would be on the side of underexposure, but preferably not more than 1/3rd or 1/2 stop. And I'd bracket anyway.
     
  3. Hertz van Rental

    Hertz van Rental TPF Noob!

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    For studio purposes you always worked on the principle that colour neg had 5 stops latitude and tranny was 3 stops - but this was because tranny was used for magazine reproduction and if you didn't flatten the image you lost highlight and shadow in print.
    B/W film latitude depends on a number of factors but the average is 7 stops.
    With PlusX film, by adjusting exposure and processing time I could expand or contract the latitude from 9 stops to 5 stops depending upon what I was shooting.
    When you do this, however, you are not actually changing what is recorded on the neg but are compressing the tonal values within the useable density range. If you do not then you will loose the information when you print as you have to take into acount the tonal range of the photographic paper. Film is actually capable of recording between 12 and 15 stops but photographic paper can't cope with this range.
     
  4. J_S

    J_S TPF Noob!

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    Firstly, thanks so much for your replies guys, you've given me a lot to think about. Thanks for clearing up the 18% gray issue, Matt, it's easy to misunderstand that 18% gray is 50% between white and black! I don't handprint from my slides and negs, only scan using a film scanner for website work etc, which probably limits the tonal range even more. But it still makes sense to ensure my slides and negs have been exposed as best I can in the key areas.

    I read abot Ansel Adam's zone system but it confused me.. the range of tones in the 11 (or 10) zones.. going from black to white.. what do they refer to? The tones that a b&w neg can record? Or the range of stops that photographic paper can record?

    So if a slide film had a latitude of 5 stops.. areas 2 stops darker and lighter than the chosen mid-point would retain 100% detail. What about areas where the film didn't record 100% detail - another 2 stops each way perhaps? So my 5-stop latitude slide film can record 9 stops tonal range in total.. does this sound about right? And b&w negs.. something like 11 stop tonal range total?

    I tried to use a printout of the 11 zone scale the other day. I was shooting a portrait of my girlfriend with b&w neg film. (http://www.joesmalley.com/images/sam.jpg) First I took a spot meter reading from her forehead, which the camera would take as 18% grey (zone 5). Then I held the scale next to her face and decided her skin tone was nearer zone 9, of white.. 4 stops greater. So I overexposed the neg by 4 stops, and it came out pretty good as a print. I'm not sure whether the photo-lab I used needed to correct exposure of the the print much, though. Is this a good way of working/metering my photos? The best way would be to use a lightmeter I guess.. but I don't have one! How many stops would you have compenstated by in this situation? If I was using colour slide film instead for the same shot.. how many stops should I overexpose by.. about two thirds of the amount id use for b&w film.. say 2+2/3 stops?

    If I was shooting b&w neg film at of a snow topped mountain.. and I wanted the white snow to appear bright white just like in real life.. I could take a spot meter reading of the snow.. then dial in 5+1/2 stops of overexposure to get it to bright white (no detail) - would this work? And what about slide film.. 4+1/2 stops?

    A lot of my photos are of friends riding bmx bikes, where nearly all the shots use a flash to expose/freeze the rider in the air. A mid-toned background competes with the action of the mid-toned rider so I like to underexpose the background by using a faster shutter speed to darken it. What i've tried to do is take a meter reading of the background and underexpose if by 2 to 3 stops to darken it (when using slide film) - what i'd like to know is how many stops can I underexpose it until it gets total black with no detail - assuming the background is plain mid (18%) gray? This will give me an idea of the range, in stops, there is between mid gray and black, so I can decide how dark my resulting photo backgrounds will be.

    You might like to check out my portfolio website:
    http://www.joesmalley.com
     
  5. Hertz van Rental

    Hertz van Rental TPF Noob!

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    To answer a lot of your questions you need to go into sensitometry.
    If you plot a graph of neg density against exposure level you get curve which is the film's Contrast Index. This CI is slightly different for each film and changes with the developer used. The steeper the curve the more contrasty the film.
    Photographic paper produces the same kind of curve so it has it's own CI. Soft paper (grade 0) has a very shallow curve as it is low contrast. Hard (grade 5) has a very steep curve.
    Most of the time you are trying to match the two curves so that the tonal range of the print matches how you saw the original scene - if you have a flat neg you tend to print it on hard paper (the tonal range has been flattened in the neg so you try and expand it again with a hard paper to actually get some blacks and whites. If you print a flat neg on soft paper you will just get greys).
    There is a limit to the maximum density (D-max) that you can get on neg and paper too.
    As you build up density on the neg grain size and printing time increases. Idealy you are trying to get shadow detail to record at the bottom of the CI curve (D-min or 'the toe') which means that all the other tones fall on the bottom two thirds of the curve. This gives the best tonal rendition and minimum grain, using the film to the optimum.
    This is the basis for the Zone System. It is just applied sensitometry.
    If this explanation seems a little confusing then I apologise but the concept and process is rather complicated and technical, not something you can easily explain in a few sentences. If I inserted some graphs it would be clearer so if anyone is really interested just ask and I will go into it in detail with a slide show. Or perhaps Matt could - he seems to be able to explain things rather better than me.
     
  6. J_S

    J_S TPF Noob!

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    Thanks again for your reply, it makes perfect sense. As suggested by Matt, I searched for the "zone system" and found this simplified zone system page which was very helpful http://www.normankoren.com/zonesystem.html. It cleared up that this zone system represents the tonal range that a NEGATIVE FILM can capture.

    Ansel Adams used an 11 zone system for negatives.. but he used a slow development process (ive never done any darkroom work so excuse me if that term's wrong!!) which allowed him to print an 11 zone tonal range. Matt talked about a 10 zone system.. but this places the mid-grey point inbetween two greys which means always compenstating in half stops using this system.. which is even more of a headache!! I've decided to use a 9-stop zone system which probably better represents the kind of (low quality / tonal range) prints I get back from my photo-lab.

    COLOUR SLIDE FILM >> That site states that Kodak colour slide film has a 5.6 stop exposure range (im trying not to use work latitude here!) and Kodak negative film 10 stops. Therefore a SLIDE FILM zone system would have less zones. The scene is still the same brightness using either film.. but the slide film renders it with greater contrast, as I understand.

    I have also found this site http://www.kenrockwell.com/tech/zone.htm which states that slide film goes clear at +2.7 and black at -4 (on Fuji Velvia), a 6.7 stop range here. So slide film has a contrast range somewhere inbetween 5.6 and 6.7 stops, depending on the film used. Let's call it a 6 stop contrast range for colour slide film then.. with a latitude of probably nearer 4-5 stops (+- 2 > 2.5).

    I've therefore devised my own zone system sheet (here) which I intend to use from now on in my photography. So if i was taking that portrait photo again.. I would take a spot meter reading of her forehead and dial in +2 stops if using negative film and +1 if using slide film. And the snow scene I was talking about.. +4 for negative and +2 for slide film.

    Is the system i've devised (im not taking full credit for it.. thanks AA!!!) going to work in practise do you think? Will I get accurate (within say.. +- 0.5 stops) results?
     
  7. J_S

    J_S TPF Noob!

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    oh, and this seems to explain why black and white photos look so much nicer than colour photos.. because the b&w neg film has nearly twice the tonal range of colour transparency film
     
  8. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Each "zone" is one stop.

    As you move past the zones that'll record 100% detail, you begin losing detail because it's too dark or too light. Think of a black sweater; you could expose to see the actual texture of the fibers (zone 3, 2 stops less than middle gray), or you could expose for the sweater to look solid black (zone 0), or you could go somewhere inbetween (zones 1 or 2, zone 2 would have more detail than zone 1).

    1) I don't know about your printer, but I wouldn't trust mine to accurately print the zones. Get a gray card.

    2) Photolabs will attempt to print the best print possible. Use slide film or access your negs to see if you have over or under exposed.

    3) It is very unlikely that your girl friend's skin tone is zone 9, that is either pure white, or darn close, like fresh house paint white. It's more like zone 6.

    Here's my advice on the above situation: have your girl friend hold up a gray card, and meter off of that(using your in-camera meter); use those settings or meter off of her skin, and over expose 1 stop (the meter reads for zone 5, you want zone 6) or meter the most important shadow details, and under expose 2 stops (the meter reads for zone 5, you want zone 3)
     
  9. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    BW vs. color is just an opinion. It probably is easier to deal with more varied lighting situations with BW, and easier to correct for problem lighting conditions. I shoot 99% BW neg film, and my buddy shoots mostly E6 (slide film). When we go out shooting landscapes in the morning he requires much lower contrast lighting than I do. As the sun comes up I can keep shooting longer as the contrast increases.
     

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