How to calculate average speed.

Discussion in 'Exposure Discussion' started by hoakin1981, Apr 29, 2014.

  1. hoakin1981

    hoakin1981 Member

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    OK this might be a stupid question (most probably is) but I thought I should confirm it just the same. Lets say that on my quest to learn how to correctly expose slide film I want to try the average rule. Using the camera spot meter I get a reading for highlights/midtones/shadows and I want to use the average.

    So, I get 1/500 for highlights, 1/125 for midtones and 1/8 for the shadows (figures theoretical) what would be the average speed to use, 1/211 (meaning the actual closest possible) correct?

    Same if I want the same for only highlights and shadows, 1/254 right?

    Thanks In advance.
     
  2. baachitraka

    baachitraka Subscriber

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    For slide film just point the incident meter towards the camera...
     
  3. wildbill

    wildbill Member

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    really?

    then pull out your spot meter to meter the highlights. then walk to the bottom of the canyon to meter the shadows with your incident meter.
     
  4. jp498

    jp498 Member

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    If it's 35mm, use a Nikon F4/F5 or equivalent as their matrix metering was made for exposing slide film. Otherwise just bracket and/or use an incident meter. The OPs light measuerments are right on the edge of what I'd expect out of slide film; any more range and I'd use negative film like Portra for it's bigger capture range. It wouldn't be a slide, but it's recorded on film.
     
  5. John Koehrer

    John Koehrer Subscriber

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    Oh please! (#3)

    The incident meter will give an average reading of light falling on the subject.

    With slides, you main concern is not overexposing the highlights. The shadows will just have to fall where they will. There's not enough latitude in slide film nor can your vary development times to extend or reduce density Zone system style.
     
  6. baachitraka

    baachitraka Subscriber

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    It is not that hard to simulate the shadow illumination with incident meter. But for slide film the range is rather limited to five stops.
     
  7. markbarendt

    markbarendt Subscriber

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    Don't think in fractions, think in stops.

    1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8

    That example averages the highlight and the shadow points from a spot meter. If you then also want to consider the mid-tone point you will need to decide if it is worth giving up some of the shadow detail to place the mid tones closer to where you measured them.

    The highlight and shadow point average in your example is asking for more exposure than your mid tone measurement. Which setting is better is simply a matter of taste. To test and see which you like better from this example I would shoot two frames, one at 1/60, the other at 1/125 then compare.
     
  8. markbarendt

    markbarendt Subscriber

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    +1

    For me there is a bit more nuance here, the exposure I want for the faces in the scene always trumps the rest of the composition.
     
  9. Doremus Scudder

    Doremus Scudder Member

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    For slide film, you need to base your exposure on the highlights, not average. Measure the brightest white that you still want a little detail in in your scene, then open up two stops. That should get you in the ball park. Make notes about your meter and adjust a bit if this isn't giving you exactly what you need in the way of highlight density. Shadows fall where they may.

    Best,

    Doremus
     
  10. TimFox

    TimFox Member

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    Another way of putting it is that the "average" is not the relevant measurement. You want to make sure that the "important" parts of the image are in the useful range of the film, which for slide film is narrower than for negative film. Depending on your image, the "important" bit might be a face. Interesting application: when shooting steam locomotives outdoors, I found that the metering the locomotive directly with a standard center-weighted meter in an SLR would render the black locomotive at 18% grey. A better metering, using only the camera meter, was to meter on the "ballast" rocks or green grass in the foreground, which would render the locomotive an appropriate shade of black with detail.
     
  11. MattKing

    MattKing Subscriber

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    Remember as well that the in prism meter on th Mamiya 645 Pro is an "averaging" meter no matter which mode you use - the only difference between the modes is that the area read is smaller in "spot".
     
  12. momus

    momus Member

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    I think humor is not a forte of the members here, unfortunately. That was pretty funny wildbill. Best rationale I've heard for using a reflective meter.

    The thing to remember about slide film is that you don't have much latitude in your exposure range. If you have a wide range between the shadows and highlights in a given scene you'll need to use a graduated filter or sacrifice one or the other.
     
  13. Chan Tran

    Chan Tran Member

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    Not warranty which one will give the exposure you like but.
    1. Midpoint between 1/500 and 1/8 is 1/60 like other have said.
    2. Average of 3 readings of 1/500, 1/125 and 1/8 is 1/80 and not 1/211.
     
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  15. RalphLambrecht

    RalphLambrecht Subscriber

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    you might be better of to turn your fractions into decimals first,then average and turn the resultsback into the closest fraction ;good luck
     
  16. Chan Tran

    Chan Tran Member

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    That would not work either Ralph. In order to average the readings in the fashion of the Olympus OM-4 or even the Sekonic meter the average has to be taken from Log of the shutter speed. With the OP example it is

    10^((log(500) + log(125) + log(8))/3)=80

    That's how meters averages multispot readings. I don't say that is the best way to arrive at a good exposure however.
     
  17. benjiboy

    benjiboy Subscriber

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    With my spotmeter to average up to 9 1° spot readings you just press the average button, but this would do you no good at all in this case because with slide film to do what you suggest you will most probably find that that the difference between the the darkest shadow and the brightest highlight is too great for the film to be able to record because of the restricted latitude of slide film (about 2 1/2 stops) , there are two options I.M.O (1) spot meter the brightest highlight and let the shadows find their own level, or (2) take an incidental light reading.
     
  18. Chan Tran

    Chan Tran Member

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    In fact if you make a lot of spot readings all over the frame and average them you will end up the result similar to that of a wide angle averaging meter.
     
  19. benjiboy

    benjiboy Subscriber

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    You're quite right Chan , you would get an O.K result this way, but would probably in a wide range subject get the best results with slide films spot metering the highlights and let the shadows find their own level even if some of them block up is the way I would do it.

    Sent from my KFOT using Tapatalk
     
  20. Bill Burk

    Bill Burk Subscriber

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    As Chan Tran pointed out in a couple different times, I wouldn't trust the "average" method. For slide film you want to hold the important highlight. Also slide film is a bit like cinematography, you may want continuity - so maybe do the metering once and set the camera f/stop and shutter speeds manually and leave them in place as you take a few shots of the same "scene". Then slides you project won't jump around in brightness.

    Use multiple spot readings to "average" a few important highlights. Then "open up" a couple stops to place the average of those readings as highlights in your calculation.

    Exactly how much to open up, I'll defer to experienced slide shooters who use spot meters to give you specific recommendations. I shoot mostly black and white so I'm usually trying to hold the shadows.
     
  21. markbarendt

    markbarendt Subscriber

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    Bill and Chan, I think y'all are underestimating the value of averaging and picking hoakin1981's "pegs" for him without understanding his intent or subject matter.

    Pegging to the highlights is one method Dunn and Wakefield talk of in Exposure Manual, pegging to mid-tones is another, and yes they do talk about pegging to the shadows but don't suggest it for slides as I remember.

    The intent of a peg is to provide a reference point that can be used to put the most important subject matter exactly where we might want it in relation to that point. For example Dunn and Wakefield make the case that whenever a face is involved that that essentially trumps everything else. When we humans, look at pictures of humans, we expect the faces to look "right".

    Pegging to the highlights is one good way to properly place a face/main subject, but it needs to be understood that even though the highlight is being used as the reference point, where the highlights fall on the film is actually being ignored. This is all about placing the main subject, the rest of the scene is allowed to fall wherever.

    Averaging is a way of finding a balance between multiple "equally important" points.

    If we decide that the highlights are important to consider as well as the face then we measure both high and mid tones and average them. The result is a reasonable compromise, neither highlight nor face ends up perfectly placed but it is as good as can be had in a single frame.

    hoakin1981 has chosen 3 "important" points. The highlight and shadow points define the background, by choosing these two points hoakin1981 is (whether he understands it or not) defining where he wants the background and telling the exposure equation "for the back ground I need to find an exposure that protects both high and low tones". By adding the mid tone point hoakin1981 tells the equation, "there is also something in the middle that is important to me and I'm willing to compromise the background exposure a bit".

    What we don't know is what hoakin1981 really thinks is important.

    Which brings us to the question:

    hoakin1981 what are you trying to accomplish, what subject matter is important?
     
    Last edited by a moderator: May 1, 2014
  22. benjiboy

    benjiboy Subscriber

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    It's no use spot metering the highlights, midtones, and shadows and averaging the readings if the resultant average is outside the film's ability to record them, which will invariably happen with slide film., as Mark writes you need to decide which tones are the most important.
     
  23. Doremus Scudder

    Doremus Scudder Member

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    Mark,
    I have to disagree a bit. In essence, basing your exposure on important subject matter is exactly right. The problem with averaging several important areas of subject matter, especially with an unforgiving medium like slide film, is that you may end up with none of them being exposed as you like. Yes, averaging works in many cases, but in cases of high contrast, we need to make a better choice.

    With black-and-white film, one "pegs" to the shadows. One develops and/or prints to get desired contrast.
    For color negative films, the same thing, but with more limitations, applies.

    For slide films and B&W reversal films, pegging to the highlights is a good practice for non-portrait scenes. In contrasty situation, the shadows, and to a lesser extent, the lower midtones, may suffer. For portraits, basing exposure on a skin tone is good. In contrasty portraits, then, the highlights and shadows would suffer. The portrait may not be acceptable; softer lighting may be the only solution.

    Average meters work well in many situations and are pretty good with films that have lots of exposure latitude. They tend to have a higher failure rate with less-forgiving materials and contrasty scenes. There isn't always "an exposure that protects both the high and low tones." Sometimes you lose them both and end up with the mid-tone somewhere you don't like.

    You can only place one value; others fall where they may. It's good to know this and use your meter help you make an informed decision.

    So, to the OP I would advise the following: Identify your most important tonality. Evaluate the contrast of the scene. Mentally place your important value and then imagine what will happen with the others. Slide film has about a five-stop range. If you place a highlight, you can see which values will go black easily. If you place a mid-tone, you can imagine what will happen with shadows and highlights equally easily.

    For low-contrast scenes, averaging often works really well. I'd use it in many cases. For scenes of higher contrast, I spend a good bit of time deciding where to place what. For landscapes with white water, it's the water: meter it and overexpose two stops. The shadows go black if the scene is too contrasty. There's nothing to do about that. Often, the shot is still acceptable. For a portrait, the skin tones and maybe let the highlights go blank white.

    Sometimes the contrast of a scene precludes making a good photograph on slide film. In that case, don't shoot and save yourself the trouble and money.

    There are some things you can do to tame the contrast of slide film: pre-exposure often helps, so does fill-flash and reflectors for portraits. For landscapes, waiting for a cloud to partially cover the sun is often gratifying.

    Hope this helps,

    Doremus
     
    Last edited by a moderator: May 1, 2014
  24. markbarendt

    markbarendt Subscriber

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    Absolutely and as you point out the fix required is often a change of lighting, whether artificial or natural.

    I will suggest though that this problem is common to all photographic mediums, the problem with slide film is that the fix has to be applied during the camera exposure because we are going straight-to-final-output. With negatives we can change the scene lighting or we can burn, dodge and manipulate when we print, or both.

    This isn't necessarily the case, in fact Dunn and Wakefield essentially puts pegging to the shadows into more of a special-case-use classification in the grand scheme of photography. It's basic advantage being minimizing exposure, which is nothing to sneeze at, but not necessarily a priority for most shooters.

    Personally I no longer peg any exposure to shadows, my choices are always centered on the mid-tones I want to print, shadows and highlights are simply allowed to fall around that come what may. I will on occasion use a high tone and an offset to peg my mid-tone. I avoid using shadows as a peg because I find judging them unreliable.

    Onward, I'm not understanding what extra limits you see with color negatives, can you elaborate.
     
  25. Bill Burk

    Bill Burk Subscriber

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    hoakin1981,

    Your sunset on the lake shot (posted on DPUG) looks great. If you wanted to have detail in the foreground, then you might try to find a Neutral Density Graduated Filter. Galen Rowell used these for exactly the problem you posed. If you can find his filters or similar, you can have both the skies and the foreground details too.
     
  26. dehk

    dehk Member

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    You guys are all wrong!
    Average speed is Total distance divided by Total time.


    I simply cannot resist. . .