depth of field?

[Spinning]

Hi,
I know I should understand this and in some respects I do. But I still seem to have issues when taking pictures of 2 to 5 people in a group.
I have a canon 50D I own an 85 prime, 18-200 and just purchased 70-300

High school season is coming up which means lots of dances and opportunity to take group photos. I do ok and have some great shots but I also seem to miss shots where only one person is in focus and the rest are blurred or not crisp. These faces are usually either taller or farther back.
So in very simple terms please explain! LOL! Thanks!

 

[photo_chick]

Many people will say depth of field is the area that is in focus. But that's not really true because there is only one plane of space that will be perfectly sharp. So we say depth of field is the area of acceptable sharpness. It's that place in space just in front of and behind the plane of focus. And what is acceptable can change depending on the size you view the image at... an example, depth of field may seem larger on a 4x6 print than it does when you pixel peep on that image. In fact, when you view an image from a high resolution camera at 100% it can look like there is barely any depth of field just because you're viewing it so large. The moral of that story is don't pixel peep, you'll never be happy if you do and your entire group will never look in focus.

OK... how it works... aperture and focal length. The wider the aperture the shallower the depth of field. Some people get tripped up on the numbers for aperture. I always equated the smaller numbers with less depth of field. It's not mathematically where the numbers come from, but it works as a way to remind yourself if you're having trouble.

Focal length also affects depth of field. The longer the focal length the shallower the depth of field. This has to do with the way the lens makes the image and how it will visually compress the space. There's a lot of technical explanations but the key thing is longer lens = shallower depth of field.

Distance to subject also affects depth of field. The closer you are the shallower the depth of field. Which means in practicality some of that long focal length and far distance to subject that's used with them can cancel each other out a bit.

A depth of field calculator can give you some guidance on what aperture to use when. There's a lot of people who say that crop or full frame will affect depth of field but it really doesn't affect the actual depth of field. The math doesn't change. Like viewing size, it can affect the perception of depth of field though.

does any of that help, was it all stuff you already knew, or do you still have questions?

 

[Spinning]

So the small the number the larger the area should be in focus? So if I was shooting 4 or 5 kids together I would go for a smaller number?
the more I zoom in the smaller the area of focus will be? This is just something I am having such a brain fart with.....

 

[Pea-n-Me]

In addition to above, I think it will help if you set your focus points to focus on all the faces, as opposed to, say, one in the middle or on one of the sides, etc.

 

[photo_chick]

So the small the number the larger the area should be in focus? So if I was shooting 4 or 5 kids together I would go for a smaller number?
the more I zoom in the smaller the area of focus will be? This is just something I am having such a brain fart with.....

You have it opposite. The smaller the number the less depth of field you have. And the more you zoom in the less depth of field you have.

 

[photo_chick]

In addition to above, I think it will help if you set your focus points to focus on all the faces, as opposed to, say, one in the middle.

This reminds me of A-dep. Canon DSLR's have an auto depth of field setting as do some others. You put the focal point on the closest thing, then on the farthest thing and it gives you the "correct" aperture to have them within the depth of field.

 

[Spinning]

I do do my focal point with the full points and not the center one.

 

[Pea-n-Me]

Face-detection technology is also helpful, if you have it. It finds faces and puts a little square around them to get them in focus.

 

[Spinning]

You have it opposite. The smaller the number the less depth of field you have. And the more you zoom in the less depth of field you have.

LOL! Well we have have found my problem!

 

[Pea-n-Me]

Well that was quick!

 

[Spinning]

now to make it work!

 

[Luvscrappin]

I was having this exact problem on vacation. I had the aperture set at f/4 and the people were blurry...and I noticed only after uploading the pics to my computer Once I figured out the problem I asked for a "re-do" the next evening (we were shooting family portraits on the beach) unfortunately my family was not as "cooperative" the 2nd night. The pictures are now sharp, but the faces aren't as "happy" Oh well, better get it right the first time next time! This is what I get for forcing myself out of the "auto" mode!

 

[MolonLabe]

Pixel peeping... as a n00b, I pixel peep on the screen after a few shots... not for perfection but to make sure I didn't do something like that.

I haven't done any exceptionally important shots yet, but until I get a lot more competent, I'll be taking multiple shots, at various exposures and some on 'auto' just to have something to fall back on. (using the Canon T4i)

 

[photo_chick]

I do do my focal point with the full points and not the center one.

I just saw you're using the 50D... my experience has been that this doesn't work well on that camera. I've been shooting with the 50D for not quite 3 years now and I've found you really have to choose the right focal point to nail the focus where you want it. It has an older AF system and does a really poor job of deciding focus on it's own. I use single point AF and toggle the point to where I need it.

 

[YesDear]

If you want to practice, put three small things in line about two inches apart on your kitchen table or somewhere convenient. Use a tripod, for consistency, if you have one. Focus on the center item. Set your camera on aperture mode and start at wide open and take a picture. then change to every fstop you have and take a picture. (another good reason to use a tripod) look at the pictures and the different depth of fields will be very evident.

Then move your camera back 5-6 feet if you can and retake You will then see what distance and compression can do to alter the depth of field.

Then go outside and do the same but put three items a foot apart and take several series.

At that point you will be a good feel for how that lens will respond for Depth of Field and compression.