Calculating focus with copy stands

[wintoid]

I'm looking for some enlightenment on what range of camera/lens/sensor distances will produce in focus results.

I have 2 setups: a Bowens illumitran and a Leica BEOON. I am using enlarger lenses for copying, and I have various sizes (50mm/75mm/80mm).

Question 1 - are enlarger lenses focused on infinity?

Question 2 - in a normal camera (not a copy stand), when you focus are you moving the lens closer/further from the sensor, or is there also internal stuff happening inside the lens (e.g. elements moving around)?

Question 3 - for a given focal length of enlarger lens, and a given subject-to-sensor distance, how can I calculate the lens-to-sensor distance which will give focus?

I've tried mucking around with this stuff manually, but my brain doesn't seem to be able to get a handle on it.

Thanks,

Simon

 

[David Hughes]

Hi,

If this will help; with a camera's 50mm lens you focus on infinity, put on it a 50mm extension tube and then use the 1:1 position. That's the old fashioned macro set-up with the lens at 2f and the subject at 2f and the same applies to the other focal lengths.

Some people use the lens stopped down to increase the DoF but, obviously, it doesn't matter for flat subjects so much.

Trouble is, I don't have a BEOON but only a BELUN and that's only got one position; meaning 1:1.

Your next problem will be exposure. It's best to look for the instruction manual but where you'll find one is beyond me at the moment.

Regards, David

 

[wintoid]

David,

Thanks for your input. I actually have the BEOON manual in the box, but in a way I'm trying to understand a bit more about the maths/physics of this stuff.

For example, I've found that if I put an 80mm lens onto the BEOON, I can't focus with any combination of extension or distance. I thought if I could understand the maths/physics, I could actually work out what was possible before trying to connect things together 🙂

I'm sure there's a good explanation of all this stuff somewhere on the internet, but I keep coming across explanations of how autofocus works, or bellows draw calculators. My google-fu is letting me down!

 

[ReeRay]

Try here

http://www.rangefinderforum.com/forums/showthread.php?t=148962

http://www.rangefinderforum.com/forums/showthread.php?t=159396

 

[David Hughes]

David,

Thanks for your input. I actually have the BEOON manual in the box, but in a way I'm trying to understand a bit more about the maths/physics of this stuff...

Hi,

Well, um, the simple stuff, based on a lens that only exists in theory, is that 1/u = 1/v = 1/f (and that's the last time you'll get f in italics). The formulae shows the relationship between the distances.

That's when the subject distance is u and the image distance is v and you ought to know what f is. And I hope that it's obvious that when the 50mm lens and ext'n tube are in place that's 2f and then the subject can be at 2f and you get 1:1 on the negative. Called macro, once upon a time.

Beyond 2f the image on the negative gets bigger and bigger and people use bellows for it. At the same time the image gets weaker and weaker in light intensity. People used to use a reversed standard lens for this micro stuff.

Anyway, at this point I thought search on "simple optics" and these two came up first:-

http://www.nightlase.com.au/education/optics/lenses.htm

http://www.physicsinsights.org/simple_optics_1.html

and so I'll leave you there...

Regards, David

PS Over 60 years since I did "A" level physics and I always wondered if it would come in handy...

 

[mdarnton]

Unlike camera lenses, which are designed to work in a certain way on certain cameras, enlarging lenses don't have to comply with any standard regarding where they sit relative to the film. Because of this, it's impossible to give you the numbers you want for various enlarging lenses. In fact, because normal lenses aren't really intended to the purpose you're putting them to in this case, making such calculations would be obtuse and backwards. If you don't want to follow the specific directions that came with the device you intend to use, your best bet is to start hooking things together and figure out what works.

 

[Dwig]

I'm looking for some enlightenment ...

Question 1 - are enlarger lenses focused on infinity?

No. enlarger lenses are not mounted using any camera register (mounting flange to film/sensor distance) distance. Instead, they are, like view camera lenses, mounted so that their flange is near the optical center(s) of the lens.

Question 2 - in a normal camera (not a copy stand), when you focus are you moving the lens closer/further from the sensor, or is there also internal stuff happening inside the lens (e.g. elements moving around)?

Yes. That is to say any one lens could work one way or the other. When focused to infinity, the distance between the film/sensor and the optical center of a lens is equal to its focal length. To change focus, you have to move the optical center of the lens further away from the film/sensor relative to its focal length. Lens can:

1. move they whole lens forward
2. move elements to shorten the focal length (typical front element focusing seen on scale focusing fixed lens cameras)
3. move elements to move the optical center forward while maintaining the same focal length.
4. some combination of the above. Internal focusing lenses are usually a mix of #2 & #3.

Question 3 - for a given focal length of enlarger lens, and a given subject-to-sensor distance, how can I calculate the lens-to-sensor distance which will give focus?

Yes, sort of. Precision is not possible, but in general you can get reasonably accurate results when dealing with simple optical designs like those found in enlarging lenses.

You will find that you actually get two answers for any one sensor to lens distance except when the distance is exactly right for 1:1 reproduction (you get one answer) or it is too small for any result. Errors result because the lens' focal length is not usually spec'd with high accuracy (~+/- 5%) and lenses actually have two "optical centers", one as seen from the rear and one from the front. You need to know the distance between these to calculate accurately.

 

[David Hughes]

...Yes, sort of. Precision is not possible, but in general you can get reasonably accurate results when dealing with simple optical designs like those found in enlarging lenses...

Hi,

I've often rigged things up using a white "screen" the lens and a spring bow compass at the nominal 50mm just to see where the node was but only in the lens-the-right-way-round position. The sun is good enough for an object at infinity although nowhere near that distance away.

Trouble is, no one gives the measured focal length, Leitz used to do so (sideways code by the ∞ mark) and the old USSR made lenses had the measured focal length and registration on the "Passports" but they are usually lost.

Regards, David

 

[Dralowid]

Somewhere on my shelves (I can't find it) is a rather dry book called The Leica Way, by Matheson. It was published for years and regularly updated. Not only does it have pages and pages of depth of field charts for focal lengths, plus charts for various Leitz copying systems but it also is handy for understanding how things work including increased exposure for extended lens to film distances. Not sure if it will help much in your case 'cos of the use of enlarger lenses etc.

I thought I could find a book on a shelf quicker than a file on a hard drive. Seemed to have proved myself wrong.

 

[Dralowid]

And there it was...Hiding behind "British Tractors 1945-1965"

For example:

dof294 by dralowid, on Flickr

 

[David Hughes]

Hi,

Everyone should have a copy, and the Leica book is quite useful too. ;-)

Trouble is, no one seems to bother, even with instruction books these days. Just look at all the cameras on ebay without them. No wonder a lot of decent cameras get broken and passed on to the next mug.

Regards, David

 

[Godfrey]

What I've used for calculating close-up focus distance, magnification, and exposure with non-TTL metering cameras for over thirty-five years is the Kodak Professional Photoguide (1977 edition). It has an extensive section on close-up photography and concepts, with easy to use paper calculator dials as well as easy to use mathematical formulae in a rugged, coated paper spiral bound book. Absolutely the most useful, practical working reference for this kind of work I've ever found.

Copies of the Kodak Professional Photoguide are easily findable on Ebay for $5 to $30. An absolute steal at those prices!

G

 

[ColSebastianMoran]

Hi,

If this will help; with a camera's 50mm lens you focus on infinity, put on it a 50mm extension tube and then use the 1:1 position. That's the old fashioned macro set-up with the lens at 2f and the subject at 2f and the same applies to the other focal lengths.

Some people use the lens stopped down to increase the DoF but, obviously, it doesn't matter for flat subjects so much.

Trouble is, I don't have a BEOON but only a BELUN and that's only got one position; meaning 1:1.

Your next problem will be exposure. It's best to look for the instruction manual but where you'll find one is beyond me at the moment.

Regards, David

For exposure: At 1:1, the effective f-stop will be 2x the nominal (shown on the lens) f-stop. Set the lens for f/4 and you will be shooting with effectively an f/8 lens.

Formula is EffectiveFStop = ( M + 1 ) * NominalFStop where M is magnification, image size divided by subject size.

 

[Dwig]

What I've used for calculating close-up focus distance, magnification, and exposure with non-TTL metering cameras for over thirty-five years is the Kodak Professional Photoguide (1977 edition). It has an extensive section on close-up photography and concepts, with easy to use paper calculator dials as well as easy to use mathematical formulae in a rugged, coated paper spiral bound book. Absolutely the most useful, practical working reference for this kind of work I've ever found.

Copies of the Kodak Professional Photoguide are easily findable on Ebay for $5 to $30. An absolute steal at those prices!

G

Quite true.

I began with my Dad's late '40s edition of Kodak's "amateur" Photoguide and found it quite valuable. I bought a Professional PHotoguide when they first came out and it was very useful. It's durable, designed to carry in your bag, and includes a lot of good reference material including an 18% grey card.

 

[wintoid]

Thanks for everyone's contributions. I may get myself a copy of that Kodak book 🙂

 

[David Hughes]

Hi,

Leica used to publish (or give away) a little card called "tips for close-ups" or something and it gave the exposure factor as (M+1)² and so for 1:1 it would be x4 and so on.

Trouble is, they all call things (including cameras) by different names and that adds to the confusion. And they all approach things from a different direction...

Regards, David

 

[ColSebastianMoran]

Hi,

Leica used to publish (or give away) a little card called "tips for close-ups" or something and it gave the exposure factor as (M+1)² and so for 1:1 it would be x4 and so on.

Right.

Exposure goes as (M+1)²

F-stop as (M+1), which is equivalent.