Depth of Field/Aperture/Flash

[Trever1t]

can you elaborate on that Keith? Why choose rear curtain over front curtain for static subjects?

 

[Josh66]

Great info all around. I see that the GN can be a good resource, but it's left me a little confused. As far as the calculation I want to use the following example.

Assuming a distance from subject 10 feet (reasonable working space outdoors), aperture of 8 (working with a consensus of what it takes to get ideal sharpness, DoF, etc). Based on this, GN should be 80? Using the table that comes in the manual for the SB600, assuming the flash head is at 50mm, I should be setting the flash to 1/2 power?

Reference: SB600 Manual pg. 35 (for the table)

Am I on the right track here? Math and equations were never my strong suit. I just want to make sure I have the variables correct.

Also, if I set aperture correct for flash exposure, will this freeze the subject regardless of having a slower shutter speed to capture more ambient light? I'm thinking for photos taken in the later afternoon/early evening.

Thanks for putting up with me... you guys have been amazing!

I'm not familiar with your flash, but it may be able to save you from doing the math in your head.

On my flash (580EX II), when in manual mode with the flash on the hot-shoe, it will read the ISO and aperture from the camera and show you on a little scale what the range will be for the power setting it's on. With custom functions, you can change the display to show feet or meters (I think meters is the default setting). Note that the flash head has to be in direct flash position for this to work.

Check your manual and see if your flash does something similar.


As an example, With my camera at ISO 100, f/8, 1/250th (shutter speed wouldn't affect this though), and the flash set to 1/4 power at 50mm zoom - the meter tells me that the flash range will be 7 feet. At full power, it's showing 15 feet. Full power, manually zoomed to 105mm, it says 20 feet.

If I switch it over to TTL, the meter shows that it will cover anything from 2.3 feet to 15 feet (50mm zoom).

(I had a 50mm lens on the camera, so that's why it picked 50mm zoom.)

You can do the same thing with the flash off camera, you just have to manually input ISO, aperture, and zoom in that case.


edit
But, yes - your calculations look right to me. Aperture = GN / Distance. So, if your GN was 80 at 1/2 power and 50mm zoom, you would want an aperture of f/8 for 10 feet.

 

[Josh66]

I have a little cheat sheet for my flash (it was like $10 at B&H) - it has pretty much everything from the manual one would need to know, and it's easy to pack. On it, there is a GN table...

Just to throw another example out there...

My flash has a GN of 48 feet at 50mm zoom and 1/8 power. Say my subject was 8 feet away... 48/8=6. So, I would probably use f/5.6 with the flash set to 1/8 (-1/3). Lets say I don't want to use f/5.6 ... I want to use f/11...

Subject is still 8 feet away. Av=GN/Dist. - I know the aperture & distance, but not the GN... So, if Av=GN/Dist., GN=Av*Dist. - 8*11=88 - So... I need a GN of 88. 1/2 power (at 50mm) has a GN of 97, and that's the closest I can get to 88 in full stops, so I would try either 1/2 power (97 is probably close enough), or 1/4 (+2/3) power (or 1/2 (-1/3), which is the same thing - it will display + or - depending on if you're adjusting up or down).

edit
Figuring out the settings this way won't be 100% perfect every time, but it will get you very close on your first shot.

 

[kirbym2]

Awesome... I feel like I'm actually getting somewhere. To be honest, I wasn't approaching my photography with any specific thought and technique into achieving my results. Yes, I often ended up with a final product I was happy with, but that was more a result of luck (and maybe a decent eye every once and a while). I feel like I can now get a more consistent result out of the camera by applying rules as they pertain to exposure, especially when using flash.

Thanks everyone - very much appreciated!