Why spot metering was invented

[ChrisLumix]

I've always liked this shot of a pair of owls in their enclosure - the colour of the plants and the mossy wall seem to be quite harmonious. Unfortunately, although the picture is well enough exposed, the main subject (the white owls) is massively over-exposed. 

 

It was taken on a Minolta XD7 with a Tamron 28-80mm lens. This camera, while very well specified - the world's first multi-mode camera, and the first to feature any kind of programmed mode - had the tried and tested 'centre-weighted' metering system.

 

The ONLY way to expose the owls properly for this composition would be to use spot metering, and under expose the rest of it. This would have resulted in a lot of fuss in the processing - masking off the owls and dodging the rest to bring out shadow details. Now of course, it's easy - just use the Shadows slider in an image editor, after taking the shot using spot metering on the owls, i.e. expose for the owls, under-expose the rest and then bring out the shadow details.

 

(Please note - this is a scan of a 6x4" print, so its technical quality is low!)

 

 

[ChrisLumix]

This is my own take on how you'd do it these days (in this wunnerful digital era!):

 

1. Select 'spot metering', assuming your camera supports it

2. Centre the 'spot' on one of the owls and lock the exposure (usually a half depression of the shutter)

3. Re-compose the shot while keeping the exposure locked, and take the picture 

 

Later, in post-processing:

 

The owls will be correctly exposed but the rest of the picture will be heavily under-exposed. The best way around this, IMO, is to use the Shadows slider (e.g. in Elements it's in the Quick menu) to bring out the detail in the under-exposed area, and if necessary increase Saturation too if desired.

 

-----------------

 

Another way to do it, is to 'guesstimate' the exposure compensation needed (in this case by dialling it down a stop or two)  or, if the camera is supported e.g. on a tripod, by taking a 'burst' of bracketed shots in quick succession. In either case you would still need to post-process to bring the rest of the shot up to scratch. 

 

If you've taken a range of bracketed shots at different exposures, there is now a way to merge them in Photoshop or Elements.

Edited September 4, 2013 by ChrisLumix

[Guest DaveW]

Spot metering is not just pointing the spot at part of a subject and using that exposure unmodified though. The spot will only give the correct exposure if the surface it is pointed at integrates to a mid grey. Point the spot at something white and it will give the exposure to render it mid grey, not white. Point at something black and again it will give the exposure to render it mid grey, not black.

 

People who spot meter need to know what the tone they point at integrates to on the exposure scale and apply the appropriate correction to the exposure. If you don't then spot metering is far more likely to produce a wrong exposure than modern integrating metering systems like matrix metering etc.

 

Really in order to spot meter you need to know a version of Ansel Adams Zone System, even if working in colour and not monochrome as he was.

 

http://clickitupanotch.com/2013/01/zone-system-the-basics/

 

Also be aware that in camera spot metering is not like using a proper separate hand held spot meter with a 1^o spot, but dependant on the lens you have fitted.  You should not spot meter with a wide angle lens fitted since the spot will probably cover the whole image and be no better than the primitive camera metering days when the scene was just integrated to mid grey and nowhere near as good as modern intelligent metering systems like matrix. 

 

In fact in camera spot metering should really be used with telephoto lenses which decrease the diameter of the spot sampled to more akin to a proper hand held spot meter.  If you want to spot meter for a wide angle lens the proper way to do it is to fit a telephoto lens and take a reading of the surface you want, apply any exposure correction needed using the Zone System, refit the wide angle and take your shot.

 

The spot size you see marked on your camera screen is only an indication of the spot area to be measured, obviously being etched there it does not alter when different lenses are fitted, but the spot the lens samples certainly does. It will be smaller than indicated when long telephoto's are fitted, but much larger with a wide angle lens, so probably is only correct for what would in the past have been called the old standard lens focal length.

 

To quote from the following link:-

 

"Step One: You Need a Reliable Hand-Held Spot Meter

A 1° measuring spot lets you measure important areas precisely and with ease.  Using a camera’s “spot” metering mode is not always practical:  the size of the spot depends on the focal length of the lens and generally requires a lot of button-pushing and pointing the camera this way and that – an exercise in frustration and wasted time."

 

http://www.alanrossphotography.com/category/tech/zonesystemandmetering/#sthash.ly7nLsuC.dpuf

 

http://spotmetering.com/spwhy.htm

 

It amazes me the number of times you hear digital photographers say I spot metered so the exposure should have been correct, without of ever applying the exposure compensation factor based on the tone of the spot they were metering. What has really happened is they have got the spot exposure wrong at the taking stage but rescued it using post processing software later. If you don't understand the Zone System stick to conventional metering.

 

This was what I was taught in night school for a spot reading off the light side of the face for most portraits.  Remember all spot readings will give an exposure equivalent to a mid grey tone, therefore apply at least the following exposure corrections:-

 

Caucasian skin = underexposes so + 1 stop to make it lighter 

 

Mid Asian skin = as per the spot reading 

 

Black African skin = overexposes so - 1 stop to make it darker

Edited September 23, 2013 by DaveW

[NannyFerret]

So if I were to spot meter a very white swan then  would I have to over-expose 1 stop the same as for caucasian skin?
I usually take the slider to the centre by either changing the aperture or shutter speed when it tells me that it's over-exposed.
 

[Guest DaveW]

Only if spot metering off the white swan Nanny, probably somewhere between one to two stops, using the "blinkies" if your camera has them would show if too much compensation was given. I would think white fur and white feathers would need similar compensation:-

 

Zone VI (6)- Average Caucasian skin tone, most pastel colours, fog, light blue sky. Will read +1 on in-camera meter.

 

Zone VII (7)- White with detail, white fur, white clouds, white sand, snow, whites in running water. Will read +2 on in-camera meter.

 

http://www.discovermirrorless.com/hybrid-photography/check-your-exposure-using-the-blinkies/

 

http://mikeatkinson.net/Tutorial-5-Exposure.htm

[NannyFerret]

Thanks, I will try to remember that  when I next come across swans,