Canon SLR & depth of field

[lcmnichols]

I am thinking about buying a Canon - EOS Digital Rebel XS 10.1-Megapixel Digital SLR Camera, with the intent of using it for portraits of my child. My goal is to get good background blur / depth of field. Will I be able to accomplish this with the lens that comes with the camera? Also, is there a lot of skill involved with achieving a good depth of field, as in manual focusing, or do you get this with just regular point & shoot techniques? I have to admit I am not an expert when it comes to cameras!

[JohnG]

The kit lens will NOT give you good background blur. I'm partial to the 85mm 1.8 for that. The 50mm 1.8 is better than the kit lens and is an inexpensive start ($85 vs. $380 for 85mm) but still not great.

The challenge with shallow DOF portraiture is getting your subject in focus. And then it's a question of how much of your subject you want in focus. For example, in this photo only half my son's face is in focus:


Here, his entire head is in focus:


BUT, here's where the trade-off comes in. If you increase the DOF so the entire subject is in focus then more of your background is in focus. So, to get a blurred background your subject needs more separation from that background. In both of the above shots the background is blurred but there's a huge difference in distance from subject to background from one photo to the other.

Also, good portrait work requires the control of light. You'll want a good external flash (430exII is a good choice) to get the best results. And you'll need to learn to use it effectively.

[TCav]

To get a shallow depth of field for portraits, you need to use large apertures (numerically smaller f-stops.) The kit lens doesn't do that. And while the Canon 50mm f/1.8 lens that JohnG mentioned is good and inexpensive, it's really not long enough for childrens portraits.

There are alternatives, like the Canon 85mm f/1.8, also that JohnG mentioned, as well as Tamron's 60mm f/2.0 and Sigma's 70mm f/2.8. Longer focal lengths and larger apertures means lots of good glass, which doesn't come cheap.

[interested_observer]

JohnG and TCav are absolutely correct, that aperature at the time you take the photo, is the best way to achieve the desired results. However, they explained the downsides - fast lenses are the tool to these results. In addition, you need to have the presence of mind to have things setup when the photo opportunity happens. Then there is always the target of opportunity - great picture, but you wish that the background was a bit too sharp or in focus....

I have however run across various examples where the desired result was done (introduced or added) after the fact with software (it is much easier when taking the picture) - post processing. I have never done it, but here is a link....

http://www.vividaspect.com/tutorials...E1_Step06.html

Now this was dated back in 2005 and software has advanced. I have not kept up on this topic, however I am sure that there has been advances made.

[dwig]

Originally Posted by lcmnichols

...My goal is to get good background blur / depth of field. Will I be able to accomplish this with the lens that comes with the camera?

No, you need a faster lens, that is one with a widere maximum aperture. Canon's 50mm f/1.8 would be a good inexpensive choice

Also, is there a lot of skill involved with achieving a good depth of field, as in manual focusing,

No, manual focusing isn't necessary

or do you get this with just regular point & shoot techniques?

No, you must take some degree of manual control. You must manually set the aperture to a very wide opening, a large f/stop like f/1.8 (f/stops are fractions, f/2 is larger than f/8 just as the fraction 1/2 is larger than 1/8th). To set the aperture you need to use the Aperture priority mode (Av in Canon terms) or manual. With some bodies you can use the Program mode (not the usually green P&S program mode) and manually alter the program to select an particualy f/stop & shutter speed combination.

[johnandscooter]

I recently (within the week) purchased the Canon XSi and I also added the 50mm F1.8 lens the guys above are talking about. I love it. I have barely used the kit lens. The background blur is easy to achieve with this lens, in fact sometimes its a bit too easy to achieve!

Here are some pics I just took with this lens:

[TCav]

Originally Posted by johnandscooter

The background blur is easy to achieve with this lens, in fact sometimes its a bit too easy to achieve!

As per johnandscooter's examples, note that, in the first photo, the front of the sandwich is in focus but the sides are blurred because they are outside the depth of field. I suspect that johnandscooter focused on the front of the sandwich, but if he or she had, instead, focused on the side of the sandwich (on the lettuce or the tomato, for instance), then locked focus and recomposed, the entire sandwich might have been within the depth of field while the background would still be blurred.

Similarly, in the second, the baby's face is in focus, but its right and left forearms are blurred, as is the hair on top of it's head.

[johnandscooter]

Originally Posted by TCav

I suspect that johnandscooter focused on the front of the sandwich, but if he or she had, instead, focused on the side of the sandwich (on the lettuce or the tomato, for instance), then locked focus and recomposed, the entire sandwich might have been within the depth of field while the background would still be blurred.

I learn something new every day! Yes you are correct, i focused on the front of the sandwich. I dont want to hijack this thread - but I'll now be learning how to lock focus and recompose!

Thanks -

John

[lcmnichols]

I'd like to learn how to lock focus and recompose as well - what's the trick?

[JohnG]

Originally Posted by lcmnichols

I'd like to learn how to lock focus and recompose as well - what's the trick?

The biggest 'trick' is understanding depth-of-field. Specifically, finding out when focus-recompose will result in your intended subject being out-of-focus. That happens when shooting at extremely thin DOF. It's simple geometry - when you focus and recompose, the slight change in angle from subject to senor will yield a slightly different distance value from original focus point to sensor. In 99% of the cases that difference won't affect whether your subject is in focus or not. But at extremely shallow depth-of-field it WILL. Whether or not the problem occurs is again a geometry issue. What is the change in the angle? And is the resulting change in distance larger than the front/back depth of field?

In practical terms - focus/ recompose works by placing the desired focus point (usually center for many cameras) over the point in the image you wish to be in-focus. You then lock focus (typically by half-pressing the shutter button) but don't FULLY compress it. Then you re-compose the shot so your area of focus is not directly under the focus point and you take the shot.