canon S90 f2 aperture, why use higher?

[marmagi]

Hello,

I have a canon S90 camera which has a lens with great F2 aperture

I often read that people use F4, 5.6 or even 8 aperture in AV mode, why this?

Why will I use higher F aperture which decreases my possible achievable speed?

I also use an INON D-2000 strobe with the S90, and usually use ISO set at 100

please tell me why I shouldnt use F2 to achieve higher speeds?

better never use F2 in neither macro or field pics? my average depth of dives is 20mt ( 60 ft) up to 40mt ( 120 ft ) which means natural light isnt in excess

please explain me

thanks

Mario

 

[M_Bipartitus]

Short response: Use f2 for wide angle, and move towards f8 in close-ups, especially if you have a strobe.

Longer response:
The main reason to use a higher f-number (often called a smaller f-stop/number as the aperture that lets light into the camera is smaller), is to achieve a larger depth of field (DOF). DOF refers to what range of distances are in focus. In a wide angle shot this effect is minimal, but it can be very important for macro. For example, if you take a photo with your subject about 10 feet away, everything from ~1.75m (5 feet) to the horizon will be in focus. If you move to f5.6, everything from ~1m (3 feet) to the horizon will be in focus. If can make a difference in the final photo, but often isn't a concern.

On a dSLR shooting macro the differences can be much more important. For example, on my Canon 5d with a 100mm macro and some subject at 1 foot (30cm) away, the depth of field at F2.8 is about 1mm ( 0.04 inch). You wouldn't be able to focus on much. At F22, there is a range is almost 10 times as much, but a strobe is necessary due to the low level of light let into the camera.

You can select the camera and settings on this calculator to play around with what the numbers are:
Online Depth of Field Calculator

 

[Skier]

There is a nice example of how DoF impacts images HERE Princples apply both above and below water.

The closer you are to the subject (and in macro mode my S90 was getting to within an inch) the higher you'll want the f stop to be otherwise the front of that nudie will be nicely in focus but the rest will be out of ficus (although that can be a nice effect).

Would suggest taking you S90 out and playing around with it in a garden or park on flowers and insects. You will come across all the issues you have in the water. Try different light conditions - your dive strobe works well out of water - so you can start making your mistakes on a lazy Sunday afternoon and not on your dive trip.

Also, start reading about photography somewhere like ephotozine. They are a friendly bunch and answer land based questions. Unless you have a good grasp of the technicalities of photography you'll struggle to get good in water results - or going to take much longer.

 

[sablefish]

f/2 is the largest aperture the lens is capable of. (it's a great lens, btw). However, the optimal range of that lens is probably f/4-f/10. Stopping down a lens improves contrast and sharpness as well as increasing DOF. Stopping down the aperture works for modern multi-coated zoom optics as well as uncoated lenses from 50 years ago. See this article on diffraction.

 

[Nemrod]

f2.0 <---- more light, less DOF, faster shutter ---- less light, more DOF, slower shutter ----> f8.0

Equivalent exposures:

f2.0 1/500
f2.8 1/250
f4.0 1/60
f5.6 1/30
f8.0 1/15

As you can see, depending upon whether it is freezing action or emphasizing depth of field or maximizing available light determines which of these example settings above are appropriate BECAUSE in terms of the exposure--they are identical.

N

 

[skynscuba]

Equivalent exposures:

f2.0 1/500
f2.8 1/250 <-----
f4.0 1/60 <-----
f5.6 1/30
f8.0 1/15

Shouldn't that be a factor of 2, not 4?

 

[Nemrod]

Shouldn't that be a factor of 2, not 4?

Yes, I apologize, it is a typo on my part, was not paying attention, I should have gone to 1/125, sorry about that.

f2.0 1/500
f2.8 1/250
f4.0 1/125
f5.6 1/60
f8.0 1/30

Is that better?

N