What ratio - Macro lens and extension tubes

oldgit

oldgit

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I've been prompted to write this by the threads by

rookies

Sigma 105 Macro Lens Users Please look in
http://www.talkphotography.co.uk/forums/showthread.php?t=81577

and

Are These.....
http://www.talkphotography.co.uk/forums/showthread.php?t=81392

because I posed myself a question and dont know / cant find the answer.

I happen to have a sigma 105 macro, and a collection of extension tubes: 13, 21 and 31mm (65mm total stacked)

With the barrel extended to give 1:1 reproduction and no extension tubes I get 1:1 on the sensor.

However IF I add one or more of the extension tubes the magnification goes up, but by how much?

Adding all 65mm now gives 1:XXXX

Anybody know how you work this out?

:thinking: I dont :thinking:
 
2:1 - I've just posted this in one of the above threads - the 2:1 image is with a full set of tubes.

If you camra's sensor is 15mm and you shoot a tape measure at 1:1 the image will be 15mm

1-1.jpg


at 1:2 (half size) you will see 30mm as twice as much tape will fit on the sensor

1-2.jpg


2:1 (2 x Life size) you will see 7.5mm as only half will fit on the sensor as its twice as big

2-1.jpg
 
Thanks, but HOW do you know?
What's the maths / logic behind it.

I could do loads of shots and measurements, but there must be some maths that says **** ***?

Ta
 
Paul it works like this I think, the total length of tube / focal length of lens so a 50mm tube on a 100mm lens gives you 1:2 but the same tube on a 200mm will only give you 1:4. hope this helps and that is how I think it works.
 
The problem with the maths for this is that the formula for each lens will be different. A 100mm lens is only 100mm when focused at infinity, the shorter the focal distance the shorter the focal length, more so with a macro lens, but by how much depends on the lens design. The Sigma 105 at min focus is actually around 75mm. That's why the answer is to take a shot of a ruler and count the mm when you want to know what mag. factor you're getting.
 
K just went and set up the copy stand and used all extension rings (36, 20 & 12mm)

Used a full frame sensor for more acuracy

K heres the method I used set sigma 105 to 1:1 moved camera until ruler in perfect focus (shows 36mm on ruler.

I added an extension ring each time in every configuration and refocused camera with copy stand, not lens, it always remains at 1:1. Here are the results

12mm 1.2:1
20mm 1.3:1
32mm 20+12 1.4:1
36mm 1.5:1
48mm 36+12 1.7:1
56mm 36+20 1.8:1
68mm 36+20+12 2:1

Friday night and I'm doing this :thinking:

I must be bored

Hope this helps

Trev
 
Ajophotog said:
Paul it works like this I think, the total length of tube / focal length of lens so a 50mm tube on a 100mm lens gives you 1:2 but the same tube on a 200mm will only give you 1:4. hope this helps and that is how I think it works.
Click to expand...

Thanks Alby, thats me somewhere down the knowledge line.

pxl8 said:
The problem with the maths for this is that the formula for each lens will be different. A 100mm lens is only 100mm when focused at infinity, the shorter the focal distance the shorter the focal length, more so with a macro lens, but by how much depends on the lens design. The Sigma 105 at min focus is actually around 75mm. That's why the answer is to take a shot of a ruler and count the mm when you want to know what mag. factor you're getting.
Click to expand...


Ooohh thats getting complicated, but then I already knew that a 105 lens works at less than 105mm when focused at 10cm not infinity, so I should expect that

TrevDR said:
K just went and set up the copy stand and used all extension rings (36, 20 & 12mm)

Used a full frame sensor for more acuracy

K heres the method I used set sigma 105 to 1:1 moved camera until ruler in perfect focus (shows 36mm on ruler)

I added an extension ring each time in every configuration and refocused camera with copy stand, here are results

12mm 1.2:1
20mm 1.3:1
32mm 20+12 1.4:1
36mm 1.5:1
48mm 36+12 1.7:1
56mm 36+20 1.8:1
64mm 36+20+12 2:1

Friday night and I'm doing this :thinking:

I must be bored

Hope this helps

Trev
Click to expand...

Very good, I'm really proud of you for actually getting off your butt and doing this. I could have but then I'm as lazy as I'm old.
Really good :thumbs:
 
smart arse :p
 
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