What is the focal length here?

[Cyclops]

Someone had a query for me about macro lenses when coupled with reversed lenses but I was unable to give an answer. Basically what is the methodf for calculating the focal length of a system when one lens is reversed onto another? For example,if I reverse mount a 50mm onto my 100mm macro lens what does the new focal length become?

[lauriek]

I think the magnification would be 2:1, so would the focal length not just be the focal length of the reversed 50? (which should be roughly 50mm!). Not sure on this, hopefully someone will come along to clarify or correct if I'm wrong!

[[Who?]]

Not the best place to put a first post ..
nevermind

you can download a excel spreadsheet for macro calculations from:
http://www.jeffree.co.uk/pages/macro-lens-calcs.html

[rjlittlefield]

The equivalent focal length is FLcombo = (FLa*FLb)/(FLa+FLb). for two lenses with Focal Lengths FLa and FLb.

In your example, the combo has equivalent focal length of (100*50)/(100+50) = 33.3 mm.

In other words, the combo acts rather like a 33.3 mm lens initially focused at infinity and then 66.6 mm of extension added.

Notice that AddedExtension/FocalLength = Magnification, in this case 66.6/33.3 = 2.

Also, AddedExtension+FocalLength = TotalDistanceFromSensor = FocalLengthOfRearLens, in this case 66.6+33.3=100 (rounded).

Now I have a question: Why is someone asking this question? What are they going to do with the answer?

--Rik

[mgoodm3]

I believe it would be 33 mm.

1/100 + 1/50 = 1/f

edit: too late, Rik got here first.

[rjlittlefield]

[Who?], welcome aboard!

a reversed 50mm is equal to a +20 diopter
(1000/FL of reversed lens)
so...

combined focal length =

1000
----------
(1000/FL prime lens) + diopter power

(i think)

That's correct. In diopters, Dcombo = Da + Db, for combining lenses of strengths Da and Db.

Doing the example as a calculation in diopters:
Da = 1000/FLa = 1000/50 = 20
Db = 1000/FLb = 1000/100 = 10
Dcombo = Da + Db = 20 + 10 = 30, and
FLcombo = 1000/Dcombo = 1000/30 = 33.3.

The calculations are equivalent -- whenever you plug in the same numbers, you get the same answer even though the arithmetic is different in the middle.

--Rik

Edited to add: mgoodm3's equation is also equivalent, with a factor of 1000 divided out of all the numerators.

Edited to add #2: we may have set a new record for overlapping edits!

[Cyclops]

The equivalent focal length is FLcombo = (FLa*FLb)/(FLa+FLb). for two lenses with Focal Lengths FLa and FLb.

In your example, the combo has equivalent focal length of (100*50)/(100+50) = 33.3 mm.

In other words, the combo acts rather like a 33.3 mm lens initially focused at infinity and then 66.6 mm of extension added.

Notice that AddedExtension/FocalLength = Magnification, in this case 66.6/33.3 = 2.

Also, AddedExtension+FocalLength = TotalDistanceFromSensor = FocalLengthOfRearLens, in this case 66.6+33.3=100 (rounded).

Now I have a question: Why is someone asking this question? What are they going to do with the answer?

--Rik

Thanks Rik,I will pass the info on. As to the reason why well he has the same camera as me,that is the Panasonic FZ7 but he bought a reverse adaptor for it and was just curious. He said he tried reversing a 100mm onto it it I think and got an effect like looking down the wrong end of binoculars,which led him to believe the focal length must be way shorter than the lens is when used normally.

[rjlittlefield]

Reversing another lens in front of an FZ7 gets complicated.

The FZ7 is actually an "SLR-like" camera with a small sensor and a complicated lens built-in. Its built-in lens zooms from 6 to 72 mm, and its sensor size is only about 5.8 x 4.3 mm.

In simplest theory, if you reverse a 100 in front of that, you'll be looking at magnifications ranging from 6/100 (0.06) up to 72/100 (0.72), depending on where you set the FZ7's zoom. Of course on the small sensor this translates into subject field widths ranging from 5.8/0.06 = 100 mm wide, down to 5.8/0.72 = 8 mm wide. So (if I've done these calculations correctly) this ought to be a respectable combo for macro.

In practice, who knows? The one time I had a camera like the FZ7 to play with, I couldn't find any lens that would work well in front of it. Everything I had produced so much chromatic aberration that I didn't consider it worth the trouble. But other people get good results, using other lenses.

About "looking down the wrong end of binoculars", I don't know exactly what that means. With the pair I have in hand at the moment, and depending on how I do it, I can get anything from a fuzzy reduced image at infinity, to a respectable magnifying glass at about 1".

BTW, it's tempting to think that the focus point with a reversed 100 mm lens ought to be 100 mm in front of the lens. That's wrong. It will actually be at the same place the film/sensor would be, if the reversed lens were used in its normal configuration. The "100 mm" is an accurate description of the refracting power of the added lens, but of course its design is such that it normally focuses just a camera's thickness away from its own lens mount.

--Rik

[mgoodm3]

i am guessing that you are getting some major vignetting with the FZ7 and a reversed lens. I have tried it before with a Nikon 5400 and got similar.

[rjlittlefield]

...major vignetting with the FZ7 and a reversed lens.

That makes sense. My previous experience was with an FZ-30. Here are my notes posted out from that experience:

I was not able to test resolution for macro work, because I did not have a suitable closeup lens for the FZ-30. I tried an old Tiffen +4, but it gave too much CA (chromatic aberration). No surprise there; that lens is almost certainly not corrected for CA. Then I tried my favorite reversed 55mm SLR lens, which has worked great for every other use I've tried. That gave way too much CA also (which was a surprise), and also I had to run the FZ-30 at full zoom to avoid vignetting. Apparently the FZ-30 is finicky about what it will work well with. Anyway, no serious macro tests, sorry.

--Rik

[Cyclops]

i am guessing that you are getting some major vignetting with the FZ7 and a reversed lens. I have tried it before with a Nikon 5400 and got similar.

I would do if I used that methodf but I have a DSLR and macro lens to play with

[Cyclops]

Reversing another lens in front of an FZ7 gets complicated.

The FZ7 is actually an "SLR-like" camera with a small sensor and a complicated lens built-in. Its built-in lens zooms from 6 to 72 mm, and its sensor size is only about 5.8 x 4.3 mm.
....


--Rik

The ting is if you think of it in 35mm terms thats a 36-432mm zoom,quite a range,and if you were to zoom it to about 100mm equivalent,say 2.5x,you could then reverse mount a 50mm leng and get 2x lifesize!

[rjlittlefield]

Reversing another lens in front of an FZ7 gets complicated.

The FZ7 is actually an "SLR-like" camera with a small sensor and a complicated lens built-in. Its built-in lens zooms from 6 to 72 mm, and its sensor size is only about 5.8 x 4.3 mm.
....


--Rik

The ting is if you think of it in 35mm terms thats a 36-432mm zoom,quite a range,and if you were to zoom it to about 100mm equivalent,say 2.5x,you could then reverse mount a 50mm leng and get 2x lifesize!

Um...sort of.

But please keep in mind that this latest "2x lifesize" has no physical meaning.

If you reverse a 50 mm in front of a real 100 mm, then the image on sensor really is 2X life size.

Out of that 2X image, you crop a piece however big the sensor happens to be. If you're shooting film, then it's 36 mm x 24 mm, and your field width at the subject is 36/2 = 18 mm wide. If you're shooting a typical DSLR, then the sensor size is around 23 mm x 15 mm, and your field width at the subject is 23/2 = 11.5 mm wide.

Already, you can see that even the usual "2X" tells you a lot about the optics but not so much about what it's good for. If you shot at 2X onto 4x5 film, the field width at subject would be 5/2 = 2.5 inches, about 63 mm.

The setup that you're proposing, as I hear it, is to reverse a 50 mm in front of an FZ7 set to an "equivalent 100 mm". What's physically happening here is that the "equivalent 100 mm" sets a focal length of about 100/6 = 16.7 mm, with 6 being the approximate ratio of sensor sizes between the FZ7 and 35mm film. Reversing the 50 in front of this 16.7 lens gives you an optical magnification of 16.7/50 = 0.3X. Out of this 0.3X image, you crop a sensor-sized piece, 5.8 mm x 4.3 mm. The resulting subject width is then 5.8 mm / 0.3X = 19 mm.

We can see in this specific case that the FZ7 combo produces the same subject width as 2X on 35 mm film. But the FZ7 combo is actually operating at 0.3X.

So...

If saying "2x lifesize" communicates to your colleagues that a full-frame subject is around 18 mm wide, then it's a useful piece of jargon. Of course a physical 2x on your Canon 10D gives a full-frame subject of only around 11.5 mm, so there's obviously some slop in what "2x" means.

This is why I start twitching when I hear things like "2x lifesize". That phrase is well defined if we're talking about two physical images --- say, the image on the sensor versus the real subject, or the image on a print versus the real subject. But when "2x lifesize" becomes shorthand for "subject width around 18 mm", or "subject width around 11.5 mm" with a different camera, it loses some of its utility.

My personal preference is to always reference against the subject size. If a camera will fill a frame with an 18 mm subject, then I have some idea what it's good for. There's obviously a lot more involved -- working distance for example -- but at least the specification is clear.

--Rik

[Cyclops]

Mostly over my head I'm afraid Rik but surely if I set the FZ 7 to around 16mm(16x6=100)and reverse a 50mm onto it I will be magnifying the sensor image by 2x

[lauriek]

When you set the FZ to full zoom, ie 16mm, it really _is_ 16mm focal length, it's no good converting that to 35mm terms. It is a real 16mm lens with a real 50mm lens reversed on the front of it!

The focal length of a lens does not change when you put it in front of a different sized sensor, the 'apparent' focal length might change but the actual focal length is what it is...

What might make sense of this is if there was a way to cut off the lens from another identical camera, and somehow fix that lens at it's '50mm equivalent' focal length, then reverse _that_ lens on the front of your FZ, then the equation would work out and you would get 2:1. (Though with a P&S this is obviously pretty much impossible to do!!) You would then have a 16mm lens with an 8mm lens reversed on the front for 2:1...