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Old Aug 21, 2006, 1:38 AM   #151
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mtclimber wrote:
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It is not an especially difficult topic, Roy. Why don't you take it on. I know that you could do a good job with it.

MT/Sarah
are you kidding?? it's one of the most difficult topics there is. i'd have no way of putting something like that together. i don't have the typing skills for it, nor any idea of a format..

roy
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Old Aug 21, 2006, 5:25 AM   #152
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Did I miss something? I am not so sure what is difficult about shutter time/aperture/ISO.

Now shoot me down in flames if I am wrong...I deserve it...:G:G

The way I understand the subject is this. The aperture setting and the shutter setting I will refer to as Av (aperture value) and Tv (time value).

Av + Tv = correct exposure

To confuse the matter when it really doesn't need to be we can add ISO into the mix.

Av + Tv + ISO = correct Exposure.

Simply put though is if we reduce the amount of light coming in we have to increase the amount of time the shutter is open to produce the same result.

Analogy. to achieve $50 we have to make $5 per hour for 10 hours or we can make $10 per hour for 5 hours.

If for example a scene has a certain amount of light that f/5.6 at 1/250sec gives correct exposure for, then ……

to change the aperture to f/8the shutter has to go to 1/125th.
to change the aperture to f/11 the shutter has to go to 1/60th
and so on.

For every stop down in Av you have to take Tv up one stop as well.

Each F stop down lets in exactly half the amount of light of the previous f stop therefore the shutter has to be open twice as long.

If we had another scene where correct exposure was say f5.6 at 1/60 then the scale would look like this



As you can see correct exposure would still be achieved with f/11 at 1/15th sec.

Now we step into another two fields.

1. Safe Focal length for hand held shooting,
RULE "Your shutter speed should always be faster than your focal length"

eg: 1/100th sec shutter, your max focal length (lens size) should not exceed 100mm if you are shooting WITHOUT a monopod or tripod. You will get blurred images. 1/250sec then you can zoom out to 250mm and so forth.

2. DOF (depth of Field) as covered elsewhere on this forum. The smaller your aperture opening (higher f stop number, ie. f32)the longer your DOF. The wider your aperture (lower f stop number, ie, f1.7) the shorter your DOF.

Short DOF is great for isolating your subject from a busy or unsightly background. Sometimes you want long DOF, eg a nice lake shot with mountains in the background, it is great to see both foreground and background ALL in focus. In that case choose an f stop of at least f8 or higher.

So, just moving your Av and Tv dials to get the meter to read 0.0 in your viewfinder is only the start. Remember your Av sometimes is more important so you can select Av mode on the dial and select a high or low aperture depending on what DOF you are trying to achieve, the camera will select the appropriate Tv to give "correct Exposure"( be careful that the shutter speed doesn't get too slow for hand held shooting, remember the Rule.)

For sports and other action shots you can select Tv mode and choose a nice fast shutter to capture the action, but remember you camera still needs a certain amount of light so watch that the camera is able to adjust the aperture sufficiently to get your shot and keep an eye on your exposure meter and make sure it isn't reading a -ve value (under exposed), also your DOF will drop down too.

This is where your ISO comes in. If it gets too dark and you can't get enough light in, you can adjust your ISO up a level or two and you will find you can choose a higher shutter speed.

ISO is simply the sensitivity of the sensor to light.

The ASA rating for film simply changed the size of the grain of the film and the film had larger grain to collect more light so was more sensitive in lower light, but more grainy when printed.

The usual happy snap film was 100ASA which gave good sharp images in good light. I use 400ASA for my night time shooting with 35mm and it is good for that, but daytime shots are quite grainy.

Back to our scene, that hypothetically measures f/5.6 at 1/250sec at ISO200 for correct exposure. If you were to change the ISO up to ISO400 then the camera's sensor is more sensitive to available light and therefore the amount of aperture required can be smaller (bigger DOF) and the amount of time the shutter needs to be opened is less (get those action shots).

The adjustment of ISO is like the adjustment of either Av or Tv. One stop difference between ISO200 and ISO400.

Each time you adjust the ISO you can reset your Av or Tv

Here is an example of correct exposure where Av is f/5.6 and Tv is 1/1000th sec

As you can see in this example shooting with a 100mm lens hand held at ISO 200 your smallest aperture would be f16 at 1/125th sec, but by going up to ISO 1600 you aperture can stay at f/16 but your shutter speed can jump up to 1/1000th sec if you are chasing those action shots and still giving you a good DOF so you can be a little off your focus and still have sharp images.

Now these charts are an example based on correct exposure for a hypothetical scene and not for all scenes. As you well know each scene has different lighting.

So next time you are playing with your camera, use manual mode, adjust your Av and Tv until you get a metering reading in your viewfinder of 0.0 and then adjust the Av up or down one stop and you will find that if you adjust your Tv one stop you will get a 0.0 meter reading again. Run out of shutter speed, then adjust your ISO up one stop to ISO400 then you have just adjusted your metering as if you had an extra f stop or extra shutter setting.

I hope this makes sense to those starting out.

Obviously if you are shooting off of a tripod then hand movement isn't the issue and very slow shutter speeds can be obtained whilst keeping a long DOF.

Crash

Note...if I am wrong on this, then start shooting:ak47::ak47::ak47:

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Old Aug 21, 2006, 11:10 AM   #153
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Well done, Crash!

You hit it right on the nail. And it was explained very nicely as well with graphics. You should be very proud of your excellent explanation.

MT/Sarah
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Old Aug 21, 2006, 11:27 AM   #154
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Great job explaining. Fits in with this thread really well.

So many people here who know their stuff AND are happy to share.

Thanks to you all



Darren
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Old Aug 21, 2006, 8:56 PM   #155
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I certainly agree, Darren-

Crash's post is excellent. It is a nice compliment to the existing Flash Thread.

So, Roy-

There it is! Crash took care of that explanation ever so nicely. Many thanks to Crash!

MT/Sarah
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Old Aug 22, 2006, 12:30 AM   #156
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I normally like to prepare when I lecture, but this was off the cuff out of my head and straight onto the keyboard.

I screwed up the fstop scales and had to make some jpegs up.

As long as it makes sense to those that are still learning and helps de-mystify photography and how to use the manual, Av and Tvsettings, thats all that counts.

As much as I have been doing photography on and off since I was a kid, and I do photography every day with my work, I still get an education on this forum.

So glad to be able to give something back. :|
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Old Aug 22, 2006, 9:00 AM   #157
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Let me chime in with thanks also. That is probably the most clear and concise explanation on the subject I have read, and I have looked quite a bit on the internet as I have been trying to learn. This was excellent and has increased my understadning.

One problem I found with learning about photography was that most of the "how to" guides assumed a certain baseline knowledge on the part of the reader . . . and I did not qualify. As I have been able to fill in the large gaps in my basic knowledge the rest of the information I am interested in has been much easier to understand.

Crashman, you have mentioned photographing accidents as part of your job. I am curious, are you a law enforcement officer or crime scene photographer or do you work for an insurance company or something else all together (if you don't mind me asking, feel free to tell me to MYOB)?

Thank,

Tim
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Old Aug 22, 2006, 11:04 AM   #158
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I am a Police Officer and have been for over 18 yrs now. I am what is referred to in the US as a Vehicle Homicide Detective.

I am also trained in Crime Scene investigation.

I do all my own photography of the crime scenes, including, night time shots or large scenes, macro work of blood/ scrapes and occasionally light bulb examination photography. It can get technical, but I love it.

I get to travel the whole State and therefore get to meet some great people and see some great places.

I am about to resign and transfer over to a Federal agency later this year and expect to be overseas for the next 2 yrs. I am taking the DL and hopefully a new K10D as well.

I plan of taking lots and lots of pics during the two years away. I WILL post often..:G

I will just have to change my name here on the forum..maybe change it to "loose cannon" :?:?


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Old Aug 22, 2006, 11:22 AM   #159
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well done crash!!! hope it helps some more people. you should start a new thread with it as having it here it isn't as visible.. why would you need to change your ID here??

roy
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Old Aug 22, 2006, 11:24 AM   #160
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Roy

When I change jobs, I won't be a crash man anymore will I? :lol::lol:
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