If I am manually setting aperture I let the camera figure the shutter speed and ISO. What I set my aperture size at depends on the effect I want. If I want the focal point to be in focus and everything past or before that to blur I use a larger aperture (smaller f#) like this. The depth of field can be adjusted by slowly choosing a smaller aperture.
If I want the whole snake in focus then I use a smaller aperture (larger f#) like this. It will have both the foreground and background in sharp focus.
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