I need a little help understanding the role of f-stop during wide angle photography. For the purposes of my questions, lets assume, WA using available light (no strobe) or large items such as wrecks, etc. I have heard others recommend a f-stop of 7 or so for WA and adjust the shutter speed as needed for proper exposure. Why would you want to use such a small aperature? (for P&S that is near the smaller limit of availablity). Most of my wrecks (here in NC) are going to be very deep, and when using available light, I would think you would want a larger aperature to best use the available light? Is my thinking wrong? I might need such a low shutter speed that I risk blur?
This is how I was thinking it through - remember I am a newbie with manual shooting.
I find the correct shutter speed for what I want my background to best the best blue that I want, then adjust f-stop to get the best exposure of the wreck?
Well you could start with not listening to all of that about settings. On your P&S you would have to stop the lens down all the way, f8 etc is near or at the upper limit. Wide angle lenses have great depth of field, even at wide apertures you should have sufficient depth of field. A wide aperture will soften edges, so what, the lens you have stacked on your P&S lens is a compromise at best.
There are no "settings," everybody always wants a magic setting, there is no such thing. Here is my scenario, with the camera in Av, set the aperture to the highest number that gives you at least 1/60th second at ISO100. If the shutter speed drops into the "shake" zone open the lens up another stop. At some point, since P&S are severely limited on both ends of the f stop range, you will have to either bump the ISO up or accept a very low shutter speed.
For any given lens, more discernible with longer focal length lenses, depth of field increases with f stop, f16 will have much greater depth of field than will f 2.8 on the same lens. Yes, real cameras with real lenses have f stops that can range from f1.0 to f32. Great depth of field is not always a good thing, if you wish to isolate the subject from the foreground and background (a candid) a photographer will choose a small f stop and if the photographer wants the foreground, subject and background in focus (a landscape) then he/she will choose a higher f stop. Additionally, short focal length lenses have proportionally greater depth of field than longer focal length lenses by the laws of optical physics.
Each full f stop is equivalent to doubling or halving the light reaching the sensor, each doubling or halving of the shutter speed does the same thing. FACT, example, f8 @ 1/60 second is the same as f5.6 @ 1/125 is the same as f16 @ 1/30 is the same as f4 @ 1/250 etc. Yes, those are all equivalent exposures, now, which "setting" do you want?
Fact, if you set your camera to manual and then use the suggested exposure setting the meter indicates you have just accomplished essentially what the camera would have done had you left it in auto mode. There is no real reason to use manual exposure except when you desire to override the cameras exposure system or operate a strobe which has limited ability to sync with shutter speeds etc.
If you are photographing a large object then choose the highest fstop that you can expose without introducing shake in order to maximize depth of field.
As well, by the laws of optical physics and the inability of humans to create perfect optical designs, most lenses are slightly softer at small f stops (f1.2, f 2.8 etc) than compared to the same lens at a higher f stop (f8, f16 etc).
Vignetting with your wet mount wide angle lens may also be more pronounced at smaller numerical f stops.
BTW, DOF and focal length are related by optical equations, I will not give them to you, you can do your own research, suffice it to say, the DOF of a P&S like your 590/P5100 etc calculates just as it would for a 35mm or a 2.25 square or a DX sensor or a telescope at your local astronomy club.
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