Exposure question - confused !

[py6km]

Just been reading Understanding Exposure, and there's something I don't get. In the section on low light/night photography, Bryan explains on most images that he meters from the dusky sky - fine. However, he meters at an aperture of, say, f/2.8 to get the corresponding correct shutter speed. He then recomposes the shot, but adjusts the aperture (to f/22 for example) and adjusts the shutter speed by a corresponding amount.

For example, if he meters off the sky at f/2.8, but then composes the shot proper at f/22, he adjusts the shutter speed given at f/2.8 by 7 stops/positions to give the correct shutter speed at f/22 (since f/22 is 7 stops from f/2.8).

My question is: why not just meter off the sky at the aperture you want to use int he shot, e.g f/22 ? Would that not give the same result ?

[Tobers]

Good question there. I'd hazard a guess that the camera meters more accurately the more light it has hitting the light meter therefore meter at 2.8 then close the aperture to what you want the shot to look like.

I think he says a few times about metering wide open then adjusting accordingly (too much maths for me though - not quite worked it all out yet).

[py6km]

Ok, yes, that makes sense. It would be interesting to see if the same results were achieved using his method and 'my' method

[Cadire]

I think that his camera did not have a shutter speed that corresponded to f2.8 @ 1/4 at f22 (30 seconds), so the exposure time could not be calculated if the metering was done at f22.

Metering first at f2.8 meant that he could calculate the correct shutter speed (for f22), and time it manually with his camera set to 'Bulb'.

[Liquid101]

This concept is a bit dated I think. There was a time when meters were more accurate when the lens was wide open - in fact some early lens/camera combinations required it.

Not sure that modern cameras and lenses would suffer this kind of thing. Might be worth experimenting though. That said, any subtle differences that these readings give would be meaning less when shooting in RAW.

[py6km]

So you think that metering at the aperture you want to use to take the shot would work, and his method is an artefact of his knackered out old camera/lenses?

[T0MAT01]

yeah dont worry about it. The lens is still wide open unless you press the depth of field preview button anyway.

[Tobers]

Well, I meter at the aperture I want so I think you're right - a historical throwback. Might be worth a few experiments though and it'll certainly get your brain around the aperture/shutter calculations.

[Cadire]

Originally Posted by py6km

So you think that metering at the aperture you want to use to take the shot would work, and his method is an artefact of his knackered out old camera/lenses?

It depends if your camera has a shutter speed that long. If a 'correct' exposure was 30 seconds @ f22, and your camera had a maximum shutter speed of 15 seconds, how would you ever meter it? Using his method allows you to do that.

[py6km]

Cheers everyone - I'll give it a go.

While I'm on the subject of exposure, and for those familiar with the book - why does a shot taken using his 'Mr Green Jeans' approach need to be exposed at -2/3 below the reading you get when you meter off any greenery ?

I've not really figured out how to do that on my D200 yet - does anyone have any experience?

[Liquid101]

Originally Posted by py6km

Cheers everyone - I'll give it a go.

While I'm on the subject of, for those familiar with the book - why does a shot taken using his 'Mr Green Jeans' approach need to be exposed at -2/3 below the reading you get when you meter off any greenery ?

I've not really figured out how to do that on my D200 yet - does anyone have any experience?

The exposure compensation button just below and the right of the shutter release. It's marked with a +/- hold this down and use the command wheel to set the compensation required. I always shoot at -0.7 - I feel the D50 exposes a little bright for my tatses. I think it roughly uses 3rd stop steps.

[py6km]

Would you adjust the exposure to -2/3 before or after you manually meter? I'm concerned that by changing the exposure manually, it will affect how the camera meters (i.e. it will try to compensate for the reduction I've made)

[Liquid101]

Originally Posted by py6km

Would you adjust the exposure to -2/3 before or after you manually meter? I'm concerned that by changing the exposure manually, it will affect how the camera meters (i.e. it will try to compensate for the reduction I've made)

I'm pretty sure it wouldn't make any difference. If you set it before it will expose with the comp - set after and it will adjust the exposure.

[Gordon @ Convergent AV]

I woudl think that modern dslr actually work out exposure with the aperture wide open then they work out the maths themselves based on your set fstop. IE When you push the button for exposure lock the aperture doesn't close down through the viewfinder......it only closes the aperture when you actually do the exposure.

Gordon

[py6km]

@ Liquid101

Maybe I'm getting mixed up here, but wouldn't that therefore mean that there was no point in adjusting the exposure ?

I mean, if you set it before, and then meter, the camera meters taking into account the exposure compensation; set it after, and the camera adjusts the shutter speed (or whatever) to maintain the exposure at the metered level - in other words, the camera works to negate the effect of the exposure compensation you've specified in order to get what it thinks is still correct ?

Would you not need to meter, and record those values, then set the exposure compensation to -2/3 and use the values for exposure obtained prior to your compensation in such a way that the camera is not able to adjust anything to try to get back to 0eV...?