Why did one image in my exposure bracket use a different aperture, and will it affect HDR?
Asked 8/21/2019
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I’m using a Fujifilm X-T20 with auto exposure bracketing set to ±2 EV. In one bracketed sequence, the first image was shot at f/2.8 while the others were at f/5.6. Why would the camera change aperture during bracketing? Is that normal, and can it affect the final HDR merge? If so, how can I prevent it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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You must have been on fully automatic exposure mode (both speed and aperture set to automatic). With f/5.6, the -2 EV image would have had to be exposed for 1/35 sec. and the camera's logic decided that that was too long (as it would have a high chance to be shaky).
So it decided to open up the aperture instead.
As you feared, this is generally not desireable for HDR merges, as the image will have a narrower DoF as the others. But in this case it doesn't look that relevant as you do not have any foreground details.
Next time, set your desired aperture value manually, letting the camera only change the time (and possibly the ISO).
Originally by user32110. Source · Licensed CC BY-SA 4.0
user32110
6y ago
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This usually happens when the camera is in a mode where both shutter speed and aperture are allowed to change automatically. For the darker/brighter bracketed frame, the camera likely decided the needed shutter speed would be too slow, so it opened the aperture from f/5.6 to f/2.8 instead.
For HDR, that’s generally not ideal. Bracketed HDR frames should normally keep the same aperture so depth of field and lens rendering stay consistent, while exposure changes are made with shutter speed (and sometimes ISO). If one frame uses a wider aperture, depth of field can differ and that can affect the merge.
In your example it may not matter much if there isn’t important foreground detail, but in general you should avoid it.
To prevent this, set the aperture manually to the value you want, and let the camera vary shutter speed for the bracketed exposures. Using a stable setup also helps if slower shutter speeds are needed.
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