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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 13 Oct 2010 (Wednesday) 11:29
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My bellows have confused me

 
The ­ Ran
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Oct 13, 2010 11:29 |  #1

Right, let me start off by saying a couple things. As far as I understand it (I've never formally studied optics, all self taught) to focus a lens you move all the lens elements together either closer or further away from the sensor, to achieve infinity focus and MFD respectively. To zoom a lens (increase or decreases the focal length) you move specific groups of lens elements closer or further away from each other. Please correct me if any of the above is wrong.

Now, when using extension tubes you're moving the entire lens (and thus all the elements) further away from the sensor which in turn decreases the MFD. Then to focus the lens you do as you normally would, turn the focusing ring to precisely alter the lens to sensor distance. So, with bellows it should be the same as they're technically just adjustable extension tubes, however because you have the ability to precisely alter the length of the bellows you can use them instead to focus, correct?

My problem is this, using my 135mm on my bellows reduces the focussing distances so much that the total range using the focussing ring is literally a couple centimetres, so instead I'd be better off racking the bellows back and fourth to focus. However when I do this the focussing distance doesn't change much (no where near enough to make them viable for focussing, it's easier to just move the whole rig forwards and backwards), instead the magnification/zoom does. I can extend and retract the bellows a few centimetres without the focus changing however the image through the viewfinder get's magnified instead. Once I reach the extremes of extension/retraction the focussing distance does indeed change, but no more than a couple centimetres.

I've tested this with the camera mounted on a tripod making sure the body and thus sensor is stationary so only the lens is moving. So, what sort of voodoo is going on here and how can I get this thing to focus like it should (to my knowledge)?


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hpulley
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Oct 13, 2010 12:15 |  #2

At these sorts of distances focusing is not simple anymore. With floating lens systems, not all elements move at the same rate when you focus a lens. This has been true for some time.

Extension tubes and bellows obviously can't do anything like floating lens systems, you just move the whole thing. To a point this will move the MFD closer until the focus is inside the lens, then you can't focus any further.

A bellows is generally mounted on a tripod and has 3 adjustments: 1) The bellows rail itself will move back and forth; 2) the position of the camera body on the bellows moves back and forth; and 3) the lens on the bellows moves back and forth. With a bellows, the 3 distances must be changed because the subject distance must change as the minimum focus distance changes and the magnification changes as well. Generally you want a given magnification like 1:1, 2:1, 1:2 or otherwise which requires a certain combination of distance from body to lens, body to subject and body to tripod which is the three distance.

Exposure is difficult here too because much of the light is not on the sensor.


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The ­ Ran
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Oct 13, 2010 12:37 |  #3

Okay, I get the bit about the floating lens systems now. With my bellows the only adjustment they have is moving the two standards closer or further apart, each one has a tripod mount so I've mounted it on the one attached to the camera body to keep that stationary. I've got no way to move the rails (and thus the whole rig itself), so how should I go about focussing instead? Using the focussing ring like I would with extension tubes just won't cut it.


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The ­ Ran
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Oct 13, 2010 12:48 |  #4

I think I may have sorted it. I decided to try mounting it by the lens side standard. Before I thought this wouldn't work, common sense tells me that as the camera moves further away the magnification decreases, but then I realised that as I adjust the bellows the magnification was increasing and thus it might offset the reduced magnification resulting from moving the sensor further from the subject, which in turn would also adjust the focus. Well, just as I thought it worked perfectly, now I can get precise framing while adjusting the focus.

I'm still confused as to why moving the sensor further from the subject changes the focus instead of the magnification now, but I'm glad that it works.


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hpulley
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Oct 13, 2010 13:14 |  #5

Moving the sensor changes the focus because you're changing either the subject distance or the sensor to lens distance. As you've discovered you are trying to solve a multivariable system as the magnification changes AND the subject distance changes. You can keep one or both constant but it requires practice. On my bellows I can adjust two of the three distances at the same time which effectively keeps one constant but mine has a rail for the body and lens which itself moves too, that makes it simpler.


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Wilt
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Oct 13, 2010 13:17 |  #6

It's easy, Ran...

Think of these distances: Subject-to-lens = A; Lens-to-FocalPlane = B

Assuming a 'normal' optic -- not retrofocus (WA FL) or telephoto (long FL)


  1. When A is infinity, B = FL
  2. When A < infinity, B > FL
  3. When B changes, magnification of the subject on sensor changes
  4. As A gets to be much less than infinity, B gets to be much greater than FL
  5. As A goes from Infinity to less than Infinity, the object magnification gets greater and greater.


Macro (subject magnification > 0.5x) is no different than with non-macro shooting (subject mangnificaton < 0.5x)

By making B a fixed distance, you change A to focus with no change to subject magnification. By making A fixed in distance, you change B to focus; but changing B also changes subject magnification.

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The ­ Ran
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Oct 13, 2010 14:07 |  #7

I know magnification changes when focussing normally, just not as drastic as when focussing so close. What I don't understand is why keeping the body/sensor stationary and moving the lens (just like how a lens normally focusses) doesn't change the focussing distance, but moving the sensor (which would usually change the magnification) does change the focus.

If I'm thinking straight this is completely backwards to how it should be. If I rigged up some sort of mount to hold a normal lens (not on bellows) by the front element to keep the lens in the same position and tried to focus I'd be moving the body/sensor forwards and backwards, this would change the magnification but not the focus (until you get to the extremes of the focus range, just like with the bellows).

So, when using a normal lens, off bellows:
Sensor to subject distance - changes magnification
Sensor to lens distance - changes focus

However when using the lens on bellows:
Sensor to subject distance - changes focus
Sensor to lens distance - changes magnification

Now to me that just doesn't seem right.


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Oct 13, 2010 17:57 |  #8

The Ran wrote in post #11090155 (external link)
So, when using a normal lens, off bellows:
Sensor to subject distance - changes magnification
Sensor to lens distance - changes focus

However when using the lens on bellows:
Sensor to subject distance - changes focus
Sensor to lens distance - changes magnification

Now to me that just doesn't seem right.

Yes that is because,when using the lens on bellows...
Sensor to subject distance - changes MAGNIFICATION...moving the sensor closer to the subject makes the subject bigger on the sensor!!!
Sensor to lens distance - changes FOCUS
...just like NON-macro shot!!!


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The ­ Ran
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Oct 13, 2010 18:47 |  #9

Wilt wrote in post #11091438 (external link)
Yes that is because,when using the lens on bellows...
Sensor to subject distance - changes MAGNIFICATION...moving the sensor closer to the subject makes the subject bigger on the sensor!!!
Sensor to lens distance - changes FOCUS
...just like NON-macro shot!!!

But that's not what it's doing. If I mount the rig on my tripod via the lens side standard the lens doesn't move, moving the body focuses the image where as it should change the magnification.


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vincent_su
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Oct 13, 2010 19:00 as a reply to  @ The Ran's post |  #10

It sounds like your rig has a fix distance from lens to subject. If you move your camera (sensor) to get magnification, you're also changing the distance between sensor and lens, am I correct?
If that's so, you'll need to adjust lens to sensor distance to get focus.
Can you show a pix of your rig?


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Oct 13, 2010 19:38 |  #11

I'll get a photo up tomorrow, it'll certainly help me explain it a bit better. Basically the lens is fixed on the tripod, so it's distance to the subject doesn't change. The sensor is then free to move forwards and backwards on the bellows which focusses the image but doesn't change the magnification. If I mount the body on the tripod so that it instead is a fixed distance to the subject then moving the lens via the bellows changes the magnification instead of focussing.


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The ­ Ran
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Oct 13, 2010 20:01 |  #12

Decided to take a photo now instead. Sorry about the ****e quality, but you should be able to make everything out.

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As you can see the front standard with the lens attached is mounted on the tripod so it's distance to the subject is fixed. Moving the rails backwards and forwards moves the sensor relative to the lens and the subject.

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stover98074
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Oct 13, 2010 20:10 |  #13

I use bellows for macro and really enjoy them. I tend to work with them this way.

1. Set the lens to infinity if your lens focuses – I use enlarging lenses and they do not have a focus capability built into the lens.
2. Set your magnification by adjusting the length of the bellows – the longer the bellows the greater the magnification.
3. Focus – My bellows (Asahi Pentax Autobellows) has a rail that allows me to move the entire bellows/camera forward and backward – this is equivalent to moving the tripod closer or father away.

Another way to think of this is with tubes.

1. Same as step one above – set the lens to infinity
2. Set magnification by the amount of tubes between your lens and camera – more tubes will increase magnification.
3. Focus – by moving the entire camera/tubes/lens closer or father away from the subject.

Hope this helps and below is a photo of the setup.

IMAGE: http://stover98074.smugmug.com/Flowers/Macro-Flowers/IMG1608/839505865_ZNWxH-M.jpg

Canon XSI, Asahi Pentax Auto Bellows, 50 Fujinon EP, 80 El Nikkor, 105 El Nikkor, 135 Fujinon EP
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The ­ Ran
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Oct 13, 2010 20:32 |  #14

So ideally I'd need another rail attached to the bottom of the bellows. Are there any disadvantages to focussing the way I am?


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stover98074
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Oct 13, 2010 21:21 |  #15

A third rail helps, macro focus adjustments are in small increments and moving a tripod can get cumbersome.

I rarely have issues focusing with a bellows if it is a still subject.

Sometimes tripod heads allow for a little forward and backward movement in the plate.

I have written about at this link.

https://sites.google.c​om …hy/5-option-b-for-bellows (external link)


I think the bellows I own would run about $80 to $100 on eBay these days.


Canon XSI, Asahi Pentax Auto Bellows, 50 Fujinon EP, 80 El Nikkor, 105 El Nikkor, 135 Fujinon EP
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My bellows have confused me
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