Sigma 400mm tele macro magnification/focal length help

Have questions about the equipment used for macro- or micro- photography? Post those questions in this forum.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

blekenbleu
Posts: 146
Joined: Sat May 10, 2008 5:37 pm
Location: U.S.
Contact:

Re: Sigma 400mm tele macro magnification/focal length help

Post by blekenbleu »

bbobby wrote:
Sat Jun 25, 2022 3:14 pm
My 2nd question is about shortening the effective focal length going close to the target - is this happen only with internal focusing lens?
So-called focus breathing is very common and not unique to internal focusing lenses,
which are preferred not least because they accumulate less dust.
https://photographylife.com/focus-breathing
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
https://blekenbleu.github.io/microscope

rjlittlefield
Site Admin
Posts: 23543
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Re: Sigma 400mm tele macro magnification/focal length help

Post by rjlittlefield »

bbobby wrote:
Sat Jun 25, 2022 3:14 pm
Many older macro lenses are going only to 1:2 and need extension tube to reach 1:1. For example Nikon PK-13 is 27.5mm and if I remember correctly it is for 55/2.8. 25.7 is half of 55mm - so my guess here is that for a lens to go from 1:2 to 1:1 it needs 50% extension. Is that always true?
That would be true if the lens retains its infinity focal length and reaches 1:2 entirely by extension. But in general not true.

What is true is that added magnification = added extension divided by effective focal length. Based on current data, your Sigma set at 1:3 has focal length only around 219 mm. Since you need 2/3 added magnification, that implies an added extension of 219*(2/3) = 146 mm total. Or, working from your observed 0.81x at 103.5 mm extension, you would need another 0.19*219 = 141.6 mm total. Two ways to get the same number, except for rounding.
Example: to get 1:1 this Sigma will need 110 mm extension - meaning its focal length is 220 mm and voila - I got the 2nd confirmation.
No, because this Sigma only reaches 1:3 without extension. (Limiting case: suppose it did not focus at all, then the extension needed to reach 1:1 would be equal to its focal length, not focal length/2.)
My 2nd question is about shortening the effective focal length going close to the target - is this happen only with internal focusing lens?
No, many lenses that obviously extend also shorten their focal length. Any lens that reaches 1:1 and auto-focuses will probably shorten its FL to focus closer, because lenses that focus entirely by extension behave very perversely around 1:1. If you slog through the math, you will find that exactly at 1:1, changing the extension on a lens with constant FL will change its magnification but not its focus! Near but not exactly 1:1 it is a little better, but still difficult to handle. Users of bellows quickly discover that around 1:1 they need to set the bellows for magnification, then focus by changing distance to subject.
Or I can be completely wrong and this would not work like that at all... and in this case better to know it beforehand and not waste time in pointless comparisons ;-)
A few hours ago I posted a new FAQ: How to find focal length and principal planes for arbitrary lens? Perhaps that will help.

--Rik

bbobby
Posts: 54
Joined: Sat Jan 15, 2022 12:40 pm
Location: Indianapolis, IN

Re: Sigma 400mm tele macro magnification/focal length help

Post by bbobby »

Just follow up... maybe my technique is not good enough, but I was not able to get exact numbers...
After 2 times I was a bit frustrated, so decided to get better ruler and run the test again... and I did... but still numbers are not exact match, actually they are worse... anyway... I will get measurements for other lenses too, but for now I guess I will have to be content with the average 215 mm.
Sigma400f5600014web.jpg
computation 1.jpg
computation 2.jpg
*****************************************************

Now I am going to do resolution tests... I did a quick one today, but have some questions about that too... any topic where I can read about the procedures how to do such test? Resolution in the center is pretty straightforward, but going to the periphery with only 1 target I will have to either move the target or the lens...
The following picture is 200% enlargement @ f/5.6 from the center, converted with Photoshop 6.0 (sharpness 75).
Sigma400f56 200 percent center.jpg

Post Reply Previous topicNext topic