Originally posted by PPPPPP42 How is that 1:1 or 1:4 ratio calculated and is it the same for all focal lengths if its a zoom?

Magnification is the ratio of the subject's size on the sensor to its life size. If a 20mm subject is 10mm on the sensor, it's 10:20 or 1:2 or 0.5x, half life-size.

Focal length matters in two ways:

1) Working distance. A lens reaches its greatest magnification when the focus distance equals the focal length, and no non-reversed lens can focus closer than its focal length. So a 50mm lens lets you work close, at up to 5cm / 50mm / 2in; a 100mm lens allows (or forces) you to work twice as far.

2) Calculating magnification from focal length and distance:

**M = F/(D-F)** where M is magnification, F is lens focal length, and D is subject-lens distance.

Quote: If I under stand correctly its the ratio of the size of the original object to the size of the image of it the lens shines on the sensor?

Just the other way around. A 15mm subject that is 5mm on the sensor is 5:15 or 1:3, 1/3 life-size magnification. A 5mm subject that's 15mm on the sensor is 15:5 or 3:1 or 3x.

Quote: So If I make the original image on my computer the exact same size as the sensor in my K20D then a picture from a 1:1 ratio lens would be life size?

Not exactly. What you see on a computer screen is an enlargement. Your camera sensor is roughly 24x16mm. (Nominal APS-C is 25.1x16.7mm. My K20D sensor is 23.4x15.6mm.) If you shoot something that's 6mm tall, and its captured image on the sensor is also 6mm tall, then you're at 1:1 or 1x magnification. If the captured image is 12mm tall, you're at 2:1 or 2x. If the captured image is only 3mm tall, you're at 1:2 or 0.5x. What you see on the computer screen is probably a huge enlargement, because "life-size" is pretty damn small.

Confused yet? Good. All this is a little tough to wrap one's head around. I still trip over it, and I started with macro many decades ago. Ay yi yi.