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# Thread: How to calculate average speed.

1. 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".

2. 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.

3. 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.

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

5. Originally Posted by RalphLambrecht
you might be better of to turn your fractions into decimals first,then average and turn the resultsback into the closest fraction ;good luck
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.

6. 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.

7. Originally Posted by benjiboy
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.
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.

8. 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.

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9. 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.

10. 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?

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