Do wide-angle adapters and focal reducers change a lens’s f-stop?
Asked 10/19/2014
2 views
2 answers
0
If a converter changes a lens’s focal length, does the effective f-number change too? For example, Sony’s wide-angle adapter for the 16mm f/2.8 turns it into about a 12mm view. Since f-stop is focal length divided by entrance pupil diameter, should the effective aperture change?
Related: products like the Metabones Speed Booster both widen the field of view and claim to make the lens “faster.” Is that just because the focal length changes, or is there a different optical reason?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
4
The ratio f/2.8 means the diameter of the entrance pupil is equal to the focal length divided by 2.8.
The key thing to note about the above is that the entrance pupil is the image of the aperture stop as seen through the front of the lens, the ratio does not depend on the physical size of the aperture itself.
A rear-mounted 2x teleconverter, such as you would use with a telephoto lens, changes the focal length whilst the entrance pupil remains the same size, so the aperture number goes from 2.8 to 5.6.
A front mounted tele/wideconverter will change the focal length, but it will also change how large the aperture appears, meaning the entrance pupil changes in line with the focal length so the f-ratio stays the same.
A rear mounted focal reducer such as the Metabones SpeedBooster will give you a brighter f-ratio as the entrance pupil remains the same size whilst the focal length gets shorter.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
11y ago
0
Generated from our catalog & community — verify before relying on it.
f-number is based on focal length divided by the diameter of the entrance pupil (the aperture as seen through the front of the lens), not just the physical iris size.
That’s why different converters behave differently:
- Rear-mounted teleconverters increase focal length while the entrance pupil stays the same, so the f-number gets larger (for example, f/2.8 becomes f/5.6 with a 2x converter).
- Front-mounted wide or tele converters change both the focal length and the apparent size of the entrance pupil by roughly the same factor, so the f-number usually stays the same.
- Rear-mounted focal reducers such as a Speed Booster do the opposite of a teleconverter: they reduce focal length and concentrate the image onto a smaller area, which makes the lens effectively faster as well as wider.
So for a front-mounted wide-angle adapter like the Sony 16mm converter, the effective f-stop is generally unchanged. For a focal reducer like a Speed Booster, the speed increase is real and comes from the optical reduction of the image, not just a trivial relabeling of focal length.
Recommended products
UniqueBot
AI11y ago
Your Answer
Related Questions
Is entrance pupil diameter the same as the effective aperture used for f-number?
Is an f-stop lower than f/1 physically possible?
Why does the same f-number give the same exposure at different focal lengths?
Why is a lens front element often larger than focal length ÷ f-number?
Does sensor size affect the maximum aperture a lens can have?