Exposure help - spot metering calculations?

beans

Hi all,

I have a problem when exposing for scenes that have a bightly lit section, and one that might be in shadow. Using aperture-priority mode, with an averaging light-meter, the shot ends up exposing correctly for the highlights and leaving the shaded area somewhat underexposed.

I was thinking of ways around this without having to spot-meter. That got me thinking - is there a way to effectively spot-meter a scene using an averaging meter by focussing on the brightest and darkest areas individually, and then doing some kind of calculation on the results

Taking my aperture-priority situation as an example, if the sunny bits were metering at 1/250 and the lowlights were metering at say 1/60, should I leave the aperture as it is and set the shutter speed at 1/125?

I will post an example of the problem in the next day or two, when I have access to a scanner

eas

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I have a problem when exposing for scenes that have a bightly lit section, and one that might be in shadow. Using aperture-priority mode, with an averaging light-meter, the shot ends up exposing correctly for the highlights and leaving the shaded area somewhat underexposed

This may be more an issue with the dynamic range of your camera more so than it's exposure system. The Camera should always try and expose for highlights, so nothing wrong there. If you're shooting a scene with extreme range of tones from black to very pure light some info is going to get lost. At this point it's where the divide between film and digital opens. Larger format film can capture a much better range that most sub €2000 digis.

Does you camera have exposure compensation? This way you could balance the exposure a bit better for your needs.

I'm not sure why you don't want to use spot metering, but the exposure averaging you explain above is basically matrix metering or average over several areas of the sensor.

For an out of camera solutuion, there is a technique where you basically take two identical shots of the same scene, one exposed for highlights and one for the shadows. You than merge these two shots in Photoshop giving the impression that it's one shot and giving an extreme dynamic range.

anyhow, that probably doesnt answer your question, but hope it helps a bit anyway.

dalk

Not an expert but...

I suppose it depends on where you want to keep detail. And the exposure latitude of the film/ccd you are using. There isn't a film or ccd that can capture all detail from very dark to very bright. Some detail might have to be lost on either end of the scale.

But generally i would expose to retain detail in the hightlights, as you can generally claw a bit of detail back from the shadows (with colour negs & shooting RAW in digital), but if you blow out the highlights theres usually nothing there (especially a problem for digital cameras).

Also with some experience you can use +/- exposure compensation for certain scenes which you know wont be metered correctly... Though invariable i mess these up. :rolleyes:

Alun

Larger format film can capture a much better range that most sub €2000 digis.

I'm confused ... what possible influence can the physical format of the film have on dynamic range? Surely it's the film type that is of concern here, and some transparency film can have just the same dynamic range problems as digital has. I mean, split ND filters were around long before digital cameras were invented specifically for this kind of problem.

FinoBlad

Unless you know the difference between the highlights and shadows in f-stops you're unlikely to get anywhere with this problem.

eas

I'm confused ... what possible influence can the physical format of the film have on dynamic range?

good point, none that I can think of. AFAIK the exposure range would be dependant on the type of fim used, not he physical size of the film, my mistake.

I retract this entire statement

Larger format film can capture a much better range that most sub €2000 digis.

Not sure why I said that.......

beans

FinoBlad wrote:

Unless you know the difference between the highlights and shadows in f-stops you're unlikely to get anywhere with this problem.

That sounds about right. Should i be aiming for a weighted average of some sort between the high and low f-stop values, or am i overcomplicating the issue somewhat?

Also, is there any point in adjusting the ISO to over/underexpose film on a few frames only, and develop the rest of the film normally? Or would you have to over/underexpose the entire film similarly and have it processed to suit? I have read somewhere that most labs will compensate to bring everything to a middle-ground of exposure.

CoolBoardr

Here's a useful guide on how to expose for highlights/shadows - its for digital cameras so im not too sure how helpful it will be with manual.

http://photography-on-the.net/forum/showthread.php?t=147144 wrote:

• set your camera to Partial Metering (where it only evaluates the center portion of the image)
• Set to Av mode and set your aperature wide open
• dial in +2 EC
• aim the camera to the bright sky
• press the * button to lock the exposure based on the sky
• aim the camera at something else (friend/landscape)
• take a picture.

Here, you should notice that your image is close to right and your sky isn't blown out.

By dialing in +2 exposure compensation and locking the meter on the sky, you effectively told the camera that what you're looking at is 2 stops brighter than mid grey. The sky is actually very close to this on a bright day.

The cool thing about this is you're telling the camera that the brightest thing in frame (the sky) is 2 stops above mid grey... which will fit into your histogram.

If you check the histogram, you should notice that it drops off at the right side and not much (if anything) is over exposed (blinkies).

In summary... when you set the EC and lock the exposure on something (*) you're telling the camera that what you're looking at should fall on the histogram where the EC bar is.

http://photography-on-the.net/forum/showthread.php?t=147144

heffsarmy

Coolboardr thats a very good tip....but for landscape shots you would want to step down the aperature to get a greater depth of filed (use hyper focal distance)...but would work the same.

FinoBlad

beans are you doing black and white?

beans

CoolBoardr - that's actually very helpful, thanks. Just what I was after infact, I'll have to burn off another test film and incorporate this

FinoBlad wrote:

beans are you doing black and white?

I'm trying to get some decent results from colour film at the moment, wrt exposure. I had planned to tackle colour first before taking what I learn and applying it to mono film, even if I'm probably going about that arseways...