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07-15-2020, 02:09 PM   #1
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Why to reverse 50mm on extension tubes?

Hello, long time no see.

I recently thought about improving my skills in macrophotography and wanted to try some techniques 'pros' use. I've stumbled upon Thomas Shahan videos on Youtube and since he uses quite motivating and cheap setup, I thought I will try to find the lens.

Today I received old and manual SMC Pentax 50mm 1.7 which was shown on the video along with old Foca extension tubes 36+20+12mm (I'm amazed on the condition of both). It appeared I'm missing the right reverse ring (guess the lense is 49mm), but I tried to just keep the lense at the end of the extension tubes just shot a few frames for test.

I've discovered... whether the lens is mounted reversed or not, there seems to be no real difference in the maginification ratio. And so I started wondering - what are the advantages of an reversed lense on an extension tube? Did I stumble upon a false positive due to the length of the extension tubes perhaps?


Last edited by geb88; 07-15-2020 at 02:11 PM. Reason: added link to the lense mentioned in post
07-15-2020, 02:16 PM   #2
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QuoteOriginally posted by geb88 Quote
Hello, long time no see.

I recently thought about improving my skills in macrophotography and wanted to try some techniques 'pros' use. I've stumbled upon Thomas Shahan videos on Youtube and since he uses quite motivating and cheap setup, I thought I will try to find the lens.

Today I received old and manual SMC Pentax 50mm 1.7 which was shown on the video along with old Foca extension tubes 36+20+12mm (I'm amazed on the condition of both). It appeared I'm missing the right reverse ring (guess the lense is 49mm), but I tried to just keep the lense at the end of the extension tubes just shot a few frames for test.

I've discovered... whether the lens is mounted reversed or not, there seems to be no real difference in the maginification ratio. And so I started wondering - what are the advantages of an reversed lense on an extension tube? Did I stumble upon a false positive due to the length of the extension tubes perhaps?
I am no expert on this technique, quite the opposite but how did you attach the lens to the extension tubes?

07-15-2020, 02:34 PM   #3
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Sorry for my English. As I tried to say, I was holding it tight to the tubes in my own hand just for the test purposes, as I'm missing the proper 49mm reverse ring.
07-15-2020, 02:35 PM   #4
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You only need to reverse a lens if you are exceeding 1:1 magnification. Wide angles lenses in the 28mm to 35mm focal lengths reversed will give you the highest magnification.

You will also need a reverse adapter at the minimum, as well as some form of extension. (regular extension tubes, helicoid extension tube or a bellows)

Phil.

07-15-2020, 02:35 PM   #5
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According to the Pentax bellows manuals you should reverse lenses for the best results when the magnification is greater than 1×. See this following post for an explanation:

Best Macro set up - PentaxForums.com
07-15-2020, 02:43 PM   #6
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If you use a reversal ring you don't need extension tubes. I reverse my M 50 1.7 directly on the camera and get about a 1:1 magnification and very good sharpness.
07-15-2020, 02:56 PM - 1 Like   #7
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Lens optical designs considers two distances: subject distance and image distance, the first from the optical center of the lens to the subject and the second from the optical center of the lens to the film/sensor plane.

On normal use, subject distance is much greater than image distance so lenses are designed for optimal results at those distances.

When doing macro at high magnification, 1X and greater, you increase image distance by using extension tubes or bellows, and at the same time reduce subject distance to a few centimeters. The image distance ends being greater than the subject distance, opposite to what the lens was designed for. In this case, using the lens inverted gives you better image quality than using it in its normal position.

To attach the lens to the extension tubes you use a reversal ring that has a filter thread on one side and a male bayonet mount on the other. You mount the ring on the filter thread of the lens so the lens ends with a bayonet mount on its front.

Last edited by CarlosU; 07-15-2020 at 03:24 PM.
07-15-2020, 03:26 PM - 1 Like   #8
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QuoteOriginally posted by CarlosU Quote
Lens optical designs considers two distances: subject distance, from the optical center of the lens to the subject, anything from a few feet to infinity; and image distance, from the optical center of the lens to the film/sensor plane, some 10 to 60 mms depending on camera and format.

On normal use, subject distance is much greater than image distance so lenses are designed for optimal results at those distances.

When doing macro at high magnification, 1X and greater, you increase image distance by using extension tubes or bellows, and at the same time reduce subject distance to a few centimeters. The image distance is now greater than the subject distance, inverse to what the lens was designed for. In this case, using the lens inverted gives you better image quality than using it in its normal position.

To attach the lens to the extension tubes you use a reversal ring that has a filter thread on one side and a male bayonet mount on the other. You mount the ring on the filter thread of the lens so the lens ends with a bayonet mount on its front.
CarlosU pretty much said it all. This is referred to as the conjugate ratio and most lenses are designed for an infinite conjugate ratio since the lens will spend most of its time focusing on subject matter far away (near infinity) while the image will be produced only a short distance from the nodal point of the lens, hence a ratio that approaches infinity. Enlarger lenses are designed with a non-infinite conjugate ratio since both distances will be far less than infinity and optimum optical performance is needed, but that means they might not be the best when it comes to scenic photography where the infinite conjugate ratio is better.

07-15-2020, 03:33 PM - 1 Like   #9
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Lens aberrations, and curvature of field in particular, can be much exaggerated at higher than normal magnifications while focussing closer than the helicoid allows for in most consumer camera lenses. Average lenses are usually corrected to about 1/10 lifesize, that is, a minimum focus of about 10 times the lens' focal length. These modern lenses often have a larger entrance pupil (front element) than exit pupil (rear element). When reversed the optical configuration allows the rear of the lens designed to best focus on a flat field (your sensor or film plane) to face the subject and the relative distances of much smaller rear element to focal plane distance compared to a greater subject to front element distance is restored. At higher magnification the Some lenses will result in a marked improvement used this way. Older style optics, macros, and adapted enlarger lenses may not show a difference because they are usually using a symmetrical design where the elements are configured the same back to front as front to back.

Last edited by From1980; 07-15-2020 at 03:49 PM.
07-15-2020, 03:49 PM   #10
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QuoteOriginally posted by CarlosU Quote
Lens optical designs considers two distances: subject distance and image distance, the first from the optical center of the lens to the subject and the second from the optical center of the lens to the film/sensor plane.

On normal use, subject distance is much greater than image distance so lenses are designed for optimal results at those distances.

When doing macro at high magnification, 1X and greater, you increase image distance by using extension tubes or bellows, and at the same time reduce subject distance to a few centimeters. The image distance ends being greater than the subject distance, opposite to what the lens was designed for. In this case, using the lens inverted gives you better image quality than using it in its normal position.

To attach the lens to the extension tubes you use a reversal ring that has a filter thread on one side and a male bayonet mount on the other. You mount the ring on the filter thread of the lens so the lens ends with a bayonet mount on its front.
Carlos really gives the answer you need (see above).

It's all about limiting lens aberrations as far as possible, really. When a lens designer does the design he or she assumes working object and image coordinates, with respect to the principal planes in the lens.

So, as Carlos points out, when you cross over the 1:1 magnification threshold (at which object and image distances are the same) you should reverse the lens, so that it then operates more closely aligned to the design's assumed coordinates.
07-15-2020, 06:19 PM   #11
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Various size reversal rings are readily available from EBAY here in the USA. Many are made in China.

I often use a 36 mm extension ring on a 300 mm lens to get 'close up' photos of wasp, bees etc. Allows a little more distance from stinging insects.

Phil

Last edited by PWhite214; 07-15-2020 at 06:23 PM. Reason: added another idea
07-15-2020, 11:40 PM   #12
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QuoteOriginally posted by From1980 Quote
Lens aberrations, and curvature of field in particular, can be much exaggerated at higher than normal magnifications while focussing closer than the helicoid allows for in most consumer camera lenses. Average lenses are usually corrected to about 1/10 lifesize, that is, a minimum focus of about 10 times the lens' focal length. These modern lenses often have a larger entrance pupil (front element) than exit pupil (rear element). When reversed the optical configuration allows the rear of the lens designed to best focus on a flat field (your sensor or film plane) to face the subject and the relative distances of much smaller rear element to focal plane distance compared to a greater subject to front element distance is restored. At higher magnification the Some lenses will result in a marked improvement used this way. Older style optics, macros, and adapted enlarger lenses may not show a difference because they are usually using a symmetrical design where the elements are configured the same back to front as front to back.
I'm guessing that's the answer I was looking for. Thank you, but also thank you all for your time and answers!
07-16-2020, 03:51 AM   #13
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if the OP hasn't found this yet, it might be wise to review

QuoteQuote:
The Advantages of a Dedicated Macro Lens
Achieving lift-size magnification
By PF Staff in Tutorial Videos on Apr 4, 2016

Read more at: The Advantages of a Dedicated Macro Lens - Tutorial Videos | PentaxForums.com

_________________

as others far more knowledgeable than I in Macro have said, the question may be what degree of " macro " is the OP seeking 1:1 or more or less

the info in the lens' review gives details on maximum magnification possible of each lens:

QuoteQuote:
Max. Magnification 1x
SMC Pentax-D FA 100mm F2.8 Macro WR Reviews - D FA Prime Lenses - Pentax Lens Reviews & Lens Database

QuoteQuote:
Max. Magnification 0.15x
https://www.pentaxforums.com/lensreviews/SMC-Pentax-M-50mm-F1.7-Lens.html

Last edited by aslyfox; 07-16-2020 at 04:01 AM.
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