How do you focus?

[emilymiller]

I have a sony DSLR, and I'm having a hard time figuring out how to focus on one object while having the rest of the photo blurry. Anyone have any advice on how to do this?

 

[Stradawhovious]

Do a little studying on something called an Aperture, and it's effects on your photo. I think you will be pleasantly surprised with your findings. You will more than likely gain some other valuable insight during the course of your journey.

Enjoy!

 

[Big Mike]

The effect you are looking for, is achieved by creating a shallow 'Depth of Field' (DOF). So there is another good topic to study.

 

[emilymiller]

Thanks, I'll look into those.

 

[mjhoward]

Do a little studying on something called an Aperture, and it's effects on your photo. I think you will be pleasantly surprised with your findings. You will more than likely gain some other valuable insight during the course of your journey.

Enjoy!

This is good advice but the Aperture is not the only factor in creating a shallow depth of field. You can also achieve this by using a longer focal length and a shorter distance from your subject. For instance, if you have say the 55-200mm kit lens, zoom to 200mm and focus on a subject from only a few feet away. Your subject will be in focus and everything in the background will be creamy.

I bring this up because it sounds like you might be a beginner and most lenses with the exception of the nifty fifty will be expensive that have large apertures.

 

[Rancor]

There are three main things to mess with:

Aperture: Bigger (f/1.8) = more BG blur, Smaller (f/5.6) = less BG blur
Focal length (Zoom): = Closer = more BG blur, Farther = less BG blur
Distance between you and subject: Closer = more BG blur, Farther = less BG blur

 

[Stradawhovious]

Do a little studying on something called an Aperture, and it's effects on your photo. I think you will be pleasantly surprised with your findings. You will more than likely gain some other valuable insight during the course of your journey.

Enjoy!

This is good advice but the Aperture is not the only factor in creating a shallow depth of field. You can also achieve this by using a longer focal length and a shorter distance from your subject. For instance, if you have say the 55-200mm kit lens, zoom to 200mm and focus on a subject from only a few feet away. Your subject will be in focus and everything in the background will be creamy.

I bring this up because it sounds like you might be a beginner and most lenses with the exception of the nifty fifty will be expensive that have large apertures.

Correct.

Sorry, I was being vague on purpose, hoping that the OP would stumble across this information in their research.

Here is a post I put together in another thread that might help visually understand how DOF works, and how its acheived...

http://www.thephotoforum.com/forum/...-gallery/240559-2-8-vs-4-5-a.html#post2212927

 

[Rancor]

If you can follow what I'm about to say, pro dudes check me if I'm off on this, I have a simple way to help noobs (like me) understand the whole DOF/OOF/Bokeh thing.

1.) Take a piece of regular printer paper and draw a line left to right. This line should be 20cm long - 1cm for each f stop in the aperture you'll be using, rounded to the nearest whole number (f/5.6 = 14cm long). This represents your aperture.
2.) Then place a dot 1cm away from the center of your line for each foot your subject will be from you.
3.) Now, using a ruler, draw a straight line that starts at each end of your aperture line, crosses the subject dot, and continues. These are your focus lines.

That's it. Now here's what this demonstrates. The closer your focus lines are to each other, the more in focus that range in the image will be, the farther they are apart, the less in focus that range in the image will be. Playing around with this by changing your aperture (the length of your aperture line) and the distance between you and your subject, will allow you to see, and better understand, how and why you achieve better blur.

I came up with this on my own, and it helped me understand.

 

[KmH]

There are three main things to mess with:

Aperture: Bigger (f/1.8) = more BG blur, Smaller (f/5.6) = less BG blur
Focal length (Zoom): = Closer = more BG blur, Farther = less BG blur
Distance between you and subject: Closer = more BG blur, Farther = less BG blur

There is a 4th main factor: how far the background is from the subject. If you want the background blurred, you want it as far behind the subject as possible.
www.cambridgeincolor.com Look at the tutorial on Depth-Of-Field.

 

[clanthar]

I see a lot of confusion show up here on this forum when this topic comes up -- to be expected as there's a lot of misinformation out there about DOF.

It's really not a good idea to identify focal length as a primary factor in determining DOF. For that matter subject distance is also less than helpful listed as a determining factor. If you can remember back to 7th grade pre-algebra we all learned a fundamental principle; simplify the equation to solve the problem.

DOF = magnification + f/stop.

That's simple and complete, two factors and one operation. Expanding it back out we want to understand: magnification = focal length + subject distance + sensor size.

(To be really thorough we'd also have to factor in print size and viewing distance, but let's hold those out for now).

How you think about what's going on is important. Run an example: Your friend is sitting on a park bench and you want to take his photo. You decide you want a head and shoulders so that you're capturing approx. 4 feet of height at your friend's position. You have that 55-200 mentioned above on a full frame sensor. With the 55mm focal length you'll get that 4 feet at a subject distance of approx. 6.2 feet. At f/3.5 you'll get 10.5 inches of DOF. If you switch to the 200mm focal length you'll get much less DOF from the same position. You'll also have a photo of your friend's nose. You really don't want to compare a head and shoulders portrait of your friend with a close-up of his nose. So you start to back up with the 200mm lens. When you get back to 22.25 feet you'll have the exact same crop you did with the 55mm lens and at f/3.5 you'll get 10.5 inches of DOF -- exactly the same as with the 55mm.

If you keep magnification constant, and seriously if your shooting a portrait for example you'll want to keep magnification constant, then focal length and subject distance aren't going to change your DOF. There are important differences in the above two photos. Longer focal length lenses distribute that DOF more evenly around the plane of focus. (Another DOF myth by the way -- it's not 1/3 front 2/3 back). Also the perspective in the two photos is very different although not because you changed lenses.

=========================

Now a true story to help explain why thinking correctly about this is important. 30 years ago I worked behind a camera sales counter. I sold the high end pro equipment like medium and large format hardware. One day a local wedding photog came in. He was trying to move up into some commercial illustration. Having a rather high opinion of himself he brought along a photo of open end wrenches laid out on a black background. He was shooting them from above at a 45 degree angle with the normal 80mm on his camera. He showed me the photo and noted that he wasn't getting enough DOF to cover front to back. Then he said it looked like it was time for him to break down and get that 50mm. A 50mm lens on a 2.25 camera is wide angle. He figured the extra DOF from the 50mm would do the trick. I asked, "aren't you going to move the camera in closer to keep the same crop?" He said, "yes." I said, "well then the DOF won't change." He knew better and told me so. Back then that lens was $900.00. I sold it to him and the DOF didn't change because DOF = magnification + f/stop and he kept the magnification constant.

Joe