Magnification APS vs Full Frame

[KeithB]

Bob:
Can you clear up something for me. The 90mm macro you just reviewed goes to 1:1. Is the magnification the same for APS-C? I know that 1:1 means the image is the same size on the sensor as in real life, so that means it *looks* bigger on APS-C, but is that all, just the relative size is larger because the sensor is smaller?

[Bob Atkins]

Yes

1:1 means that the area covered is the same size as the sensor, so it's 24x36mm for full frame and about 21x14mm for APS-C. 1:1 on four-thirds would cover an area of 18x12mm.

The APS-C shot "looks bigger" because it covers a smaller area and the 4/3 shot looks even bigger.

I think I did cover this in the article, or at least if I didn't, I meant to! It can be a little confusing.

[KeithB]

"1:1 means that the area covered is the same size as the sensor" - I don't think that is quite correct. I think it is better to say that the image fomed on the sensor is the same size as the subject , no matter how big the sensor is.

[Bob Atkins]

Both are true.

1:1 means a direct mapping of the sensor on the image plane (and vice-versa). That means that you image an area which is exactly the same size as the image sensor.

It's equally true to say that the image formed on the sensor is exactly the same size as the subject.

I think the former definition is a little more useful in practice since it gives you an idea of how small a subject will fill the frame, which is what you're usually interested in when it comes to macro work.

For example an image of an 10mm long ant at 1:1 wouldn't fill much of a 24x36mm 35mm full frame sensor, but at the same 1:1 you could barely squeeze the ant's head on a typical small digicam sensor measuring maybe 4x3mm.