Fuji X-E3: different exposures whether using Focus-Check or not

[Karlson-B]

Hi there,

I realized that the exposure measuring is different when I use the focus check (rear command dial) to zoom in and then pressing and releasing the shutter in opposition to not use the focus check and taking a picture.

The effect is easily to figure out:

1. Place a white paper on a dark underground.
2. Zoom in with the focus check (pressing rear command dial) and let the white paper just fill out the view finder.
3. Take the shot.
4. do not reposition the camera
5. Take a second shot without focus check, so that the surrounding dark background is in the frame to.
6. The exposures will be noticeable different.

Question:

1. Is this by intention?
2. Is this only with the X-E3?
3. Do I always have to AE-lock the original exposure before zooming in with focus check? --> would be very inconvenient and counterintuitive...

normal exposure without focus check




exposed when using focus check

 

[nuzzart]

That is an interesting effect. I don't see the issue on my X-T20. I would find it hard to believe that the Fuji software would use only part of the sensor for focus check rather that just change what is displayed in the viewfinder. Having done software programming for many years I would see that as real newbie programming error and I don't think Fuji makes that kind of errors. Also it would probably show up on all models. Now of course, I never programmed a digital camera so I could be totally wrong and there could be a very good reason to use only part of the sensor for focus check.

My guess is if the multi metering is uses the focus points in its algorithm to set the exposure then the second image focus locked on a slightly different place. One way to test this would be to use average metering. Since average metering looks at the full sensor if it still does it then what the meter is looking at changes with focus check. If with average metering the exposure doesn't change then it might be related to the multi-metering.

Well I just confirmed that the metering changes when image magnification is engaged on both my X-T20 and GFX. When you think about how the exposure system works this makes perfect sense as with magnification set you are essentially switching to a spot meter. With a mirrorless camera there is no separate exposure sensor it comes from the sensor. I suspect that it would be a very heavy lift (if even possible) to have two simultaneous feeds from the sensor, one full frame for metering and one for magnified view. I mostly shoot full manual so it is not an issue for me but I don't see that it would that much of an issue to not lock metering when when magnification is engaged.

I agree everything comes from the sensor. What I am suggesting is that the sensor data is mapped to memory locations. Then for speed pointers are used to read that data. Changing the value of the start and end pointer changes what is viewed. When you change the magnification of the focus check, all you are doing is changing what data is used by the viewfinder by changing the value of the pointers. When you have a spot meter it just uses a different value for its pointers for its data set. There would be no reason to use the same data for both the meter and the viewfinder. The same applies to when you view a stored image and then zoom or pan. All you are doing is changing the start and finish pointers. Like I said before, I could be totally wrong since I never programmed a digital camera but that is how I would do it.

My guess was they use the focus points as an input to the multi-meter and the point changed. They already do that where you can tie the spot meter to the spot AF point. Also they do it for face recognition. I had an old Minolta 7Xi that did the same using its focus point in part in setting the matrix metering.

 

[giarc]

To see this effect you have to set Button/Dial -> Shutter AE -> AF-S/MF to OFF.

Looks like a defect. Good catch, hopefully somebody from Fuji is watching.

Yeah, I agree that this feels like a bug in how the metering is performed. I wouldn't expect the scene to be metered any differently just because I zoom in to check focus.

I tried this on my XT-2, with auto shutter speed, matrix metering and on a tripod, not re-focusing between shots, the photos are exposed with different shutter speeds depending on the level of 'zoom'. It does happen in other metering modes to a lesser degree, except in spot, which intuitively makes sense.

I'm not sure how much of an impact it has in actual shooting, and I certainly won't worry about it, but it is somewhat surprising.

--

Craig

Attached the matrix metered set:




No zoom




Partial zoom




Full zoom