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05-13-2011, 01:34 AM   #31
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I guess it might actually be quite a logical that an enlarger lens would work better when used reversed. Think of the subject you are photographing as the negative and think of your camera's sensor (or film) as the paper onto which the enlarged image of the subject is projected. The enlarger lens is optimized with the assumption that light enters from the side that is facing the negative. With that in mind reversal of an EL seems like the way to go.


Last edited by lars.o; 05-13-2011 at 06:26 AM.
05-13-2011, 07:13 AM   #32
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I advocate reversing camera lenses (on extension) for macro work because 1) reversal works, and 2) I understand the subject-field vs image-field relationship. At least, I *think* I understand it!

The image field -- the image that's projected onto the frame (film or digital) by the lens -- *MUST* be flat, or the lens is worthless for standard use. The subject field -- the light that the lens gathers -- *SHOULD* be fairly flat. But since reality is rarely flat, most standard (non-macro nor -enlarger) camera lenses can show some subject-field curvature and still deliver acceptable results.

When a standard camera lens is reversed, the subject field becomes flat, but the working distance also decreases, down to the register (flange-focal distance) of that lens. And because the reversed image field is so close to the frame, that field is essentially flat also.

But a macro or enlarger lens is designed so BOTH subject and image fields are flat. And an enlarger lens *is* designed for fairly close work, with the image usually projected no more than 1m. Infinity is not an issue. I do not doubt those who say that close-work optics improve when an enlarger lens is reversed. I've not tried it yet; and experimentation must await my return home in a few weeks, where I have zillions of step-rings and adapters to play with.

What I would like to see: A technical reference about reversing enlarger lenses. My rough gargling over a slow net connexion with my limited-capacity travel sub-notebook reveals no explanation of *WHY* reversing enlarger lenses is beneficial. Can anyone point to a reference?
05-13-2011, 08:22 AM   #33
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Three simple relationships..

QuoteOriginally posted by lars.o Quote
Hello...

I want to get more serious with macro photography (mostly plants and insects). And I'm wondering what lens to get for my K20D. One thing I would be very much interested in is achieving magnification greater than 1:1 (say 2:1 or even 4:1). I imagine that would be possible by using a 1:1 macro lens with extension tubes provided that the focal length was long enough to maintain a viable working distance. Say at least 100mm.

I understand that getting more DoF than a sheet of thin paper will be a big issue when trying to get to larger magnification. That means that I will need to stop down quite a bit.

Is using extension tubes to go beyond 1:1 even a good idea? Are there other options? And if you know of a lens you would recommend for this sort of scenario, I'd love to hear about it.

Thanks,
Lars
Only a few simple relationships are needed to describe macro photography set-ups. I know that many people are repelled by equations, but these are actually pretty simple and useful.

The first is the focal length of combined lenses of focal lengths f' and f" spaced d apart (usually d ~0).

f=f'f"/(f'+f"+d)

When a short focal length lens is reversed on a long focal length lens a new lens is created with a focal length shorter than the shortest lens in the stack..

Regarding distances and magnification, m:

extension (distance from sensor to lens) = f(1+m)
working distance (from lens to subject) =f(1+1/m)

That's all; many important practical conclusions follow from these three.

for example: If you want a working distance of at least 100mm from a subject and a magnification of 2:1 there is no escape from needing a focal length that satisfies the working distance equation:

100mm > f(1+1/2) or f > 67mm.

Stacking a 28mm lens on a 200mm lens won't do it for you because the focal length of the stacked combo is about 200*28/(28+200) = 24.6mm

I'm not saying you shouldn't stack lenses but just pointing out there are no magic solutions to the working distance - magnification problem. I think that for high mag work it is reasonable to set up for a working distance of about 40-50mm. This gives you room enough to get light on the subject and get your hand in there if necessary.

EDIT: here is one way to get working distance beyond the limit above; use a transfer lens. Say you take a good 10omm lens and position it 200mm from a subject. That lens will create a "real image" of the subject floating in space. You can then use another lens set-up to look at the real image as close as you want.

Stacking lenses has some benefits in providing brighter images for focusing than simply extending a longer lens further: the effective f-stop for a macro image is f(1+m) therefore making f small by stacking can increase image brightness. Unfortunately stacking may result in too much contrast loss; you just have to try. Stacked lenses can also be sharper than either of the original lenses (they can compensate for each other's problems.)

There is no optical solution to the DOF - high mag problem. Increasing f-stop much beyond about f:5.6 will most likely cause diffraction softening that will wipe out any increase in DOF

Last edited by newarts; 05-20-2011 at 04:39 PM.
05-13-2011, 03:14 PM   #34
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I tried experimenting using my 100 to 300mm zoom lens along with the full set of Pentax extension tubes and a 2x converter to come up with the results shown on picture that is attached. I am getting slightly larger than life with this set-up. My working distance is about 4 feet away. With stock off the shelf camera parts, I think this is all I can get for getting close up to my subjects. The 300mm lens allows me to keep some distance from the subject matter while getting a nice close picture. The drawback is the amount of weight of the camera. The tripod I was using really was not suited to this set-up. I do have a heavier type that can support this lens combo. The exposure for the finished picture is 1/125th at f8. ISO is set at 400.



05-20-2011, 04:28 PM   #35
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Finally most of the stuff I needed arrived. A used Ashai Pentax m42 Bellows II, some adapter rings, etc... Unfortunately, the guy I got the bellows from did not include the part that connects the bellows to the camera. So I had to try and build a temporary solution that will hold up until he finds the part and ships it to me (if he finds it...)
Anyway... With the bellows fully extended and the Rodagon N 50mm reversed I get to a bit more then 4:1 at around 55mm working distance. (Photographing a ruler, I can fit 6mm on the k20d sensor which I believe is around 25mm wide, 25:6 = 4.16:1)
Here is a quick test shot, at f8 and 1s exposure time (not enough light at hand for the quick test). The little blossom has a diameter of around 7mm, so this is not the full 4:1 but close. The other image is a 100% crop...

I think this isn't bad at all for a really quick test and improvised mount between camera and bellows. Hopefully I get the mount issue resolved soon so I can be sure camera and lens are correctly aligned.

PS. Looks like my improvised camera mount got me quite a bit of dust on the sensor

Cheers,
Lars
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05-20-2011, 04:30 PM   #36
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QuoteOriginally posted by lars.o Quote
Finally most of the stuff I needed arrived. A used Ashai Pentax m42 Bellows II, some adapter rings, etc... Unfortunately, the guy I got the bellows from did not include the part that connects the bellows to the camera. So I had to try and build a temporary solution that will hold up until he finds the part and ships it to me (if he finds it...)
Anyway... With the bellows fully extended and the Rodagon N 50mm reversed I get to a bit more then 4:1 at around 55mm working distance. (Photographing a ruler, I can fit 6mm on the k20d sensor which I believe is around 25mm wide, 25:6 = 4.16:1)
Here is a quick test shot, at f8 and 1s exposure time (not enough light at hand for the quick test). The little blossom has a diameter of around 7mm, so this is not the full 4:1 but close. The other image is a 100% crop...

I think this isn't bad at all for a really quick test and improvised mount between camera and bellows. Hopefully I get the mount issue resolved soon so I can be sure camera and lens are correctly aligned.

Cheers,
Lars
Set it to F/5.6 and try again
05-22-2011, 12:26 AM   #37
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QuoteOriginally posted by yeatzee Quote
Yes there is, but not in PK mount The canon MPE-65mm can go from 1x - 5x all by itself.

I shoot above 1:1 exclusively (I personally hate 1:1) and use extension tubes most of the time to do it. If you want 100mm of working distance I cant help you.

But what I will say is that if the objects you shoot are inanimate, look into focus stacking. Stopping down a ton will result in diffraction making the image soft.
Agree +1

A point about focus stacking, unless you have a telecentric lens you will get various problems with the image as you change focus to get the stack. I am in the first stages of trying to construct a telecentric lens to get round these problems, which are basically around the change of magnification and therefore perspective or field of view as you alter focus to get the stack.
05-28-2011, 05:38 AM   #38
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Here's one with the 80mm Rodagon reversed at f/8... (not cropped)





Last edited by lars.o; 05-28-2011 at 05:44 AM.
05-28-2011, 10:06 AM   #39
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QuoteOriginally posted by waxsage Quote
Agree +1

A point about focus stacking, unless you have a telecentric lens you will get various problems with the image as you change focus to get the stack. I am in the first stages of trying to construct a telecentric lens to get round these problems, which are basically around the change of magnification and therefore perspective or field of view as you alter focus to get the stack.
Wait - this should only be an issue if you change the film-plane to lens distance ( focus with the bellows or helical ) rather than moving the camera and lens as a unit, correct?
05-28-2011, 10:42 AM   #40
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QuoteOriginally posted by jstevewhite Quote
Wait - this should only be an issue if you change the film-plane to lens distance ( focus with the bellows or helical ) rather than moving the camera and lens as a unit, correct?
But moving the camera and lens as a unit, changes the scale of the subject, doesn't it (i.e. I'm getting closer). Or is this something the software can take care of by transforming the images that make up the stack? I really need to try this out...
05-28-2011, 10:54 AM   #41
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QuoteOriginally posted by newarts Quote
No. In general if either object or image distance changes an off-axis out-of-focus disk changes both diameter and distance from the axis

Therefore when the camera or subject is moved the magnification of what's in-focus stays constant but what's out-of-focus shrinks or grows depending on whether it is under- or over- focus.

One set-up where this is not true is when the lens is telecentric from both sides (the afocal condition).
http://www.schneiderkreuznach.com/pdf/div/optical_measurement_techniques_wit...ric_lenses.pdf see section 4.2.2

Note: It was surprising to me that Afocal ("A" = without) lenses could make real images but they do...I even set one up to see for myself.
Oh, right, that's what I meant; the in-focus bit retains the same magnification, yes? If you're stacking focus, isn't that what's important? I'm confused about what problem is being solved here...
05-28-2011, 11:02 AM   #42
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QuoteOriginally posted by jstevewhite Quote
Oh, right, that's what I meant; the in-focus bit retains the same magnification, yes? If you're stacking focus, isn't that what's important? I'm confused about what problem is being solved here...
Yes the in-focus bit is the same magnification --- but now I'm not sure about the out-of-focus bit so I deleted my post until I'm certain.

I looked further and for an "object side telecentric lens"* the image disk in a fixed image plane for does not move towards or away from the optic axis as the object is moved (ie. the object's magnification does not change as the object is moved towards or away from the camera.)

*"Object side telecentric" puts an aperture at the focal point on the image side.

Last edited by newarts; 05-28-2011 at 11:53 AM.
05-28-2011, 11:04 AM   #43
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QuoteOriginally posted by newarts Quote
Yes the in-focus bit is the same magnification --- but now I'm not sure about the out-of-focus bit so I deleted my post until I'm certain.
No, I think you'd be right about the out of focus part because the distance between the lens and the subject (oof bit) changes. I'm just having trouble understanding why that would be a problem for someone doing focus-stacking.
05-28-2011, 12:34 PM   #44
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Moving back and forth the entire camera+lens seems to work just fine at least in my case. Here's the first try with just 3 layers at f/5.6 with the 80mm lens. I should have done more but I couldn't convince the spider to stay still...



Last edited by lars.o; 05-28-2011 at 12:48 PM.
05-28-2011, 12:49 PM   #45
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Here's what I think is going on with an object side telecentric lens. The top figure shows ray tracing for a normal lens setup; the out-of-focus image of an object moved away from the lens is smaller in the original image plane:


The aperture at the image side focal point of the telecentric lens restricts the ray pencil such that the projection of the out-of-focus image of the moved object overlays the original image.

Therefore moving the camera with respect to the subject in macrophotography with an object side telecentric lens will cause less out-of-focus image magnification change than with a normal lens. This in-turn might increase the effectiveness of image stacking software.

To do it place an aperture one focal length behind the lens being used... this is hard to fit in for normal work but not hard for macro work using tubes & bellows.

For example, put a 45.5mm extension tube with a 10mm aperture on its camera end on a Pentax 50mm lens to make a 50:5 telecentric lens. I've an Iris diaphragm on order and will try to make a telecentric 50mm macro lens when I receive the iris.

It follows that if an aperture is placed at the focal point on the object side of the lens image magnification will not change as focus is changed using the lens' focus ring.

Last edited by newarts; 05-28-2011 at 02:43 PM.
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