Nikon Live View and aperture setting

Started Apr 28, 2011 | Discussions thread
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iso rivolta
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Nikon Live View and aperture setting
Apr 28, 2011

In a number of recent reviews, those of D3100, D5100 and D7000 there is included this or a similar paragraph:

The D5100's Live View aperture control behavior also shows an odd quirk that first appeared way back on the D300. When you enter Live View, the camera will stop down to the currently set or metered value (offering an undocumented depth-of-field preview in the process), but what it won't then do is readjust the diaphragm 'live' if you change the aperture setting. It will honour the set aperture when actually making an exposure, and indeed return to live view at that aperture afterwards; alternatively you can force it to readjust by exiting and re-entering live view.

This behavior is, simply, a bug (albeit a long-standing one) rather than some fundamental limitation of the F-mount; the D3 series bodies can continuously adjust aperture in live view, exactly as you'd expect. And it does have a couple of practical consequences - it can give a misleading impression of the depth of field you'll end up getting, and it further complicates manual focus as you have to remember to always initiate live view at an appropriate aperture (normally with the lens wide open). Also, in lower light conditions the display can appear dark and/or grainy if you enter live view with the camera set to a small aperture. As we'll see later, this bug has a curious knock-on effect when shooting movies too.

In the conclusion page of the D5100's review there is even a line regarding the "buggy live view".

OK, this behavior is not what an user would expect, but is not the result of a software bug.

Nikon, Pentax and Minolta/Sony have mechanical levers for controlling the lens diaphragm. Canon and Olympus DSLRs and all mirrorless cameras use electrical contacts for this. OT, it is strange that Minolta chose a mechanical lever in the '80s when the all new A mount was created.

The most simple and cheap implementation of the mechanical aperture lever is by a mechanical coupling with the mirror mechanism. Live view is obtained in these cameras using the same sequence as when taking a picture. OK, the new D3100, D7000 and D5100 have the means of keeping the mirror locked up during live view shooting, but the rest of the mechanics are working like during regular shooting.

The sequence when entering live view is:

1. Mirror is up, aperture is stopped down to the shooting value (like when taking a picture, that gives an unplanned DOF preview)
2. First curtain of the shutter travels down

Suppose now that you want to set a different aperture value to the one set before entering live view. You cannot vary the aperture during the taking of a picture with these cameras. The mechanical lever does not have a separate motor, it must wait for a new mirror & shutter gearing cycle. That means completing the cycle, i.e. closing the second shutter curtain and taking a shot, which is in fact almost the same thing as exiting and re-entering live view. You can see the aperture control lever moving only during these steps.

So, even if the mirror is kept up in LV, the aperture is behaving like during normal shooting, showing that the rest of the mechanical connections are well in place.

When taking a picture in LV the sequence is:
0. (Mirror is up, the aperture is stopped down.)
1. Shutter closes (second curtain travels down)

2. Aperture opens all the way (like you have taken a shot and you need to look in the OVF between shots) - now the shutter is re-cocked

3. Aperture closes back down. (now it will close at the value set previously when in LV)
4. Shutter opens and closes.
5. Aperture opens all the way back up (again)
6. Aperture closes back down.
7. Shutter opens, live view resumes.

The only Nikon/Pentax/Sony DSLRs that can vary the aperture in live view are the Nikon D3 family and Pentax K-7 & K-5. These more expensive cameras have a separate electromagnetic actuator for the mechanical lever.
Quote from K-7's press release:

Thanks to the newly developed diaphragm-control mechanism, the Live View function can also be used in the continuous shooting mode with the mirror at lock-up position.

Nikon found a way to decouple the mirror from the mirror-shutter-diaphragm_lever gearings (using one shared motor) during live view in the entry and mid-level DSLRs. But the aperture cannot function independently.
A quote from Nikon:

D3100 employs a newly developed shutter that is ideal for both Live View and movie shooting. The lightweight and compact design coupled with the ability to operate while the mirror is up during Live View shooting...

It's not a bug. There is no possible firmware fix.

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