Symptom: I can’t focus my lens to infinity

The lens works fine on other cameras

The cause of the symptom is likely to be the camera you are using.

Please check the cameras and find out what the differences between this camera and the other cameras are.

The camera has smaller pixels than the other cameras

It may be that the lens used is still just about good enough for the “old” cameras, but not good enough for this camera. This could be the case if you have just changed to a 5 megapixel camera, for example. This assumption would be supported if there is a an equal amount of blurring when focusing on both long and short distances.

The malfunctioning camera has a different mechanism than the other cameras

Mechanical obstacles – especially filters – could prevent the lens from being screwed in far  enough. This can be confirmed for c-mount lenses like this :  A small gap remains between the lens and the camera after completely screwing in the lens.

Never apply force to mount the lens; this could destroy  the lens or the camera.

The camera image is totally blurred

… i.e. not just ‘slightly’ out of focus.
It could be that you are using a cs-mount lens on a c-mount camera (see: What are the differences between c-mount and cs-mount?)
You should first check whether both cameras are of c-mount or  cs-mount type. (see: How can you tell if you have a c-mount or cs-mount camera.)

Since it is assumed here that the camera and the other cameras are different, a quick test would be to  unscrew the lens and have a view to the front of the camera. The distance from the front edge of the emechanics to the sensor is about 5mm less with cs-mount cameras than with c-mount cameras. You should be able to see significant differences.

Different filters

If the cameras have different / too many / too few filters, this leads to different optical behavior. Check that the path between the sensor and the lens looks the same on both cameras. If the IR cut filter has been removed from the camera, we have found the problem (see Problems by removing the IR cut filter).

The reason that the IR cut filter was removed, is most likely because the camera user wanted to see IR light and it is the task of the IR cut filter, to prevent exactly this. If you really have to remove the IR cut filter from a camera, then it is essential to use optical glass or optical filters with equal refractive index and thickness, or else this phenomenon occurs where you can focus on shorter working distances, but not to infinity

The lens does not work on other cameras

Here the fault probably lies with the lens.

Depending on the type of lens you can either adjust the back focus, or the lens will need to be replaced. Please contact the lens supplier.

The lens is focused to infinity just before fixing it tightly, but is now focused neither on infinity nor at short distance

The cause could be the filter.

This phenomenon can occur when the original IR-cut filter of the camera has been replaced by a filter with thicker glass or of a different refraction index.

It may also occur with imprecisely produced camera bodies, particularly in combination with a wide angle lens.

Smaller focal length lenses have the nice property of having a high depth of field, but of course, this is only if they are focused. Depending on the lens design, it may be necessary that required displacement along the optical axis for focusing is, for example, only 0.2mm to focus from  20cm to oo.
Some camera manufacturers produce their camera housings 0.3mm shorter as a precaution (so that the customer can use thin distance rings in case of an emergency). In this case, the lens could be focused from a few meters to infinity. If the user adds a 0.5mm distance ring however, the same camera focuses on close distances, but not on infinity.

The IR-cut filter was removed and replaced with the same thick glass with the same refractive index

Here the cause is probably the wavelengths of light that occur.

Typical would be the simultaneous occurrence of IR and visible light. The whole picture would be slightly out of focus  when both types occur simultaneously. For outdoor shots with a color camera, a typical symptom that can occur during the day time is that trees can be almost white in color and the whole picture in pastel colors.

This zoom lens won’t focus, but another zoom lens will

You probably have a varifocal lens as opposed to a proper zoom lens. (See What is the difference between zoom lenses and varifocal lenses?)
A zoom lens would keep the focus on all the focal lengths. Therefore, if it isn’t possible to focus with one of the focal lengths to infinity, then it wouldn’t be possible for any of the other focal lengths either.
In a varifocal lens, however, the position of the image relative to the lens housing changes. This allows for an fixed focus to be found for almost any distance, so that the image is sharp. The longest and narrowest opening angle probably won’t focus to infinity, however. The symptom is therefore most likely caused by the camera than by the lens.

The camera mechanism seems fine, another lens is not available for comparison

Some lenses have a flange adjustment option. It might be possible that someone has adjusted it incorrectly.

The lens is a ‘factory automation’ lens or macro lenses

Some “Factory Automation” lenses, such as the CMFA-Series of Lensation GmbH and also ‘macro’ lenses and ‘telecentric‘ lenses have been designed for specific short working distances, for example below 1m. Please check whether according to the data sheet focusing to infinity should be possible.

The lens is an S-mount lens (M12x0.5)

  • The lens may be mounted in an adapter that’s too long and does not allow the lens to be brought close enough to the camera sensor.
  • Another possibility is that there could be an IR cut filter in the lens holder that doesn’t allow the lens to get close enough.
  • The lens holder could also have a blocking mechanism that doesn’t allow the lens to get close enough to the sensor.
  • Some CCD sensors have an extra thick cover glass. If the lens has a very small back focal length, then  it might not get close enough to the sensor.
  • If you use a lock-ring with the lens, remove the lock-ring and try to focus again. Once again,  the mechanics (not the lens) were the cause.

To localize the cause :

  • remove any existing lock-ring and try to focus . If you can now focus, could use a thinner lock-ring, or a suitable glue and no lock-ring at all.
  • Alternatively screw the lens (very carefully) in the direction of the sensor until you reach a mechanical limit:
    – if there is a gap between the lens head and the holder, the likely cause is a mechanical obstacle that is blocking the lens from getting close enough the the sensor (a filter, the shape of the holder, the CCD’s  glass surface)
    -if there is no gap, then the holder is too long for the lens

 

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