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Old Aug 24, 2006, 11:30 AM   #1
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The following is a copy and paste from another thread - it was written by the Chrashman
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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
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Old Aug 24, 2006, 11:36 AM   #2
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I've taken the opportunity of copying the above from Sarah Flash thread so like Roy says, it won't get lost. Hopefully the Crachman won't mind - let me know and I will delete it if that's the case.

My question: Setting the dial on Av will let me choose my own speed? or vise versa? And if I want blurred-out background, can I use f2.8 and set the speed according to your diagram?

You see Crashman, there are still questions to be asked.


Edited the f stop
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Old Aug 24, 2006, 12:05 PM   #3
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if you want blurred out back ground due to DOF then you need to go the other way with the aperture, say f2.8-4. the smaller the aperture(bigger number) the more DOF you will get. that's why we use f16+ when shooting macros. it did not help me today tho..

roy
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Old Aug 24, 2006, 12:08 PM   #4
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nlp239 wrote:
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I've taken the opportunity of copying the above from Sarah Flash thread so like Roy says, it won't get lost. Hopefully the Crachman won't mind - let me know and I will delete it if that's the case.

My question: Setting the dial on Av will let me choose my own speed? or vise versa? And if I want blurred-out background, can I use f22 and set the speed according to your diagram?

You see Crashman, there are still questions to be asked.
jc, hope you didn't make him mad.... he's interpol or didn't you know that. he'll put the thread stealin cops on you..
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Old Aug 24, 2006, 12:15 PM   #5
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Ooops! I meant 2.8 or there abouts - I've edited the fstop in the previous thread.Thanks Roy.

Yeah, I hope I didn't make him angry but he ain't Interpol yet.
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Old Aug 24, 2006, 12:23 PM   #6
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Setting camet to Av lets you choose aperture and camera sets shutter speed. Tv lets you choose shutter speed and camera chooses aperture.

The DOF is decided with a combination of aperture and distance to your subject. when shooting landscape at f22 you'll get everything in your photo sharp. when shooting macro at f22 you will get very short DOF. Basically the closer you get to your subject the shorter DOF gets at chosen aperture.

So for ladscape we usually use high f number (like 16-32) and focus relatively close from camera (for kit lens at 18mm and f22 to get everything sharp in landscape the focus distance is around 2.5m).

for poirtarits relatively small f number (like 5.6-8 )to get nice blurred dackground.

for macro to get everything sharp high f number.

Hope this helps and I didn't mix up everything.
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Old Aug 24, 2006, 12:24 PM   #7
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Av means Aperture Value. In Av mode you select the aperture and the camera looks after everything else. So where DOF is key factor in the shot you are making then select Av and select the aperture that will give YOU the DOF you want.

F22 will give you a very long DOF. If you want a blurred background then select something under f/4 preferably f2.8 or less. This will give a very short DOF. The camera will adjust the shutter speed to ensure correct exposure , right up to 1/3200sec, if there is too much light still coming in then your shot will be over exposed and you may have to reduce your Av setting to bring the light down to a reasonable level.

That is why Neutral Density filters are used. They reduce the incomng light, like sunglasses do for your eyes. In doing so it allows you to select a wide aperture and get that short DOF without over exposing.

It can also help when you are trying to get a very very slow shutter during the day....Why would you want a slow shutter? perfect example is waterfalls and creeks. By having a very slow shutter say 1/15sec, 1/3 sec or even 1 sec exposure the water becomes blurred because of its movement.

Obviously the amount of light entering onto the sensor in 1 sec is huge and the aperture can only reduce the incomng light some much at say f/22 or f/32, so you use a ND filter to reduce it even more.

Tv mode is the opposite, YOU set the shutter speed to what you want and the camera will auto select the aperture to give the correct exposure, obviously within reason, it can't let in more light than is allowed by the lens attached to the camera.

This is why they talk about fast lenses. a 75mm f/2.8 lens is twice as fast as a 75mm f/4. it lets in twice the amout of light at its widest aperture than the f/4 which allows a faster shutter to be used in the same lighting conditions.

Tv mode is good when you are hand shooting and want to take your shots at say 1/125th or 1/250th and not have to worry about hand shake and subject movement. as long as you don't walk into a very dark area, you will find you can just point/ focus and shoot. The camera will look after the rest.

Lastly the diagrams above are based on hypothetical scenes. Don't use them to set your Av or Tv. A simple way to learn is to set your camera to Av, point it at a subject, half press the shutter release and read off the shutter reading. Write them down, now adjust the Av up or down one stop and reframe the shot and repeat the half press, you will see the shutter will have adjusted one stop to compensate.

Techinically you could (for THAT scene) write out the table like I have above with the values in it. Start with your smalllest aperture for your lens, i.e. f32 and list all the f stops all the way to your widest. Then write in against them the shutter speed from your first reading, it might have been f6.7 and 1/250th sec so write the 1/250 next to the f6.7 then simply write in the other shutter settings and you then have the scale of exposures for THAT scene.

Remember the f stop number go up as the aperture gets smaller, so as the f stop number goes up, the speed has to slow down, 1/30th is slower than 1/60th.

Most people do not do this (writing out a scele) when shooting, but it is good to know that if you want to adjust something in your shot, you can stop down the aperture 3 stops and know you have to slow down your shutter by 3 stops as well to get your correct exposure again.

Crash
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Old Aug 24, 2006, 12:25 PM   #8
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robar:I thought somebody will be faster than me with the answer.:-)
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Old Aug 24, 2006, 12:27 PM   #9
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I see everyone posted whilst I was writing "War and Peace" :lol::lol:

All postings are public forum and don't belong to me, do as you wish with them. :blah::blah:
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Old Aug 24, 2006, 1:28 PM   #10
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we jus all wanted to help :blah:
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