UNDERSTANDING CAMERA LENSES
Understanding camera lenses can help add more creative control to digital photography. Choosing the right lens for the task can become a complex trade-off between cost, size, weight, lens speed and image quality. This tutorial aims to improve understanding by providing an introductory overview of concepts relating to image quality, focal length, perspective, prime vs. zoom lenses and aperture or f-number.
LENS ELEMENTS & IMAGE QUALITY
All but the simplest cameras contain lenses which are actually comprised of several “lens elements.” Each of these elements directs the path of light rays to recreate the image as accurately as possible on the digital sensor. The goal is to minimize aberrations, while still utilizing the fewest and least expensive elements.
Optical aberrations occur when points in the image do not translate back onto single points after passing through the lens — causing image blurring, reduced contrast or misalignment of colors (chromatic aberration). Lenses may also suffer from uneven, radially decreasing image brightness (vignetting) or distortion. Move your mouse over each of the options below to see how these can impact image quality in extreme cases:
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| Original Image | Loss of Contrast | Blurring |
| Chromatic Aberration | Distortion | |
| Vignetting | Original | |
Any of the above problems is present to some degree with any lens. In the rest of this tutorial,when a lens is referred to as having lower optical quality than another lens, this is manifested as some combination of the above artifacts. Some of these lens artifacts may not be as objectionable as others, depending on the subject matter.
Note: For a more quantitative and technical discussion of the above topic, please see the
tutorial on camera lens quality: MTF, resolution & contrast.
INFLUENCE OF LENS FOCAL LENGTH
The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position. Wide angle lenses have short focal lengths, while telephoto lenses have longer corresponding focal lengths.
Note: The location where light rays cross is not necessarily equal to the focal length,
as shown above, but is instead roughly proportional to this distance.
Required Focal Length Calculator
Subject Distance
Subject Size
Camera Type
Note: Calculator assumes that camera is oriented such that the maximum
subject dimension given by “subject size” is in the camera’s longest dimension.
Calculator not intended for use in extreme macro photography.
Many will say that focal length also determines the perspective of an image, but strictly speaking, perspective only changes with one’s location relative to their subject. If one tries to fill the frame with the same subjects using both a wide angle and telephoto lens, then perspective does indeed change, because one is forced to move closer or further from their subject. For these scenarios only, the wide angle lens exaggerates or stretches perspective, whereas the telephoto lens compresses or flattens perspective.
Perspective control can be a powerful compositional tool in photography, and often determines one’s choice in focal length (when one can photograph from any position). Move your mouse over the above image to view an exaggerated perspective due to a wider angle lens. Note how the subjects within the frame remain nearly identical — therefore requiring a closer position for the wider angle lens. The relative sizes of objects change such that the distant doorway becomes smaller relative to the nearby lamps.
The following table provides an overview of what focal lengths are required to be considered a wide angle or telephoto lens, in addition to their typical uses. Please note that focal lengths listed are just rough ranges, and actual uses may vary considerably; many use telephoto lenses in distant landscapes to compress perspective, for example.
| Lens Focal Length* | Terminology | Typical Photography |
|---|---|---|
| Less than 21 mm | Extreme Wide Angle | Architecture |
| 21-35 mm | Wide Angle | Landscape |
| 35-70 mm | Normal | Street & Documentary |
| 70-135 mm | Medium Telephoto | Portraiture |
| 135-300+ mm | Telephoto | Sports, Bird & Wildlife |
*Note: Lens focal lengths are for 35 mm equivalent cameras. If you have a compact or digital SLR camera, then you likely have a different sensor size. To adjust the above numbers for your camera, please use the focal length converter in the tutorial on digital camera sensor sizes.
Other factors may also be influenced by lens focal length. Telephoto lenses are more susceptible to camera shake since small hand movements become magnified, similar to the shakiness experience while trying to look through binoculars. Wide angle lenses are generally more resistant to flare, in part because the designers assume that the sun is more likely to be within the frame. A final consideration is that medium and telephoto lenses generally yield better optical quality for similar price ranges.
FOCAL LENGTH & HANDHELD PHOTOS
The focal length of a lens may also have a significant impact on how easy it is to achieve a sharp handheld photograph. Longer focal lengths require shorter exposure times to minimize blurring caused by shaky hands. Think of this as if one were trying to hold a laser pointer steady; when shining this pointer at a nearby object its bright spot ordinarily jumps around less than for objects further away.
This is primarily because slight rotational vibrations are magnified greatly with distance, whereas if only up and down or side to side vibrations were present, the laser’s bright spot would not change with distance.
A common rule of thumb for estimating how fast the exposure needs to be for a given focal length is the one over focal length rule. This states that for a 35 mm camera, the exposure time needs to be at least as fast as one over the focal length in seconds. In other words, when using a 200 mm focal length on a 35 mm camera, the exposure time needs to be at least 1/200 seconds — otherwise blurring may be hard to avoid. See the tutorial on reducing camera shake with hand-held photos for more on this topic.
Keep in mind that this rule is just for rough guidance; some may be able to hand hold a shot for much longer or shorter times. For users of digital cameras with cropped sensors, one needs to convert into a 35 mm equivalent focal length.
ZOOM LENSES vs. PRIME LENSES
A zoom lens is one where the photographer can vary the focal length within a pre-defined range, whereas this cannot be changed with a “prime” or fixed focal length lens. The primary advantage of a zoom lens is that it is easier to achieve a variety of compositions or perspectives (since lens changes are not necessary). This advantage is often critical for dynamic subject matter, such as in photojournalism and children’s photography.
Keep in mind that using a zoom lens does not necessarily mean that one no longer has to change their position; zooms just increase flexibility. In the example below, the original position is shown along with two alternatives using a zoom lens. If a prime lens were used, then a change of composition would not have been possible without cropping the image (if a tighter composition were desirable). Similar to the example in the previous section, the change of perspective was achieved by zooming out and getting closer to the subject. Alternatively, to achieve the opposite perspective effect, one could have zoomed in and moved further from the subject.
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| Two Options Available with a Zoom Lens: | |
| Change of Composition | Change of Perspective |
Why would one intentionally restrict their options by using a prime lens?Prime lenses existed long before zoom lenses were available, and still offer many advantages over their more modern counterparts. When zoom lenses first arrived on the market, one often had to be willing to sacrifice a significant amount of optical quality. However, more recent high-end zoom lenses generally do not produce noticeably lower image quality, unless scrutinized by the trained eye (or in a very large print).
The primary advantages of prime lenses are in cost, weight and speed. An inexpensive prime lens can generally provide as good (or better) image quality as a high-end zoom lens. Additionally, if only a small fraction of the focal length range is necessary for a zoom lens, then a prime lens with a similar focal length will be significantly smaller and lighter. Finally, the best prime lenses almost always offer better light-gathering ability (larger maximum aperture) than the fastest zoom lenses — often critical for low-light sports/theater photography, and when ashallow depth of field is necessary.
For compact digital cameras, lenses listed with a 3X, 4X, etc. zoom designation refer to the ratio between the longest and shortest focal lengths. Therefore, a larger zoom designation does not necessarily mean that the image can be magnified any more (since that zoom may just have a wider angle of view when fully zoomed out). Additionally, digital zoom is not the same as optical zoom, as the former only enlarges the image through interpolation. Read the fine-print to ensure you are not misled.
INFLUENCE OF LENS APERTURE OR F-NUMBER
The aperture range of a lens refers to the amount that the lens can open up or close down to let in more or less light, respectively. Apertures are listed in terms of f-numbers, which quantitatively describe relative light-gathering area (depicted below).
Note: Aperture opening (iris) is rarely a perfect circle,
due to the presence of 5-8 blade-like lens diaphragms.
Note that larger aperture openings are defined to have lower f-numbers (often very confusing). These two terms are often mistakenly interchanged; the rest of this tutorial refers to lenses in terms of their aperture size. Lenses with larger apertures are also described as being “faster,” because for a given ISO speed, the shutter speed can be made faster for the same exposure. Additionally, a smaller aperture means that objects can be in focus over a wider range of distance, a concept also termed the depth of field.
| f-# | Corresponding Impact on Other Properties: | |||
|---|---|---|---|---|
| Light-Gathering Area (Aperture Size) |
Required Shutter Speed | Depth of Field | ||
| Higher | Smaller | Slower | Wider | |
| Lower | Larger | Faster | Narrower | |
When one is considering purchasing a lens, specifications ordinarily list the maximum (and maybe minimum) available apertures. Lenses with a greater range of aperture settings provide greater artistic flexibility, in terms of both exposure options and depth of field. The maximum aperture is perhaps the most important lens aperture specification, which is often listed on the box along with focal length(s).
An f-number of X may also be displayed as 1:X (instead of f/X), as shown below for the Canon 70-200 f/2.8 lens (whose box is also shown above and lists f/2.8).
Portrait and indoor sports/theater photography often requires lenses with very large maximum apertures, in order to be capable of a narrower depth of field or a faster shutter speed, respectively. The narrow depth of field in a portrait helps isolate the subject from their background. For digital SLR cameras, lenses with larger maximum apertures provide significantly brighter viewfinder images — possibly critical for night and low-light photography. These also often give faster and more accurate auto-focusing in low-light.Manual focusing is also easier because the image in the viewfinder has a narrower depth of field (thus making it more visible when objects come into or out of focus).
| Typical Maximum Apertures | Relative Light-Gathering Ability | Typical Lens Types |
|---|---|---|
| f/1.0 | 32X | Fastest Available Prime Lenses (for Consumer Use) |
| f/1.4 | 16X | Fast Prime Lenses |
| f/2.0 | 8X | |
| f/2.8 | 4X | Fastest Zoom Lenses (for Constant Aperture) |
| f/4.0 | 2X | Light Weight Zoom Lenses or Extreme Telephoto Primes |
| f/5.6 | 1X |
Minimum apertures for lenses are generally nowhere near as important as maximum apertures. This is primarily because the minimum apertures are rarely used due to photo blurring from lens diffraction, and because these may require prohibitively long exposure times. For cases where extreme depth of field is desired, then smaller minimum aperture (larger maximum f-number) lenses allow for a wider depth of field.
Finally, some zoom lenses on digital SLR and compact digital cameras often list a range of maximum aperture, because this may depend on how far one has zoomed in or out. These aperture ranges therefore refer only to the range of maximum aperture, not overall range. A range of f/2.0-3.0 would mean that the maximum available aperture gradually changes from f/2.0 (fully zoomed out) to f/3.0 (at full zoom). The primary benefit of having a zoom lens with a constant maximum aperture is that exposure settings are more predictable, regardless of focal length.
Also note that just because the maximum aperture of a lens may not be used, this does not necessarily mean that this lens is not necessary. Lenses typically have fewer aberrations when they perform the exposure stopped down one or two f-stops from their maximum aperture (such as using a setting of f/4.0 on a lens with a maximum aperture of f/2.0). This *may* therefore mean that if one wanted the best quality f/2.8 photograph, a f/2.0 or f/1.4 lens may yield higher quality than a lens with a maximum aperture of f/2.8.
Other considerations include cost, size and weight. Lenses with larger maximum apertures are typically much heavier, larger and more expensive. Size/weight may be critical for wildlife, hiking and travel photography because all of these often utilize heavier lenses, or require carrying equipment for extended periods of time.
FURTHER READING
For more on camera lenses, also visit the following tutorials:
- Using Wide Angle Lenses
- Using Telephoto Lenses
- Macro Lenses: Magnification, Depth of Field & Effective F-Stop
Want to learn more? Discuss this and other articles in our digital photography forums.
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Source: http://www.cambridgeincolour.com/tutorials/camera-lenses.htm
Camera lens cleaning
It is very normal that the lens of a camera gets dirty… especially for TLRs that are at least 20 years old.
Very often photographers need to clean their lens. Many time i have seen newbies that using inappropriate method to clean lenses that almost destroy the surface coating.
Below is an articles from bhphotovideo on how to properly clean a lens.
The first question most people ask when shopping for a new lens is “Is it sharp?”, yet if you were to pop the lens cap off that new lens a month down the line there’s a 50-50 chance you’ll find dust and a fingerprint or two on it. And if it’s not on the lens, there’s a 50-50 chance there’s dust or a fingerprint or two on the “protective filter,” which was purchased to keep dust and fingerprints off that new lens. And that’s on a DSLR.
If it’s a point-and-shoot camera, there’s even a greater chance of dust or fingerprints on the lens because when you turn the camera off, the lens usually slips behind little louver blades that A) protect the lens, and B) hide the fact there’s dust or a fingerprint on the lens.
Checking your lens for dust and smudges is something you should do on a regular basis, not only to ensure you’re getting the sharpest possible pictures, but also because a fingerprint or alien smudge left to “ripen” can cause permanent damage to the lens coatings. Keeping your lenses clean isn’t rocket science, but nonetheless, is a chore that should be performed carefully, thoughtfully and with the proper tools and techniques.
Dust and smudges on the front element of your lens (or your filter) are the easiest to spot because they’re front and center. While dust and smudges on the front element can diminish sharpness and contrast levels, you lose greater levels of sharpness and contrast when you have dust or smudges on the rear lens element because that’s the one that ultimately projects your image onto the camera’s sensor (or film). If your “projector lens” isn’t clean, your pictures won’t be sharp as they can be.
The Proper Tools for Cleaning Lenses and Filters
- Air blower (Not canned air!)
- Camel-hair brush
- Lens tissue or micro-fiber cloth
- Methyl alcohol (aka methanol or wood alcohol)
The Proper Technique for Cleaning Your Lens or Filter
The surface of your camera’s lens has special coatings designed to maximize contrast, color saturation and color fidelity as well as minimize flare. These coatings can be scratched easily, and as such, when cleaning your lens it’s always a good idea to keep things simple. If dust or loose grit is the only villain, the best way to get rid of it is to gently brush the surface of the lens with a soft, camel-hair brush or give it a few blasts of air using a bulb-style air blower. Avoid using pressurized canned air. Avoid grinding grit into the lens surface with a cleaning cloth.
Smudges and fingerprints take a bit more effort, and here too, you should be as gentle as possible. Start by taking a soft micro-fiber cloth or a piece of lens tissue (folded, not bunched up), breathe onto the lens surface (never dry-clean a lens) and gently wipe the lens surface in a circular motion. Repeat if needed using a fresh piece of lens tissue or clean portion of the micro fiber cloth. If this doesn’t work, try dampening the tissue or cloth with a few drops of methanol (wood alcohol) or alcohol based lens-cleaning fluid and try again by gently wiping the lens in a circular motion.
Alcohol or lens-cleaning fluid should never be applied directly onto the lens surface. Doing so can possibly harm the lens coatings and/or compromise the adhesives that hold the lens elements in place.
If you’re out on a shoot and need to clean dust or smudges off your lens, and do not have a micro-fiber cloth or lens tissue with you, a cotton t-shirt or similar cotton-based material (preferably old and not freshly starched) should do the job equally well. What you never want to use is facial tissue, paper towels, polyester-based material, or any type of coarse or abrasive fabric or paper surface.
If the above procedures fail to do the job, or if gritty particles that can scratch the lens coatings are embedded in the smudge, you’d be wise to have a qualified technician address the problem.
If you can see a few bits of dust floating around between your lens’s inner elements, they’re not worth fretting over, as they will have little if any visible impact on the sharpness levels of your photographs, and are certainly not worth the time, trouble, or expense of having the lens taken apart, cleaned and reassembled.
Dust on your Mirror and Imaging Sensor
A common misconception about dust is that you can see it in your camera’s viewfinder. The truth is the specs of dust you see in your viewfinder are not on your lens, but on your camera’s mirror. These dust marks, as distracting as they may be, do not affect your picture quality. Now before you take your lens off and try to clean your mirror, be advised the mirror in your camera is a surface-coated mirror finish, which can be permanently scratched with little effort on your part. Never, ever try blasting dust off with canned air, because you’ll most likely pit or scar the mirrored coating, or even blow the mirror off its hinges altogether.
The most drastic action you should take is to try removing the dust particles by gently blowing them off with a bulb-type air blower. If this doesn’t do the trick, bring the camera to a trained technician or simply live with it because as noted above, dust on the mirror is annoying, but will not affect your picture quality.
Dust marks on your image files―specifically, blurry smudge-like marks that appear repeatedly on the same portions of all of your image files―are caused by dust on your camera’s imaging sensor. Here too, the most drastic and least invasive action you should try on your own is to remove the lens, set your camera to the mirror-lock position, and gently blow it off with a bulb-style air blower while holding the camera face down. If this does not work, it’s recommended that you have a trained technician clean your camera’s sensor. And just as you should never blast your mirror with canned air, the same goes for your camera’s imaging sensor.
Ciro-flex TLR Camera Repair/ restoration
The Ciro-flex is an American made TLR manufactured during the 1940s and ’50s. The Ciro camera company was purchased by Graflex and the Ciro-flex continued to be manufactured as the Graflex 22. Altogether, there were six different Ciro-flex models, A through F. The model A has a slightly different focusing mechanism, and is not quite as common. The B and C models have a non-synchronized shutter while the D, E, and F models have flash synchronization. The B and D models use the simple Alphax shutter while the C, E, and F models use the high-quality Rapax shutter. Early versions of these cameras have a parallax correction frame under the ground glass. In later models, the frame was replaced with a fresnel lens in order to help brighten the focussing screen. Overall, these are no-frills cameras with only a simple knob winder, no double exposure prevention or automatic film counter. However, they are sturdy cameras with good lenses and shutters. The model F has an excellent four-element lens that is quite sharp. All of these cameras are capable of taking very nice pictures and are still very usable for low-budget medium format photography.
Unfortunately, you will almost always find these cameras in terrible condition. The lenses, reflex mirror and ground glass are usually hazy and the shutter sticky or inoperative. In addition, the steel body will rust anywhere the paint has chipped off, and, often these cameras look like they are well beyond repair. However, as long as the lens isn’t damaged from fungus, a Ciro-flex can almost always be brought back to life. The rust on the body doesn’t really affect the performance of the camera; it just looks bad. These are easy cameras to work on and make an excellent first project if you are interested in learing how to repair a TLR. In addition, these cameras can be bought for very little money (often $20-30) and so if you are willing to put in a little work, you can have a nice medium-format camera for a very low price.
In this article, I show disassembly of the focus mechanism. Usually, this isn’t necessary. The focus cam can be cleaned using cotton swabs dampened with cigartte lighter fluid by reaching in from the back of the camera. On this camera, there was some dried out grease and I needed to soak the parts in solvent to get everything clean. As you can see, there isn’t much else to disassemble on the Ciro-flex.
A very good book on the Ciro-flex camera, and TLR usage in general, is PHOTOGRAPHY with the Ciro-flex by Bruce Downes. I have uploaded a 300dpi scan of the information in a Ciro-flex brochure.This describes the features of models B through F and shows the original prices.
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To clean the focusing screen and/or reflex mirror, remove the four screws around the top cover and lift the cover off.
NOTE: on early versions of the Ciro-flex, with the parallax correction frame, you will not be able to lift the viewing hood off until the frame’s connecting lever is disconnected. In order to disconnect the lever, you need to remove the front panel, rotate the focus to the shortest distance and then remove the screw located on the right side of the lens standard. |
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The focusing screen and fresnel lens are held in place by two spring clips. Turn the hood over and remove the two clips. You will need to then slightly spread the frame to allow the screen and lens to fall out.
There are also four tabs on the frame that may be bent inward slightly to help hold everything in place. On cameras with the parallax frame, these tabs hold the frame together. Bend the tabs straight to allow the frame to separate. |
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With the hood off, you can reach the rear element of the viewing lens for cleaning. However, these lenses seem to get a coating of haze on the inner surfaces, and may need to be disassembled to get them clean. In addition, if you need to disassemble the focus, you will need to remove this lens in order to get the front panel off.
To remove the viewing lens, loosen the retaining ring at the base of the lens, then unscrew the lens assembly from the body. This ring is often tight. You can use a soft-jaw plier to remove it. It is easier to remove this lens after the shutter is removed from the camera. If you are going to remove the shutter, wait until the shutter has been removed before removing the viewing lens. The outer lens elements are held in place by screw-in retaining rings that have no slots for a spanner. Use a rubber furniture leg cup to unscrew the rings. These rings may be stuck from oxidation. A small amount of solvent, such as alcohol or laquer thinner, will help loosen the rings. To adjust the focus on the viewing lens, just loosen the retaining ring and rotate the lens until the image on the focusing screen matches the taking lens focus. Retighten the retaining ring when done. |
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The shutter is held in place by a retaining ring accessible through the back. On models without a flash connector, just unscrew the retaining ring and the shutter will lift out.
On models with flash, you need to disconnect the flash wiring before removing the shutter. Open the back and unsolder the flash connector you find there. There is another flash wire under the front panel that will need to be unsoldered as well. Remove the front panel to gain access to the second wire. |
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To remove the front panel, first peel off the leatherette from the front. The leatherette is made of paper and you should not use any liquid to try and loosen the glue. It is quite common for the leatherette to disintegrate when you try to remove it, so take that into consideration before deciding the remove the front panel. Remove the four screws and the front panel can then be lifted off. On the model F, you may need to loosen the shutter retaining ring slightly in order to work the panel around the shutter.
On flash models, you can now reach the body connecting wire to unsolder it. |
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To remove the focus for cleaning, first remove the front cover, shutter and viewing lens. Unscrew the hex nut and the lens mounting plate will lift off. |
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The next panel rarely needs to be removed. You can remove it for cleaning if needed, or if you want to remove all the old leatherette. Remove the four screws and the plate will lift off. There is a felt washer underneath the panel that goes around the hole that the guide post for the lens plate moves in. |
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The focus bracket is held to the body with four screws located underneath the leatherette. You need to cut into the leatherette to expose the screws and then remove them. It usually isn’t necessary to remove this bracket since it can be cleaned once the forcus shaft is pulled out from the back. (See next picture.) However, if you need to remove the focus cam, this bracket has to come out first.
In the previous picture, note the four holes for the screws that hold the focus bracket. These screws can be removed without removing this panel. |
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Remove the screw in the end of the focus shaft and lift the guide and shaft out.
Note that the guide that sits against the focus cam is not exactly perpendicular to the camera body. It must be positioned so that the guide does not bind against the cam when the cam is rotated to the short focus position. This is usually when the guide is at an angle of about five degrees to the camera body. To reinstall the focus shaft, insert the shaft and guide but don’t tighten the hex nut on the front panel. Rotate the focus to the short position and allow the guide to rotate to where it doesn’t bind. Hold the guide in place while tightening the hex nut on the front.
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To remove the focus cam, loosen the screw in the center of the focus knob and lift the knob off. The cam can be removed from the inside of the camera.
The focus cam needs considerable lubrication on both the inside and outside surfaces. To adjust the focus, set the focus so that the image at the film plane is sharp at infinity, then loosen the screw in the middle of the knob and turn the knob against the infinity stop. |
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This picture shows the parts of the focus shaft and guide. There is a spring tensioner in the guide that puts pressure on the guide to prevent slap-back of the focus.
Lightly lubricate the shaft where it goes through the mounting bracket |
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To remove the wind knob, push out the tapered pin in the spool shaft and then pull the collar off, then reach in and pull the coiled spring out.. The knob and shaft come out together. There is a spring inside the knob that prevents the winder from being turned backwards. One end of this spring is bent so that it fits into the slot in the tube the winder goes through.
You will need to put a liberal coating of grease on the inside of the knob. This will give the knob a nice smooth feel and will also stop the winder from squeaking. |
Notes
If you need to remove the back, push out the steel rod in the hinge and the back will separate.
The two film guide rollers may have rust on them and need to be removed for cleaning. Slightly bend the bracket that holds the ends of rollers and then pop the rollers out.
TLRgraphy 