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01-13-2011, 01:58 AM   #16
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QuoteQuote:
One point that has not been made yet is that all a close-up filter gives you is closer focus (in fact, they give you a *fixed* focus distance). If that distance is not closer than the "native" one of the macro lens, you gain nothing and lose IQ in the process.
QuoteQuote:
Thanks for clearing this up, I wasn't quite understanding how they worked. My initial assumption was that they were essentially a "magnifying glass", so they'd just increase the magnification of a macro lens....
Your initial assumption was correct. A close-up lens always increases the magnification of a camera lens, macro or not. In this case, the minimum magnification for the DFA 50 with a 125mm close-up lens is about 0.4x and when fully extended about 1.8x.

Details: A 50mm lens focused at infinity has a magnification of zero because things at infinity are infinitely small. When a 125mm close-up lens is added the focus distance changes from infinity to 125mm and the magnification is about* m" = (f_original/f_close-up)=(50/125)=0.4x

Extending a lens to a magnification of m' then adding a close-up lens with a magnification m" results in a total magnification m of:

m=(1+m')(1+m")-1 (if the actual focal length of the original lens does not change due to extension)

Pentax says the minimum focusing distance is 7.67" or 195mm; this implies the actual focal length at 1X is close to 50mm. Digital Cameras and Accessories - Official PENTAX Imaging Web Site. (minimum_focusing_distance=f(1+m)(1+m)/m).

When this lens is extended to 1x and a 125mm close-up lens is added the expected magnification is about* m= (1+1)(1+.4)-1=1.8x.


Also the working distance between the lens and the subject is not fixed by adding a close-up lens; in this case, it is nominally** 125mm when the original lens is focused at infinity (magnification 0.4x) and reduces to about 56mm when the original lens is fully extended.

Details: The ratio of working distances w' and w" for magnifications m' & m" due to extension is approximately:

w'/w" = (1+1/m')/(1+1/m") (if the actual focal length of the original lens does not change due to extension)


Dave

*I say "about" because these are approximations based on what's called Thin Lens optics. In the thin lens case the magnification due to adding a close-up lens to an original lens with a spacing between them results in a lower magnification than if they were touching; the formula is:

m_close-up = (focal_length_original - spacing)/(focal_length_close-up)

**I say "nominally" in the above because of the thin-lens approximations used and possible lens recesses.



Last edited by newarts; 01-13-2011 at 02:34 AM.
01-13-2011, 03:19 AM   #17
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Here's one I did with a D/FA100mm and a Pentax close-up filter (a tricky combo):



I tried it on my DA35mm Ltd, but it was a no-go due to the impossibly close focal plane.
01-13-2011, 06:05 AM   #18
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QuoteOriginally posted by newarts Quote
Granted image quality likey doesn' t increase using a Raynox or equivalent quality closeup lens but it is still excellent in the many cases reported here in the " Raynox Club" thread.
Let me prefix what follows by saying that you don't need to convince me of the DCR-250's quality, I own one and I really like it.

However, the OP's question had to do with using a close-up filter *with* a dedicated macro lens.

QuoteQuote:
The closest focusing distance using a close-up lens attachment is always smaller than without it.
Definitely not always, and most certainly not with dedicated macro lenses as per the OP's question.

The working distance with the DCR-250, regardless of the lens it is attached to, is 11cm (it is a +9 diopter, working distance = 1/9 meters). That is the distance from the DCR-250 itself to the subject.

For example, the Vivitar 105/2.5's minimum focusing distance is 351mm. That is measured from the sensor, and translates to a working distance of 13.8 cm at 1:1 (the lens measures 17cm, and the registration distance is 4.3cm. 35.1 - 17 - 4.3 = 13.8). In this case, the gain in magnification is insignificant.

With shorter macro lenses, which have even closer minimum focus distances, the DCR-250 would actually *decrease* magnification because it would force you to focus *farther*.
01-13-2011, 07:37 AM   #19
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QuoteOriginally posted by RBellavance Quote
....

The working distance with the DCR-250, regardless of the lens it is attached to, is 11cm (it is a +9 diopter, working distance = 1/9 meters). That is the distance from the DCR-250 itself to the subject.

For example, the Vivitar 105/2.5's minimum focusing distance is 351mm. That is measured from the sensor, and translates to a working distance of 13.8 cm at 1:1 (the lens measures 17cm, and the registration distance is 4.3cm. 35.1 - 17 - 4.3 = 13.8). In this case, the gain in magnification is insignificant........
I'm sorry, but your assumption is incorrect. A close-up lens is like any other lens; it does not have a fixed working distance, it has a focal length. In the case of the Raynox DCR 250 (an 8 diopter lens) this focal length is 125mm. See: DCR-250 Super Macro conversion lens for D-SLR camera

Here's what's actually going on when a close-up lens is added to a camera lens, macro or not (some simple algebra is required):

(1) When a lens of focal length f' is placed next to a lens with focal length f" the combination has a focal length given by:

f=f'f"/(f'+f") if there's a distance d between the lenses the formula is f=f'f"/(f'+f"-d)

The equation can be re-arranged to yield

f=f'/(1+(f'-d)/f") = f'/(1+m_close-up) where m_closeup = (f'-d)/f" - we'll use this later

Putting a close-up lens close to a camera lens always results in a "new" lens with a shorter focal length. If the distance to the sensor doesn't change, the image gets larger.

A lens projects an image on a sensor according to the following rule:

(2) when a lens is positioned at a distance from a sensor, the magnification, m, focal length, f, and distance, s, from the sensor are related as:

s=f(1+m) - when the lens is focused at infinity, f=s, ie the lens is one focal length from the sensor, and m=0

Now when a close-up lens f" is added to a camera lens of focal length f' already focused at infinity, the distance from the sensor doesn't change but the focal length does:

s=f'=f(1+m)

Substituting equation (1) for the new focal length f results in:

s=f'=f'f"(1+m)/(f'+f"-d)=f'(1+m)/(1+m_close-up)

solving for m results in m_close-up=(f'-d)/f". the magnification due to the close-up lens is (f'-d)/f" when the primary lens is focused at infinity.

Now let's start with the same f' lens, this time positioned such that the magnification due to extension is m_extension. The distance from the sensor is now:

s_extended=f'(1+m_extension)

Now we add a close-up lens f" to the original lens; the distance from the sensor doesn't change, so

f'(1+m_extension) = f(1+m)

Substituting for the new focal length due to the close-up lens gives

f'(1+m_extension)=f'f"(1+m)/(f"+f"-d) = f'(1+m)/(1+(f'-d)/f") = f'(1+m)/(1+m_close-up)

Solving for total magnification m yields a useful, simple result

m=(1+m_extension)(1+m_close-up)-1

I am sorry about the algebra, but it is needed to explain what's going on.

To understand what happens with working distance (distance from lens to subject) or closest focus distance (distance from sensor to subject) use:

working_distance = f(1+1/m)

closest_focus_distance = f(1+m)(1+m)/m

and make the appropriate substitution for f as modified by the addition of a close-up lens.

Dave

01-13-2011, 07:51 AM   #20
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I agree with Dave (newarts). Adding a Raynox macro adapter to any lens will increase the magnification, regardless of focal length.

To answer the OP's question, yes, you can add a macro adapter to a macro lens. The downside is that it's harder to handle due to decreased DOF and shorter working distance. You need LOTS of light. Here's an example:

https://www.pentaxforums.com/forums/753591-post16.html
01-13-2011, 08:17 AM   #21
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QuoteQuote:
For example, the Vivitar 105/2.5's minimum focusing distance is 351mm. That is measured from the sensor, and translates to a working distance of 13.8 cm at 1:1 (the lens measures 17cm, and the registration distance is 4.3cm. 35.1 - 17 - 4.3 = 13.8). In this case, the gain in magnification is insignificant.
With the Vivitar focused at infinity, put on the Raynox. It will focus somewhere around 125mm away from the front of the lens as you say. Now extend the Vivitar lens by turning its focusing ring with the Raynox lens in place; the subject will come into focus at a larger magnification and will be closer to the front of the lens when it does. This demonstrates the working distance is not constant.


Here's what thin lens optic theory has to say:

You say the closest focus distance for the bare 105 lens is 351mm; focal length, magnification and closest_focus_distance are related by:

Closest_focal_distance= f(1+m)(1+m)/m
351=f(4) so f ~ 88mm (this is off a little more than expected).

This implies the lens distance from the sensor at 1X is about

s=f(1+m) or s=88(1+1)=178mm.

If a 125mm close-up lens were placed in contact with the Vivitar the new focal length will be 88/(1+88/125) ~ 52mm (see earlier post)

The lens' distance from the sensor hasn't changed so the new magnification is calculated from
s= 178=52(1+m) or m ~ 2.4.

The closest focus distance will change to about

closest_focus_distance = f*(1+m)(1+m)/m ~ 52(3.4)(3.4)/2.4 = 250mm

It may be the case that this brings the subject too close to the lens, maybe not. Perhaps you could try it and let us know? You should find that putting the close-up lens on the Vivitar increases the magnification for every extension of the Vivitar.

You say the lens is 170mm extended and the registration distance is 45mm for a total of 215 mm so there may be sufficient working distance to use the close-up lens fully extended, (but it'll be close )

Dave

PS Thin lens optics equations are not precise in a case like this but offer good guidance. They are not precise because real lenses aren't thin.

Last edited by newarts; 01-13-2011 at 08:24 AM.
01-13-2011, 09:32 AM   #22
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Well, I just tried with a +4 diopter on the Vivitar (I loaned my DCR-250 to a friend), and I was indeed wrong: the diopter make the Vivitar focus even closer than normal, and magnification is therefore increased.

Apologies to anyone I may have mislead with my earlier statements in this thread, and thanks to Dave (newarts) for setting the record straight.
01-13-2011, 09:56 AM   #23
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QuoteOriginally posted by RBellavance Quote
Well, I just tried with a +4 diopter on the Vivitar (I loaned my DCR-250 to a friend), and I was indeed wrong: the diopter make the Vivitar focus even closer than normal, and magnification is therefore increased.

Apologies to anyone I may have mislead with my earlier statements in this thread, and thanks to Dave (newarts) for setting the record straight.
Thanks for your gracious response.

It was only recently I realized how the working distance of a close-up lens/camera lens combination must change as the main lens is extended.

The logical barrier to me was thinking of the close-up lens by itself. By itself, a lens' focus can not be closer than its focal length.

It is just a coincidence that the working distance is equal to the close-up lens' focal length when the camera lens (with attached close-up lens) is focused at infinity.

Dave

01-13-2011, 07:17 PM   #24
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QuoteOriginally posted by miltllama Quote
but it's sounding like the best option for greater than 1:1 would be extension tubes with my macro lens. Unless that won't work either?!
Depends what you mean by 'best', eh? How much magnification do you want? How close do you want to work to a subject? Do you use flash, or controlled (studio) lighting, or ambient light? For flash, some aperture-linked tubes or de-glassed TC's are the best option for your DFA50. With the lens at full extension (closest focus), 50mm of tubes will take you to 2:1, 100mm to 3:1, 150mm to 4:1, etc.

Without flash (or with lots of diddling with exposures), lens-stacking gives the most magnification. A manual 24mm secondary reversed on a 200mm primary gives over 8:1 -- umma gumma! As with any lens reversal, working distance is under 2 inches.

If you need more working distance than a stack or a 50mm gives, you need a longer macro lens. That's why I often use a ~160mm enlarger lens on bellows+tubes. But for more magnification, MUCH more extension is needed. With a 100mm macro, you'd need to double the numbers I gave above for the DFA50. And dealing with 300mm of extension is, ah, cumbersome at best. And to retain aperture automation for easy flash work, many many not-cheap tubes or TC's are needed. Ay yi yi.

That's part of what makes this fun -- there is no 'best', just whatever is suitable. Again, see Blaker's FIELD PHOTOGRAPHY for a thorough discussion of tools and techniques. Good luck!
01-13-2011, 09:10 PM   #25
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QuoteOriginally posted by miltllama:
but it's sounding like the best option for greater than 1:1 would be extension tubes with my macro lens. Unless that won't work either?!
I began writing this post to recommend the use of Raynox type close-up lenses. Then I looked at a Pentax website for your lens' specifications. PentaxWebstore smcP D-FA 50mm f2.8 Macro As I read the specs, the working distance with the lens fully extended is only 47mm, which isn't much and implies the front glass element is fairly well recessed.

I'm afraid that adding a close-up lens will decrease the working distance too much for practical use. Further, extending the lens with bellows or tubes will also reduce the working distance; extending the lens to achieve 2:1 will put the subject inside the lens tube if the 47mm working distance at 1:1 is correct.

The only practical approach I can think of that I'm sure will work is to mount the lens backwards, perhaps on tubes if necessary to get the magnification you want. If you do this, you'd have to find a way to control the aperture lever. Fortunately, this is not hard to do - here's a scheme that works applied to a kit lens:


The tube holds the aperture lever in place by friction. You would set up with the aperture open, then move the plastic tube to close the aperture to the value you've preset with the aperture ring. This approach works well in my experience.

The photo strips below the camera/lens image are of a laptop screen; the pattern pitch is about 0.25mm.

Dave

PS please let us know if the 1:1 working distance of the DFA 50 is not about 47mm, as longer distances may change the recommendations.

PPS consider putting your A 50:2 on a Bellows or tubes, maybe reversed (aperture control is still a problem with this approach.)

Last edited by newarts; 01-13-2011 at 09:30 PM.
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