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04-14-2020, 08:37 PM - 1 Like   #16
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No, I won't order this 645 bellows unit ... I have a 645 FA 120 mm f/4 macro that fulfills all my Macro needs as of now (some flowers only, no more insects except butterflies but then all the Botanical Centers are closed because of the pandemic).

Regards.

04-15-2020, 06:30 AM   #17
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QuoteOriginally posted by barondla Quote
Wow, that is fast shipping. Besides the 75mm, what other lens are you going to reverse? Don't think the 120 macro will gain anything. Usually normal and wideangle lenses are reversed. The wider the lens, the greater the magnification.
...
Since last writing, I have been looking further into the subject, and now understand that longer lenses are not suitable.

I still have some questions about the optics theory being applied. With a theoretical thin lens, it should not matter whether the lens is reversed or not, due to symmetry. And using a thin lens, increased magnification occurs from using extension tubes. I have not yet found a good explanation of the value of lens reversal with extension tubes vs. normal mounting with extension tubes, although it might be the result of the front principal plane being farther from the filter threads than the rear principle plane is from the rear mounting flange. If it is due to some subtle factor in lens correction going one way vs. the other, I have yet to find mention of it.

I can also try my FA 45mm which has a 58mm filter thread. It will be a while before I start thinking about 'D' lenses and '67' lenses. I believe all '67' lenses will require step-up rings per the 67 Interchangeable Lenses booklet. I am unaware of a Pentax 645 DA/DFA Interchangeable Lenses booklet, so I will have to examine those lenses I have to see what the filter threads are.
04-15-2020, 07:49 AM   #18
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QuoteOriginally posted by kaseki Quote
Since last writing, I have been looking further into the subject, and now understand that longer lenses are not suitable.

I still have some questions about the optics theory being applied. With a theoretical thin lens, it should not matter whether the lens is reversed or not, due to symmetry. And using a thin lens, increased magnification occurs from using extension tubes. I have not yet found a good explanation of the value of lens reversal with extension tubes vs. normal mounting with extension tubes, although it might be the result of the front principal plane being farther from the filter threads than the rear principle plane is from the rear mounting flange. If it is due to some subtle factor in lens correction going one way vs. the other, I have yet to find mention of it.

I can also try my FA 45mm which has a 58mm filter thread. It will be a while before I start thinking about 'D' lenses and '67' lenses. I believe all '67' lenses will require step-up rings per the 67 Interchangeable Lenses booklet. I am unaware of a Pentax 645 DA/DFA Interchangeable Lenses booklet, so I will have to examine those lenses I have to see what the filter threads are.
I started off with the DFA 55 because I'm doing the Single In challenge where the same lens is used for a month. A cool thing about reveresing is any brand lens can be used. It doesn't
have to be Pentax or even medium format. An aperture ring does make things easier. So feel free to try some of full frame or APS-c normal and wide angle primes. It is fun.

I don't know optical theory to be able to answer your questions. If you don't find answers forum member B dery might help. He is an optician.

Thanks,
barondla
04-15-2020, 09:51 AM - 3 Likes   #19
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using reversed lenses that lack aperture rings

Remember that even a “thin” lens focuses such that the distance to the subject is greater than the distance to the film. Most lenses are optimized for a 10:1 difference or greater (magnifications of 1:10 or less).

Also, remember that even a symmetrical lens is positioned in front of a mirror box. That means that reversing them positions the optical center farther out from the film, simply because of the geometry of the mount.

But most shorter lenses are retrofocus, meaning the glass is in front of the rear node. Reversing them puts the glass behind the rear node, which reversed the retrofocus effect, increasing magnification rather than reducing it. Retrofocus design used to be called “reverse telephoto”.

Reversing the lens 1.) positions glass farther from the sensor, 2.) reverses the retrofocus effect to increase magnification, and 3.) keeps the optimization favorable by providing a much greater distance to the focus plane in front of the lens vs. behind it.

True copy lenses are absolutely symmetrical and are therefore optimized for 1:1. (Think of the lenses in old analog copy machines.) Reversing them has no effect. Lenses for SLRs that also focus to infinity do this by use of floating elements to maintain optimality when focused closely. The first lenses to do so have a separate physical setting for manually repositioning elements based on focus distance. Nowadays it’s done mechanically. Lenses designed for greater magnification than 1:1 are optimized for it, and you see bellows lenses of old stating they are designed for, say, 2:1 to 5:1.

Putting a regular lens on a bellows is therefore sub-optimal, unless you reverse it.

Rick “but a reversed lens requires a very close subject distance” Denney

04-15-2020, 12:44 PM - 1 Like   #20
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QuoteOriginally posted by barondla Quote
A cool thing about reveresing is any brand lens can be used. It doesn't
have to be Pentax or even medium format. An aperture ring does make things easier. So feel free to try some of full frame or APS-c normal and wide angle primes. It is fun.
Yeah I reverse my Takumar 28/3.5 and use it on my Pentax 67II, via the 6x7 49mm reverse adapter and the 6x7 helicoid extension tube. Gives me around 5:1 magnification.

Phil.
04-15-2020, 04:25 PM   #21
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QuoteOriginally posted by rdenney Quote
Remember that even a “thin” lens focuses such that the distance to the subject is greater than the distance to the film. Most lenses are optimized for a 10:1 difference or greater (magnifications of 1:10 or less).

Also, remember that even a symmetrical lens is positioned in front of a mirror box. That means that reversing them positions the optical center farther out from the film, simply because of the geometry of the mount.

But most shorter lenses are retrofocus, meaning the glass is in front of the rear node. Reversing them puts the glass behind the rear node, which reversed the retrofocus effect, increasing magnification rather than reducing it. Retrofocus design used to be called “reverse telephoto”.

Reversing the lens 1.) positions glass farther from the sensor, 2.) reverses the retrofocus effect to increase magnification, and 3.) keeps the optimization favorable by providing a much greater distance to the focus plane in front of the lens vs. behind it.

True copy lenses are absolutely symmetrical and are therefore optimized for 1:1. (Think of the lenses in old analog copy machines.) Reversing them has no effect. Lenses for SLRs that also focus to infinity do this by use of floating elements to maintain optimality when focused closely. The first lenses to do so have a separate physical setting for manually repositioning elements based on focus distance. Nowadays it’s done mechanically. Lenses designed for greater magnification than 1:1 are optimized for it, and you see bellows lenses of old stating they are designed for, say, 2:1 to 5:1.

Putting a regular lens on a bellows is therefore sub-optimal, unless you reverse it.

Rick “but a reversed lens requires a very close subject distance” Denney

What a delightful explanation - clear, logical and practical. Thanks for that :-)

Ed "still not usually inserting phrases into the middle of my name" Hurst

Last edited by Ed Hurst; 04-15-2020 at 05:23 PM.
04-16-2020, 11:48 AM   #22
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QuoteOriginally posted by Ed Hurst Quote
What a delightful explanation - clear, logical and practical. Thanks for that :-)

Ed "still not usually inserting phrases into the middle of my name" Hurst
Thanks, Ed.

Rick "rarely able to improve on a dedicated macro lens with floating elements, such as the 645 A and FA 120" Denney
04-18-2020, 06:33 PM - 1 Like   #23
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QuoteOriginally posted by kaseki Quote
I can also try my FA 45mm which has a 58mm filter thread.
Bzzzt. The 45mm has a 67mm filter thread, so no go without a step-up ring. I may have one in a ring light kit, but that will be another day to look for.

Two images are shown below using all three 645-A extension tubes and the 75mm standard lens. The first is with the lens reversed using the adapter/attachment kit, and the second is without using the lens not reversed. The object is an avatar on my BenQ monitor screen. The avatar is of a Windows aquarium program that can be made to run on Linux. The width of the mini-aquarium is 19.5mm, approximately. Images are PF reductions from the 645Z jpg files. No post processing. Parameters (available to the camera) are in the exif files.

This is only a functional test. Target objects of finer detail need to be used to see if there is any difference in apparent resolution.

In my view, the slight gain in magnification from lens reversal for this lens is hardly worth the loss in camera control of the lens. Also, focusing via the focus ring seemed to be numb for the reversed configuration requiring tripod displacement to achieve focus, whereas the normal configuration, also with the extension tubes, allowed easy manual focus.

Attached Images
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PENTAX 645Z  Photo 
View Picture EXIF
PENTAX 645Z  Photo 
04-22-2020, 04:18 PM - 1 Like   #24
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Found a 58 --> 67 mm ring I had bought for use with my AF-140C. This allowed a brief test with the 45mm/f2.8 FA lens. The first two images below are (a) for this lens reversed as above with the full set of 645A extension tubes, and (b) in normal orientation with the same extension tubes. One issue with normal orientation is that the front focal distance is so small that difficulties arise with some target objects. And as before, focus in the reversed orientation calls for tripod movement. I can see why the bellows was invented.

The second set are portions of a Paph. orchid flower with the lens in normal orientation and all three extension tubes. Focus was difficult because the petals interfered with the lens housing and so the object moved with focus attempts. The lens was at f/2.8 so depth of field is as minimal as one might expect.
Attached Images
View Picture EXIF
PENTAX 645Z  Photo 
View Picture EXIF
PENTAX 645Z  Photo 
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PENTAX 645Z  Photo 
View Picture EXIF
PENTAX 645Z  Photo 
06-22-2020, 09:20 PM   #25
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Update on the hot spot. The reversed 645 DFA 55 lens does not have a hot spot. The Fotodiox 645>67 adapter causes the problem. Figured it out while using the Pentax 67 165 f2.8 lens. Many of the landscape photos had significant flare or hot spots.

Applied felt to the inner ring and it helped slightly, but in bright conditions flare remained. Then I noticed the two coupling levers were bright silver. Blackened them and the flare vanished. Fotodiox should be ashamed their brand new adapter performs so poorly.

Thanks,
barondla
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