Tip of the week: Depth of field, how to choose the aperture
You must have heard a million times: “Man, that is great lens it can achieve so shallow DOF”. Today I’ll try to point out few things regarding DOF and give you some guidelines on using it, so stay with me.
DOF stands for Depth Of Field and it represents area of image with acceptable sharpness that will appear in focus. Actually when you focus with your camera on subject DOF spreads 1/3 of depth in front of point where you focused and 2/3 behind that point. In that area you will have zone of acceptable sharpness. Some situations can be critical regarding focus so it is good to know how that focus area is distributed.
What are the main factors that affect DOF:
- Subject distance from the camera
- Focal length
Aperture is controlling amount of light that is reaching camera sensor through the lens. The wider the aperture is (lower f number) the DOF will be shallower and vice versa. So when you need to isolate your subject from the background and put it out of context, lenses that have large aperture are more than welcome. Standard prime lenses usually have maximum aperture of f1.8 and professional primes are usually f1.4 or even f1.2. Be aware that pulling subject out of context is not always a good thing to do. I’ll give you an example. You always wanted to go to Paris, and after some time you manage to visit it, stand on place where you can have image with entire Eiffel tower behind you and your friend takes his great pro camera with 85mm 1.2 lens and take a shot on 1.2 aperture, and what a creamy bokeh he made behind you, but wait, how will anyone know that it was in Paris? Maybe it is your backyard since everything but you is blurred on that picture? In such occasions, pay attention and close that aperture down so you can make it obvious where the photo was taken. So what aperture should you set? I can’t tell you exactly since aperture is just one factor. It depends on what focal length you are using and on distance from the camera.
Focal length makes a huge difference in DOF if used with same aperture. For instance f2.8 aperture on 14mm full frame lens when focused on subject that is 2m away will have DOF 13.2m deep, and if you do the same with 50mm lens you will have DOF that is just 0.27m deep. Moving forward with focal lengths, DoF is becoming shallower and shallower (the above Paris example shot from a distance of 3m from the subject would give you DOF of only 9cm).
So, after killing you with technical stuff I will give you few advices:
When shooting portraits where you want to pull your subject from the background, use very fast aperture (based on framing and focal length it can vary from 1.2 to 2.8).
When shooting portraits where you need to have context on photo, close it down to f4 or even f5.6 (sometimes even more stopping is needed believe it or not).
Usual apertures used for landscape and architecture photography are from f8 through f11 and sometimes f16 since we want our entire frame to be sharp.
Usual street shooting - well no strict rules apply, I shoot them from f1.4 to f8 depending on light conditions and framing (most of the time 2.8 to 4).
Macro photography is pretty hard since more closer you are to subject, the DOF is being shallower. In many situations I found myself having f16 aperture on my 105mm lens very shallow, so I had to do focus stacking from few images to get that image sharp all the way.
Keep in mind also that if you have a moving subject, shallow DOF is usually your enemy since you might end up with a lot of photos being out of focus due to the fact that DOF is sometimes very shallow and that autofocus system is not product of miracles but a product of technology that has some limitations.
There are various sites and mobile apps with DOF calculators which can provide you with the info how deep is DOF based on focal length, aperture and distance from subject, so use them to help you understand how it works and keep shooting until you master it.
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