How can a Milky Way photo be made from 100 untracked 4-second exposures at f/5?
Asked 11/23/2014
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I saw a Milky Way image described as: 100 × 4-second exposures at f/5, ISO 12800, shot on a Canon 5D Mark III with a 24-105mm lens at 105mm, with no tracking—just a tripod. Most Milky Way advice recommends very wide focal lengths and fast apertures, so I’m confused how this works at 105mm and f/5. Is the result mainly because of stacking many high-ISO frames, and does a darker location help significantly?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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Everyone talks about shortest focal length and lower aperture when shooting Milky Way, and this guy does it with f5 How is that possible?
It is possible to shoot at f/5 because he is also shooting at ISO12800. A single image at that ISO would be extremely noisy, but stacking 100 images at that ISO allows the random noise from each image to be averaged out. The constant noise has probably been dealt with by dark frame subtraction.
Most stacking software allows for the changes in alignment from frame to frame. Some will even do alignment automatically for you. Of course you need to compose the shot a little wider to allow for the edges that will be cropped in the alignment process.
Likewise, most stacking software allows you to use a single dark frame to be applied to all of the images in a stack. This eliminates the requirement of taking a dark frame after every single 4 second exposure.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
11y ago
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Yes—stacking is the key. A single 4-second frame at 105mm, f/5, ISO 12800 would be very noisy and limited, but combining 100 frames lets the real signal add up while much of the random noise averages out. That makes settings that seem impractical for a single exposure more workable.
The stars also shift slightly between frames, so stacking software aligns the images before combining them. Because alignment changes the framing a bit, you usually shoot a little wider and accept some edge cropping.
Fixed-pattern noise and hot pixels can also be reduced with dark-frame subtraction, and many stacking programs can apply one dark frame to the whole set.
A dark sky absolutely helps: less light pollution improves contrast and makes the Milky Way easier to capture. But the main reason this setup works is not just location—it’s the combination of many short, high-ISO exposures, alignment, and noise reduction through stacking.
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