Aperture and shutter speed

tomwazza

tomwazza

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Tom
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Hi Everyone,

Today I've been trying to grasp the theory behind the manual functions of my camera. This is what I've worked out from watching some tutorials and playing with my camera:

The higher the F-Stop number the longer the depth of field is (i.e. more is focused), however because the diameter of the diaphragm is so small the shutter speed/exposure time must be increased to allow for more light to be processed. This means that the subject must be still and the camera must be mounted on a tri-pod otherwise the photo will be blurred. So this is ideal for landscapes and macro shots.

For images of things that move, lower F-Stop numbers must be used meaning that only the focused subject will be sharp and anything in the foreground or background will be blurred. The diameter of the diaphragm will be large allowing for a faster shutter speed. Ideal for cars, animals, people, sports, etc, etc.)

My first question is am I correct with that theory?

My second is could people post other instances when a larger or smaller aperture can be used.

Also what is ISO and how it is related to the aperture and exposure time.

Thanks,

Tom.
 
more or less.

you are correct with depth of field etc, and some landscapes lend themselves to a high DOF.

Basiclaly Aperture and Shutter speed are used to create a result you like. Some people like portraits at say f2.8 which gives a very shallow DOF, others prefer to retain more detail over a wide DOF.

ISO - comes from teh days of film where films would have differnet sensitivities to light. Eg. ISO 100, would be used in sunnier conditions than say ISO400. I believe in sunny places like Oz, you could even get ISO25.

In the digital world, think of it as changing the sensors sensitivity to light. Pretty much doubling the ISO will halve the shutter speed - so if you are indoors and trying to photograph a moving object or freeze something, you might want to increase the ISO to reduce shutter speed whilst maintaining the same aperture.

However, once (on a 30D,) if you go much above ISO 800 you will get some "noise" - loss of image quality. - it will look grainy.

This noise can be removed (to an extent) post processing but depending on the severity of the noise, results can be mixed.
 
Your pointing in the right direction. In effect the combination of aperture , shutter and iso all do the same thing…they provide a way to control the amount of light hitting the sensor to record a good exposure.

The bonus within that range is cameras variable aperture, which I think of as a kind of filter to change the proportions of wide rays (out of focus) against narrow rays (in focus) that are able to hit a sensor ….The shutter speed being the compensator to give a good exposure as the proportions change.
The wider side will render details away from the focal plane (where you’ve focused) as increasingly out of focus … and in the opposite the narrower side will render more detail in focus either side of the focal plane… Your aperture gives you the choice of how much or how little of the image you your able to emphasise by the use of this variable depth of field..
….the choice is there for any shot circumstances allowing.

The iso and shutter speed can be said to do the same thing in one effect ..that being the iso affects how sensitive/fast the sensor is to respond to an amount of light. The shutter controls how long this amount of light will hit the sensor.
As Lynton says, you could also think of the shutters speed as a time capturer. In that it can either freeze a moment or capture it as it streams passed.
 
I think that you understand it OK.

The aperture and the shutter control the light reaching the sensor.

ISO can be viewed as an amplification of the sensor output. By increasing the ISO you can use either faster shutter speeds or smaller apertures, or a combination of both, for any given situation.

As the shutter speed can affect motion blur of the subject you can either use a fast shutter speed, to capture a bird in flight for example, or you can use a slow shutter speed to show the motion of a bird in flight for example :)
Same situation but you decide what effect you are after. A classic example of a slow shutter speed on a moving subject is a waterfall, these are often pictured with blurry, milky water. Car lights at night are shot with a slow shutter speed to show light trails.

If you are shooting hand held you need to keep the shutter speed fast enough to prevent camera movement blurring the picture. If you cannot achieve a fast enough shutter speed with a low ISO then you can increase the ISO.

As aperture affects depth of field most portraits are taken with a large aperture so that the background is out of focus, however you may wish to include the background if you want to show where the person lives or works etc. in that case choose a smaller aperture for a greater depth of field.

I work in quite a simple way.

If I want to isolate any subject I use a large aperture, if I want to show the subject with the foreground and background I use a small aperture. The shutter speed is chosen to give the correct exposure.

If the subject is moving I decide if I want motion blur or not and choose the shutter speed accordingly. The aperture is chosen to give the correct exposure.

If I am not that worried I use f5.6 or f8 as my lenses perform better at these apertures (or so I am told - it is hard to see really)

I have my ISO set to Auto with a maximum ISO of 800 and a minimum shutter speed of 1/60. This means that if my shutter speed is going to be slower than 1/60 the ISO is automatically increased i.e. the sensor output is amplified, and the shutter speed will increase. I set the max ISO to 800 because above that I get noise in the pictures - they look a bit grainy. If I cannot get faster than 1/60 with the ISO at 800 I decide whether to use a tripod, rest the camera on a bean bag or wall etc or whether to increase the ISO and try to reduce the noise either in camera or on the computer.
 
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