Speed booster confusion in DOF

Started Jun 6, 2013 | Discussions thread
EinsteinsGhost
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Re: Speed booster confusion in DOF
In reply to kkx, Jun 7, 2013

kkx wrote:

EinsteinsGhost wrote:

kkx wrote:

EinsteinsGhost wrote:

kkx wrote:

Hi,

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).

Since I did not change the framing, I assume the DOF will be the same.

For example, if my photo is of a ruller, if in the FF ref, the DOF cover inch 2 to inch 4 making (I focus on inch 3 and the DOF is 2 inches)

The crop camera photo will still show that inch 2 to inch 4 is in focus. The framing has change, but I still focus on inch 3 and DOF is (absolute) still 2 inches center around the marking of inch 3.

I know you can assume that you have an "effective" 150mm F4.2 lens, but since I did not change the framing, relative (to FL)distance to subject for the FF and crop photo is different. And this result in the same area in focus.

Please correct me if I have any misunderstanding.

Here I was addressing your point on framing being different. To have the same framing (FOV) as 100mm f/2.8 on a FF and the same lens on APS-C, your focusing distance will have to change (increase) accordingly which will affect DOF.

I see. My bad. I use the term "framing" inconsistently.

In my original post, I said "framing will be different" but just now I said "framing will be the same".

In both cases, I mean to say that, I do not reframe the photo between FF and crop. Keeping the focus distant the same, and just let the photo from crop sensor be a crop from photo of the FF sensor/camera.

Sorry for the confusion.

If you keep the focus distance the same, you will get shallower DOF with APS-C (and narrower FOV).

If you replace non-optical (non-SB) adapter with optical (SB), and do the same, you will get the same DOF and FOV, but gain about a stop in exposure.

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