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 p.2 #20 · who still uses a light meter? | |
KEllis wrote:
Interesting article in the November issue of Studio Photography, It was basically talking about using the histogram or light meter. My impression of the article can be summed up by this base ball analogy. A histogram can get you on first, a flash meter gets you a home run. Article can be read here, the article is entitled its all about accuracy.
All tools have their uses. A knife is good for cutting up stuff, but only if it is sharp. Same is true with either a meter or histogram.
A meter will only produce correct results after it has been matched the actual ISO of the camera it is being used with. The camera dial might say ISO 100 but the actual ISO may be higher or lower.
To test and compensate the meter one first needs to define what constitutes nominally correct exposure. For digital it is the point where more exposure causes a loss of detail in the highlights. To measure when that point is reached it is necessary to use an object with texture. A white terry towel is ideal for this.
Put a towel in flat even lighting. Even in flat light the loops in the fabric will create texture via subtle shadowing. Take a meter reading (e.g. f/5.6) then shoot a bracketed series around that reading (-/+ 1 stop in 1/3 stop increments). Open the files and look at the towel image. Find the one that shows the first loss of detail. The nominally correct exposure will be the file taken with 1/3 less exposure.
More than likely the file which looks best exposed will not be the one taken at the indicated meter reading. Canon camera ISOs are typically 1/3 stop faster than a hand held better. So if the meter reading is f/8 the file shot at f/9 will typically be better exposed.
Sekonic meters anticipate this an provide a simple way to enter a compensation factor. One the L-358 both ISO buttons are held down, then exposure comp. factor is entered via the adjustment wheel on the side. With compensation entered the display meter, in this example, would change from f/8 to f/9. Once compensated in that manner the meter readings will produce correct results in that camera.
As for the histogram it is impossible to interpret unless it can be related to the content of the scene. Again the white towel is useful as a benchmark. On a dark background the towel will create a blip on the histogram. Where that blip falls on the histogram will indicate whether or not exposure is perfect.
To discover what position the blip represents perfect exposure one simply needs to perform the same test as for the meter, shooting a bracketed series of exposure and looking at the image of the towel in Photoshop, picking the best exposed version of the towel. Once you find it, simply go back and scroll through the same files on the camera, training your brain recognize where textured highlights fall when correctly exposed (base of histogram kissing right edge) overexposed (piled up and running off the right) or underexposed (gap on the right).
A histogram is of little help on a white background, but there is another in-camera aid with does, the over-exposure (clipping) indicator. It not only shows when highlights are clipping, it also shows where.
So if you start by adjusting the test target towel with key and fill until its 1/3 stop below clipping in the OEW, matching the background is simply a matter of bringing up its lighting to the same point, 1/3 stop below clipping to get good perceptual balance while maintain specular highlight contrast
A well compensated incident meter is very convenient tool for getting the studio lights into the correct zipcode and street, but the best way to get the exposure correct on the front porch is to measure what the camera actually recorded on the last exposure. The OEW does not show what is in the RAW file, but if you compare the point where the white towel clips in the camera OEW with where the detail of the towel in the RAW file disappears you can corollate the two. I find with my camera adjusting exposure 1/3 stop below the point of clipping of the textured white highlights based on the camera OEW results in optimally exposed RAW files.
I find the quickest path to optimal exposure is to use all four tools: meter, towel target, histogram, OEW for the part of the process they are best suited for:
Meter: Setting ratios, getting the exposure close
OEW + towel: Precisely adjusting exposure, balancing foreground and white background, checking if background exposure is even (just open the lens and watch where it clips first).
Histogram + black test object: When exposure is nominal per the highlights (set via OEW) the left side of the histogram will show whether or not shadow detail is being lost. A histogram running off the left side indicates the need for more fill.
Chuck
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