Have you ever been frustrated because you look at other peoples aperture settings, and when you try them they are WAY too shallow or not shallow enough?
You can really only compare someones f stop setting to yours if:
1) you have the same size camera sensor as they do
2) you are the same distance from the subject as they are
This is because DISTANCE AFFECTS THE AMOUNT OF DEPTH OF FIELD EVEN WITH THE SAME APERTURE, and smaller sensor cameras often make you stand farther away.
Here is a DOF calculator: http://www.dofmaster.com/dofjs.html
ok lets talk cameras first.
full frame cameras: canon 5D, 5dII, and 1DsIII, and nikon D3/3x/3s and D700
smaller sensor cameras: everything but whats above (so rebels, D90, d3000, etc)
With a smaller sensor camera, you cannot go as wide as a full frame (it automatically zooms you in more) and to fill the frame in the same way as a full frame camera, you actually stand FARTHER away from the subject.
Now, your depth of field (how much is in focus) is actually based on two things…the f stop (for example f1.4, f2, f5, etc) and your distance from the subject. The farther you are from the subject, the more will be in focus.
Here are some examples
Small sensor @ f1.8
distance: 3ft = total depth: 1 inch
distance: 5ft = total depth: 3 inch
distance: 7ft = total depth: 5 inch
distance: 9ft = total depth: 8 inch
However your distance from the subject will be different. These are approximate examples:
full frame: 3 ft = small sensor: 4.5 feet
ff: 6 ft = ss: 9 ft
ff: 8 ft = ss: 12 ft
So because you are farther away from your subject when you use a small sensor, you are automatically getting more of the pic in focus.
And if you are using a full frame sensor, and you think to yourself “why cant i get both eyes in focus at f1.8 when the subject is slightly turned away from the camera, when my friend with the rebel did,” it’s because you are closer and therefore the depth of field is razor thin.
Here is a rough explanation of the difference between using a full frame 5D and using a camera like a rebel or 7D if you want to get roughly the same amount of the pic in focus.
5D (full frame)
focal length: 50 mm
subject distance: 4 ft
total depth of field: 7 in
7D (not full frame)
focal length: 50 mm (so thats a 80mm equivalent in full frame)
subject distance: 6 ft (changed because you have to be further away to get this same shot)
total depth of field: 6.3 in
so you can see that f2.8 on a smaller sensor camera the same depth of field as f5 on a full frame larger sensor camera…
I highly suggest an iphone app called “photocalc” because you can put in your camera, lens, and distance info, and it will tell you your total depth of field. or there are other online, printed, or app calculators out there that will give you the same info.
Here is a visual to help you understand.
The top is a small sensor. The bottom is a full frame sensor. The distance from the subject is the same in these two photos.
So if you want to make the small sensor look like the bottom photo (with that extra stuff on the side) you have to stand farther back.
Also, it’s a semi rule that when you focus on a subject, 1/3 of the in focus area is in front, and 2/3 is behind. That is why you should focus on the eye closest to the camera if someones head is slightly turned. Though check with a DOF calculator because it will tell you the exact percentage.