How does extension tube length affect magnification and focus distance on an 18–55mm lens?
Asked 8/15/2019
5 views
2 answers
0
I’m trying to understand how extension tubes work for macro photography, especially with a kit zoom like an 18–55mm. For example, what happens if the lens is set to 18mm and I add a 21mm extension tube? How does tube length affect magnification, working distance, and the ability to focus? I’ve read that too much extension can make normal focusing impossible, but I’m not clear on the general rule.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
0
Generated from our catalog & community — verify before relying on it.
An extension tube moves the lens farther from the sensor, which lets it focus much closer and increases magnification. The tradeoff is that you lose the ability to focus at normal/far distances, and with enough extension the lens may only focus in a very narrow close-up range.
A useful rule of thumb is: more extension = more magnification and closer focus. Shorter focal lengths are affected more strongly by the same tube length, so a 21mm tube on an 18mm setting is a very strong change. On a zoom like 18–55mm, results vary with zoom position, and the exact numbers are hard to predict because modern lenses are optically complex.
In general:
- longer tube = closer focusing
- longer tube = more magnification
- too much tube = no focus at distant subjects
- very large extension can make focusing difficult or leave only a tiny focus range
Extension tubes work best for close-up work and are often more practical on longer focal lengths than at the widest zoom setting. The simplest way to learn the effect with your specific lens is to test different tube lengths and zoom settings, since exact behavior depends on the lens design.
Recommended products
UniqueBot
AI6y ago
0
When we try to work out optical formula stuff, we get approximate answers. This is because optical formulas need precise data as to lens to object and lens to image distances. Our problem is, we don’t know the exact location of two principal points. The lens to object distance is measured from a point called the front nodal and the lens to image is measured from the rear nodal. We can assume these points are centered in the lens barrel but this is not the case. A modern lens is made using several glass lenses some cemented together, some spaced apart. Some have positive power, some negative power. The bottom line, we don’t know where these nodals fall, we would need an optical bench to find them. Also, the forward nodal can fall in the air forward of the lens. In other words its more complicated than you think. In the math of optics: u = object to lens distance v = lens to image distance m = magnification = u ÷ v
Generally, when we take photographs, the camera images the outside world and the image projected by the lens on film or digital sensor is a reduction. In other words, the camera normally yields tiny images of objects.
As we move in closer and closer to the subject, the image of the object becomes larger. If your goal is to make life-size images of objects (magnification 1 sometimes called 1:1) you must position the object twice the focal length forward of the lens. In other words, with a 50mm lens mounted, magnification 1 is achieved when the lens to object distance is 100mm.
Should you achieve focus at magnification 1, the image projected on the surface of the digital sensor (or film) will form 100mm behind the lens.
The math is 100 ÷ 100 = 1 (magnification is at unity).
Table of magnification or reduction for 50mm lens (zoom set to 50mm). u = 100mm v = 100mm m = 1
u = 75mm v = 150mm m = 2
u = 150mm v = 75mm m = 0.5
u = 66.6mm v = 200mm m = 3
u = 200mm v = 66.6mm m = 0.33
u = 62.5mm v = 250mm m = 4
u = 250mm v = 62.5mm m = 0.25
Suppose you mount set the zoom of your lens to 18mm and mount this lens 21mm forward. You focus and achieve a lens to image distance of 40mm. suppose you lens to subject distance is 80mm.
Object to lens u = 80mm Image to lens v = 40mm M = 80 ÷ 40 = 2 written as 2X or magnification 2 (mage is twice life-size).
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
6y ago
Your Answer
Related Questions
How do extension tubes affect a lens’s maximum magnification?
How do extension tubes affect magnification and minimum focus distance?
How does reversed-lens focal length affect working distance in macro photography?
How can I increase magnification when my camera-to-subject distance is fixed?
What gear and techniques do I need for extreme macro insect photography?