FFD on my Completely 3d Printed 16mm Film Camera - Looking for advice

[Henry kidman]

Hey guys, 

Firstly, I have only shot on film once, and I don’t own a camera, so forgive me if my terminology is not perfect! 

 

Im sorry if this is the wrong place to post this. im in the process of designing and building my own 16mm film camera. I have the transport reasonably sorted (ignore the take up spool spinning in the wrong direction and not at the correct speed, still need to code that part out), however I still want to refine the pressure plate and add a spring on the rail for lateral support. 

 

However, at the moment I’m trying to dial in the flange focal distance. I’m using a Sony e mount on the camera so it has a very short FFD of 18mm. I understand that the depth should be set a slightly short of that so it ends up focusing just past the emulsion. After reading, I’m thinking I get a depth micrometer and put a gauge block against the gate then take the measurement that way. Is a depth micrometer that does 0.01 mm fine enough or do I need to be investing in a 0.001mm? Is the depth micrometer the best way? How do actual camera techs do it?

 

Also if you have any other tips for the camera please feel free to let me know!! I will link a little video of it running - 

 

Thank you in advance:) 

Henry

 

[Tyler Purcell]

So 3D printed material, no matter what it is, will not have the tolerances for proper FFD, never going to happen. Manufacturers spend an ungodly amount of time engineering the gate to flange distance and making it perfect. The gate and lens mount, really need to all be made of metal and somehow connected. 

So the tolerances are based directly on film channel/float. Some cameras have a pretty large film channel, this is the gap between the pressure plate and aperture plate. Metal gates are used because you can polish the side rails and the film can be at the same depth as the physical aperture. The pressure plate, shouldn't actually be putting pressure on the film itself, it should be very smooth but held firmly in place by the gate laterally to prevent wobble. 

Yea, you will need gauges that go down to .01, as the FFD range is .00-.03mm, depending on how much float the film has. Some cameras can be run at .03 and not be a big deal, others need to be spot on .00 and that's the tricky part. Engineering the pulldown, the consistency with the FFD across the frame, film channel, a spring loaded side rail gate, timing with the shutter, all of these things are very challenging to get right and I'm afraid, no way a 3D printed camera would actually create proper images. Someone tried it with a 2 perf 35mm 3D printed camera and it was basically unusable. The tolerances are incredible, .01mm off and you go from working to not working. As someone who services film cameras for a living, it's a miracle any motion picture camera works at all. 

I commend the work tho, I'm glad to see you playing around with it. I'd love to see if any images come out. 

 

[Henry kidman]

Hey Tyler, 

thanks so much for your input. Yeah the timing of the shutter with the film movement mechanism was a little bit of a headache, but it now works in such a way that whilst they aren’t mechanically linked as each has its own stepper motor, the film advance sprocket waits until the motor has completed a specific amount of its rotation before it will move, this way it’s not relaying on perfect time keeping or complex mechanical links. 
 

yes I did fear the ffd would be the part of this that stumps me. With that being said, I have ordered a depth micromiter so I’m just going to try to get it dialled as best I can. If you were servicing a camera and the ffd was out, how would you adjust it? I’m sure that varies from camera to camera but is it a shimming set up, or are you adjusting set screws? 
 

and as for the pressure plate its interesting you say it’s not meant to apply any pressure on the film it’s self. So should I be making a tiny shelf that the pressure plate pushes against that is 0.15mm above the height of the gate so the film can just fit through the gap? 
 

One final question, if I were to purchase a piece of ground glass and place it on the film gate to be able to visually check focus while I’m adjusting the ffd, would the result be indicative of the result when using film as far as focus goes? 

[Aapo Lettinen]

1 hour ago, Henry kidman said:

Hey Tyler, 

thanks so much for your input. Yeah the timing of the shutter with the film movement mechanism was a little bit of a headache, but it now works in such a way that whilst they aren’t mechanically linked as each has its own stepper motor, the film advance sprocket waits until the motor has completed a specific amount of its rotation before it will move, this way it’s not relaying on perfect time keeping or complex mechanical links. 
 

yes I did fear the ffd would be the part of this that stumps me. With that being said, I have ordered a depth micromiter so I’m just going to try to get it dialled as best I can. If you were servicing a camera and the ffd was out, how would you adjust it? I’m sure that varies from camera to camera but is it a shimming set up, or are you adjusting set screws? 
 

and as for the pressure plate its interesting you say it’s not meant to apply any pressure on the film it’s self. So should I be making a tiny shelf that the pressure plate pushes against that is 0.15mm above the height of the gate so the film can just fit through the gap? 
 

One final question, if I were to purchase a piece of ground glass and place it on the film gate to be able to visually check focus while I’m adjusting the ffd, would the result be indicative of the result when using film as far as focus goes? 

There has been lots of discussion here on the forums about the limitations of materials and 3d printing and electronic vs mechanical links. 

3d printing is just not a good technology to make every and all parts of a movie camera, and electronic links are not good idea if one needs high precision, high speed and reliability at the same time. 

3d printing out of metal like stainless steel is possible but cnc machining is cheaper on most type of parts with potentially better end results. Compared to 3d printed plastic, ANY old camera including the cheapest Soviet models has better quality film transport and gate than a 3d printed plastic one would be.

Fully 3d printing a camera out of plastic does not make sense, it will be a sub par product whatever you do and any 50 bucks old camera will blow it out of the water image quality wise. I think you have confined yourself inside a "I must make this entirely with 3d printing" box which limits your engineering process too much and you will never make a good working camera for actual filming unless changing some goals here. Allow yourself to use proper metal for dimensionally critical parts and a mechanical link for movement for example. You will get much better camera that way. Stainless steel, aluminium, brass, spring steel in selected critical applications. Then rest can be 3d printed if necessary. But allow yourself to consider on every single component: does this need to be metal? Can it be printed or is cnc machining and polishing and coating better option? How much each option affects the price and reliability? 

I would highly recommend getting some old movie cameras for evaluation how they did things. And if you find a good working used part just scavenging it from old camera instead of making basic stuff like rollers and sprocket wheels and gears from scratch 🙂

[Henry kidman]

Hey Aapo,

Yes very good points. And I would agree, cameras made by companies with budget, knowledge, talent and machines that far surpass my own will obviously be better, however that isn’t really the point. I want to make one my self. A modern digital camera is objectively cleaner and more practical in every way than a 16mm camera, and yet here you are. There is a charm about the process and the imperfections.

the 3d printing constraint isn’t some arbitrary challenge I have given my self, it’s out of necessity. I sadly don’t have access to a cnc machine, or the plethora of required supporting tools and knowledge, but I do have a 3d printer. Almost everything in the camera would benefit from being metal, but I’m curious how far I can make it with what I have.
 

Borrowing parts from old cameras is a good idea, specifically parts like the sprockets where dimensions are very important, however, I have done the work to get these sprockets of my own to fit properly so I’m pretty happy with them now!  And once again, I’m stubborn, I want to do it my self!! 
 

I would also have to disagree with electrical connections being less accurate than mechanical connections. We are constantly seeing mechanical links being swapped for electrical, think aircraft flight controls, throttles, electronic digital shutters. they offer many advantages, cost saving, accuracy and most importantly simplicity - fewer moving parts means less compounding play through small tolerance inaccuracy's.

I also think 3d printing might surprise you now, it has come a long way, and whilst far from having the dimensional properties of metal, it’s pretty cool to see how affordable it has become to be able to print things with a resolution of 0.017 mm. 
 

if you have any tips on measuring FFD, let me know!! 
 

cheers, 

henry 

[Aapo Lettinen]

1 hour ago, Henry kidman said:

the 3d printing constraint isn’t some arbitrary challenge I have given my self, it’s out of necessity. I sadly don’t have access to a cnc machine, or the plethora of required supporting tools and knowledge, but I do have a 3d printer. Almost everything in the camera would benefit from being metal, but I’m curious how far I can make it with what I have.
-----

if you have any tips on measuring FFD, let me know!!

Most of other people don't have personal access to high quality cnc machines either. We design the parts so that the first test design can be 3d printed to see if it is approximately correct design. Then when the design is finessed enough, it is time to look for a outside company to do a test piece from the file for reasonable price. When the test piece arrives and has been thoroughly evaluated, it is possible to decide if the design needs some final touches or if it is already possible to order the final parts from the outside company, machined to final tolerances with final surface finish.

it is not necessary to purchase a 300k+ priced cnc machine when one can hire a company to make the part for you from the step file. Just design the part so that it is possible to cnc machine and then quote from various places how much it would cost to machine it out of aluminium or steel etc.

you can check for example pcbway and jlcpcb for cnc quotes if can't get good quotes from AU.

 

the ffd needs to just be calculated close enough when designing parts, then manually tuning when fitting. and when almost there, sand down and polish the shim until you get it perfect. you may need to make your own ffd gauge if there is no readily made available. the gauge would be a good project to test outsourced cnc machining, you need to make it out of metal. get a suitable dial gauge and design the rest of the parts by yourself

 

[Aapo Lettinen]

Here is an  example of stuff I made last year. the first test pieces are 3d printed to save costs, then when the design is final I will check various places how much they charge for final one made. The first print is home printed from pla and not sturdy enough for use but works for optical testing. The final part is cnc machined from stainless steel by pcbway with basic surface finish. the final part cost a little over 100usd with shipping costs to make which is why I had to be sure the dimensions were at least close to correct ones, thus made multiple home 3d prints to finesse it (prints cost maybe from 1 to 3 usd a piece so one saves a lot if can find basic errors with printing before ordering 100usd+ final parts made out of metal)

The camera handle made the same way btw, it is cnc machined aluminum. I was so sure about the design that did not bother to 3d print a test piece. the handle has 1/4" and 3/8" threads for mounting accessories. I think it was something around 100usd with shipping costs, made by jlcpcb. black anodizing too

Edited April 21 by Aapo Lettinen

[Aapo Lettinen]

personally I would make the gate from stainless steel, the pulldown claw spring steel, pulldown axles some kind of steel which is available, the pressure plate aluminium, the shutter aluminium. All of these can be cnc machined.

then the main support structure connecting the gate and pulldown+shutter assembly to the lens mount would be aluminium on basic designs and stainless steel on very high quality designs. I would use cnc machined aluminium on this one for the much lower costs. Basically it would be a kind of U-shaped structure which seamlessly connects the gate and lens mount together to control the ffd precisely. a shim under the lens mount is used to fine tune the distance. you can get the shim cnc made too though would be cheaper to use some available Chinese shims from lens adapters and just design the mount so that they can be used.

film transport depends, the rollers are perfectly fine if plastic. the sprocket wheel depends but often it can be plastic if the sprockets themselves are metal. for example a metal ring with sprockets which is attached on a plastic roller to form a full sprocket assembly.

All the other parts of the body which don't have high durability and / or dimensional demands are made out of plastic. on low quantity it is economic to 3d print. if making like 20 cameras it is often more cost effective and higher quality to cnc machine the plastic body too if the shape is such that it is possible to machine

[Henry kidman]

That is very impressive! 
Who knows, it may come to the point that I need to send parts off to be done properly, however, the goals for this camera are very modest, it’s not trying to be better, or more practical then any of the other 16mm cameras available, it doesn’t have a view finder and it’s missing a lot of the other niceties. I just want it to take a nice clean image, and for it to be able to reasonably accurately focus. 
 

so we will see when I run some film through how far off I am from that. Depending on how that goes I might have to step off my high horse and get the parts machined!! 
 

thanks for the examples, they look very professional 

[Aapo Lettinen]

4 minutes ago, Henry kidman said:

That is very impressive! 
Who knows, it may come to the point that I need to send parts off to be done properly, however, the goals for this camera are very modest, it’s not trying to be better, or more practical then any of the other 16mm cameras available, it doesn’t have a view finder and it’s missing a lot of the other niceties. I just want it to take a nice clean image, and for it to be able to reasonably accurately focus. 
 

so we will see when I run some film through how far off I am from that. Depending on how that goes I might have to step off my high horse and get the parts machined!! 
 

thanks for the examples, they look very professional 

thanks! yes it is often better to machine stuff for final parts but 3d printing is very good technique for first prototypes and saves lots of costs on prototypes. You can work very efficiently by replacing one part at a time with cnc machined metal version when you are sure that part is final. I would first replace the gate and pulldown with metal ones so that you have possibility to get higher quality images to begin with. then can replace other parts later on one at a time when feeling necessary and have collected resources for it 🙂

cnc machining has some limitations with sharp corners and one may need to have additional finishing ordered for parts which for example need sharp cornered hole cut. For example it is possible to machine the film gate with rounded corners on the gate opening but if needing to have as sharp corners as possible one may need to order additional wire saw or EDM finishing to get them as small radius as possible. machining cavities has some limitations too.  For the gate I would use stainless steel or chromed brass if steel is too expensive. the surface needs to be smooth and polished on rails but other parts are possible to leave a bit rough if they are dimensionally accurate. the Soviets only polished the parts which touched film surface and other as left unfinished... most Western cameras are finished all the way through.

with cnc and metal it is easy to get high quality threads on the parts which is often important when assembling stuff.

top:  Eclair NPR 

bottom: Kinor 16cx-2m

[Dom Jaeger]

Hi Henry, I’m a Melbourne based cine camera and lens technician, roughly 30 years experience. There are not many movie cameras I haven’t worked on over the years, though some I’m much more familiar with than others. Great to see Aussie ingenuity and curiosity at work, good on you! 

While I agree with other comments here that 3D printing isn’t really suitable for the critical parts of a movie camera, if you’re just experimenting and playing around with what you have, then go for it. Nobody invented anything by listening to people who told them something can’t be done. 

To measure flange depth we use a depth gauge with 0.01mm accuracy, along with a backing block that is placed in the gate. We also use a cylinder blank accurately machined to the nominal flange depth in order to zero the depth gauge. To adjust flange depth, we use shims of various thicknesses. They can be metal or plastic, so you could cut your own from plastic shim stock. 

While 35mm cine cameras often utilise a very precise film channel with a pressure plate that doesn’t press on the film, the pressure plates on most 16mm cameras actually do exert pressure. That might make it a bit easier to manufacture, though you need to get the pressure right. I would suggest looking at something like a Bolex, which uses a very simple mechanism that still produces quite stable images. 

I have manufactured my own S16 Bolex gates out of steel, which could possibly be of use to you. Or you could get some bits out of an old junker Bolex. You do need to make sure the lens mount is quite parallel to the gate, ideally within 0.01mm across the diameter. The flange depth should also ideally be within that tolerance, though all this accuracy is somewhat contingent on other factors, including your expectations. I don’t want to go down that rabbit hole yet however.. the question I have for you is, if you don’t have a viewfinder system, how do you plan to focus a lens? There have been a few systems invented over the years, the most common one before reflex viewing was a rack-over system, where the lens mount is shifted over to a ground glass and viewfinder for focussing, then shifted back to the gate for filming. 

If you ever come to Melbourne, I’d be happy to show you my service department and some of the tools and techniques we use for camera checks. 
 

 

[Tyler Purcell]

13 hours ago, Henry kidman said:

Hey Tyler, 

thanks so much for your input. Yeah the timing of the shutter with the film movement mechanism was a little bit of a headache, but it now works in such a way that whilst they aren’t mechanically linked as each has its own stepper motor, the film advance sprocket waits until the motor has completed a specific amount of its rotation before it will move, this way it’s not relaying on perfect time keeping or complex mechanical links. 
 

Using electronics to join multiple motors together is fine, but with 16mm cameras especially, you can't use a sprocket intermittent to pull the film down, you will need some sort of pulldown claw. A sprocket, won't work. The reason cameras use pulldown claws, is because they can have very tight tolerances. A sprocket has very bad tolerances, which would cause lots of up and down movement in the image. Even if you were to reduce that with a spring loaded side rail, it would still be an extremely unusable/unstable image. I know it runs through the camera ok, but those micro movements can't be seen by the naked eye. 

As Aapo suggested, the best thing to do is start with a movement from another camera. So that all your main components are made of metal already. I really like the ACL movement (minus the mirror) as it's mounted to a nice thick piece of metal, that you could easily find a front housing for, even perhaps an ACL one. To save even more money, re-working a K3 movement, is probably pretty straight forward. 

13 hours ago, Henry kidman said:

yes I did fear the ffd would be the part of this that stumps me. With that being said, I have ordered a depth micromiter so I’m just going to try to get it dialled as best I can. If you were servicing a camera and the ffd was out, how would you adjust it? I’m sure that varies from camera to camera but is it a shimming set up, or are you adjusting set screws? 

Shims yea, that's the way most cameras work. Arri has a few cameras that the entire movement can move back and forward but for the most part on 16mm cameras, it's done with shims between the lens mount and the body. 

13 hours ago, Henry kidman said:

and as for the pressure plate its interesting you say it’s not meant to apply any pressure on the film it’s self. So should I be making a tiny shelf that the pressure plate pushes against that is 0.15mm above the height of the gate so the film can just fit through the gap? 

The kit to measure would have a flat piece of metal for measuring that is the width of the 16mm gate. A FFD gauge, which usually has a flange that you would push onto the lens mount. Then a tester tool, so you can set the FFD gauge to zero before putting it onto the camera. FFD tools aren't horribly expensive and are available for sale, but the 16mm width flat piece of metal, maybe harder to find. I had to buy one from another tech. 

13 hours ago, Henry kidman said:

One final question, if I were to purchase a piece of ground glass and place it on the film gate to be able to visually check focus while I’m adjusting the ffd, would the result be indicative of the result when using film as far as focus goes? 

You can't use the ground glass method because it adds too much depth. A very thin piece of smoked paper can work on 35mm cameras to see if you're in the ballpark, because the image is so much bigger, but on 16mm cameras with that small of an image, it's just impossible with out proper FFD tools. No matter what, to "dial it in" you will 100% need the tools, there is nothing you can do about that. A collimator could be used theoretically, but they're more expensive. 

[Aapo Lettinen]

It is entirely possible to make a simple pulldown system from scratch. simplest cameras like Krasnogorsks use this style of very simple movement which should be pretty "easy" to make to work OK.

there is a cam driving the end of the pulldown claw up-down-towards gate -away from gate. The claw has a guide pin in the middle-ish which slides on guide rails, translating the movement from the cam to the desired type of "pull-the-film-down, retract and go back up, repeat" type of movement.

It is not perfect but it is simple and relatively easy to make to work well enough for home use. needs to be made out of proper metal though, no plastic can tolerate this use.

I quickly modeled some reference images, nothing is in scale and you may want another set of guide rails on top to add stability but the basic principle is easy to see.

 

[Henry kidman]

10 hours ago, Dom Jaeger said:

Hi Henry, I’m a Melbourne based cine camera and lens technician, roughly 30 years experience. There are not many movie cameras I haven’t worked on over the years, though some I’m much more familiar with than others. Great to see Aussie ingenuity and curiosity at work, good on you! 

While I agree with other comments here that 3D printing isn’t really suitable for the critical parts of a movie camera, if you’re just experimenting and playing around with what you have, then go for it. Nobody invented anything by listening to people who told them something can’t be done. 

To measure flange depth we use a depth gauge with 0.01mm accuracy, along with a backing block that is placed in the gate. We also use a cylinder blank accurately machined to the nominal flange depth in order to zero the depth gauge. To adjust flange depth, we use shims of various thicknesses. They can be metal or plastic, so you could cut your own from plastic shim stock. 

While 35mm cine cameras often utilise a very precise film channel with a pressure plate that doesn’t press on the film, the pressure plates on most 16mm cameras actually do exert pressure. That might make it a bit easier to manufacture, though you need to get the pressure right. I would suggest looking at something like a Bolex, which uses a very simple mechanism that still produces quite stable images. 

I have manufactured my own S16 Bolex gates out of steel, which could possibly be of use to you. Or you could get some bits out of an old junker Bolex. You do need to make sure the lens mount is quite parallel to the gate, ideally within 0.01mm across the diameter. The flange depth should also ideally be within that tolerance, though all this accuracy is somewhat contingent on other factors, including your expectations. I don’t want to go down that rabbit hole yet however.. the question I have for you is, if you don’t have a viewfinder system, how do you plan to focus a lens? There have been a few systems invented over the years, the most common one before reflex viewing was a rack-over system, where the lens mount is shifted over to a ground glass and viewfinder for focussing, then shifted back to the gate for filming. 

If you ever come to Melbourne, I’d be happy to show you my service department and some of the tools and techniques we use for camera checks. 
 

 

Hey Dom, 

Thank you so much for your response, and so cool that you are in Australia!

I just got my self a depth mic, a gauge block and a piece of granite for something flat so lets see how i go! Do you have a link to where i can find some blank plastic shim stock, I cant seem to find any,  just pieces that are already cut to shape.

Yes that was my understanding when i used the k3, that the pressure plate was pushing directly on the film. My current pressure plate set up is a big janky, however it does hold the film with just a little bit of force, so it may work. I'm using this sprung plunger design, which is a product of implementing it in the design last, and now having to work around things. Its biggest problem is that there is a little bit of lateral wobble in it which I'm sure is not ideal, if it is causing too much trouble i will do a proper re design.

Thats so cool that you have made gates out of steel, where do i go to have a look at these? did you machine them your self or have them sent off? 

Thats a good question about focusing. When i was doing research, many people were saying the view finders on the 16mm cameras (the more budget ones anyway) are not to be trusted for focus, so instead they suggested its best to use a tape measure and the markings on the lens. Whether this is true or not i don't know, however,  knowing that people do focus just using the marks on the lens gave me the idea to ditch the view finder as it was a lot of extra complexity.

I would love to have a look at your service department, I'm in Melbourne every couple of months for work so if you don't mind I might send you a message next time as it would be fascinating to see how you do it, and would definitely help me with my cam!!

Edited April 22 by Henry kidman
add photos

[Tyler Purcell]

6 hours ago, Aapo Lettinen said:

It is entirely possible to make a simple pulldown system from scratch. simplest cameras like Krasnogorsks use this style of very simple movement which should be pretty "easy" to make to work OK.

Yea, your drawings are basically similar to what I would do, even if it was 3D printed. One COULD theoretically just yank that assembly out of a K3, maybe cut the front of the K3 off and just power it by a motor, it maybe the easiest way to go about it. The nice big flange the movement is mounted to in the ACL, that's just grand. I really like that design and it's why I'm gonna be using it for my prototype. We still have to get the metal machines in, but with our current commander and fascist, there is probably no way we'll ever be able to afford them OR the raw materials anymore. We ordered a new style of 3D printer (at great expense) two months ago, but seeing as they're coming from China, I doubt we will see it in the next month or two. But theoretically it can print more accurate parts for prototyping that would allow us to make the thing 3D properly (with a copied movement) and then send it out for metal manufacturing. In the US however, there is very little metal prototyping going on, they only want contracts for big jobs. So we would have to send over to Asia, but now that we're cut off from them, I have no idea what to do. The entire project is sidelined. 

[Dom Jaeger]

Apart from a drawful of metal shim stock I've accumulated I've been using a book of Artus shim stock for about ten years, it goes from 0.01mm up to 0.75mm.. but I'm not sure where you'd get it now. This is part of the problem of trying to make something with very fine tolerances, you end up needing specialised tools and materials. There's this from RS Australia, but it only starts at 0.025mm and is rather expensive:

https://au.rs-online.com/web/p/shim-kits/0518778

Apart from ensuring the film is flat against the gate, a pressure plate also helps to keep it stable while the sprockets are still feeding the film through. The way movie cameras work is the sprockets are continuously driving the film, creating a fixed loop, while a pull-down mechanism creates the intermittent movement required for exposure and then moving to the next frame. If you don't have something like a registration pin you need another method to stabilise the film during exposure. Usually it's a combination of a pressure plate and a side spring (which is used to maintain lateral steadiness). If you're trying to use an intermittent sprocket drive to advance the film, I think you will find it both not accurate enough, and too aggressive on the film perfs. A small fixed loop is much less inertia to start and stop in such short time frames. But see how you go I guess.

I had my S16 gates made, they're quite accurate, but specific to the Bolex design. You could scavenge a standard 16 one from an old non-reflex Bolex pretty cheaply, and get the associated claw and side spring/lateral cage along with it. Or use parts from a K3  if you have one of those.

For focusing, if you rely only on lens marks and distance measurement then you need a few things to be right. You need the flange depth to be spot on, and you need the lens to be perfectly collimated. And if you want to use telephotos, then it becomes very hard to just use distance marks and still get a sharp image. It could work for wides and mediums, especially if you are stopped down a lot. If you are using stills lenses however, which are usually not very accurately marked with distances, and can have short focus throws, you might struggle to consistently get sharp focus. In the professional film industry where I work, crews do use distance marks, as well as focus aids and monitors, but the lenses are all thoroughly checked for mark accuracy, and they are proper cine lenses with long focus throws, many finely scaled distance marks and no internal backlash. 

You also have no way of knowing what you're framing without some kind of viewfinder. There have been different solutions to this problem used over the years, from a simple tube with framelines for different focal lengths, to more complex zooming viewfinders with parallax correction, to viewfinders with turrets and a selection of matching lenses.