Thursday, August 7, 2008

Never Surrender - Understanding Exposure and Why it's Important

Everyone knows it happens, but most people don't know how it happens or why it happens.

The automation of cameras has made users lazy. You can just set it to "Automatic" or "Program", press the shutter button, and voila, a properly exposed image.

Q. "Why do I need to know all of this stuff if the camera will figure it out for me?"

A. Because it gives you control, and surrendering creative control of your work to a machine is never a good idea.

Exposure is the process of allowing the appropriate amount of light into your camera for the appropriate amount of time to properly expose your cameras sensor or film. To understand exposure, you need to understand three things; ISO, Aperture, and Shutter Speed. Contrary to popular belief, there are a hundred different combinations for any given scene that will give you an identical and proper exposure.

Let's talk about these three elements individually.

ISO

ISO is the same as "Film Speed". I know, there is no film in a digital camera. But there is a sensor and when you adjust the ISO, you are adjusting the sensors sensitivity to light, the same way that changing film to a film with a different film speed would make the film either more or less sensitive to light.

Common ISO's are:

100
200
400
800
1600
3200

100 is the "slowest" or least sensitive to light, and 3200 is the "fastest" or most sensitve to light. If you look at these numbers closely, you'll see a pattern here. Each number is either double or half of the number on either side of it. This means that each ISO is either twice as sensitive to light, or half as sensitive to light, as the ones right above and below it.

Shutter Speed

Shutter Speed is the amount of time that the shutter is open and allowing light to pass through onto the film or sensor.

Common shutter speeds are:

1/2
1/4
1/8
1/15
1/30
1/60
1/125
1/250
1/500
1/1000
1/2000
1/4000

Please note that these are all fractions of a second. Some cameras eliminate the fraction, just using the denominator as an indicator. I once had a student proclaim "There is no way that I can hold still for 125 seconds!" She was both right and wrong. She was right that she could not hold still for 125 seconds. She was wrong in thinking that the proper exposure time was 125 seconds. It was 1/125th of a second. It's much easier to stay still for 1/125th of a second.

Now look at this list of numbers.... do you see the pattern? Each shutter speed is either twice as fast or half as fast as the one on either side of it. Each time you move the shutter speed one step, you are either letting light in for twice as long or half as long as the previous shutter speed you were using.

Aperture

Aperture is the number that refers to the size of the opening in your lens that controls the amount of light that is let through the lens. This light is stopped prior to reaching the film or sensor by the shutter. Do not confuse "aperture" with "f-stop". They are similar and related, but not the same thing.

Common apertures are:

1.4
2.0
2.8
4
5.6
8
11
16
22
32

This is where it can get tricky. The smaller numbers at the top of the list are the larger openings, letting in more light. The larger numbers at the bottom of the list are smaller, letting in less light. The aperture of a lens is very similar to the pupil of your eye, expanding and contracting to let in the proper amount of light for any given subject. While it may not be readily apparent, each aperture in this list is either twice as large or half as large as the aperture on either side of it, letting in either twice as much light or half as much light as the neighboring openings.

Smaller Number = Larger Opening
Larger Number = Smaller Opening


Putting it All Together

It is the combination of these three things that gives you a properly exposed image. The "Sunny 16" rule states that on a bright sunny day outdoors, if you set you camera to f16 and your shutter speed to the inverse of your ISO (or as close as you can), you will get a proper exposure. The red lines on the chart below show the exposure recommended by the "Sunny 16" rule. From that point, all of the other exposures are extrapolated. All of the white lines on the charts are also proper exposures.


Scott Bulger Photography

Starting from the suggested red line exposure, if you want a faster shutter speed, all you have to do is adjust your aperture the same amount of steps in the opposite direction. If you want a different aperture, all you have to do is adjust your shutter speed the same amount of steps in the opposite direction. See how easy that is?

Why do you need all of these options? Control. You want to control how your final image looks. Shutter Speed controls motion, and Aperture controls Depth of Field, so by adjusting your exposure based on these factors, you can control how much Depth of Field you have or you can control how much Motion is shown in your photographs.

In my next installment of "Never Surrender", we'll discuss how controlling the Depth of Field and Motion will affect your final image.

15 comments:

KIM said...

EXCELLENTE!!!!!!!!!!!!! I can NEVER see this too many times! You've broken it down to such an understandable level it's ridiculous! LOL! Off to PRINT this out and carry it around.
THANKS Scotty!

Crash said...

Scott, this is phenomenal. You answered so many questions of mine with this one post. Thank you so so so much! Sue (from Buzznet and myspace)

Sewlutions' World said...

Wow! Very informative, I thank you for that.

LazyTcrochet said...

Thanks for the refresher course. I will be coming back to study some more!

pollyhyper said...

Terrific post! You've done a great job of illustrating the points.
I'm so glad I learned to shoot with a fully manual camera (my first was a Pentax K1000) so that I was forced to learn all of this so that now it comes naturally. You're right - so many people rely on "auto" now. They miss out on so much!

lizness said...

Thank you soooooo much, Scott! I've been trying to master exposure (which means wrapping my head around what I see vs. what I get). Your charts are so helpful.

Cherry Lane Jane said...

Very informative! I really need to get a better hand of the camera.

Peter said...

thanks, Scott!

simple, clear, informative. i hadn't heard of the "sunny 16" rule, now i have i will be trying it for sure!

Judi FitzPatrick said...

Scott, thanks for the reminder about taking control. Since "going digital" I've been guilty of using program mode most of the time, seems like I need to get a lot more intimate with my DSLR and learn how to change those settings for myself. Thanks again for all the great posts. Peace, Judi

Sarah McBride said...

thank you so much for this post! I am bookmarking it as a favorite. Thank you for breaking down the basics. I am always stumped with it comes to shutter speed, aperture, etc. Thank you thank you thank you!!

Karen Casey-Smith said...

Thank you for sharing your knowledge, Scott. You are a wonderful teacher and such an accomplished photographer. I always look forward to reading your posts.

woolies said...

awesome, learned something new!
thank you!

Grasshopper said...

Excellent information!!!

The relationship between shutter speed, ISO and aperture was the HARDEST thing for me to understand. All of my books talk about it but for the life of me I could never "Get it." It took a long time for me to finally understand it and I still don't FULLY understand it. Your charts with the red & white lines are awesome....a great gift. I am going to have to print this out and stuff it in my camera bag for my next field trip. Hopefully within time I will have it all memorized as to where I can explain it to someone as well as you can.

I do have one question. You mantioned F-stop and aperture. I know they are closely related and you mentioned that they are not the same. Could you fill us in on that part sometime? I still haven't grasped F-stop in relation to aperture.

Thanks again Teach!!!

Chantal Powell said...

Just stumbled across your blog - fantastic! Thank you so much for sharing your knowledge. I've not long had my DSLR and I'm really enjoying learning with it - your site will be checked by me frequently!
I've been posting some of the self portrait shots I've been trying out recently on my blog and if you had a moment to look I would really appreciate any feedback!
http://chantalpowell.wordpress.com/
Also would it be okay to add your site to my blogroll? I always figure its polite to ask first!
Thanks, Chantal.

Chantal Powell said...

Just stumbled across your blog - fantastic! Thank you so much for sharing your knowledge. I've not long had my DSLR and I'm really enjoying learning with it - your site will be checked by me frequently!
I've been posting some of the self portrait shots I've been trying out recently on my blog and if you had a moment to look I would really appreciate any feedback!
http://chantalpowell.wordpress.com/
Also would it be okay to add your site to my blogroll? I always figure its polite to ask first!
Thanks, Chantal.