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AndrewC

Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

Posted: Sat Feb 27, 2010 5:14 am    Post subject:

lauriek wrote:
 AndrewC wrote: For the calculations it doesn't matter if the lens is reversed or not, so long as it is a simple symmetric lens.

This comes up quite often. How would one know if a lens is a simple symmetric lens or not, examine the apparent aperture size from both sides?

That and does it "look simple" If you have a lens with IF and multiple lens groups moving internally it isn't simple ! On the other hand, set it at infinity focus and try it and see. Personally, my approach / attitude is why bother trying to reverse a heavy expensive lens when you can just use a simple cheap and very effective simple enlarger lens ?

Andrew
bvalente

Joined: 18 Jan 2010
Posts: 69
Location: Los Angeles, CA

 Posted: Sat Feb 27, 2010 8:41 am    Post subject: Is it fair to say enlarger lenses are generally symmetric? The prime EL lenses Cheers Brian
AndrewC

Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

Posted: Sat Feb 27, 2010 9:00 am    Post subject:

 bvalente wrote: Is it fair to say enlarger lenses are generally symmetric? The prime EL lenses Cheers Brian

Yes
_________________
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes
bvalente

Joined: 18 Jan 2010
Posts: 69
Location: Los Angeles, CA

 Posted: Sat Feb 27, 2010 9:18 am    Post subject: sweet! Well that makes that part simple enough Cheers Brian
rjlittlefield

Joined: 01 Aug 2006
Posts: 20494
Location: Richland, Washington State, USA

Posted: Sat Feb 27, 2010 11:19 am    Post subject:

 AndrewC wrote: I'll add a picture sometime soon showing where you need to measure to for the extension - basically it is the shoulder of the enlarger screw mount, so if you are using the lens reversed it seems as if you are measuring to the front of the lens facing the subject.

A bit of clarification...

Extension is measured from infinity focus with the lens oriented as it will be used. Normal and reversed orientations have different reference points. Here are reference points that I just now roughly measured for my EL Nikkor 50 mm f/2.8:

Brian, I'm curious about background knowledge. Are you comfortable with the basic lens equation 1/f = 1/o + 1/i ? Do you know about "principal planes"?

--Rik
bvalente

Joined: 18 Jan 2010
Posts: 69
Location: Los Angeles, CA

 Posted: Sat Feb 27, 2010 11:58 am    Post subject: Rik I am a landscape/fine art photographer. While my colleagues tell me I'm one of the most technically proficient photographers they know (For example I teach photoshop master classes and have contributed articles to OP and DPP mags) I am definitely not familiar with equations or optical theory - at least to the level discussed here. But I am also happy to research and understand the concepts. My hope would be to eventually boil down some of the more technical discussion to something that would be a little more friendly to traditional (and by traditional I mean greater than 1x magnification )photographers Hope that answers your question, and you're not about to tell me I'm about to get into some serious trouble! Cheers Brian
AndrewC

Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

Posted: Sat Feb 27, 2010 1:12 pm    Post subject:

rjlittlefield wrote:
 AndrewC wrote: I'll add a picture sometime soon showing where you need to measure to for the extension - basically it is the shoulder of the enlarger screw mount, so if you are using the lens reversed it seems as if you are measuring to the front of the lens facing the subject.

A bit of clarification...

Extension is measured from infinity focus with the lens oriented as it will be used. Normal and reversed orientations have different reference points. Here are reference points that I just now roughly measured for my EL Nikkor 50 mm f/2.8:

--Rik

Interesting, last time there was a long discussion about the niceties of reversed El-Nikkors

http://www.photomacrography.net/forum/viewtopic.php?t=7340&postdays=0&postorder=asc&start=15

you seemed pretty confident that the reference plane for the image was the H plane - at the shoulder. You now seem to have migrated to the H' plane closer to the lens physical centre. I guess you measure by looking at magnification ratios or if not, how ?

If I play with some measured versus theoretical numbers of working distance as a function of extension at about 150mm and 250mm extension

Use H plane as the image reference plane (shoulder):
The Newtonian model is about +3mm

Use H' plane as the image reference plane (internal):
Both models give the same error but it now changes from -2mm to +2mm at 150mm and 250mm extension respectively.

Note: I am using an f2.8N whereas you are looking at the earlier f2.8

Kind of interesting, and suggests the H' plane is the correct one, but not important enough to alter the path of global events

Andrew
rjlittlefield

Joined: 01 Aug 2006
Posts: 20494
Location: Richland, Washington State, USA

Posted: Sat Feb 27, 2010 11:29 pm    Post subject:

Andrew,

For starters, perhaps this illustration will help:

The situation is confused because we have not been precise and consistent about our terms.

In the other thread, first post, you defined "extension" as being measured from the sensor to some lens plane, and you asked where that plane was.

Measuring from the sensor, the reference plane in reversed orientation is indeed the one labeled "H", which is close to the mounting flange. In this case Magnification = (E/f)-1.

But in your web page calculator, you are now using "extension" in a different way: the amount added, beyond infinity focus. That's the only way that "Magnification = E/f".

The user must have some way to measure this extension.

I casually and perhaps incorrectly assumed that you intended it to be measured from the camera lens flange to some point on the lens.

It happens, with a Canon EOS, that both styles of EL Nikkor 50 mm f/2.8 reach infinity focus in normal orientation when the shoulder of the mounting threads is roughly lined up with the front of the camera's lens mount.

So to a user measuring extensions from the camera's lens mount, the proper reference point on the lens in normal orientation is roughly the shoulder of the mounting threads, as I have it diagrammed. Around 0 mm of "extension" will give infinity focus.

When the lens is reversed, it cannot actually reach infinity focus because the lens would have to push into the camera body.

If one could push the lens into the camera body, then infinity focus would occur when the plane that I marked "Reversed" is lined up with the front of the camera lens mount. If you want 2X magnification, then you need that point to be 2*50 = 100 mm away from the front of the camera lens mount. Measuring 100 mm from the lens mount to any other reference point on the lens will give some different magnification. For example, measuring 100 mm from the lens mount to the shoulder of the normal mounting threads will give a magnification closer to 1.72X, not the desired 2X. Measured from the sensor to the shoulder can give the right result, but in that case the extension would have to be 150 mm ((mag+1)*50), not 100 mm.

If you want to use the H and H' planes, that's fine, but then you'll have to define extension=(magnification+1)*focalLength, and measure from the focal plane.

An alternative approach is to just say
 Quote: 1/f = 1/o + 1/i will get me in the ballpark most of the time if I just use the sensor and physical center of the lens as reference points. If I need anything more accurate, then I'll measure the magnification and use the formulas based on magnification rather than focal lengths and extensions. If that's not accurate enough, well, then clearly things are getting complicated and it's safer and easier to just rely on experiment rather than theory.

That approach has served me very well for several decades. Despite that I now understand about principal planes and how they relate to reference points for measuring extensions, I generally do not think about all that when I'm going after a photo.

Wearing my scientist hat, I feel compelled to work theory until it matches observations. Wearing my photographer hat, I really don't care about lens theory beyond what it takes to get me in the right ballpark.

The beauty of analysis using principal planes H and H' is that it gives results that are accurate within measurement error. The ugliness is that there's a big learning curve to use them correctly, and if you use them not correctly you can easily get results that are worse than not using them at all.

I notice that Lefkowitz doesn't even index the term "principal plane", and I can't recall seeing it mentioned in his text. Regarding reversal, what he says is this (page 68 ):
 Quote: And if the lens has been reversed at high magnification to improve image resolution, the exact image size will be anywhere from 20-100 percent smaller or greater than calculated from the aforementioned formula. [E.D = m x F.L.] The exact amount depends on lens design, focal length, magnification, reversing-ring thickness, and even how deeply the lens is recessed in its mount. The best thing to do is to try it, and if an exact magnification figure is required, use the ruler technique...

For most beginners working with bellows, I'm inclined to recommend taking the approach outlined in the first quote above, "...use the sensor and physical center of the lens as reference points...", and just acknowledge that it's a ballpark calculation.

--Rik
AndrewC

Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

 Posted: Sun Feb 28, 2010 2:57 am    Post subject: I agree - sloppy nomenclature from me The numbers that make sense to me when I'm working are: Si = sensor to lens distance (I've been sloppily thinking of that as "extension" when it is really extension + focal length). Working distance = subject to lens distance Magnification = optical magnification on the sensor I've changed the calculations and added some "words" on my calculator. Also fixed a silly Javascript error. Now I'm going to go and change my bandsaw blade and slice some veneers - very therapeutic ! Andrew
rjlittlefield

Joined: 01 Aug 2006
Posts: 20494
Location: Richland, Washington State, USA

 Posted: Sun Feb 28, 2010 12:18 pm    Post subject: Andrew, the update to your calculator page is good. The picture helps a lot. One possible confusion: "working distance" is diagrammed as being from the subject to some point inside the lens. Isn't it usually measured from the front-most part of the lens? --Rik
AndrewC

Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

Posted: Sun Feb 28, 2010 2:09 pm    Post subject:

 rjlittlefield wrote: Andrew, the update to your calculator page is good. The picture helps a lot. One possible confusion: "working distance" is diagrammed as being from the subject to some point inside the lens. Isn't it usually measured from the front-most part of the lens? --Rik

... I guess it would vary from lens to lens, that's where I'm calculating it from for the El-Nik 50mm. For that lens I could easily give the distance from the front (actually rear ! ) of the lens but I think I'll leave it as it is just now. Most people have some sort of lens hood slipped over the very front so they couldn't see that reference point anyway.I use it as guideline to know where the front standard of my bellows should be when the subject is hidden inside diffusers, lens shields, etc.

Andrew
ChrisR

Joined: 14 Mar 2009
Posts: 8496
Location: Near London, UK

 Posted: Sun Feb 28, 2010 6:04 pm    Post subject: It's good to see CofC as a separate input, though I think the 1/1000 rule is out of date. Cheap lenses can out-resolve that, as we have seen. If I had paid for a sensor 6000+ pixels wide, letting a point spread to 6 pixels, (before the low-pass filter gets to spread it more), wouldn't please me. (This is one of the fallacies in the dof/sensor-size argument. ) A really useful input might be one which asked about the final application - whether it be eg an 800 pixel web image, or a 300dpi A3 spread.
AndrewC

Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

Posted: Mon Mar 01, 2010 1:16 am    Post subject:

 ChrisR wrote: ... A really useful input might be one which asked about the final application - whether it be eg an 800 pixel web image, or a 300dpi A3 spread.

Don't agree on that one Chris - I work from "start high and end low", as in even though 95% of my stuff just ends up as a 600px web image I do all my capture / edit in 16bit, max resolution, etc.

Andrew
bvalente

Joined: 18 Jan 2010
Posts: 69
Location: Los Angeles, CA

 Posted: Mon Mar 01, 2010 10:48 am    Post subject: Andrew two quick questions on your online calc: 1. on the enlarger lenses SI is the idea that we would measure the SI by hand using a ruler or calipers, or that we would somehow know or calculate it? 2. on the microscope objectives, I may understand it wrong, but as I change the NA the effective aperture seems to go in the opposite direction from what I'd expect. For example, a 4x at NA .21 yields effective aperture of 11.9, but when I change NA to .4 effective aperture goes up to 6.3. I would have expected it to go in the opposite direction? Cheers Brian
AndrewC

Joined: 14 Feb 2008
Posts: 1436
Location: Belgium

 Posted: Mon Mar 01, 2010 10:58 am    Post subject: Hi Brian, NA is the numerical aperture of a lens, the aperture of f number is inversely proportional to it. Increase NA and aperture goes down. Si = image distance = lens to sensor : typically you would measure that by hand when you set your bellows extension. Just added some extra script - should preselect some values and run the calculations when the page first loads. Also tells you if you don't have enough "extension". Andrew_________________rgds, Andrew "Is that an accurate dictionary ? Charlie Eppes
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