Slow shutter speeds indoors while using Av mode

BigRed

Hello @Moose! I am fairly new to the SLR world (couple months), but have a pretty good understanding of it already. I'm exclusively using the Av, Tv and M modes.

My question is about the shutter speed (seen thru the viewfinder) while in Av mode. Last night I was studying up to learn more about my camera and I was sitting in my office which is brightly lit, facing my very bright kitchen (all lights on).

I took a bunch of photos using different settings from f/1.8 to f/11 with the ISO between 100-400. The shutter speeds "appeared" to be in a feet/inches measurement, like 15" and 30". I am going to go ahead and assume that that this means 15 seconds and 30 seconds?

Why would it automatically set to such a slow shutter speed? I mean, I wasn't outside in the sun or anything, but I also wasn't in a dark room either. Sorry if this is a dumb question, I couldn't find the answer anywhere on my own. Thanks in advance! :-)

PBked

Hi @BigRed - You correctly answered your first question. 15" does indeed mean 15 seconds.

What may seem to be a bright room to you will not necessarily be a bright room to your cameras image sensor. The human eye adjusts constantly to account for the amount of light reaching the retina.

With Av selected, the T2i was determining how much light would reach the sensor. At f/1.8 you probably had an exposure time of about 1/60th, but as you closed the aperture (larger f-number) the amount of light reaching the sensor would have diminished, therefore longer shutter speeds would be needed to make an accurate exposure.

Try taking the same pictures tonight, but with 'P' mode and note the settings it gives you. Hope this helps.

BigRed

Oh! Great idea @PBked! Thanks, I wouldn't have thought to do that. I will try tonight.

So those 15" and 30" were seconds. That confused me quite a bit last night, I'd never seen it do that. Thank you so much for your insight, very much appreciated! :-)

PBked

For a fuller explanation of the relationship between aperture, shutter speed and ISO and also many other useful articles, check out Cambridge in Colour (www.cambridgeincolour.com). Happy shooting!

BigRed

Will do! Thank you again. :-)

Moose

Howdy @BigRed - Looks like @PBked got to you before I could. :) He's 100% correct. I'll just add that your aperture is like a flood gate. The lower the f-number, the wider the aperture opens up. The higher the f-number, the smaller your aperture becomes.

With a wide open aperture, you're allowing more light to pass through the lens onto your image sensor. This in turn speeds up the shutter speed. That's why you hear about lenses that are better suited for low light situations.

For example, the Canon 50mm f/1.8 II (see my visual guide here) would allow you to obtain a maximum aperture of f/1.8, while your kit lens would only allow you to go as low as f/3.5 towards the wide-end (18mm) and f/5.6 towards the long-end (55mm).

Side by side, the 50mm f/1.8 would give you faster shutter speeds which would effectively reduce the chances of subject blur and/or camera shake when shooting hand-held in low light.

Hope that makes sense and happy shooting! :)

BigRed

Yes, that makes much sense @Moose! Thank you both for your help... this is my first post in any forum about photography. It's really nice to have people to turn to! :-)