Complete Noob Blurred Background Question

[gismcieri]

Hi,

I bought the canon T1i for my wife for her BDay and it came with the 18-55 lens. I am trying to take a picture of a person with a blurred background and I can't get the background to blur for anything. This is ability is one of the major reasons she wanted a camera like this (she also wanted a faster shot our one year old doesn't sit still very much.) I have set the appature to 3.5 the smallest I can get and I set the subject a few feet away from the background. I move myself forward so that i just got her shoulders and head. I focused on her eyes and took the picture. The background was crystal clear.

I have been doing a lot of reading on performing this task and I can do it if I am looking a something small like a flower bud. But I can't get it on a person for the life of me. I am pretty sure I can get a prime lens 50mm or greater with a f/1.8 or less to make this easier but I should be able to do this with my current lens. Any idea what I am missing? I am assuming I am just not understanding DOF.

Thanks in advance,
Matt

[teaking]

When you see a blurred back ground it is caused by circles of confusion where the light travels through the lens but the focus point crosses before it hits the sensor and causes it to be out of focus.

To increase this effect you use a smaller aperture which as you said in your case is 3.5 and move your subject further from the background the further the better. Your background may still be in the depth of field for your focus range. If possible move the subject further from the back ground this causes the circles of confusion.

Then by moving your self further from the subject you increase the effect slightly and finally if you increase magnification by zooming you increase the circles of confusion and increase the effect of Depth of field.

[Sterling]

If you are shooting at the wide end (18mm) of the lens then it is much tougher to get a blurred background because the depth of field is so much deeper. For example, at f/3.5, if your subject is 10 feet away, everything from about 6 feet to 21 feet will be in focus. Obviously, if the background is 12 feet away then it will be in the zone of sharp focus. If you take the same shot zoomed in to 55mm then everything from about 10 feet to 11 feet will be in focus. So the background at 12 feet will be blurry (slightly).

[gismcieri]

Wow thanks for the quick replies. The f 3.5 is only available at the 18mm. f5.6 seems to be the smallest at 55mm. So about where would I have to put the subject from the background to get it to blur. What I still don't quite understand is why can I get the background of a close up flower to blur but not a person.

Thanks,
Matt

[BCampbell]

Can you post an example or two with EXIF data? Would help narrow things down.

[Sterling]

Originally Posted by gismcieri

Wow thanks for the quick replies. The f 3.5 is only available at the 18mm. f5.6 seems to be the smallest at 55mm. So about where would I have to put the subject from the background to get it to blur. What I still don't quite understand is why can I get the background of a close up flower to blur but not a person.

Thanks,
Matt

It's because the zone of focus gets shallower as you move closer to the subject. How far were you from the flower? Less than a foot? If you shot your portrait from the same distance as you shot the flower you'd get the same blurring of the background. The numbers I gave above were for a subject at 10 feet using f/3.5 at 18mm. If your subject was only 2 feet away then the focus zone would only be from about 1.8 feet - 2.25 feet. So only a depth of about 6 inches would be in focus for a subject 2 feet away versus about 15 feet in focus if the subject is 10 feet away.

If you shot your portrait at f/5.6 and 55mm and your subject is 5 feet away, then the focus depth will extend from about 4.8 feet to about 5.3 feet. if your background is at 7 feet it will definitely start to get more blurry. Increase the distance between the subject and the background to increase the blurriness.

[Samanax]

Depth Of Field Tutorial

Example For Understanding Depth Of Field

[gismcieri]

How do you export EXIF data from a camera with image. I have a canon T1i.

Matt