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08-11-2010, 09:17 AM   #1
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Lenses, Bellows and Magnification

I'm looking into purchasing my first set of bellows for macro and had a question on the usefulness of some lenses. I noticed in the instruction manual for the Auto Bellows M that the close tables on page 33 (http://www.pentaximaging.com/files/manual/Auto_Bellows-M__Slide_Copier-M.pdf) , for instance, only lista the 50mm f1.7 as going up to 1.15x magnification with a bellows extension of 60mm. Does that mean lenses, when mounted the right way around, are limited to a "maximum" useful magnification? and if so, what happens if the bellows are extended beyond the useful point (in this case 60mm)?

And yes, I know to reverse the lens to push beyond.

Thanks

08-11-2010, 09:51 AM   #2
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There isn't really a maximum, but the more you magnify, the less light there will be, and the closer you'll have to be to your subject, so eventually your photos will just become unusable.

With a 50mm lens, the rule of thumb is that each 50mm of extension will increase the magnification factor by 1.

You can see more info on Pentax bellows in our accessory database.

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08-11-2010, 11:01 AM   #3
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This thread may give you some info you're looking for.

Also, note that with bellows, you have to stop down the lens yourself. The camera body can not do that. A lens with an aperture ring is a must.

Last edited by SOldBear; 08-11-2010 at 11:06 AM.
08-11-2010, 03:12 PM   #4
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With any lens, on any bellows or tubes, the formula for magnification is:

M = (TE-FL) / FL

where M is magnification, TE is total extension in mm, and FL is focal length in mm.

A camera 50mm lens already has 50mm of extension when focused to infinity. If you add 50mm of bellows or tube extension for a TE of 100, the magnification is (100-50) / 50 = 1:1. If you add 75mm of extension, M = (125-50)/50 = 1.5:1 and so forth. The only real limit is how much extension you have available.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

As mentioned, adding extension cuts down on light. Calculated in f-stops, the formula is:

EA = NA * (M+1)

where EA is effective aperture, NA is nominal aperture, and M is magnification.

Adding 50mm of extension to a 50mm camera lens for 1:1 magnification, with the aperture set to f/8 (the NA), gives an EA of 8*(1+1) = f/16, a loss of 2 f-stops. With 2:1 magnification at f/11, the EA is 11*(2+1) = f/33, a loss of 3 f-stops. You compensate by using a slower shutter.

08-11-2010, 03:42 PM   #5
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QuoteOriginally posted by RioRico Quote
With any lens, on any bellows or tubes, the formula for magnification is:

M = (TE-FL) / FL

where M is magnification, TE is total extension in mm, and FL is focal length in mm.

A camera 50mm lens already has 50mm of extension when focused to infinity. If you add 50mm of bellows or tube extension for a TE of 100, the magnification is (100-50) / 50 = 1:1. If you add 75mm of extension, M = (125-50)/50 = 1.5:1 and so forth. The only real limit is how much extension you have available.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

As mentioned, adding extension cuts down on light. Calculated in f-stops, the formula is:

EA = NA * (M+1)

where EA is effective aperture, NA is nominal aperture, and M is magnification.

Adding 50mm of extension to a 50mm camera lens for 1:1 magnification, with the aperture set to f/8 (the NA), gives an EA of 8*(1+1) = f/16, a loss of 2 f-stops. With 2:1 magnification at f/11, the EA is 11*(2+1) = f/33, a loss of 3 f-stops. You compensate by using a slower shutter.
WOW !!! Very clearly explainations .

Thank you much RioRico .
08-11-2010, 05:05 PM   #6
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QuoteOriginally posted by Ken T Quote
WOW !!! Very clearly explainations .

Thank you much RioRico .
De nada, amigo. My sacred text when dealing with macro, closeups, exposure, optics, and much other tech stuff, is FIELD PHOTOGRAPHY: Beginning and Advanced Techniques, by Alfred A. Blaker (WH Freeman, 1976), a Scientific American book. Something newer and better may exist, but I haven't found it yet.
08-12-2010, 08:42 AM   #7
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Great information! Thanks everyone!
08-12-2010, 09:01 AM   #8
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Enlarging Lenses

I use a pentax auto bellows with several focal lengths and very much enjoy working with a bellows. I use enlarging lenses and have found them sharp and cost effective.

08-12-2010, 09:10 AM   #9
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QuoteOriginally posted by RioRico Quote
With any lens, on any bellows or tubes, the formula for magnification is:

M = (TE-FL) / FL

where M is magnification, TE is total extension in mm, and FL is focal length in mm.

A camera 50mm lens already has 50mm of extension when focused to infinity. If you add 50mm of bellows or tube extension for a TE of 100, the magnification is (100-50) / 50 = 1:1. If you add 75mm of extension, M = (125-50)/50 = 1.5:1 and so forth. The only real limit is how much extension you have available.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

As mentioned, adding extension cuts down on light. Calculated in f-stops, the formula is:

EA = NA * (M+1)

where EA is effective aperture, NA is nominal aperture, and M is magnification.

Adding 50mm of extension to a 50mm camera lens for 1:1 magnification, with the aperture set to f/8 (the NA), gives an EA of 8*(1+1) = f/16, a loss of 2 f-stops. With 2:1 magnification at f/11, the EA is 11*(2+1) = f/33, a loss of 3 f-stops. You compensate by using a slower shutter.
Interesting, but what happens if you add tubes to a, let's say, 50mm 1:1 macro lens? And how do you calculate then?
08-12-2010, 10:24 AM   #10
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You reach 1:1 with the lend fully extended so it is easy

A 50 mm 1:1 has 100 mm total extension capability (50 mm for infinity focus plus another 50 for 1:1)

Just add any additional extension tube length to this
08-12-2010, 11:50 AM   #11
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Nothing better than Blaker that I've found. A must if you can find it.
08-13-2010, 12:44 AM   #12
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QuoteOriginally posted by Lowell Goudge Quote
You reach 1:1 with the lend fully extended so it is easy

A 50 mm 1:1 has 100 mm total extension capability (50 mm for infinity focus plus another 50 for 1:1)

Just add any additional extension tube length to this
Quite so. With any prime lens, the non-tube extension can be found by just measuring the difference between the close-focus point (extended) and the infinity-focus point (all the way in). With wide lenses, that won't be much -- and there's not much call for putting a 15mm lens on tubes anyway. With a 'macro' lens, it's so simple; at 1:1 the total extension is 2x the focal length, at 1:2 it's 1.5x the focal length, etc.

Partly in response to another thread featuring contention about "crop factor" and magnification, I'll say that in practice, we rarely worry about the exact amount of extension or magnification. We're generally more interested in filling the frame nicely. There are exceptions, of course. With stacked lenses, we always know the magnification -- it's the ratio of primary to secondary. With much technical work, knowing the magnification is vital; if dimensions are crucial, incorporate a ruler or other reference. But with macro-for-fun, such detail may be unnecessary.

QuoteOriginally posted by jbinpg Quote
Nothing better than Blaker that I've found.
I was afraid of that. I mean, it's been ~35 years.
08-13-2010, 03:11 AM   #13
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Lowell, Rico, thanks guys. These forums are just a treasure of knowledge.

So, if I did calculate right, I would need something in the area of a 50 - 100mm extension for making any significant difference in magnification with at 105mm 1:1 macro lens. Would that even work without vignetting, etc? Would it change the working distance?

I will keep an eye out for that Blaker book, I need to read up on these things.
08-13-2010, 10:28 PM   #14
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QuoteOriginally posted by wowtip Quote
So, if I did calculate right, I would need something in the area of a 50 - 100mm extension for making any significant difference in magnification with at 105mm 1:1 macro lens. Would that even work without vignetting, etc? Would it change the working distance?
EXTENSION: A 105mm macro lens fully screwed-out to 1:1 has 210mm total extension. To reach 2:1, you would need to add another 105mm of extension. Macro lenses longer than 50mm are usually preferred for longer working distance, not for huge magnification. For instance, suppose I had a 400mm macro lens. Ooops -- ain't none! But if I put a 400mm tele on 200mm of bellows and tubes, I can only reach 1:2 magnification, but I can work about 1m from the subject. (Blaker might have a formula, but I just tried it empirically.)

With extensions alone, shorter lenses provide more magnification. A 35mm macro lens at 1:1 has a total extension of 70mm. If put on a 140mm bellows for a TE of 210mm, the magnification is (210-35)/35 = 5:1. There's no vignetting, but it gets dark down there. Why no vignetting? Because the image circle goes WAY beyond the sensor/film frame. Why dark? Because only a portion of the image circle makes it all the way to the frame.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

WORKING DISTANCE: With extensions alone and any un-reversed lens, there is a limit: you cannot focus closer than the focal lenth of the lens. I tested this earlier today. I put a normal 50mm lens onto 50mm, 100mm, and 150mm of extension. In each case, my close-focus point (closest working distance) was... 50mm. If you put a 105mm macro lens on 1155mm of extension for 10:1 magnification, your close-focus point would be... 105mm. The closest a 28mm macro lens can focus is... 28mm. Et cetera.

Notice that I mentioned un-reversed lenses. Working distance for a reversed lens is quite different -- it's the register of that lens type, which for Pentax is about 46mm. A reversed 28mm or 20mm or 10mm Pentax- or Nikon- or OM-type lens will have a working distance of: ~46mm. (See Camera Mounts Sorted by Register for details.) A reversed non-macro prime lens on tubes or bellows is often sharper for macro work than if it's not reversed, but that varies with lens design. With a (cheap) mount-reversal ring, you can use almost ANY lens from ANY lensmaker, because you're mounting via the front threads, not the proprietary mount system.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

SUPPLEMENTALS: Extending a lens, either 'macro' or normal, straight or reversed, isn't the only way to do macro. There are also supplemental lenses, of 3 sorts:

1) Simple +diopter glass, cheap and easy, but they distort.
2) Two-element corrected magnifiers, like the Raynox adapters.
3) A standard prime 'stacked' (reversed) onto a longer lens.

Stacking requires a (cheap) thread-reversal ring. I have several 49mm-thread Takumars and the like, so I use a 49mm male-male ring. I'll put a 100mm lens (the primary) on the camera, then the 49mm ring, then a reversed 25mm lens (the secondary). Magnification is the ratio of primary to secondary: 100/25= 4:1. And the working distance is... the register of the secondary, the same as if I'd reversed that secondary on a bellows or tube instead.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

And that's the basic poop on macro magnification and working distances. There are nuances and tricks related to stacking, but I've blathered enough already.

Last edited by RioRico; 08-13-2010 at 10:36 PM.
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