Speed booster confusion in DOF

Started Jun 6, 2013 | Discussions thread
EinsteinsGhost Forum Pro • Posts: 11,977
Re: Speed booster confusion in DOF

kkx wrote:


I am trying to understand this before buying the adapter.

Read many posts and articles here and on the net in general, but still not certain.

I hope someone who understand this can help me out. Best of all, if you have the adaptor, so test will be fantastic.

Say I have 100mm 2.8 lens mounted on tri-pod. Focus and framing is fixed.

If I mount a FF camera (say 6D?) and expose at ISO 100 f2.8 1/1200 I will get a photo that I will use as reference.

Now swapping the camera (550D or Nex via simple no lens adaptor) and expose at ISO 100 f2.8 1/1200, I will theoretically get a photo with the same exposure (brightness), absolute DOF as the ref photo above, but the framing will be different, it will be cropped (1.6 or 1.5).

Same exposure - Yes. Narrower field of view (comparable to 150mm on FF) - Yes. Absolute DOF? Depends on what you mean by it. But the DOF will actually be shallower (150mm f/4.2 lens on a FF camera will have a shallower DOF compared to 100mm f/2.8 lens on that camera).

If I know swap the camera with nex + the speech booster and I expose at ISO 100 f2.8 1/2400: I will get a photo that is exactly the same (brightness, DOF and framing) as the first reference photo.

I.e, I can use faster shutter because of the 1-stop of light increase, and the framing is the same as FF. But the DOF is aslo the same as FF.


There are a lot of "info" saying that the lens will be effectively f2.0 because of the light/speed increase of 1-stop. So I am confused.

That is reflected in faster shutter speed (1/2400s versus 1/1200s). However, DOF will be the same. All of this assumes that f-stop is as good representative of t-stop as well.

Is my expectation correct? From what I have "learn" from internet with 2x tele-converter, if you don't change the framing, using the same lens with and with-out the tele-converter will mean exposure difference of 2-stops, but DOF is not changed. DOF is only affected by FL vs entry pupil size.

Can anyone help me confirm this?

DOF will decrease. The effect of FL is greater than that of f-stop. If you use a 2X TC on 100mm/2.8, you'd have 200mm/5.6. But just because 2.8 is now 5.6 does not compensate for the effect of doubling of focal length. In fact, doubling the focal length reduces by the DOF by four times (the factor is: square). Whereas, f-stop is linear (increase from 2.8 to 5.6 will increase the DOF by 2 times). So, your effective reduction in DOF will be by a factor of 2.

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