Barack Obama Shot ...

Since we're being geeky here, isn't it the focus distance, not the focal length, that effects DOF secondary to aperture?

ie.: if you're using 55mm at f/5.6 and you focus on something that is physically 20 feet away from you, the focus distance will be close to the far end, causing a larger DOF. But if you're standing in the same spot, focusing on same subject 20 feet away from you, but you're using 200mm at f/5.6, the focus distance will close to the near end, causing a shallower DOF.

ie.2: if you now stand 5 feet from the subject (I'm guessing at the math here) so that the subject at 55mm fills the same the same way that it does at 20 feet and 200mm (both still using the same aperture), the depth of field will be the same.

...right?
 
There are tree factors that alone decide the depth of field.

1. Focal length
2. Aperture
3. Focus distance.

To get a shallow DOF, you need to have 1. a reasonably long focal length, 2. a large aperture and 3. a reasonably short focus distance.

This also explains why cropped bodies has more DOF in practice compared to full frame bodies (and medium format has shallower DOF than full frame, same story with big format compared to medium format, of course).

The reason you get more DOF is that when using a 50mm lens on a fourthirds body, that give you the field of view of a 100mm lens on a full frame body. Given the same aperture as the 100mm on the 5D, the E-400 will give about the same field of view, and the same lighting gathering abilities with a 50/2.0 as a 100/2.0 would do on a 5D.

Therefore, you stand at the same distance (and therefore get the same focus distance) with the 100/2.0 and the 50/2.0, but the focal length is shorter on the E-400 than it is on the 5D.

If you use the same logic about an E-400 with a 50/2.0 and 5D with a 50/1.8, you'll get the same focal length, the same aperture, but to frame the same shot, you need to get closer, thereby, the 5D will get less DOF than the E-400.

As long as both cameras use the same focal length, the same focus distance and the same aperture, the DOF will be equal. Although, it would mean either a very tight indeed portrait on the E-400, or a very distanced portrait on the 5D.

:)
 
And why the hell are you using a 105mm lens for portraits on a FF cam? 75-80 on FF or 50 on APS is the rule.

The hell kind of rule is that?
 
A very silly rule. Anything between 14 to 600 (EFL) is an appropriate focal length for portraits.
 
The mental image of someone using a 400 for portraiture just made me giggle. I'm picturing one of those sports photographers in a cramped studio with a HUGE lens on, "Okay, SMILE!"
 
A very silly rule. Anything between 14 to 600 (EFL) is an appropriate focal length for portraits.

When I shoot portraits I usually hangout with my 50 and my 80-200. If I was really into portraits I'd look into the Tamron 90 or Nikkor 85.
 
The "rule" is more of a "guideline" for standard-type portraits. Too close and too wide will exaggerate perspective, weight and facial features. Too far and too long and you'll get either depths of field that are too shallow or you'll have to use such large apertures that you'll get diffraction and softness. THERE IS NOTHING WRONG WITH THESE EFFECTS, they are just starting points. Experienced photographers can use these effects to their advantage and style.
 
It's what portrait photographers have preferred and used forever. But you can use anything.
 
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