Flange focal distance / DOF question

[Daniel Browning]

Brian explained things very well, but I'd like to add my two cents anyway.

Originally Posted by Don Parrish

I remember the XL1S as not having great bokeh because it had 1/3 inch chips.

Bokeh is not the same thing as thin depth of field (or defocus blur). Bokeh is the quality of the blur, not the quantity. So even a 1/3" chip, with only a slight amount of defocus blur, can have good bokeh. While a large 35mm camera, with a huge amount of defocus blur, can have bad bokeh.

Originally Posted by Don Parrish

This placed the flange distance closer to the senor correct ??

No. First, 3-chip cameras have to have a relatively large flange distance in order to provide room for the prism.

Originally Posted by Don Parrish

Larger chips require the lens be farther away in order to cover the sensor correct?

No. 35mm rangefinder lenses can be (and sometimes are) just a few millimeters away from the sensor.

Originally Posted by Don Parrish

Such as the 7D vs the 5D Mk II.

The flange distance is identical between those two, but the *backfocus* distance is ever-so-slightly different.

Originally Posted by Don Parrish

Doesn't a larger sensor automatically force a longer flange distance ?

No.

Originally Posted by Don Parrish

What creates the razor sharp DOF on the F3, 5D MK II etc.

Wide iris diameters (i.e. long focal length and slow f-numbers or short focal lengths and fast f-numbers)

Hope that helps.

[Don Parrish]

Thanks for the replies,

by "bokeh" I meant it was very hard to get an XL1S to blur the background (thus very little bokeh ).

Razor sharp DOF, we all know large aperture creates blur, what I was asking is why the F3 etc has it, most prior pro sumer camcorders could not achieve this thin DOF.

[Daniel Browning]

Originally Posted by Don Parrish

Razor sharp DOF, we all know large aperture creates blur, what I was asking is why the F3 etc has it, most prior pro sumer camcorders could not achieve this thin DOF.

Some will say that the reason is because of the larger sensor, but I think a better answer is the lenses that are available. By this, I mean that for any given angle of view and focus distance, the F3 has lenses available that are capable of wider iris diameters. That's why it can achieve thinner DOF than other camcorders.

For example, when I zoom my XHA1 to a 40-degree angle of view (the so-called "normal" focal length), the focal length is 7mm, and the widest f-number is f/1.6. The iris diameter is 7 divided 1.6, which is 4.4mm.

With the F3, a 40-degree angle of view requires a 32mm lens. You can find lenses in this range with an f-number as wide as f/1.2, but even if you only use f/2.8, the iris diameter still results in 11.4mm. That is still much wider than the 4.4mm maximum diameter possible with the XH-A1 set to f/1.6. That is why it's possible to have a depth of field so much thinner.

In general, larger sensors tend to have lenses available with wider iris diameters. However, that's not always true. For example, many digital Medium Format cameras have lenses with such slow f-numbers (e.g. f/2.8) that they cannot achieve the same iris diameter as lenses on the 36x24mm FF35 format (which open to f/1.0, f/1.2, etc.).