Bracketing - Old & New


By ralph

It seems the glitzy 21st century with its super technology is no time to be fussing about bad exposures but heck, them varmints just never seem to go away.
Many new photographers who are new to shooting slides (like negative shooters ) seem puzzled when they get bad results. They wonder how they went wrong even though they used all the right numbers --- like the ISO film speed setting. Welcome to the real world where camera calibrations are often far from perfect and subjects with atypical brightness patterns can throw the meter even further into error.
Unfortunately some of the makers don't realize they're over or under because they've never shot any perfect exposures which could have made excellent standards of comparison. To an untrained eye without a good aim point those washed out slides look okay, and a slide seems never too dense if you can hold it against a 150 watt bulb and see through it.
It surely makes no sense to travel a thousand miles and then to let a great picture slip through your fingers because you didn't shoot a couple extra frames of film. Extra exposures on the under and over side are what you call an exposure bracket. Of course you can't bracket action subjects but when I see badly exposed scenic or still life shots I have to assume the maker just wrongly assumed they "couldn't miss".
Some of the newer cameras that can be set to bracket automatically (but hopefully only when wanted) make the process easy. But If you do it manually remember that the only way to change exposure in a fully automatic camera is to vary the ISO rating of the film, since just changing a speed or aperture will be counteracted by the camera making an opposite change. ( Telling the camera the film is slower fools it into giving more exposure and telling it the film is faster tells it to give less). If you have a camera with no manual override of auto DX film indexing, it probably cannot be coaxed into shooting optimal slides --- save it for color negatives.
A good spacing of exposures is "on it", then 2 notches (2/3 of a stop) over and under. If you go a full stop the best shot may fall in between exposures. If you're shooting purely in manual (full control of stops and speeds), and you're pretty sure you're close, plus or minus a half stop would give you finer control -- and then another half stop for the really exceptional subjects (for a total of five). It's a good idea to shoot the best guess, or meter reading exposure first, just in case the town garbage truck parks in the middle of your scene after one pop. When you check your slides see if the first shots are generally good. If the best shot tends to be at the darker or lighter end it's time to admit your camera is missing the mark and you have to crank in a correction. You can move the compensation to plus or minus or always add or subtract a correction factor to the stated ISO rating. Aim for a long term situation where the first shot is usually right. This lengthy procedure is the only way to handle the occasional need to shoot without brackets, like action or people.
"Chromazones" was an exposure system developed by Collins to address the limited seven stop range of slide film. Even so, the lightest and darkest ends have very little detail so the more practical range is taken as four or five stops. In fact four stops is about all that the super boosted, brilliant films like Velvia and Kodak E100VS (Elite EX aka Extra Color) can cover, while the average films like E100SW will cover five, and a lower contrast film like E200 can pretty well reach seven
The glib advice you often hear on the subject of reading exposures slyly implies that all tonal problems can be solved with the right exposure, but it's just not so; some subjects cover too long a range of brightness for one
ex
posure to cover. If you can't do any better, favor the highlights (since the eye always sees highlight detail but does see some darker areas as black.)
In ˜99 I was still shooting with little thought about computer manipulation, but that transition was right around the corner. I found that some extra long brackets (lets say 5 exposures over a 2 to 2 1/2 stop range) still failed to deliver any single exposure that could cover the whole range of some subjects but they turned out to be great starting points for blending the light and dark tones in Photoshop. Surprisingly, such blends of light and dark images retain, to an amazing degree, the overall saturation and brilliance that we associate with slides. The "compression" of all the tonal information does not produce a duller image the way a B&W negative, underdeveloped to handle an extreme density range would.
It's even possible (in fact one of the easier tricks) to take a single exposure, which shows too little separation in its light and dark tones, split it into two identical images, radically darken one, and lighten the other, and recombine them to keep the best of all the tones. Just open the image (and all my directions speak the Photoshop "language") in your image software. If the layers palette isn't visible, click on "window" in the top title bar, go down to "show layers" and let go of the mouse clicker. The image that you opened is already there in the layers palette. Now click & drag that layer down to the little "new layer" button, the middle of 3 on the bottom of the palette. Lo and behold, you now have two identical layers to play with. Make the top one dark enough to show rich highlight detail, and the bottom one light enough to open up shadow areas. (click on the layer to make it active, then go to Image- Adjust- Levels. When levels opens you'll see three little slider points on the bottom of the squiggly histogram. Just click and drag the center point to the right to darken your image and to the left to lighten it) .
You view from the top down. A word of caution here; the little eyeball at the left of each layer says that you are seeing that layer but if the top layer is visible you can't see through it to the bottom one, so to see the bottom layer click the top eye off to render the top layer transparent. You'll do this many times as you keep checking your progress (If ever you are doing something to an image and nothing seems to be happening, clicking off the top layer is usually the answer.)
So right now you're looking down at a darkish sandwich of images, though it should have excellent highlight tone. Go to the toolbox (vertical palette), and about 5 tools down you'll find the eraser. Despite its shape,( which takes you right back to fourth grade), when used it's just another variation of the brush tool, where you find a wide choice of sizes. If you're cautious like me you'll set the Opacity at a fairly low setting like 20% --- a good hedge against inexperience. That way each sweep has minimal effect. Okay, now take your brush and just erase away the excessively dark areas of the top layer to reveal the brighter tones in the lower layer. Do it gradually until you achieve a smooth range of tones from enhanced shadows to detailed highlights. Don't forget that the layer you want to work on, not only has to be visible but also active. Just click on it to make it so.
Erasing seems to be a slow process so if you move fast the effect may fall behind real time. Just stop and let it catch up. The beauty of this technique is that there is no worry about the two images being in register. Each layer takes up a lot of memory so when you're done playing with the image you may want to go to the bottom of the Layer menu and hit "flatten image" That returns the file size to its starting point.
Let's say no single slide could hold all the highlight and shadow detail that you want. Hopefully you have a widely spaced bracket of exposures,
and h
opefully the camera was secured to a tripod so that the images will be easy to register. You can blend images with rather dramatically different exposures, like say 1 1/2 to 2 stops apart, and get good results. There are a couple more steps at first. Open both images separately. Now Select one of them (Select- All), go to the top bar, click "file" and go down to "copy" and click on it. Now push this image aside and make the other one active (click it),then using the move tool from the toolbar slide the first one back over the second till they line up neatly( hold the shift key down while moving and release when you feel you're in register. It should jump into place), go back down from file and click Paste. That will create the second layer and put the image into it. In case you didn't get the dark one on top double-click the background layer to make it a movable one, and just click-drag the top layer below it.
There's question about whether registration with these separate images takes care of itself or requires careful matching. I recently did such a sandwich by just quickly lining up the margins of the images and when it was done registration looked perfect even at high magnification. I previously did one where I enlarged both images hugely (to see every pixel), made the top one 50% transparent, so I could line it up with the bottom image, and nudged them into place pixel by pixel with the arrow keys. It may have been unnecessary --- or maybe the film was mounted differently in each slide.
The above, somewhat technical ways of expanding the limited tonal range of slide film shows how the computer can be a useful tool for overcoming that film's rather rigid shortcomings. Still it's almost laughable how simple it is to achieve similar control when printing in the darkroom from a negative. More exposure on the film (with a bit less development) easily covers a huge tonal range. If the image looks a bit flat when printed straight, just up the contrast, take a one cent dodging tool ( a disc of cardboard taped to a bit of hangar wire) and hold back those areas which might get too black. It's the exact equivalent of that layer trick in Photoshop.