Calculating for long exposures (or how Shutter Speed and Aperture relate Part II)

This article is kindof an extension of this one here. You may want to read the previous one before continuing.

When you want to take photos at night that require shutter speeds of over 30 seconds, you're left to calculate the settings on your own. If you grab your camera and put it on Manual Mode, set the shutter speed to 30sec, and then go one more setting down, you'll see "Bulb" or "B". When you put the camera on "Bulb", you'll should notice that it won't calculate aperture or give you a meter to read to see what your exposure will be. This is because with "bulb", the shutter speed depends on you to open and close the shutter manually. You do this with a bulb cable that will allow you to push a button down to open the shutter, and then lock it down to keep the shutter open until you release the button to close the shutter.

So, how do you know how long to keep the shutter open? Well, this is where the ability to add and subtract stops of light comes in. If you understood the previous article, then it should be easy:
* set your camera to aperture priority and dial in the aperture priority mode
* dial in the aperture you want to use
* set the ISO to 800 or 1600 -whatever will give you a shutter speed of 30sec or close to it
* take a test shot -does it look close to what you were hoping for? If not, use exposure compensation to tweak it now instead of tweaking it when you're waiting 5min between shots (I never remember to do this, but often kick myself for not doing it after the first shot :-P)
* when you get a good shot, write down or remember the settings you now have for shutter speed, aperture, and ISO
* switch to manual mode and choose "bulb" for your shutter speed
* do the math to dial the ISO down to reduce noise while using longer shutter speeds to keep the exposure the same

The following will all give you the same exposure:
30sec f/16 ISO 1600
1min f/16 ISO 800 <-- subtract 1stop of light w/ ISO, add 1stop w/ shutter speed 2min f/16 ISO 400 <-- ditto, on down the list 4min f/16 ISO 200 8min f/16 ISO 100 If it's really dark or you're trying for longer shutter speeds, to get star trails for example, then you may need to use a smaller aperture for your first 30sec exposure as well, then calculate a smaller aperture for more depth of field as well as for reducing ISO. It makes it a bit more complicated, but not by much: And here's an example for even darker photos (like star trails): 30sec f/4 ISO 1600 <-- subtract 2 stops of light w/ aperture, add 2 stops w/ shutter speed 2min f/16 ISO 1600 <-- ditto 4min f/16 ISO 800 <-- subtract 1stop of light w/ ISO, add 1stop w/ shutter speed 8min f/16 ISO 400 16min f/16 ISO 200 32min f/16 ISO 100 I literally write this out if I can't do it in my head while doing long exposure shots. There may be an easier way (tables?), but a quick google search of this question on Flickr and several forums reveals that most people just calculate it out this way. After you get the idea of adding a stop and subtracting a stop, than it's just a matter of remembering how the settings effect light. If you aren't 100% sure about it, it's easy enough to write/print out the following on a small slip of paper, "DIY laminate" it with packing tape, and keep it in your camera bag: * Shutter speed: more time = more light * Aperture: lower number = more light * ISO: higher number = more light

* For shutter speed: 1 stop = half or double the number

* For aperture: 2 stops = half or double the number

* For ISO: 1 stop = half or double the number

How aperture relates to shutter speed and vice versa

Learning how aperture and shutter speed (and ISO) relate to one another is an important thing to know when you're learning to use manual mode as well as the priority modes. But, this too is easy!

If you find a combination of settings that gives you the magic "0" reading on your meter (which means the photo should be exposed properly), and then you want to change the aperture value to get more depth of field, it's fine -all you have to do is some elementary math:

You may have heard of a "stop" of light? That's what those numbers on your exposure meter are -"stops" of light.

(Your exposure meter looks like this: [-2...-1...0..+1..+2] although Nikon flips the numbers around.)

The handy thing about measuring light in "stops" is the fact that when you half or double the number you use for ISO, aperture, or shutter speed -that's a "stop". So, in this example, if you remove a "stop" of light by using f/8 instead of f/4 your photo will come out too dark (at -1 on your exposure meter). But, by doubling the shutter speed to ADD a stop, you'll be back at "0" (correct exposure)! But if we want f/16, then you're doubling the number twice, so we have to half the shutter speed twice, which gives us 1/250 at f/16.

All you have to remember is how the settings effect the light that hits your film or sensor:
* Shutter speed: you gain a stop of light by halving the shutter speed.
* Aperture: you gain TWO stops of light by halving the number.
* ISO: you gain a stop of light by doubling your ISO.

So, as an example assuming your ISO is set to 100 and doesn't change, 1/1000th at f/4 would be the same as all of the following:
* 1/500 at f/5.6
* 1/250 at f/8
* 1/125 at f/11
* 1/60 at f/16
* 1/30 at f/22
...if you wanted a shutter speed of 1/15 you can't double your aperture to 1/32 -that setting isn't available on most lenses. So, you'd have to get that extra stop of light by doubling your ISO instead.

So, assuming your aperture was set to f/4 and doesn't change, 1/1000 at ISO 800 would be the same as all of the following:
* 1/500 at ISO 400
* 1/250 at ISO 200
* 1/125 at ISO 100

If you're not one for math you could even just count the clicks up from f/4 to f/16, then change the shutter speed down by the same number of clicks to get back to correct exposure. ...the math really isn't hard though.

This may be hard to read and understand, but it's really easy once you do it a few times. A great way to see this for yourself is with this camera simulator:
http://www.photonhead.com/simcam/shutteraperture.php


One reason that you may want to know how to do this is night shots. For exposures longer than 30 seconds, you need to calculate the settings yourself. More info on this can be read in Part II of this article here.

Technical Photography Videos

Generally this blog's been aimed at beginners, but I found this Google TechTalk series on photography on YouTube and I had to share it. It's definitely advanced stuff, talking about exactly how lenses work, how a digital camera sensor works down to the atomic level at times. There's also some stuff on printing, HDR, and other topics as well (even one about eyesight).

There are 26 videos in all, each about an hour long, so if you watch all of them (like I've been doing), it'll take you awhile. Some presenters are good, others are quite boring, but all-in-all I've found it very interesting and I thought I'd share. I even rolled them into a playlist for you so you don' t have to hunt through them to watch them in order like I did ;-).