Calculate magnification for reversed lenses

[Cunha]

Dear friends,
is there a way, a formula or at least an approximation to know beforehand the magnification expected from reversed lenses based on their focal length.

An example:
50mm and 85mm: if reversed I´ll achieve (expect to) a magnification of 1:?

Thanks a lot.
Regards

[rjlittlefield]

I assume you're talking about reversing the short lens in front of the long one, such as a reversed 50 in front of a normal 85.

In this case, when the rear lens is focused at infinity there is a simple formula:

magnification = rear_focal_length / front_focal_length

So, reversed 50 in front of an infinity-focused 85 gives magnification 85/50= 1.7 .

This formula is exact only when the rear lens is focused at infinity. Usually you will get more magnification by focusing the rear lens closer than infinity, but results vary widely depending on internal details of the lenses that are hard to know.

--Rik

[Cunha]

I assume you're talking about reversing the short lens in front of the long one, such as a reversed 50 in front of a normal 85.

In this case, when the rear lens is focused at infinity there is a simple formula:

magnification = rear_focal_length / front_focal_length

So, reversed 50 in front of an infinity-focused 85 gives magnification 85/50= 1.7 .

This formula is exact only when the rear lens is focused at infinity. Usually you will get more magnification by focusing the rear lens closer than infinity, but results vary widely depending on internal details of the lenses that are hard to know.

--Rik

Sorry my mistake.
I meant one of those lenses reverse alone.
Using 50mm reversed.
Using 85mm reversed.

What magnification to expect for each.
Thank you.

[Pau]

When you already have the equipment, usually it's much more easy (and accurate) to measure the magnifiction than to calculate it.

Just take a picture of a rule in landscape orientation and divide the horizontal lenght of your sensor by the rule's lenght in the image. In any case you want to test your equipment before using it for real life pictures.

( I know I'm not answering your question as Rik does)

[Cunha]

When you already have the equipment, usually it's much more easy (and accurate) to measure the magnifiction than to calculate it.

Just take a picture of a rule in landscape orientation and divide the horizontal lenght of your sensor by the rule's lenght in the image. In any case you want to test your equipment before using it for real life pictures.

( I know I'm not answering your question as Rik does)

Anyway thank you.
I´m trying to understand if makes sense to buy for-reversed rings with electric contacts because I´ve EOS lenses without aperture ring. If the magnification I´ll have, worth the investment.

[Cunha]

I assume you're talking about reversing the short lens in front of the long one, such as a reversed 50 in front of a normal 85.

In this case, when the rear lens is focused at infinity there is a simple formula:

magnification = rear_focal_length / front_focal_length

So, reversed 50 in front of an infinity-focused 85 gives magnification 85/50= 1.7 .

This formula is exact only when the rear lens is focused at infinity. Usually you will get more magnification by focusing the rear lens closer than infinity, but results vary widely depending on internal details of the lenses that are hard to know.

--Rik

Sorry my mistake.
I meant one of those lenses reverse alone.
Using 50mm reversed.
Using 85mm reversed.

What magnification to expect for each.
Thank you.

Please.
Any further help to know what magnification to expect from a 50mm and a 85mm reversed lenses. For one and the other (not together).
To close this topic.
Thank you very much.

[rjlittlefield]

It varies some depending on the lens design. I don't have a Canon fixed-length 50 or 85 so I can't measure.

Perhaps there is some info for you at Shooting with a reversed 18-55 mm Canon kit lens. One snippet says "so magnifications here range from about 5X to 1.5X". Later with a different 18-55 lens and adapter, "field width goes to as much as 37 mm" which would be 0.6X .

Novoflex's description of their EOS-RETROReverse Adapter for Canon EOS notes that

Example: With a Canon EOS 28-105 mm zoom lens in reverse position, you get an image ratio of 1:7 with a focal length of 105 mm from 1:7 to 2.8:1 at 28 mm.

This is a wider setting range than is possible with a macro lens.

--Rik

[Cunha]

It varies some depending on the lens design. I don't have a Canon fixed-length 50 or 85 so I can't measure.

Perhaps there is some info for you at Shooting with a reversed 18-55 mm Canon kit lens. One snippet says "so magnifications here range from about 5X to 1.5X". Later with a different 18-55 lens and adapter, "field width goes to as much as 37 mm" which would be 0.6X .

Novoflex's description of their EOS-RETROReverse Adapter for Canon EOS notes that

Example: With a Canon EOS 28-105 mm zoom lens in reverse position, you get an image ratio of 1:7 with a focal length of 105 mm from 1:7 to 2.8:1 at 28 mm.

This is a wider setting range than is possible with a macro lens.

--Rik

Thank you.
One lens is a Canon EF 50mm f/1.4 USM.
The other is the Zeiss ZE Planar T 85mm f/1.4.