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PostPosted: Tue Feb 01, 2011 10:41 pm 
Hi all,

I've just purchased a DSLR as I am going travelling soon. I've had it a week and am just starting to get my head around what everything is, but I have a question - if the light in a picture is overpowering and you want it to be dimmer, what is the difference between 1. lowering the ISO, 2. speeding up the shutter speed or 3. under-exposing the picture on the EV scale?

from what I've read all of these three things make your pictures darker, but on what basis do you decide which one or which two etc to employ in different situations? Do they all 'darken' images in different ways and if so how?

Apologies if this question doesn't make any sense, it is a strong possibility that I've totally misunderstood everything! I am an absolute beginner so please excuse my ignorance!

Many thanks

Isaac


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PostPosted: Tue Feb 01, 2011 11:14 pm 
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iso is a norm value for the sensitivity of your sensor, the higher it is your sensor will need less light to form an equal illuminated photo, this has its downside because the higher the iso value means that built imperfections on your sensor will jump up to hence applying noise and reducing detail in your photo (lower iso the better :) ).

speeding up your shutter speed will decrease the time during witch the sensor takes in light. the longer it is the more light enters your camera and brighter the photo is, this usually has no effect on image quality, but be aware that slow shutter speed will pick up motion of your hand and cause motion blur.
this is the most common way to control your exposure

another way to control exposure is setting the aperture, the higher the f number, more light enters the camera since the opening in your lens is bigger (and vice verse), this has a side effect of applying different depth of field to your image.
most commonly aperture is used to control depth of field, and than shutter speed gets set for the exposure.

now if you are in a, s or p mode you can dial + or - EV to change your exposure
in p and a mode dialing the EV adjusts your shutter speed (faster or slower) just as you can do it manually.
in s mode you set the shutter speed and the camera will choose aperture, and this might be tricky because dialing EV changes your aperture and affects the depth of field in your image

hope i cleared it a bit

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PostPosted: Wed Feb 02, 2011 5:07 am 
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Hello ipalmer, and welcome to the Cameralabs forums!

Bakica did a very good job explaining. Shutter speed basically controls the amount of time that the digital sensor is exposed to light to form an image. For example, a shutter speed of 1/30 would mean that the camera's sensor is exposed to light for one-thirtieth of a second. This may sound like a short amount of time, but it can cause motion blur if your hands move during the shot. Because there is more light outdoors, generally you use a faster shutter speed (Such as 1/250 of a second) and indoors you'd need to use a slower speed because there is less light (Such as 1/15 of a second).

ISO is basically how sensitive your camera is to light. Back in the days of film, it was the film speed or "ASA". The lower value of your ISO (Such as a low value of ISO 100 or 200) the less sensitive the camera is to light. Lower numbers such as 100 or 200 should generally be reserved to outdoor photography or if you have a tripod. Indoors, you'd need a shutter speed such as 1/3 if you used ISO 100, meaning that you should boost it. For every doubling of the ISO, the shutter speed is halved. For example, if at ISO 400 you need a shutter speed of 1/5, at ISO 800 you'll only need a shutter speed of 1/10, etc. You may be thinking, "So why not just use higher ISO values?" The answer is simple, at higher ISO values you get digital "Noise", which appears as large, colourful pixels that can ruin an image. You should generally want to keep your ISO value under 1600 or 3200 for most cases, however depending on your camera and the environment you may want to use something different.

EV, or "Exposure Value", is simply another way to change the shutter speed. I generally don't like to use it unless I'm in P (Program) or A (Aperture Priority) mode. As Bakica mentioned, in S (Shutter Priority, sometimes known as TV which is Time Priority) it will change your aperture, which is why I generally like to stay away from it there. I also generally simply change the shutter speed when I shoot in full manual, I don't usually change the EV.

I hope that I helped!

-Evan

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PostPosted: Wed Feb 02, 2011 5:47 am 
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All very well explained by people on here so far.

One other thing to note. As your shutter speed gets close to the reciprocal of your focal length you may get blurry pictures.

Sounds complicated, but it's not really.

Shooting at 60mm focal length, then try to keep the shutter speed at a slowest level of 1/60th. Keeping it faster will increase your chances of getting a sharp shot. If you can't get away with increasing the shutter speed, then try taking several shots at the same time to increase your chances of getting a sharp shot.

For example if you're shooting at 35mm and have a shutter speed of 1/100th of a second, then you should get a sharp shot. If the shutter speed is 1/30th of a second, then you may get a bit of blurring due to camera shake. Also at that speed you may get movement of the subject, which will induce a different kind of blurring.

You should be able, by reading the manual, to set your camera up to maintain a certain minimum shutter speed. Rather than select a shutter speed below the value you've set, it will increase the ISO (up to a maximum value you set to insure you don't get too much noise) to make sure you have more chance of getting a sharp shot.

For example, if you've set the minimum shutter speed to 1/60th of a second and the light level drops so that it would select 1/30th, the camera will up the ISO automatically to keep your shutter at 1/60th instead.

There are plenty of resources online to help you learn about exposure as well.


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PostPosted: Wed Feb 02, 2011 7:22 am 
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Just to clear up

Quote:
another way to control exposure is setting the aperture, the higher the f number, more light enters the camera since the opening in your lens is bigger (and vice verse), this has a side effect of applying different depth of field to your image.


it's the opposite way. High f number: small lens opening, less light enters, more DoF

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PostPosted: Wed Feb 02, 2011 11:40 am 
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thanks janren, never noticed the mistake.. (higher the number, smaller the entrance to your lens..)

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PostPosted: Mon Feb 07, 2011 6:51 pm 
you could put a polarizer filter on, give you at least 1 stop -EV.


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