jackfarrell wrote:
How does a flash effect DOF and how should you compensate for it?
I am in the process of trying to understand the relationships among the various camera and flash settings and their affect on the final exposure.
Because the original question appears to address the DOF and flash relationship, I am not addressing shutter speed for freezing of motion or providing for the appearance there is motion.
I believe there are two exposure components to taking a picture using a flash or additional artificial lighting. The total exposure has an ambient light component and the flash component.
The aperture affects the depth of field and controls the ambient light. If you want a narrow DOF to blur the back ground use a small aperture number i.e., f/1.4. If you want a wide DOF for max depth of field use a large aperture number, i.e., f/22 and the hyper focal distance focusing. In setting this up I have identified the exposure I want for the ambient light part, i.e., the ISO, shutter speed, the aperture.
Because flash units fire a pulse of light that is much shorter than the duration the lens is open (determine by the shutter speed) it is typically easily to add the additional lighting desired. There are some issues to resolve with respect to sync shutter speed and when to fire the flash during the lens opening, which typically can be resolved in reading the manual.
Now, after I have setup the camera for the ambient light exposure and if I then determine I want some additional lighting or lighting effect, then without changing the ISO, shutter speed and the aperture setting above; I then determine the flash distance, power output, and location to achieve the effect I want.
Note: This does not change the DOF it just adds additional lighting.
I then fire off a test and adjust from there.
I hope this helps in understanding the relationship, effect and process I use for setting the ISO, Aperture, shutter speed and using a flash.
By putting this out there I have now put on my flack jacket for the incoming comments.
Have a good day.
dave