metering the sky at Apeture 2.8 and recompose

[hao]

I have been reading "Understanding Exposures" , which is a good book btw.
In the book, the author metions a technique for shooting backlight, snow, or sunset. What you do is that you meter the sky to get your aperture and shutter speed first then recompose to get a proper exposure.

What confuses me is that he likes to meter the sky with an 2.8 aperture, and adjust the shutter speed accordingly. Then it comes to recomposing. A bigger aperture number, say 16, is neededfor landscape, so aperture is adjusted to 16 (5 stops) and the shutter speed needs to be decreased by 5 stops accordinly for the correct exposure.

My question is:

Why do we need to meter the sky at aperture 2.8 instead of the aperture we need, 16,at the first place? Does it give a better metering result?
thanks!

 

[Tiberius47]

I have no idea why he would do this. You can set your aperture to whatever you want and then adjust your shutter speed to correctly expose for the sky.

 

[PhotoXopher]

He just uses the sky to meter the light, aperture has nothing to do with it other than coincidence that he's shooting wide open with his 35-70 a lot in that book.

 

[Josh66]

He just uses the sky to meter the light, aperture has nothing to do with it other than coincidence that he's shooting wide open with his 35-70 a lot in that book.

Yeah, but he's metering, then adjusting his settings (that he just got done setting) by 5 stops!

I think I would just meter in whatever aperture you plan on taking the picture at... Less to remember that way. "Now, how many stops did I change the aperture...? Oh, crap - what did it say the shutter speed was before I started changing everything...?"


I'm sure he has a reason for doing it his way, but I have no clue what it could be.

 

[Josh66]

The ONLY time I do something like what he's doing is if I'm going to have to go into Bulb.

Let's say I want a picture of an old barn in the middle of the night... (That I still haven't made it back to, BTW...)

You can't really meter past 30 seconds, so you have to do a little math in your head to figure out what it needs to be.

Let's say f/1.4 is 2 stops underexposed at 30 seconds. That means you need an exposure of 120 seconds (30*2*2). But - what if you don't want to shoot at f/1.4?

Let's say you want to shoot at f/8. That's another 5 stops. 120*2=240*2=480*2=960*2=1920*2=3840.

Get your watch ready, because that's 64 minutes.

OK, so you don't want to sit there that long... f/4 (2 stops less) sounds more reasonable. That would be 16 minutes (960 seconds). You could drop down to f/2.8 and only sit there for 8 minutes too.


Anyway... I think it's important to know how to figure it out - but don't make it harder than it has to be.


edit
(Maybe that's why he says to do it that way in his book - so you'll know how when you need to.)

 

[Big]

I actually saw a guy do this one day. I was taking a walk on the coast and he was photographing a girl at the ocean and I noticed he kept pointing it up at the sky and then back to her.

 

[PhotoXopher]

That'll be the next big feature - exposure preview

 

[Dao]

Here is the reason why Bryan Peter do that in the cityscape type shot.

He metered the sky. Then change to F/11 and 10s shutter speed based on the metering.

 

[bhop]

Here is the reason why Bryan Peter do that in the cityscape type shot.

He metered the sky. Then change to F/11 and 10s shutter speed based on the metering.
...

Actually, in the video he mentioned that since the subjects were far away, f/stop didn't matter, so he chooses f/8 or f/11, then adjusts his shutter speed (in this case 10 sec), he never actually said why he was metering the sky.

As a side note.. fancy hairstyle he's got.

 

[Garbz]

That'll be the next big feature - exposure preview

Ha sucker. My D200 has this feature already. I click the shutter button and on the screen comes an exposed preview.

 

[musicaleCA]

Considering it's a cityscape, metering off anything else is kinda useless. If you meter the buildings, you might get a reading that turns the lights to neutral, except that's not what you want at all. He's just deciding that the sky should be in the middle of the dynamic range of the image. If you think about it, the why becomes clear.

 

[KmH]

Here is the reason why Bryan Peter do that in the cityscape type shot.

He metered the sky. Then change to F/11 and 10s shutter speed based on the metering.
...

Actually, in the video he mentioned that since the subjects were far away, f/stop didn't matter, so he chooses f/8 or f/11, then adjusts his shutter speed (in this case 10 sec), he never actually said why he was metering the sky.

As a side note.. fancy hairstyle he's got.

But, everyone here understands why he metered the sky, right?

The exercise with calculating a change in stops the OP mentiones was just to show the relationship of aperture to shutter speed. To maintain the same exposure, if you change one, you have to change the other an equal amount.

 

[musicaleCA]

Here is the reason why Bryan Peter do that in the cityscape type shot.

He metered the sky. Then change to F/11 and 10s shutter speed based on the metering.
...

Actually, in the video he mentioned that since the subjects were far away, f/stop didn't matter, so he chooses f/8 or f/11, then adjusts his shutter speed (in this case 10 sec), he never actually said why he was metering the sky.

As a side note.. fancy hairstyle he's got.

But, everyone here understands why he metered the sky, right?

The exercise with calculating a change in stops the OP mentiones was just to show the relationship of aperture to shutter speed. To maintain the same exposure, if you change one, you have to change the other an equal amount.

I wonder: does anyone else just count the number of notches you travel on the dial, and then go the other way with the other setting, ignoring calculating stops altogether? Works a heck of a lot faster with me (I mean, I guestimate first where my shutter speed or aperture is going to land, and adjust ISO to compensate if either goes somewhere I don't want, but still).

 

[Garbz]

Exactly. I only ever calculate things when i'm sitting here posting on this forum. Every other time it's 3 stops ok that's 9 clicks on the wheel.

 

[musicaleCA]

Exactly. I only ever calculate things when i'm sitting here posting on this forum. Every other time it's 3 stops ok that's 9 clicks on the wheel.

Yeah. Or when the exposure is super long, but then it's a piece of cake to keep multiplying by 2.