Queries about projection lenses used as taking lenses.

dan_ · Expire: 2016-12-19

As most of you, I have a lot of projection and enlarger lenses and I really enjoy using them as taking lenses. But is something I really don't understand about that. Let me explain.

A lens is designed and calculated for a certain light path. When a projection/enlarger lens is used for photography that path is reversed.
If the lens has a symmetrical design it shouldn't be very important but if the design is asymmetrical then the IQ of a reversed projection/enlarger lens used as taking lens should be significantly greater then it's IQ when used not-reversed.
But It seems that this is not the case and most of us use the projection/enlarger lenses as taking lenses not reversed.

More then that, when a certain design is used for both projection and taking lenses it's schema is not reversed. Let's consider the classical Petzval design used in both taking and projection lenses (I know, it's almost symmetrical, but...). The Achromatic lens is in the front group in both cases and the Crown/Flint glass order is the same even the light path is reversed. But the light path in an Achromatic lens shouldn't be reversed or it looses it's Achromatic character. Yet the projection Petzvals have the same design as taking Petzvals.

Could, please, someone explain that?

bernhardas · Expire: 2017-05-23

Edited

ZoneV · Expire: 2011-12-02

As optics engineer I could say:

Good explanation bernhardas!
It does not matter which direction the light has, light rays could be reversed with no harm.
The magnification ratio is still the same, the large size and distance is at the lens front, the small size and distance at the film gate/sensor.

calvin83 · Joined: 12 Apr 2009 Posts: 7576 Location: Hong Kong

I use my Yashinon 1.4/50 in reverse direction as magnifier. It is much better than a cheap magnifying glass. Smile

kds315* · Expire: 2021-03-09

The light direction indeed does not matter, fully agreed what has been said already, what matters is the optical paths it was designed for
and from that no reversion is needed.

dan_ · Expire: 2016-12-19

Thanks for the explanations.
I lived under the impression that, as long as some lenses are added to an optical system to correct the aberrations induced by other lenses, the light direction should count. I mean that the light path in the order LENS ---> CORRECTOR LENS (specially computed to correct the aberrations induced by the first) should produce a better corrected image (better IQ) than in the inverse direction, CORRECTOR LENS--->LENS.

kds315* · Expire: 2021-03-09

Try playing with raytracing software (free on the net), it is very instructional!
Winlens for instance http://www.winlens.de/

Lightshow · Joined: 04 Nov 2011 Posts: 3666 Location: Calgary

Photons don't care which direction they travel through a lens, they only care about the transitions and how much they have to bend.

dan_ · Expire: 2016-12-19

Thanks, I think I get it now.
Light is reversible and therefor if through a perfect optical system a point in reality is transformed into an image point(=no aberrations) than, if the light direction is reversed and the distances remain the same, a point is transformed into an image point with no aberrations as well.
In fact in the LENS-CORRECTOR LENS optical system, if reversed, the LENS corrects for the aberrations induced by the CORRECTOR LENS. Therefore the light direction doesn't matter and what's important is the geometry of the system.
I suppose that the reason why an Achromatic lens is recommended to be used only in one direction should be a great difference between the 2 distances: real point - lens and lens - image point (= geometry).
For the same reason an inverted normal photographic lens, corrected for infinity, is better for macro (>=1/1) than it is if used with bellows but in it's normal position.

ZoneV · Expire: 2011-12-02

Yes, but I want to add that there is not one lens correction systems in most lenses.

All the lenses and their aberrations adds up, and the lens designers work is to have some negative aberrations and some positive so that they add up to more or less zero. Lens design is a very complex work, I see this when I open a lens in Zemax lens design software, there are a lot of dependencies.
This is the reason why a big part of lens design is try and error - the program tests many thousands posibilities to optimize the design. This is something that is not implemented for example in the free version of Winlens :-/

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

dan_ · Expire: 2016-12-19

Thanks Gerald for the more scientific explanation.