Exposure changing with focus point when focus stacking

[Ian L]

I was wondering if anyone could help. I have a Sony A7CR (with 20-70mm lens). I like to focus stack some of my landscape shots. The problem I'm having is when I take two shots, one focusing on the foreground and one focusing on the background, the exposure changes almost every time, even though the light in the scene is not changing.

I have the following set

Single Shot AF, Focus Area Spot (S)

I also tried setting Spot Metering Point = Center (although I'm using Multi Metering)

Has anyone else had this problem, and am I missing another setting? Thanks

[Pieter]

Why don't you use manual exposure? When not in manual mode, every time the camera takes a photo, it meters for correct exposure. Odds are high that it picks slightly different exposure parameters between shots.

What you could also try is to map Auto Exposure Lock (AEL) to one of your custom buttons:

https://helpguide.sony.net/ilc/2370/v1/en/contents/0406_ae_lock.html

Edited October 26, 2023 by Pieter

[Ian L]

Hi Pieter,

Thank you for your advice. I like to shoot in aperture priority mode, but what you say about using manual mode makes sense. I never had this problem with my previous camera (Fujifilm X-T30 II) in aperture priority mode. The exposures were always the same when focus stacking, except in situations of fast changing light, which I could accept.

I think I may have resolved my issue though, by changing the metering mode from multi to center. Now the exposures are all the same at different focus points. Very odd why this would fix the problem, but it did.

I think there may be a bug when using multi metering mode. When I put the focus point on a dark area of the scene it will always take a longer exposure than when I put the focus point on a lighter area. So I think it is trying to use spot metering even though multi metering is selected.

Edited October 26, 2023 by Ian L

[FunWithCameras]

12 hours ago, Ian L said:

Hi Pieter,

Thank you for your advice. I like to shoot in aperture priority mode, but what you say about using manual mode makes sense. I never had this problem with my previous camera (Fujifilm X-T30 II) in aperture priority mode. The exposures were always the same when focus stacking, except in situations of fast changing light, which I could accept.

I think I may have resolved my issue though, by changing the metering mode from multi to center. Now the exposures are all the same at different focus points. Very odd why this would fix the problem, but it did.

I think there may be a bug when using multi metering mode. When I put the focus point on a dark area of the scene it will always take a longer exposure than when I put the focus point on a lighter area. So I think it is trying to use spot metering even though multi metering is selected.

There is a setting which tells the camera to link metering to the focus point - not sure if it biases the metering to that point or not - the matrix metering may give done extra weight to the metering at that point.

If you spot meter at the centre, and the camera is on a tripod, and the light is not changing, then yes, you’ll get the same exposure for all the frames in the bracket.

Edited October 27, 2023 by FunWithCameras
Adding reply to second quedtion

[Ian L]

Thanks FunWithCameras. I made sure that setting to link metering to the focus point was turned off. I'm convinced there is a bug here. It's like it's ignoring I've set multi metering mode and operating with spot metering mode regardless of not linking metering to the focus point. I can live with using center metering for now.

[FunWithCameras]

13 hours ago, Ian L said:

Thanks FunWithCameras. I made sure that setting to link metering to the focus point was turned off. I'm convinced there is a bug here. It's like it's ignoring I've set multi metering mode and operating with spot metering mode regardless of not linking metering to the focus point. I can live with using center metering for now.

It was just a guess. So it wasn't helpful.

One other, possibly irrelevant, question - does the lens you were using exhibit much focus breathing? if it does, and the focus was moving a fair way, then the width of the scene would be changing with the focus, and maybe it's including something that changes the matrix metering result? If that were the case, then centre metering would be a simple and effective fix.

[Ian L]

I don't think so. I also tested in a controlled environment in my house. It's defo forcing spot metering. Focusing on dark areas always produces a longer exposure than focusing on light areas.