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DPChallenge Forums >> Tips, Tricks, and Q&A >> Just read 'Understanding Exposure"..some questions!
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 11/06/2006 06:56:28 AM · #1
I just finished reading 'Understanding Exposure' and had some questions!

1) Starting on page 128, "Night and Low-Light Photography," he meters off the sky using an aperture of f2.8. He then recomposes the shot and adjusts the aperture & exposure to f/22. Why doesn't he just meter the sky at f/22 if that is the aperture he wanted to use to begin with?

2) In many places in the book, he talks about focusing which I do not understand. Here are some examples:
-Page 114: "...I chose and aperture of f/22 and preset my focus via the distance scale on the front of the lens. I knew before I took the exposure that everything from two feet to infinity would be in focus."
-Page 41: "..preset the focus via the depth of field scale.

Are these things done in manual focus? Can you explain the infinity thing?

Thanks for any help :) It is a pretty good book. However, I wish he covered flash photography and exposure ;o)
 11/06/2006 07:22:43 AM · #2
Just a quick answer for the focus question - he's using the hyperfocal scale. Do a google for "hyperfocal" and you'll find some info. Sorry I don't have more time to respond right now, but it's a wonderful thing that most autofocus lenses and nearly all zooms no longer have. It can be done, but the scale is not there, as it's dependent on the focal length of the lens. Very often a prime lens feature. Also used more in the film days than now, it seems.

He possibly metered at f2.8 because metering at f22 would have resulted in a longer exposure time than the camera meter would indicate. (This is usually handled by using a "bulb" setting and manually timing.)
 11/06/2006 03:19:49 PM · #3
*bump* :o)
 11/06/2006 03:22:57 PM · #4
Originally posted by nards656:
Just a quick answer for the focus question - he's using the hyperfocal scale. Do a google for "hyperfocal" and you'll find some info.

He possibly metered at f2.8 because metering at f22 would have resulted in a longer exposure time than the camera meter would indicate. (This is usually handled by using a "bulb" setting and manually timing.)


Thanks. I did a search and so many of the site assume you already know how to set the lens for hyperfocal focusing. \

Can anyone explain this more with the lenses I have?

As far as metering, there is no reason I couldn't meter at night at f/22 then? His camera just had limitations?

11/06/2006 03:34:32 PM · #5
There's no reason you couldn't meter at f22 however, the meter may be flashing at the far left showing that you need a longer exposure.

Metering with the lens wide open gives you the starting point from which you work out what exposure will be at the aperture of your choosing.

example: Let's say at f2.8 the meter says the exposure should be 10 seconds. What would that be at f22?

f2.8 10s
f5.6 20s
f11 40s
f22 80s

By f11 you're into bulb mode (in the example)

Remember by closing the aperture a full stop you are letting in 1/2 the amount of light therefore you need to double the exposure time.

Message edited by author 2006-11-06 15:34:59.
 11/06/2006 03:35:25 PM · #6
Originally posted by JRalston:
I just finished reading 'Understanding Exposure' and had some questions!

1) Starting on page 128, "Night and Low-Light Photography," he meters off the sky using an aperture of f2.8. He then recomposes the shot and adjusts the aperture & exposure to f/22. Why doesn't he just meter the sky at f/22 if that is the aperture he wanted to use to begin with?

Not sure on that one...
Originally posted by JRalston:

2) In many places in the book, he talks about focusing which I do not understand. Here are some examples:
-Page 114: "...I chose and aperture of f/22 and preset my focus via the distance scale on the front of the lens. I knew before I took the exposure that everything from two feet to infinity would be in focus."
-Page 41: "..preset the focus via the depth of field scale.

Are these things done in manual focus? Can you explain the infinity thing?

Thanks for any help :) It is a pretty good book. However, I wish he covered flash photography and exposure ;o)


The second part is for manual and mostly non-zoom prime lenses. Many of the good ones will have a hyperfocal scale set on the side of the lens. You can also get some hyperfocal info at //www.dofmaster.com since most newer and zoom lenses do not have that feature.
 11/06/2006 03:48:26 PM · #7
Originally posted by wavelength:
Originally posted by JRalston:
I just finished reading 'Understanding Exposure' and had some questions!

1) Starting on page 128, "Night and Low-Light Photography," he meters off the sky using an aperture of f2.8. He then recomposes the shot and adjusts the aperture & exposure to f/22. Why doesn't he just meter the sky at f/22 if that is the aperture he wanted to use to begin with?

Not sure on that one...


Because typically at night, you'll be off the scale of your meter at f22. For night shooting I quite often switch to ISO 3200, lens wide open and meter. You'll get something in the 1-30 seconds range, (i.e., within the range of the meter)

You can then half the ISO and double the time repeatedly to get to a suitable low noise ISO and correct exposure time. Then you can stop down the lens and double the time again for each aperture stop you change.

That way you'll get to a low ISO, smaller aperture exposure, with a suitable time, that lets you take a decent night shot. You don't need another light meter other than your camera with this method.

I also typically shoot the wide open, high ISO version to get a composition reference, prior to shooting the 'real' exposure. You can see the composition from the preview and approximate exposure level from the histogram.

I then usually end up shooting at about ISO 400 and f4 or f5.6 for star trails. Anything more stopped down than that and you don't get enough light in to record much of a trail. But higher apertures are more useful for architectural night work and similar.


11/06/2006 03:49:51 PM · #8
Originally posted by cpanaioti:
There's no reason you couldn't meter at f22 however, the meter may be flashing at the far left showing that you need a longer exposure.

...

Remember by closing the aperture a full stop you are letting in 1/2 the amount of light therefore you need to double the exposure time.


Thank you. I think I was confused because he didn't use that slow of a shutter speed in the picture on p129. It really confused me..LOL

Originally posted by from Understanding Exposure:
I set the focal length to 35mm and the aperture to f/2.8, and then raised the camera to the dusky blue sky above the mountain range. I adjusted the shutter speed until the meter indicated 1/8sec as the correct exposure. I then recomposed, stopped the lens down to 5 stops to f/16 and increased my exposure time by five stops. With my exposure time now set for f/16 for 4 seconds, I tripped the shutter with my cameras self-timer.

 11/06/2006 03:50:52 PM · #9
Originally posted by wavelength:

The second part is for manual and mostly non-zoom prime lenses. Many of the good ones will have a hyperfocal scale set on the side of the lens. You can also get some hyperfocal info at //www.dofmaster.com since most newer and zoom lenses do not have that feature.


Thank you for the link :)
 11/06/2006 03:51:51 PM · #10
The book was written based on film cameras.

1) He didn't want to overexpose the sky?

2) Older lens frequently had distance scales - given a focal length and aperture what is the range of acceptable focus. The newer lenses that I have for my digital cameras don't have this distance scale. You can find DOF charts on the web if you need to know this distance.
 11/06/2006 03:57:03 PM · #11
Thank you all.

So being that I am a digital user, what is the best way to maximize DOF? I know to use a small aperture, but where do I focus? Let me pullup a couple pictures for discussion...



Say I am using an aperture of f/22, do I focus 1/3 of the way from the bottom as the book suggests? The one with the wall, it is probably more important that the wall is in sharp focus, but what about the road one? I know the pictures turned out fine, but I want to feel confident I am doing things right when I take the picture.

I use my Canon 10-22mm and my Canon 24-70 2.8L most often for landscapes.

Message edited by author 2006-11-06 15:57:57.
 11/06/2006 04:05:47 PM · #12
1/3 of the way into the scene (from the bottom of the frame) is about right for f22. This will probably be a good focus point for f16 as well.

11/06/2006 04:06:15 PM · #13
Originally posted by JRalston:
Thanks. I did a search and so many of the site assume you already know how to set the lens for hyperfocal focusing. \

Can anyone explain this more with the lenses I have?


Remember that when you focus at say, 10 feet, only the stuff at exactly 10 feet is *technically* in focus. For practical purposes, though, things that are both in front and behind this "plane of focus" are so close to "in focus" that the camera cannot tell the difference. They are within the DoF. As we know, the DoF changes with both f/ratio and focal length; as f/ratio is increased (smaller aperture size) or focal length is decreased, the DoF becomes larger. Following?
Now, if we wish far-off objects to be in sharp focus, we can set our focus on them. We'll still have some nearer stuff in focus, due to the near portion of the DoF. We can, however, set teh focus somewhat closer, and still keep even the most distant objects in focus, since there is some DoF available *behind* the focus plane!. The focus distance that places very distant objects (objects "at infinity") at the far end of the DoF *behind* the plane of focus is the "hyperfocal distance." It varies with aperture and focal length, just like the DoF. You also need to assume a value for the circle of confusion (CoC); don't worry too much about understanding this, just use twice your pixel pitch, about 0.014mm. Here is an on-line calculator for DoF which also calculates the hyperfocal distance. Playing around with it may give you an idea of how to utilize hyperfocal distance. Selecting the 20D from the cameras list uses a larger number for the CoC, and if you shoot using this assumption, you'll find that very distant objects are slightly blurry. If you scroll way down on the cameras list, you can set a CoC for any of the listed values.

Originally posted by JRalston:
As far as metering, there is no reason I couldn't meter at night at f/22 then? His camera just had limitations?


You surely can meter at any aperture that yields a valid meter reading for your camera. You will find that for very dark scenes, you'll only be able to meter at larger apertures. For a moonlit night, for instance, exposures of >30 seconds are normal, even with very fast glass metered wide-open.
In my experience, it's often useful to take a test exposure wide open at 1600 ISO, look at the image results and histogram and calculate the required exposure at the desired ISO and aperture from that. Much faster than pure trial and error, and more accurate than metering one area.
 11/06/2006 05:30:58 PM · #14
Originally posted by cpanaioti:
1/3 of the way into the scene (from the bottom of the frame) is about right for f22. This will probably be a good focus point for f16 as well.


This is often quoted but as often misunderstood I think.
It is actually 1/3rd of the depth of the area you want in focus, as for non-macro scales, the acceptable focal range extends about 1/3rd of the way infront of the actual focus point and 2/3rds of the way beyond. It only makes sense to say '1/3rd from the bottom' if the scene is a flat plane leading away from you.

The hyperfocal point is that point where the 2/3rds behind the focal point reaches to infinity.
11/06/2006 06:45:17 PM · #15
Originally posted by JRalston:
I use my Canon 10-22mm and my Canon 24-70 2.8L most often for landscapes.


At focal lengths 10-22mm you won't NEED to use f/22 to get everything in focus.

At the long end of your lens (22mm), you can get everything in focus from 4' to infinity just by selecting f/11 as your aperture and focusing on something 10 feet away. And at the 10mm super wide end, it gets even better. At f/4 you'll have everything in focus from 3' to infinity by focusing on an object 10 feet away.

My point is .. don't go overboard on your aperture. There is a law of diminishing returns. At apertures greater than f/11 diffraction becomes a factor (i.e. the pixels on your sensor and the optical quality of your lens become limiting factors for how "sharp" the image may be). For sharp images, I try to never go beyond f/11 for that reason. Instead, I reserve the smaller apertures for longish exposures where I expect to see some fuzziness - perhaps due to movement.

You can read more about the topic of diffraction here.


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