Technical: relationship between aperture and shutter speeds

I have been using a digital compact camera (Canon G6) for 4 years, and planning to buy my first DSLR (Canon 450D).

I am thinking about getting a Canon 50mm 1.8 lens, but there are 2 things I don’t understand, about how aperture affects shutter speeds.

What I didn’t like about my compact camera was that in low-light situations, such as indoors, the shutter speeds were too long. So typically, in a room with lights on in the evening, the automated programme on my camera would choose something like f/2.0 (maximum available aperture) and shutter = 1/15 sec, which resulted in not sharp images. The new lens (50mm 1.8) is supposed to be better in low-light situations like this, but since there isn’t much difference between f/2.0 and f/1.8, does that mean that in the above example, my shutter speed would need to be only slightly shorter e.g. around 1/20 sec? This doesn’t help a lot. I was hoping to reduce the shutter down to maybe 1/60, or 1/100, to get nice and sharp images, do I really have to buy a f/1.4 lens for this?

I guess my question is generally about the relationship between aperture, and shutter speeds. Assuming that there is same amount of light available, how will shutter speeds differ between a 50mm f/1.8 and f/1.4 lenses?
Is the shutter speed identical for two different lenses, if they are set to identical apertures?


Thanks a lot
Pawel

The basic answer to your question is yes they would be the same. However you have one other setting to consider and that is ISO. In order to increase the your shutter speed time you would also need to increase the ISO. This may result in more noise int he image but will get you a higher shutter speed. I can not find the link right now but I think it was Grandpa that had a great link to illustrate the all three work in coordination to give you proper exposure, hopefully he will see this post and send it your way.

I found the link I was looking for once I got home. Hope this helps.

http://dryreading.com/camera/index.html

That link is a gem for anyone looking to better understand the relationship between shutter speed, aperture, and ISO.

Cheers,

Quote:

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Originally Posted by Old Timer

In order to increase the your shutter speed time you would also need to increase the ISO.

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Ah, that seems backwards.
To decrease the time (make the number under the 1/ bigger) so you can freeze motion, or use a smaller aperture to give greater depth of focus, you increase the ISO.

Increasing the time means decreasing the speed.
Increasing the speed means decreasing the time.