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I am fairly new to photography and I have read many "beginners guides" and watched youtube tutorials. One thing that remains elusive to me is how to properly pick what f-stop to use.

I use a 35mm, 55mm f1.8, and a 24-70mm f4 lens with my Sony A7.

For portraits, I generally try to pick lower f-stop like between (1.8-6f). For more general use like taking pictures of animals I usually pick between (6f-10f).

The issue I primarily run into is that when I take a picture with a lower f-stop, it looks great in my viewfinder. However when I go home to load it on a larger screen, I find that a lot of elements I am trying to capture is actually out of focus.

How do I know how much room I have to play with before a subject goes out of focus? It seems to change with f stop, how close I am to the subject(s), and the focal length I am using.

I understand the concept that I am basically focusing on a "slice" of space in front of me and when I lower the f-stop, this slice of space becomes narrower. I can't figure it out and I end up getting disappointed when I view the pictures on a large screen.

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    \$\begingroup\$ Have you seen this question? How to estimate depth of field ? It can be calculated, but mainly you learn from experience roughly what aperture you can get away with for a given shot. \$\endgroup\$
    – MikeW ♦
    May 11, 2016 at 4:48
  • \$\begingroup\$ "It seems to change with f stop, how close I am to the subject(s), and the focal length I am using." You nailed it. You are only missing one variant. How far the background is. So it is a 4 variables game. \$\endgroup\$
    – Rafael
    May 11, 2016 at 18:14
  • \$\begingroup\$ To get a better feeling, choose a patient enough subject (like a flower), and do the same shoot with multiple settings. Write them down if you cannot see it in the software you use to see the pictures (but Windows generally allows to see the parameters at the JPG even). Then you can compare the effects and learn what to use for what look. \$\endgroup\$
    – Aganju
    May 13, 2016 at 21:13
  • \$\begingroup\$ One of the things that finally "clicked" with me and DOF is to think of DOF as dependent on magnification -- not distance or focal length. For example, 20' at 100mm has the same DOF as 10' at 200mm. Think about macro, distance doesn't change the DOF -- the magnification of the subject on the sensor does. What this does for me is I think about the size of the person in the frame and mostly ignore the focal length in my f-stop estimates. \$\endgroup\$
    – rrauenza
    May 18, 2016 at 22:07

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Depth-of-field (DOF) We focus our camera to obtain a sharp image of an object at a specific distance. We know from practical experience that objects before and behind the distance focused upon will appear sharp. This span of acceptable sharpness is what we term depth-of-field (DOF).

The DOF span is variable based on several factors:

Focal length of the lens – shorter more DOF – longer less DOF 1. f/# - Lower aperture number (larger opening) less DOF 2. f/# - Higher aperture numbers (smaller opening) more DOF 3. Subject distance – The further the subject the more DOF 4. Viewing distance – The closer the displayed image is to the observer the less DOF 5. Acceptance of what is sharp is a subjective decision 6.
DOF charts and calculators abound, but likely we can’t consult during the shoot. Additionally, the DOF viewfinder preview falls short; we must fall back on experience and instinct.

Some rules-of-thumb:
DOF is not split down the middle; it extends about 1/3 back towards the camera and 2/3 behind the distance focused upon. Portraiture likely is best if DOF is shallow; use larger lens openings Landscapes likely best if DOF is expanded; use tiny lens openings. Best if the focus distance and the aperture used match the hyperfocal distance. Hyperfocal distance: Maximizes DOF – all distances acceptably sharp from infinity ∞ to half the distance focused upon. Use tables / charts to find hyperfocal distance. A math formula that will calculate: Focal length X 3.3 ÷ f/number = Hyperfocal Distance in feet. Example: 50mm lens mounted and set to f/16 Hyperfocal Distance = 50 X 3.3 ÷ 16 = 10 feet. Set focus to 10 feet and aperture to f/16 then DOF extends from 5 feet to infinity ∞ .

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  • \$\begingroup\$ Is there a difference between "3. Subject distance – The further the subject the more DOF 4. Viewing distance – The closer the displayed image is to the observer the less DOF 5" \$\endgroup\$
    – Alan
    May 11, 2016 at 18:47
  • \$\begingroup\$ 3. DOF is profoundly influenced by subject distance. Double the distance focused upon, the result will be a fourfold increase in the span of DOF. 5. Distance image to observer – The lens images by creating countless tiny circles that make up the image. These circles are called circles of confusion because they are huddled together and they have indistinct bounders. A measure of their size is how we gage sharpness. The image appears sharp when these circles are so small we see them as a point and not a disk. This occurs when viewed from a distance 1000 times there diameter or more. \$\endgroup\$
    – Alan Marcus
    May 11, 2016 at 20:21

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