www.photomacrography.net :: View topic - Zerene stacking step size table question
www.photomacrography.net Forum Index
An online community dedicated to the practices of photomacrography, close-up and macro photography, and photomicrography.
Photomacrography Front Page Amateurmicrography Front Page
Old Forums/Galleries
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Zerene stacking step size table question

 
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions
View previous topic :: View next topic  
Author Message
Lou Jost



Joined: 04 Sep 2015
Posts: 2635
Location: Ecuador

PostPosted: Mon Jul 30, 2018 9:19 am    Post subject: Zerene stacking step size table question Reply with quote

I am testing a medium format lens combo: the Mamiya 250mm f/4.5 apo lens in the rear, and Mamiya 120mm f/4.0 macro lens reversed in front, so m=2.

If I look at the Zerene step-size table for lens combos stopped down in front, Table 2-A, I get a step size of 80 micrometers for a lens at f/4.

If I look in Table 2-B for lenses stepped down in back, I get a step size of just 9 micrometers.

I wonder which one I should use, with both lenses wide open? They are very different.
_________________
Lou Jost
www.ecomingafoundation.wordpress.com
www.loujost.com
Back to top
View user's profile Send private message Visit poster's website
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 19177
Location: Richland, Washington State, USA

PostPosted: Mon Jul 30, 2018 10:40 am    Post subject: Reply with quote

I'll have to review the situation for Table 2-A, but at this moment I think that it's only an approximation for combos focused in the front. Tables 2-B and 2-C are exact (within the limitations of the model, of course).

I would analyze as follows...

The exit pupil of the front lens will be 120/4 = 30 mm diameter.

The entrance pupil of the rear lens will be 250/4.5 = 55.6 mm diameter.

The front lens will be the limiting aperture.

That being the case, there are two ways to go.

One is to convert f/4.0 to NA 0.125, and use Table 2-C or the formula that lies behind it.

On that path, the closest value in the table would be for NA 0.14, DOF = 0.028 mm.

The formula sitting behind the table is DOF = lambda/NA^2, so more accurately the DOF would be 0.00055/0.125^2 = 0.035 .

Alternatively, knowing that Feff_rear = Feff_front * m, conclude that the effective aperture at the sensor is f/8 and look that up in Table 2-B, where the value again shows 0.035 .

Does that make sense? Then more interesting, does it match experiment?

--Rik
Back to top
View user's profile Send private message Visit poster's website
Lou Jost



Joined: 04 Sep 2015
Posts: 2635
Location: Ecuador

PostPosted: Mon Jul 30, 2018 11:33 am    Post subject: Reply with quote

Thanks for showing how to think about this! I'll try some experiments.
_________________
Lou Jost
www.ecomingafoundation.wordpress.com
www.loujost.com
Back to top
View user's profile Send private message Visit poster's website
Lou Jost



Joined: 04 Sep 2015
Posts: 2635
Location: Ecuador

PostPosted: Tue Jul 31, 2018 7:41 am    Post subject: Reply with quote

I looked at how exposure changes as the rear aperture is closed (front aperture is wide open throughout). The exposure does not change until the rear aperture is closed down to about halfway between f/11 and f/16. From f/22 onward, one stop of aperture closure = one stop less light reaching the sensor.

So this suggests the front aperture is limiting until just past f/11, so EA of the front lens may be higher than what the theory calculated.

I can probably safely use your step value, as a conservative choice, until f/8, and from there on use your table 2-B.
_________________
Lou Jost
www.ecomingafoundation.wordpress.com
www.loujost.com
Back to top
View user's profile Send private message Visit poster's website
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 19177
Location: Richland, Washington State, USA

PostPosted: Tue Jul 31, 2018 7:49 am    Post subject: Reply with quote

Just to check... The analysis method that I used assumes that the rear lens is focused at infinity. Was that the case in your experiment?

--Rik
Back to top
View user's profile Send private message Visit poster's website
Lou Jost



Joined: 04 Sep 2015
Posts: 2635
Location: Ecuador

PostPosted: Tue Jul 31, 2018 7:59 am    Post subject: Reply with quote

Yes, as was the front lens.
_________________
Lou Jost
www.ecomingafoundation.wordpress.com
www.loujost.com
Back to top
View user's profile Send private message Visit poster's website
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 19177
Location: Richland, Washington State, USA

PostPosted: Tue Jul 31, 2018 9:54 am    Post subject: Reply with quote

OK, then one simple and interesting check is to just measure the pupil diameters. Are they 30 mm and 55.6 mm? Both measurements should be taken in the infinity space between lenses, so looking from the rear into the reversed front lens, and looking from the front into the rear lens.

--Rik
Back to top
View user's profile Send private message Visit poster's website
Lou Jost



Joined: 04 Sep 2015
Posts: 2635
Location: Ecuador

PostPosted: Tue Jul 31, 2018 9:58 am    Post subject: Reply with quote

I'll take it apart and look later, but when I put an adjustable 46mm iris in between the two lenses to run a second set of tests just now, the exposure did not change as I closed down that iris, until it reached about 28-29mm. My exposure meter only registers in 1/3 stop increments, so this is consistent with an exit pupil of 30mm.
_________________
Lou Jost
www.ecomingafoundation.wordpress.com
www.loujost.com
Back to top
View user's profile Send private message Visit poster's website
perdu34



Joined: 25 Nov 2016
Posts: 50

PostPosted: Tue Jul 31, 2018 2:23 pm    Post subject: Reply with quote

rjlittlefield wrote:

The formula sitting behind the table is DOF = lambda/NA^2, so more accurately the DOF would be 0.00055/0.125^2 = 0.035 .


Hi Rik, A couple of, probably irrelevant questions, about this equation.

Why is lambda in mm and not nm? Is it just to give an output that is also in mm?

Why is lambda green? For violet DOF is 24um and red it's 40um. Am I misunderstanding something about colour correction in lenses?
_________________
www.lucasblack.net
Back to top
View user's profile Send private message
Lou Jost



Joined: 04 Sep 2015
Posts: 2635
Location: Ecuador

PostPosted: Tue Jul 31, 2018 2:49 pm    Post subject: Reply with quote

Every formula has to have all its variables in consistent units or it will require extra factors in the formula to make them so.

Humans and cameras are most sensitive to green light, so it is normal to use that. If you are using monochromatic light of a different color, that should be lambda.

I ran some stacks with my lens combo at different apertures, and compared stopping down the rear lens versus stopping down an aperture between the lenses. It turns out that using the rear lens' aperture caused chromatic aberrations, which got worse as the lens was stopped down further. Stopping down the lens with an added aperture between the two lenses did not cause aberrations. For this combination, it was definitely superior to stopping down the rear lens. This is in line with Rik's analyses of how to stop down a lens combo.
_________________
Lou Jost
www.ecomingafoundation.wordpress.com
www.loujost.com
Back to top
View user's profile Send private message Visit poster's website
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 19177
Location: Richland, Washington State, USA

PostPosted: Tue Jul 31, 2018 4:28 pm    Post subject: Reply with quote

perdu34 wrote:
Why is lambda in mm and not nm? Is it just to give an output that is also in mm?

Yes. NA is just a number, with no units attached. So the calculation lambda/NA^2 gives a value that has the same units as lambda. If you plug in lambda in nm, you get DOF in nm.

Quote:
Why is lambda green? For violet DOF is 24um and red it's 40um. Am I misunderstanding something about colour correction in lenses?

No, what you've said is correct -- red has more DOF than violet. Resolution varies the same way -- violet can resolve more detail than red. Both of those less/more relationships are easy to confirm by experiment. The exact values are much harder to determine, for the simple reason that sharpness various smoothly with distance from perfect focus. See http://www.photomacrography.net/forum/viewtopic.php?t=23751 for a lot more about that. For most stacking operations, you want to be running in a regime where the MTF curve just sags more with larger step size, rather than dropping clear to zero at some lower cutoff frequency. As a result, for Lou's application the differences between say 0.030, 0.035, and 0.040 would be a matter of subtle differences in sharpness at the midpoints between steps, rather than clear and obvious focus banding.

In practice there are a lot more considerations that affect choice of "best" step size. A big one is the tendency of features to "squirm" laterally as focus is changed, due to the "utilized aperture" effects. Sometimes linear features on shiny subjects require a lot finer step size to avoid artifacts from squirming, than they would just to maintain sharpness from diffraction-limited DOF. See for example http://www.photomacrography.net/forum/viewtopic.php?p=149187#149187 .

The choice of green for lambda is motivated by human vision, as Lou said.

--Rik
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions All times are GMT - 7 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group