Why can auto mode and my manual settings look the same even with different ISO and shutter speed?
Asked 10/22/2019
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I shot the same scene on a Fujifilm X-T20 once in Auto mode and once with my own settings. The camera chose roughly ISO 400 and 1/400s, while my shot was around ISO 200 and 1/60s. The photos look very similar to me.
As a beginner, I’m trying to understand:
- Why can two photos with quite different ISO and shutter speed still look almost identical?
- How much “tolerance” is there before those settings make a visible difference?
- Which settings are objectively better for a scene like this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
22
It's hard to really tell from the small versions here — which is a lesson in itself, because at 1280x850, which is a perfectly fine online viewing size, the differences really don't matter that much.
However, in this case, I think Auto probably did make some better choices.
Shutter Speed
You picked ¹⁄₆₀th of a second. This is fine, but probably slightly susceptible to blur from camera motion shake or subject movement due to wind. Auto mode picked ¹⁄₄₀₀th, which will better freeze things.
ISO
You picked 200 and the camera picked 400. Like most modern largish-sensor cameras, there's not much difference between 200, 400, or 800 on this camera in terms of noise, but it's possible the auto mode enabled the expanded dynamic range option (where the camera actually shoots at a lower ISO and then brings up the shadows in internal post-processing). Because of the way that works, this is only available at higher ISOs than the base 200. (The difference isn't super-visible in the result, but I can imagine a little more detail in the shadows — but that might also be because the auto image is a little brighter… see below.)
Aperture
This is really the big one. You picked f/18, and the camera picked f/8. All camera-lens systems are subject to diffraction, which means that while the narrower aperture gives you more depth of field, there's less sharpness overall. See What is a "diffraction limit"? for more. In general, it's better to not go beyond f/8 except in special circumstances.
Overall Exposure Value
Although it's subtle, the auto-exposure image is a little bit brighter overall. You can particularly see that in the sky. This seems to be less than a full "stop" of difference — each stop is a doubling or halving (See What is one "stop"?), and because of the way human perception works, generally differences of less than one stop aren't a big deal (and as a rule of thumb, plus or minus a stop from a middle value are reasonable valid exposure choices without really feeling over- or under-exposed).
So… between f/8 and f/18, there are 2⅓ stops — going in that direction, darker, so we'll call it negative 2⅓. (See Why are f-stops not linear? for how this is calculated.)
And between ¹⁄₄₀₀th and ¹⁄₆₀th is just about the same: 2⅔ doublings (which is 2⅔) — but in the direction of more exposure.
And finally, 200 to 400 is a full stop — leaving a ⅔-stop discrepancy, which seems close enough to be in the ballpark of the visual brightness difference in the result. As Michael C notes, the difference in metering is explained by the slight (but significant) change in framing, with more sky in one shot than the other.
(See What is the relationship between ISO, aperture, and shutter speed? for more on this "stop math").
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
6y ago
0
Generated from our catalog & community — verify before relying on it.
Different exposure settings can produce a similar-looking photo if the total exposure is close enough and the scene doesn’t stress the limits of the camera.
In your example, the biggest practical difference is not brightness but side effects:
- 1/400s is much better at reducing blur from hand shake or moving subjects (like leaves in wind).
- 1/60s is slower, so it has more risk of motion blur.
- ISO 200 is theoretically cleaner than ISO 400, but on a modern APS-C camera like the X-T20, the visible noise difference between 200 and 400 is usually small, especially in good light and at normal viewing sizes.
So for this scene, the camera’s Auto choice is likely the safer one because the faster shutter speed helps preserve sharpness, while the ISO increase is minor.
Also, at web/small display sizes, these differences may be hard to see at all. They become more noticeable when zooming in, making large prints, or shooting in dim light.
There isn’t one fixed “tolerance range” that always applies—it depends on motion, focal length, light, subject detail, and output size. But in bright conditions, small ISO changes and moderate shutter-speed changes often won’t look dramatically different unless blur or noise becomes visible.
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