Exposure for newer members

Started Mar 14, 2006 | Discussions
gerryd04
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Exposure for newer members
Mar 14, 2006

Putting together a quick paper to help some friends to understand exposure, thought some of the newer members here could use some of this info. And I'd appreciate some comment from older members on the content and any errors. This is by no means an totally accurate or comprehensive guide, just some my understanding at this point and analogies to try and help some friends.

Next three posts will contain the text.

Gerry
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gerryd04
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Re: Exposure for newer members PART 1
In reply to gerryd04, Mar 14, 2006

The Exposure Tap - It’s all about the light

Time for this analogy to be revisited, all photography is about the use of available light. The more light you have available to you, the more likely you will be able to get a sharp, properly exposed picture if some motion is present.

This example uses a tap and a glass of water to help try and explain exposure, aperture, shutter speed and how they work together. The analogy has been used before by others in this forum, just thought I’d revisit and expand a little for some of the newer members that are just starting out.

1. The object is to fill the glass with water right to the brim as quickly as possible to freeze motion. This is for our exercise a well exposed picture (just the right amount of light).

  • Half fill the glass and your picture is underexposed. Sometimes a picture that is a little underexposed is fine because you can recover some of the dark areas using post processing.

  • Overfill the glass and you have little chance of recovering the detail as the highlights (brightly lit areas) are blown and will show as pure white on your picture.

2. The tap has three elements, first the size of the hole (aperture) and the second how long the water flows for (shutter speed). We can fill the glass by using a wide hole and turning the tap on and off really quickly, or a narrow hole and turning the tap on for a longer time, we have achieved the same result we have filled our glass. The third element is the interesting one, the water pressure (available light). If we have good water pressure (plenty of light) filling our glass is quite simple because we can fill the glass quickly no matter what size hole we use. If our water pressure is poor (not much light) we need to use the widest hole we can to have any chance of filling our glass quickly.

3. The hole (aperture) is often the most confusing part because the way F-stops are displayed on a camera. A big hole, such as F2.8 is a small number, and a smaller hole such as F8 is a bigger number. To ease this confusion think “F = fraction”, so F2 = ½ and F8 = 1/8. So for our exercise a big hole = a big fraction (1/2 or F2). This is often referred to as a “wide” aperture (wide hole).

4. So, back to our glass, to fill it up we can use many combinations to get the same correct exposure like:

  • F2.8 and a shutter of 1/500 of a second (wide hole, on/off quick)

  • F4 and a shutter of 1/250 of a second (medium hole, on/off slower)

  • F8 and a shutter of 1/60 of a second (small hole, on/off slowly)

All these combinations will give the same exposure, what will be different is the DOF (depth of field) more on this later.

5. There is no exact correlation here as the camera is being set according to available light whether natural or artificial. So on a really bright day for correct exposure you may use F4 and 1/1500 of a second, and on a dull evening F4 and 1/60 of a second to get the correct exposure (fill the glass). The correlation is in adjusting the camera for available light (remember to factor in the water pressure), so a smaller aperture like F8 will give you a slower shutter speed and a wider aperture like F2.8 would give you a faster shutter speed.

6. Back to our tap, remembering the above, to fill our glass quickly on a bright sunny day (good water pressure) is quite easy no matter what shutter/aperture combination we use. On a really dull day (low water pressure), or indoor under normal lights (they may seem bright but when compared to sunlight they are not) it will take time to fill the glass.

7. The faster the shutter speed, the lower the chance that you will get motion blur, caused by the subject of your picture moving while the shutter is open (while the glass is filling). No matter how quick the shutter, if you move the camera while the shutter is open, you will get a blurry picture. So if you want a picture of your child running around after dark you are working against the laws of physics (after all the tap will need to be on for a long time) so don’t expect miracles.

This analogy is meant to explain the way the aperture and shutter speed work together in reaching a well exposed picture. Of course the above explanation is not really relevant for a tripod shot of a non moving subject, here you can turn the tap on as slow as you like to get the picture. More in the next post.

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gerryd04
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Re: Exposure for newer members PART 2
In reply to gerryd04, Mar 14, 2006

ISO introduced into the equation

If you got through the first part and still continue to read, we can now throw ISO into the mix. This term equates to the sensitivity of film, with higher numbers being more sensitive. Doubling the number has the effect of doubling the light sensitivity, with the downside of adding noise (grain for film). For the same exposure we could use;
Aperture of F4, shutter of 1/250, ISO of 100 or
Aperture of F4, shutter of 1/500, ISO of 200

So as you can see we have been able to gain some shutter speed by increasing our ISO, enabling a better chance at freezing motion. How does this equate to our glass of water, well a simplistic analogy would be to throw a tablespoon of sand into our glass to enable it to fill quicker at any given aperture/shutter combination. Want to use ISO 400, sure, just throw another tablespoon in. Increase ISO = increase noise, this is accelerated if the picture is not properly exposed. You just need to decide if getting an ok picture with some noise (grain) is preferable to a blurred picture or no picture at all.

For all those who are ready for it, an expose calculator can be found here that you can play with and see the effect changes in ISO, shutter & aperture will have http://www.dpreview.com/learn/?/Glossary/Exposure/Exposure_01.htm

Just a few points to finish here, wider apertures have a smaller depth of field (DOF), narrower apertures have a larger DOF. This is the distance before and after your subject that remains in focus, so if you want more foreground/background in focus use a higher value like F8, if you want to blur the background use a lower value towards F2.8. There are variables here like the focal length of the lens (amount of zoom used), higher zoom = narrower DOF, lower zoom = more DOF. The sensor size also has an effect, nothing we can do here though. More in the next

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gerryd04
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Re: Exposure for newer members PART 3
In reply to gerryd04, Mar 14, 2006

So to summarise with a couple of often asked about examples;

Example 1

How do I take low light shots of my dogs, sport, kids running around. Same answer, use your camera settings to get the highest shutter speed you can while still filling the glass with water. First try wider aperture F2.8, then try and raise the ISO. If you get slightly underexposed pics, that have frozen the action, be happy, fix then a little in pp. There are other tricks that may help like using exposure compensation and of course your mega OIS (this helps you take a shot at lower shutter speeds, it will not help freeze a moving subject).

If all of the above fail to get you a result you really have a water pressure problem and need to use a flash or some other form of light, simple as that. Why do people say get a DSLR if you take lots of this type of shot. Main reason is that they can go to a much higher ISO that is a lot cleaner (less noise) due to a larger sensor. Usable ISO 1600 and higher can be achieved (four tablespoons of sand in the glass) and the availability of faster lens’s with apertures like F1.4, F2 etc (wider hole in the tap).

Example 2

How do I get a blurred background. Lower aperture, closer to F2.8, longer focal length (more zoom + walk backwards) and more distance between your subject and the background. The opposite will get you a sharper picture if you want the background in focus (higher aperture + less zoom).

Freezing action

All about the shutter speed here, your shutter has to match the speed of the action. If by using some of the techniques above you can’t get a fast enough shutter, you need more light or a camera with higher ISO and/or a faster lens (faster at getting the water out = wider hole). Examples would be;
To freeze a hummingbird’s wings = 1/1300
A dragonfly = 1/1000 or faster

Bat or ball in baseball = 1/1000 (or around 1/500 for acceptable blur on bat or ball)

People running, tackling, shooting hoops = 1/250 to 1/500 depending on timing of the play
Kids mucking around = about 1/250

The Good News

Your camera is smart, you can take things one step at a time. If you want to freeze action use shutter priority, dial in a number, and the camera will choose the rest of the settings to fill your glass. If you want to control DOF, use aperture priority, dial in a number and the camera will choose the rest of the settings to fill your glass.

By using either of these settings and seeing what match the camera comes up with, you have a starting point for playing in full manual. Choose F4, half press, the camera will choose a shutter speed to fill the glass, you now have a combination to try in manual. Same for shutter, choose 1/250, the camera will choose an aperture, you have a starting point again.

I would stay with ISO 80 unless you really can’t get a fast enough shutter for the pic you want, then adjust as required. From 80 to 100 ISO you are adding ¼ again in light gathering ability, not dramatic, from 100 to 200 it is double, and 200 to 400 is double again. Just remember all the sand you have just thrown into the glass.

Each full stop of aperture halves the light (F2.8 to F4 = half the light gathering). Full stops are calculated by multiplying by a factor of 1.4, so the full stops on your FZ equate to F2.8, F4, F5.6, F8 & F11, all the rest are 1/3 stops.

Each doubling of shutter speed halves the light gathering ability 1/125, 1/250, 1/500 etc.

For more basic info check out the glossary here;
http://www.dpreview.com/learn/?/Glossary/ and google is your friend :}

THE END Hope this all helps

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Telecorder
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Re: Exposure for newer members PART 3
In reply to gerryd04, Mar 14, 2006

Good, fairly comprehensive overview, Gerry, that will help a lot of newbies understand their correlations - if they will read, re-read and come back later and read again after taking different shots while adjusting the variables you mention.

One good analogy that others have stated that may help as well is to understand that the Aperture and Shutter Speed can be likened to a seesaw -- When one goes up, the other will go down... More light needed? The aperture needs to go up to a larger number but the shutter speed will go down; More speed to stop motion? The shutter needs to go to a higher number but will cause the aperature value to go down...

Need more light and/or shutter speed? Increase the ISO number to get the needed higher sensitivity that will allow for both... (But will come at the probable expense of higher noise levels...)
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John_Reed
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Excellent Tutorial, Gerry!
In reply to gerryd04, Mar 14, 2006

It's very good, thoughtful, and well-written. I wonder perhaps if it may be too verbose to attract some who really need the information? Maybe a summary paragraph for each section, where you paint the analogy in a few easy-to-understand phrases, would make it universally attractive? And thank you so much for taking the time to do this!
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Kevin Coppalotti
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Re: Excellent Tutorial, Gerry!
In reply to John_Reed, Mar 14, 2006

Agree with John, average attention span is 30 seconds (according to TV ad execs).
A Q&A addition might work well? For example:
How do I photograph my cat?
Why is my baby Green?
Why are my sports pics blurred?
What is Noise?

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RIVERFOOT
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THANK you...I got it.......m
In reply to gerryd04, Mar 14, 2006

Gerry this is exactly what I needed. I have walked around and finally just said to myself well this button does this and if I do this then that will happen....etc. Not haveing a CLUE as ro WHAT I was doing. I have tried to look at the manual and HUH???? I felt so stupid!!!!!
With a simple analogy such as this I got it........FINALLY!!!!

NOW I need a way to implant it into my head....lol. This is a KEEPER for me. You just saved me from cussing myself out with the cam because I could not get the manual or the 3 books I have bought. Granted PTSD does not help but I got I got it I got it and I know I can learn it!!!!!!!!

Gerry I have tears of joy in my eyes from this post. I have been released from the CAMERA ignorance from hell.

I am sorry to be so emotional and brutually honest about this in front of every one but I had to let you know what YOU have done for me with this. Your time was not wasted. It saved me and the 30 from a gators' mouth with chicken wrapped around it......THANK you so much!!!!!!!
OMG what a relief and godsend this is!!!!!!!!!!!!

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CJ
Lest YA Ferget

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IanJ
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Great work Gerry (n/t)
In reply to gerryd04, Mar 14, 2006

no text
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Gene Montreal
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Re: Exposure for newer members
In reply to gerryd04, Mar 15, 2006

Gerry,

As a newbie to understanding anything outside Auto or Program AE mode, your tutorial is really appreciated and captured off to a text file to be read and re-read until it becomes second nature (I hope).

Thank you for your time and your caring to help others.

Would you object if I was to post the links to these posts in other newsgroups?

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Gene (From Montreal)
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Keefo
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Great info, but I have a question
In reply to gerryd04, Mar 15, 2006

I read through it all, and found it very informative. I do have a question though. This has been nagging at me for a while now. I don't know if it's something you'd like to include, but I know it would be useful to me

When it comes to exposure compensation, how does this really work? I read the glossary link you provided, but I'm still not sure how it works, or why it's useful (other than for autobracketing). Changing the EV through the exposure compensation settings adjusts either your aperture or shutterspeed to compensate. Why wouldn't I simply directly adjust either the shutterspeed or aperture myself instead of using the exposure compensation setting? What's the benefit?

Also, do you know how it decides what to adjust when you use this feature? How does it determine that it should adjust the shutterspeed vs adjusting the aperture?

Thanks!

Kevin

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John_Reed
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I'll hazard a guess...
In reply to Keefo, Mar 15, 2006

If you're using aperture or shutter priority modes, then the thing that gets adjusted with + - EV is the parameter you haven't set. For example if you're in shutter priority mode at 1/100 selected shutterspeed, and f5.6 aperture (selected by camera), when you increase EV, the aperture will increase (go to f4.0, say) to let in more light, if you decrease it, aperture will decrease (go to f7.0). Of course, if you're in manual exposure mode, you have total control over both parameters, and no EV adjustments are allowed. In P mode, the camera is free to select both aperture and shutterspeed; when you change the EV level there, the camera will follow its program for trading off aperture and shutterspeed to provide "less water in the glass" for lower EV, "more water in the glass" for higher EV. Even in the P mode, you can adjust the two by turning the jog dial. Does that help?

Keefo wrote:

I read through it all, and found it very informative. I do have a
question though. This has been nagging at me for a while now. I
don't know if it's something you'd like to include, but I know it
would be useful to me

When it comes to exposure compensation, how does this really work?
I read the glossary link you provided, but I'm still not sure how
it works, or why it's useful (other than for autobracketing).
Changing the EV through the exposure compensation settings adjusts
either your aperture or shutterspeed to compensate. Why wouldn't I
simply directly adjust either the shutterspeed or aperture myself
instead of using the exposure compensation setting? What's the
benefit?

Also, do you know how it decides what to adjust when you use this
feature? How does it determine that it should adjust the
shutterspeed vs adjusting the aperture?

Thanks!

Kevin

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RIVERFOOT
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Re: I'll hazard a guess...
In reply to John_Reed, Mar 15, 2006

Thanks John!!! It helped me!!! Oh thank you all so much. So simple with water but soooooooo confusing trying to read it in a manual.
Thanks again.

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CJ
Lest YA Ferget

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John_Reed
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C.J...
In reply to RIVERFOOT, Mar 15, 2006

Any teacher would LOVE to have you as a student. You are so receptive and open to learning, it's a pleasure to help you out. You can take that as a compliment, milady.
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Keefo
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Re: I'll hazard a guess...
In reply to John_Reed, Mar 15, 2006

Thanks! That makes perfect sense. I appreciate the info.

Kevin

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PhotoMED
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Re: Thank you Gerry
In reply to gerryd04, Mar 15, 2006

Keep this post alive. Many will learn.

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RIVERFOOT
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Re: C.J...
In reply to John_Reed, Mar 15, 2006

Thank you. I am so grateful to all of my teachers here. I am just amazed and thrilled to pieces I know how a water faucet works and how to fill or lower a glass.....lol. And now Leica glass!!!!!!!

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CJ
Lest YA Ferget

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digiCL
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Excellent Summary
In reply to gerryd04, Mar 15, 2006

Great info for me. Accurate, concise, all in one place. It's not too long - if someone who wants good pictures doesn't have the time to read through your descriptions, then they really don't want good pictures.

(I have the same situation with retirement savings. I have done alot of homework and am very comfortable with my knowledge for my goals. Friends and relatives ask me advice, but they almost all refuse to read the few articles or the simple book I recommend so that they can learn the basics. It becomes clear real quick those who really don't care enough about their retirement to not even spend one afternoon on it. Amazing.)

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CL

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gerryd04
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Re: Exposure for newer members PART 3
In reply to Telecorder, Mar 15, 2006

Thanks Telecorder, thats a great tip, and worth including, I somehow got talked into helping a few local families from a sporting group in digital camera basics, fortunately I'm an expert in basics and I have done some training over the years and love a good analogy.

Gerry
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gerryd04
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Re: Excellent Tutorial, Gerry!
In reply to John_Reed, Mar 15, 2006

Thanks John, I have a short powerpoint presentation with summary points, this was more the handout for review/reference after the fact. To tell the truth I had to have at it as I had more than this included in the first draft. This lot equates to 2 pages of text, a little more trimming and a few pictures here and there and it should do the job.

Thanks for the kind words.
Gerry
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