Depth of field

[gaterbite]

Ive been playing around with my Rebel XT trying to figure out depth of field. I found a focus chart on line and was trying to see the differances using differant aperture settings. All things being equal, the photos of the chart get darker. Im a little confused because shutter was set at 1/60, iso 400 and the telephoto lens focal length was at 27mm. I get it that less light gets in because of the smaller aperture but wouldnt the slower shutter ,1/60, still allow enough light in for the exposure?? Oh, I was using the built in flash on the camera. If needs be I can post some of the pictures.

[Robert_Lay]

First, you need my PhotoTool program in order to learn about the 3 things that govern exposure - ISO, shutter, and aperture. The program displays the shade of gray that is registered by the camera for a given ISO, aperture and shutter settings with a given reflected brightness from the subject.
PhotoTool Program is free and available in zip file, ready for installation on any Windows machine:
http://www.zaffora.com/W9DMK/Programs/PhotoTool.zip

Next, you need the on-line Depth of Field tool:
http://www.dofmaster.com/dofjs.html

[Longwatcher]

For those that don't like downloading things

If given that at F8, 1/250, and ISO 400 as the correct exposure.

If you change aperture to f5.6 (1-stop) and nothing else then you will be over-exposing by one stop, if to f4 then 2-stops over. f5.6 will be a narrower DoF then the f8 you started with and f4 even narrower, but it really gets noticably narrow (meaning background goes very blurry ) at say f1.4, which is 5-stops over-exposure off your starting point, which means almost everything will be whited out.

Thus you need to adjust something so
First you can adjust the shutter speed. since increasing speed lessens the amount of light you can increase the shutter to 1/500 to get the f5.6 correct then 1/1000 to get the f4 correct. to get that f1.4 you need 1/8000 shutter speed.

Now if you want a larger DoF (or sharper through out the picture front to back ) then you need to narrow the aperture (bigger number) so you go from f8 to f11 and then f16, which will make your pictures under-exposed if you don't change anything. So for say f16 you need to slow the shutter down by 3-stops or in this case to 1/30th of a second.

Next we add in that maybe there is even less light to start with so you starting point is ISO 400, f8 and your shutter needs to be at 1/30th to start. well if this is the case this is just too slow for people normally, so you have to take the shutter to at least 1/60th or better 1/125th. thus at 1/125 you are now 2-stops under exposed. at which point you have two options Aperture or ISO. If you like the depth of field, then that leaves ISO since you started at ISO 400, to get more light you need to increase the ISO so 2-stops means ISO 1600.

Shortening it down:
Aperture:
- smaller number more light - narrower DoF (blurry background )
- bigger numbers more light - wider Dof (shaper background )
[technically the numbers go the other way because it f8 is really 1/8, but it "looks" like a bigger number then f2 (which is really 1/2)

Shutter
- Slower more light - may lose sharpness due to camera or subject blur
- Faster less light

ISO
- smaller number less light
- Larger number More light - may cause too much noise at highest numbers

So the object is to balance all three of these factors based on the availlable light to get the correct exposure.

In lesson two we cover changing the light instead of the camera. :) 8) :)