Lens mount adaptors

Liney · 11-10-2019, 02:06 AM

I was musing this afternoon while going to the green waste tip (don't worry, the wife was driving...} and I had a thought which I hope someone can answer for me. It's a bit of an optics / physics question but I look forward to the responses.

I'd been looking at second hand lenses recently and found an advert for lens mount adaptors. Just to clarify there are the things that look like very thin lenses with no glass in that fit between the lens and the camera body.

My understanding is that a lens focuses so that the image is clear and sharp on the mirror and then the sensor. if I fit a lens adaptor between the lens and the camera body, then I assume that the lens will still focus on the mirror / sensor even with the extended distance from the lens to the mirror / sensor, but will the size of the image change?

Let me put that another way, when the image has been focused by the lens, is the top / bottom and left / right edge of the image parallel? If not parallel then my assumption is that the increased distance between lens and sensor will mean the image will increase or decrease in size?

Sandy Hancock · 11-10-2019, 02:17 AM - **1 Like**

It sounds like you are describing one of two things.

1) extension tubes. They allow a lens to be used for extremely close focus for macro photography, but you lose the ability to focus on more distant objects.

2) the other possibility is that they are adaptors for larger format lenses to a smaller format body. I have one for 645 lenses on a K-mount body, and one for K-mount lenses on a Q-mount body. These are designed to provide the correct registration distance to allow infinity focus - so you can essentially use the lens over its full focal range, but only a small part of its image circle.

Bertrand3000 · 11-10-2019, 02:19 AM - **1 Like**

It all depends of flange distance, that is the distance expected between the lens mount and the sensor. For each mount (K is one), there is a well defined distance flange. Flange focal distance - Wikipedia

When you use an adapter (without glass), you add to the distance flange. It is not a problem when the lens you try to adapt expect a larger distance flange than the camera is expecting. For example, you can adapt a Nikon F lens (46.50mm flange distance) to Pentax K camera (45.46mm) without limitation (not counting automatism) as long as the adapter is not larger than 1.04mm.

If the lens + adapter (with no glass) has a distance flange greater than the one expected by the camera, the main problem is that you will loose the ability to focus to infinity. That's why such adapter usually uses glass to allow focusing to infinity, but adding an optical element can add their own problem (flare, chromatic abberrations, ...).

zapp · 11-10-2019, 03:30 AM - **1 Like**

It is the whole idea of a mount adaptor to align a lens of different mount parallel to the sensor plane. The lens defines angle of view and image circle. A small format lens with small image circle and too short flange distance will not fit at all.
Just make sure that the adaptor spec sheet mentions focus to infinity and proud should be physically ok.

**PJ1** · 11-10-2019, 03:33 AM - **1 Like**

If a lens adapter is properly engineered (and most of them are) it will focus from its normal short focus to infinity without issues. Unfortunately, there are some cheap M42 to K mount adapters that have a flange. It may only be about 1mm thick, but that flange increases the registration distance and you lose infinity focus. The nature of the helicoid means that a lot of focusing distance is covered by the final 1mm or so of travel provided by the helicoid thread. The registration distances for Nikon and K mount were mentioned above. As far as I know you cannot get an adapter for Nikon F to K mount, not just because there is only 1.04mm of distance to play with, but because the throat size of the Nikon mount doesn't allow you to adapt to K mount. There is a system (the Leitax adapter) which allows the mounts to be swapped on some Nikon lenses, but that is a fairly expensive way to go. With Canon EOS lenses the problem is that the EOS registration distance (44mm) is shorter than K mount. Therefore, to get infinity focus, the rear of the lens would end up inside the camera body (that is presuming that a K mount system could be grafted on to it. There are Nikon F adapters which are like an extension tube with a glass element. Obviously, such adapters will change the character of the lens (including the effective focal length).
To give you a direct answer to the question about using an M42 to K mount adapter - if it is properly engineered (like the Pentax proprietary adapter, although I have cheap ones that work just fine) there is no change in registration distance and really you wouldn't know you were not using a K mount lens in full manual mode. I also have a couple of Pentacon 6 mounts to K mount adapters that I use on old Zeiss glass. I use them without even having to think that they are not native K mount. I hope this satisfies some of your curiousity.

Wasp · 11-10-2019, 04:24 AM - **1 Like**

Regarding Pentax K mount bodies, about the only easy answer is the M42 mount. There are also adapters for medium format lenses, e.g. Pentax 645, Pentax 6x7, Hasselblad V etc. These only really make sense if you already have one of these lenses. Some of the adapters can do the tilt/shift thing, which looks like fun. These particular adapters don't come cheap, though.

There are adapters available to adapt a wide range lenses to Canon EF or EF-S mount bodies. In one of life's ironies, the old Canon FD mount from the film era is not among them. Don't even think about K mount lenses on Canon full frame bodies. The mirror will strike the aperture lever of the lens. Yes, you can snap off or grind down that lever, but the lens will be fully never functional on a Pentax body ever again.That is too ghastly to contemplate. Like I said, don't even think about it. AFAIK crop frame bodies are OK in this respect but I haven't fact checked myself on that.

Nikon bodies have one of the longest flange distances in the business. Adapting lenses is therefore not big with Nikonians.

If you have a mirrorless body, the world is your oyster. The much reduced flange distance means that any lens made for any 35mm film SLR can be made to fit any modern MILC. Any exceptions to this is more likely to be due to low demand than technical limitations.

Finally, check out Leitax. They manufacture a wide variety of adapter kits to facilitate cross breeding. This is a semi-permanent mod. For example, you remove the Nikon F mount and fit a Pentax K mount. You then have a Nikon lens that fits Pentax bodies. In most cases the mod can be undone, but it is not quick or easy.

**UncleVanya** · 11-10-2019, 07:12 AM - **1 Like**

The answers above have probably clarified this but I will try in my own words as well.

If the tube is one without lenses as you describe, then it is either an adapter or an extension tube. The former allows lenses to be mounted to the camera that weren't designed to fit it. The ideal situation is one where the original flange focal distance of the adapted lens is longer than the original lenses that fit the camera. By inserting the tube with mount adapters the adapted lens is positioned at the correct (longer than the norm for the body's native lenses) distance to achieve infinity focus.

There are times people do not use an adapter that adjusts the distance to the correct distances. These still permit the lens to work, but due to limitations of the two systems, there may be insufficient distance to adapt the lens - either because the correct distance is smaller than the body is designed for, or because it is too similar so that by the time the mounts and a small bit of time are put together the adapter fails to be short enough. In these cases the adapter acts like a small extension tube limiting the lens to shorter distance shots just like an extension tube would. A common example of this is a flanged m42 adapter for K mount bodies. The flanged adapter pushes the lens out a few mm which makes infinity focus impossible since the m42 optical distance required is the same as K mount. The normal m42 adapter sits inside the bayonet mount allowing the lens to sit at the correct distance it is designed for.

Using adapters on many mirrorless digital cameras is easier than most slr style cameras since the native lenses have short flange focal distances. This short native mount distance leaves room for adapters of various length needed to position adapted lenses at their original distances to permit infinity focus.

Wasp · 11-10-2019, 07:59 AM - **1 Like**

This is an adapter to put a K mount on an EF body. This one has a chip to fool the body into thinking that a lens is mounted, hence the contacts. This means that focus confimartion on the camera will work.

As can be seen, it is not very thick. There isn't a whole lot of room in the flange distance differences between brands.

**othar** · 11-10-2019, 09:43 AM - **1 Like**

Originally posted by Liney

My understanding is that a lens focuses so that the image is clear and sharp on the mirror and then the sensor. if I fit a lens adaptor between the lens and the camera body, then I assume that the lens will still focus on the mirror / sensor even with the extended distance from the lens to the mirror / sensor, but will the size of the image change?

A lens can only produce sharp images for one distance -> the distance from the lens to the sensor just happens to be the same as from the lens to the matt screen/focusing screen

When increasing the distance between lens and camera (with extension tubes for instance) you will be able to focus on closer objects but loose infinity focus instead, the only exception beeing tele converters because they have an additional lens element to correct this shortsightness of the lens.

**UncleVanya** · 11-10-2019, 10:50 AM - **1 Like**

Originally posted by othar

A lens can only produce sharp images for one distance -> the distance from the lens to the sensor just happens to be the same as from the lens to the matt screen/focusing screen

When increasing the distance between lens and camera (with extension tubes for instance) you will be able to focus on closer objects but loose infinity focus instead, the only exception beeing tele converters because they have an additional lens element to correct this shortsightness of the lens.

Small correction: it can provide sharp images from many distances but not over the full focusing range. Extension tubes work for example but you lose distance focus.

**othar** · 11-10-2019, 01:43 PM - **1 Like**

Originally posted by UncleVanya

Small correction: it can provide sharp images from many distances but not over the full focusing range.

I kind of see your point, but not quite. Let's try it this way:
The mirror is just redirecting the light passing through the lens in the camera to the focusing screen (on the top of the mirror box) but there won't be a sharp image on the mirror plane itself. The distance camera mount to the sensor is the same as the sum of the distances from the mount to the mirror plus the distance mirror to matt screen.

Originally posted by UncleVanya

Extension tubes work for example but you lose distance focus.

I did mention that in my second sentence.

y0chang · 11-10-2019, 02:09 PM - **1 Like**

Every mount has a different lens distance where the image is formed, this is called the lens registration distance. Some adapter required optics to focus the image to the correct mount registration distance,while others just need a spacer. This depends on the difference in distance being adapted. There are also other speedbooster adapter that act as reverse teleconverters that actually shrink the image to adapt larger format lenses to smaller formats. These mounts always have glass in them and may or may not degrade the image depending on the lens combination with the speedbooster.

**UncleVanya** · 11-10-2019, 02:34 PM - **1 Like**

Originally posted by othar

I kind of see your point, but not quite. Let's try it this way:
The mirror is just redirecting the light passing through the lens in the camera to the focusing screen (on the top of the mirror box) but there won't be a sharp image on the mirror plane itself. The distance camera mount to the sensor is the same as the sum of the distances from the mount to the mirror plus the distance mirror to matt screen.

I did mention that in my second sentence.

I just misread you I think.

Liney · 11-12-2019, 02:40 AM

Well thank you all for your responses, very educational. I own and use extension tubes so I was aware of the effect of those (focusing to infinity is something I tend to get over by changing the distance to the object, with so shallow a DoF I'm more concerned with getting anything in focus!}.

I must admit when I asked the question my focus (if you excuse the pun} was around image size rather than ficus, but I can see now that it will be at least a consideration I have to be aware of.

I have been tempted in the past to pick up some cheap lenses which may not originally be suited to a K mount, but have shied away due to nto being aware of issue around mount adaptors. I can see now that it may well be feasible but I need to be mindful of a couple of things.

Thanks agin for your time

**UncleVanya** · 11-12-2019, 07:36 AM - **1 Like**

Originally posted by Liney

Well thank you all for your responses, very educational. I own and use extension tubes so I was aware of the effect of those (focusing to infinity is something I tend to get over by changing the distance to the object, with so shallow a DoF I'm more concerned with getting anything in focus!}.

I must admit when I asked the question my focus (if you excuse the pun} was around image size rather than ficus, but I can see now that it will be at least a consideration I have to be aware of.

I have been tempted in the past to pick up some cheap lenses which may not originally be suited to a K mount, but have shied away due to nto being aware of issue around mount adaptors. I can see now that it may well be feasible but I need to be mindful of a couple of things.

Thanks agin for your time

Pentax has a fairly long flange focal distance which makes adapting other lenses complicated. Nikon for example is slightly longer but not long enough for a traditional adapter.

11-10-2019, 02:06 AM	#1
Liney Veteran Member Join Date: Apr 2013 Posts: 1,237	Lens mount adaptors I was musing this afternoon while going to the green waste tip (don't worry, the wife was driving...} and I had a thought which I hope someone can answer for me. It's a bit of an optics / physics question but I look forward to the responses. I'd been looking at second hand lenses recently and found an advert for lens mount adaptors. Just to clarify there are the things that look like very thin lenses with no glass in that fit between the lens and the camera body. My understanding is that a lens focuses so that the image is clear and sharp on the mirror and then the sensor. if I fit a lens adaptor between the lens and the camera body, then I assume that the lens will still focus on the mirror / sensor even with the extended distance from the lens to the mirror / sensor, but will the size of the image change? Let me put that another way, when the image has been focused by the lens, is the top / bottom and left / right edge of the image parallel? If not parallel then my assumption is that the increased distance between lens and sensor will mean the image will increase or decrease in size?

11-10-2019, 04:24 AM - 1 Like	#6
Wasp Pentaxian Join Date: Mar 2017 Location: Pretoria Photos: Gallery Posts: 4,660	Regarding Pentax K mount bodies, about the only easy answer is the M42 mount. There are also adapters for medium format lenses, e.g. Pentax 645, Pentax 6x7, Hasselblad V etc. These only really make sense if you already have one of these lenses. Some of the adapters can do the tilt/shift thing, which looks like fun. These particular adapters don't come cheap, though. There are adapters available to adapt a wide range lenses to Canon EF or EF-S mount bodies. In one of life's ironies, the old Canon FD mount from the film era is not among them. Don't even think about K mount lenses on Canon full frame bodies. The mirror will strike the aperture lever of the lens. Yes, you can snap off or grind down that lever, but the lens will be fully never functional on a Pentax body ever again.That is too ghastly to contemplate. Like I said, don't even think about it. AFAIK crop frame bodies are OK in this respect but I haven't fact checked myself on that. Nikon bodies have one of the longest flange distances in the business. Adapting lenses is therefore not big with Nikonians. If you have a mirrorless body, the world is your oyster. The much reduced flange distance means that any lens made for any 35mm film SLR can be made to fit any modern MILC. Any exceptions to this is more likely to be due to low demand than technical limitations. Finally, check out Leitax. They manufacture a wide variety of adapter kits to facilitate cross breeding. This is a semi-permanent mod. For example, you remove the Nikon F mount and fit a Pentax K mount. You then have a Nikon lens that fits Pentax bodies. In most cases the mod can be undone, but it is not quick or easy. Last edited by Wasp; 11-10-2019 at 04:30 AM.

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11-10-2019, 02:17 AM - 1 Like	#2
Sandy Hancock Moderator Site Supporter Join Date: Feb 2012 Location: Adelaide Hills, South Australia Photos: Gallery \| Albums Posts: 11,272	It sounds like you are describing one of two things. 1) extension tubes. They allow a lens to be used for extremely close focus for macro photography, but you lose the ability to focus on more distant objects. 2) the other possibility is that they are adaptors for larger format lenses to a smaller format body. I have one for 645 lenses on a K-mount body, and one for K-mount lenses on a Q-mount body. These are designed to provide the correct registration distance to allow infinity focus - so you can essentially use the lens over its full focal range, but only a small part of its image circle.

11-10-2019, 02:19 AM - 1 Like	#3
Bertrand3000 Veteran Member Join Date: Feb 2019 Posts: 2,009	It all depends of flange distance, that is the distance expected between the lens mount and the sensor. For each mount (K is one), there is a well defined distance flange. Flange focal distance - Wikipedia When you use an adapter (without glass), you add to the distance flange. It is not a problem when the lens you try to adapt expect a larger distance flange than the camera is expecting. For example, you can adapt a Nikon F lens (46.50mm flange distance) to Pentax K camera (45.46mm) without limitation (not counting automatism) as long as the adapter is not larger than 1.04mm. If the lens + adapter (with no glass) has a distance flange greater than the one expected by the camera, the main problem is that you will loose the ability to focus to infinity. That's why such adapter usually uses glass to allow focusing to infinity, but adding an optical element can add their own problem (flare, chromatic abberrations, ...).

11-10-2019, 03:30 AM - 1 Like	#4
zapp Pentaxian Join Date: Jul 2011 Posts: 2,385	It is the whole idea of a mount adaptor to align a lens of different mount parallel to the sensor plane. The lens defines angle of view and image circle. A small format lens with small image circle and too short flange distance will not fit at all. Just make sure that the adaptor spec sheet mentions focus to infinity and proud should be physically ok.

11-10-2019, 03:33 AM - 1 Like	#5
PJ1 Loyal Site Supporter Join Date: Jul 2012 Location: Toowoomba, Australia Photos: Gallery \| Albums Posts: 5,484	If a lens adapter is properly engineered (and most of them are) it will focus from its normal short focus to infinity without issues. Unfortunately, there are some cheap M42 to K mount adapters that have a flange. It may only be about 1mm thick, but that flange increases the registration distance and you lose infinity focus. The nature of the helicoid means that a lot of focusing distance is covered by the final 1mm or so of travel provided by the helicoid thread. The registration distances for Nikon and K mount were mentioned above. As far as I know you cannot get an adapter for Nikon F to K mount, not just because there is only 1.04mm of distance to play with, but because the throat size of the Nikon mount doesn't allow you to adapt to K mount. There is a system (the Leitax adapter) which allows the mounts to be swapped on some Nikon lenses, but that is a fairly expensive way to go. With Canon EOS lenses the problem is that the EOS registration distance (44mm) is shorter than K mount. Therefore, to get infinity focus, the rear of the lens would end up inside the camera body (that is presuming that a K mount system could be grafted on to it. There are Nikon F adapters which are like an extension tube with a glass element. Obviously, such adapters will change the character of the lens (including the effective focal length). To give you a direct answer to the question about using an M42 to K mount adapter - if it is properly engineered (like the Pentax proprietary adapter, although I have cheap ones that work just fine) there is no change in registration distance and really you wouldn't know you were not using a K mount lens in full manual mode. I also have a couple of Pentacon 6 mounts to K mount adapters that I use on old Zeiss glass. I use them without even having to think that they are not native K mount. I hope this satisfies some of your curiousity.

11-10-2019, 07:12 AM - 1 Like	#7
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	The answers above have probably clarified this but I will try in my own words as well. If the tube is one without lenses as you describe, then it is either an adapter or an extension tube. The former allows lenses to be mounted to the camera that weren't designed to fit it. The ideal situation is one where the original flange focal distance of the adapted lens is longer than the original lenses that fit the camera. By inserting the tube with mount adapters the adapted lens is positioned at the correct (longer than the norm for the body's native lenses) distance to achieve infinity focus. There are times people do not use an adapter that adjusts the distance to the correct distances. These still permit the lens to work, but due to limitations of the two systems, there may be insufficient distance to adapt the lens - either because the correct distance is smaller than the body is designed for, or because it is too similar so that by the time the mounts and a small bit of time are put together the adapter fails to be short enough. In these cases the adapter acts like a small extension tube limiting the lens to shorter distance shots just like an extension tube would. A common example of this is a flanged m42 adapter for K mount bodies. The flanged adapter pushes the lens out a few mm which makes infinity focus impossible since the m42 optical distance required is the same as K mount. The normal m42 adapter sits inside the bayonet mount allowing the lens to sit at the correct distance it is designed for. Using adapters on many mirrorless digital cameras is easier than most slr style cameras since the native lenses have short flange focal distances. This short native mount distance leaves room for adapters of various length needed to position adapted lenses at their original distances to permit infinity focus.

11-10-2019, 07:59 AM - 1 Like	#8
Wasp Pentaxian Join Date: Mar 2017 Location: Pretoria Photos: Gallery Posts: 4,660	This is an adapter to put a K mount on an EF body. This one has a chip to fool the body into thinking that a lens is mounted, hence the contacts. This means that focus confimartion on the camera will work. As can be seen, it is not very thick. There isn't a whole lot of room in the flange distance differences between brands.

11-10-2019, 09:43 AM - 1 Like	#9
othar Site Supporter Join Date: May 2014 Location: Linz Photos: Albums Posts: 3,098	Originally posted by Liney My understanding is that a lens focuses so that the image is clear and sharp on the mirror and then the sensor. if I fit a lens adaptor between the lens and the camera body, then I assume that the lens will still focus on the mirror / sensor even with the extended distance from the lens to the mirror / sensor, but will the size of the image change? A lens can only produce sharp images for one distance -> the distance from the lens to the sensor just happens to be the same as from the lens to the matt screen/focusing screen When increasing the distance between lens and camera (with extension tubes for instance) you will be able to focus on closer objects but loose infinity focus instead, the only exception beeing tele converters because they have an additional lens element to correct this shortsightness of the lens.

11-10-2019, 10:50 AM - 1 Like	#10
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by othar A lens can only produce sharp images for one distance -> the distance from the lens to the sensor just happens to be the same as from the lens to the matt screen/focusing screen When increasing the distance between lens and camera (with extension tubes for instance) you will be able to focus on closer objects but loose infinity focus instead, the only exception beeing tele converters because they have an additional lens element to correct this shortsightness of the lens. Small correction: it can provide sharp images from many distances but not over the full focusing range. Extension tubes work for example but you lose distance focus.

11-10-2019, 01:43 PM - 1 Like	#11
othar Site Supporter Join Date: May 2014 Location: Linz Photos: Albums Posts: 3,098	Originally posted by UncleVanya Small correction: it can provide sharp images from many distances but not over the full focusing range. I kind of see your point, but not quite. Let's try it this way: The mirror is just redirecting the light passing through the lens in the camera to the focusing screen (on the top of the mirror box) but there won't be a sharp image on the mirror plane itself. The distance camera mount to the sensor is the same as the sum of the distances from the mount to the mirror plus the distance mirror to matt screen. Originally posted by UncleVanya Extension tubes work for example but you lose distance focus. I did mention that in my second sentence.

11-10-2019, 02:09 PM - 1 Like	#12
y0chang Veteran Member Join Date: Jan 2012 Location: United States Posts: 793	Every mount has a different lens distance where the image is formed, this is called the lens registration distance. Some adapter required optics to focus the image to the correct mount registration distance,while others just need a spacer. This depends on the difference in distance being adapted. There are also other speedbooster adapter that act as reverse teleconverters that actually shrink the image to adapt larger format lenses to smaller formats. These mounts always have glass in them and may or may not degrade the image depending on the lens combination with the speedbooster.

11-10-2019, 02:34 PM - 1 Like	#13
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by othar I kind of see your point, but not quite. Let's try it this way: The mirror is just redirecting the light passing through the lens in the camera to the focusing screen (on the top of the mirror box) but there won't be a sharp image on the mirror plane itself. The distance camera mount to the sensor is the same as the sum of the distances from the mount to the mirror plus the distance mirror to matt screen. I did mention that in my second sentence. I just misread you I think.

11-12-2019, 02:40 AM	#14
Liney Veteran Member Join Date: Apr 2013 Posts: 1,237 Original Poster	Well thank you all for your responses, very educational. I own and use extension tubes so I was aware of the effect of those (focusing to infinity is something I tend to get over by changing the distance to the object, with so shallow a DoF I'm more concerned with getting anything in focus!}. I must admit when I asked the question my focus (if you excuse the pun} was around image size rather than ficus, but I can see now that it will be at least a consideration I have to be aware of. I have been tempted in the past to pick up some cheap lenses which may not originally be suited to a K mount, but have shied away due to nto being aware of issue around mount adaptors. I can see now that it may well be feasible but I need to be mindful of a couple of things. Thanks agin for your time

11-12-2019, 07:36 AM - 1 Like	#15
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by Liney Well thank you all for your responses, very educational. I own and use extension tubes so I was aware of the effect of those (focusing to infinity is something I tend to get over by changing the distance to the object, with so shallow a DoF I'm more concerned with getting anything in focus!}. I must admit when I asked the question my focus (if you excuse the pun} was around image size rather than ficus, but I can see now that it will be at least a consideration I have to be aware of. I have been tempted in the past to pick up some cheap lenses which may not originally be suited to a K mount, but have shied away due to nto being aware of issue around mount adaptors. I can see now that it may well be feasible but I need to be mindful of a couple of things. Thanks agin for your time Pentax has a fairly long flange focal distance which makes adapting other lenses complicated. Nikon for example is slightly longer but not long enough for a traditional adapter.