Moving the focus area...

BradUF

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It is a bleak day here in g-ville and I wanted to get some shots of the railroad. I have an idea what I wanted to do but not sure how to achieve the effect the best way. I want to catch one of the railroad stop lights to the left of the photo and have the rest blurry or not. Not really sure yet.. But to do this would I move the focus area to the left and make the appertarue really small, with longer shutter speed? How slow can I go with shutter speed until I need a tripod?

Whats the difference between moving the focus area around from the center to just moving the camera to change focus area?

Usually I would just take the shots and see how they turn out but it is going to be a drive across town and I would like to get it right.

I have a 50mm 1.8 lens...
 

iflynething

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When you move the camera, you will affect exposure. It would be best to just shift the focus area through the camera (to the left I guess) and then use the aperature at 1.8 if you want alot of blurr in the background.

The less blur you want, the higher that aperture will be and give the background more in focus.

~Michael~
 

Big Mike

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You can use a shallow DOF, which will have a small range in focus and make things that are not in that range, out of focus.

As for how to focus on the sign, you can move your active focus point to one of the side points (if your camera has multiple points)...or you can just use the centre point and do what I do...focus and recompose. Focus on the sign, lock the focus, then change the view.

You get a shallow DOF by using a large aperture (F1.8 for example). That will give you a faster shutter speed than a smaller aperture...so you might not have to worry about the shutter speed.

Just for reference, the rule of thumb for when you need a tripod...is that to hand hold a shot and have it be sharp, you want a shutter speed that is 1/focal length. Some people also say to include the crop factor, if you are using a 'crop' camera body. So for a 50mm lens, you would want to use a shutter speed of at least 1/50 (1/60)...and 1/80 (1/90) on a Canon Digital Rebel etc. Faster is better though and there are other factors...like how steady you are and that comes down to good posture and technique.
 

azruial

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When you move the camera, you will affect exposure. It would be best to just shift the focus area through the camera (to the left I guess) and then use the aperature at 1.8 if you want alot of blurr in the background.

I thought that AE always calculates at the last moment, just between full press and actual shutter release... Using focus lock and recomposing should not have any effect on Auto Exposure, unless you have changed the configuration to use a half-press on the shutter button as AE Lock, but I don't think that's usually the default...?
 

iflynething

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I guess waht I meant by the exposure is when you're trying to lock the focus as that left side and you press halfway down, most cameras by default will lock the focus as well as the focus. It, of course, can be changed though

~Michael~
 

azruial

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Wow yeah, I was totally wrong :blushing:, and that is really annoying... The camera wants you to AE lock on the scene, focus on the subject and then compose again for the scene in order to get it right!?! I wonder why that is the default?
I just fixed my camera's settings :)
 
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BradUF

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Ah, I see it looks like Focus Lock will be useful also but if I am going to use Focus lock why would I need to move the focus area to the left anymore? Doesn't focus lock allow you to pick any area and moving the focus area is a more automatic way of selecting focus area?
 

Big Mike

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but if I am going to use Focus lock why would I need to move the focus area to the left anymore?
You don't. I haven't used anything besides the centre point in years. Also, my particular camera's center point is better than the other points because it's a cross type sensor (rather than just a line type) when used with fast lenses.

Another good tip...I have changed the Auto focus activation on my camera so that it's not activated with the shutter button. It's activated by the * button that is behind the top control dial (by my right thumb). This way, I can activate the AF, focus on what I want to, then release the button and not have to worry about the focus when I recompose.
 

Happy Hour

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one more thing is. If you don't want the hassle of setting up a tripod you can get a mono pod. they even sell them at wal-mart and they are under $20.00 I seem to use this allot more than my tripod. The only downside is you can't take a sideways pic.
 

RyanLilly

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one more thing is. If you don't want the hassle of setting up a tripod you can get a mono pod. they even sell them at wal-mart and they are under $20.00 I seem to use this allot more than my tripod. The only downside is you can't take a sideways pic.

Unless you put a head on it! :thumbup:
 

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