What is a mount adapter?

About the Mount Adapter and Flange Back

(The following are explanations of the relationship between the mount adapter and the flange back and the quality of the mount adapter.)



1. What is a mount adapter?
2. About the flange back
3. About the thickness of the adapter
4. Accuracy of the adapter (thickness / parallelism)
5. Quality of the adapter (internal reflection, feel of attachment, robustness)

 


1. What is a mount adapter?

An interchangeable lens camera has a joint between the lens and the camera to allow the interchange of lenses. This joint is also called a mount part and is manufactured according to the mount standards standardized by various manufacturers.



For 35 mm format range finders, there are the Leica screw and M mounts, for 35 mm format single-lens reflex cameras, there are M42, Pentax PK, Canon FD and EF, and the Nikon F.

For mirrorless cameras, Sony αE, Fuji X, and Micro Four Thirds. There are many mount standards for medium format cameras and recently introduced medium format mirrorless digital cameras.

Mount standards strictly define the shape of the joint between the lens and the camera body and the flange back dimension, and there is no compatibility between different mount standards. Therefore, a lens and camera body of different standards cannot be attached and used.



A mount adapter is used by inserting it between a lens and a camera of different mount standards and is an item that greatly expands compatibility between the lenses and the cameras, which cannot be originally attached.

By using a mount adapter, you can enjoy a variety of shooting methods, such as combining your favorite lens with a particular camera or combining a lens used in your memorable film camera with an up-to-date mirrorless camera.

 

 

2. About the flange back

Flange back is the distance from the mount surface of the camera body (the surface on which the lens is attached) to the sensor surface (the film surface in the case of a film camera) and is strictly defined by each mount standard. The flange back of a lens or camera body is manufactured with high precision in accordance with the mount standard.

Even with a combination of genuine products, if there is an error or looseness in the flange back, the focus accuracy at infinity gets out of order or results in a photo out of focus on one side (the degree of blur differs at the upper limit of the screen or between the left and right). Therefore, it is an important part that affects the depiction.

 

・ Flange back dimension (as a rough indication)

Medium format camera (Brownie version) - - - 60–89 mm
35 mm format single-lens reflex camera - - - 40–49 mm
35 mm format range finder camera - - - 20–39 mm
Mirrorless camera - - - 10–29 mm

[Example] Flange back of PK mount (Pentax PK) and SαE mount (Sony αE)


 

3. About the thickness of the adapter

The thickness of an adapter is determined by the difference between the flange backs of the lens and the camera body to be combined

Flange back of lens - Flange back of body = Thickness of the adapter

[Example] When attaching a PK mount lens to a Sony αE mount body

Flange back of PK mount - Flange back of SαE mount = Thickness of the adapter

 

Since mirrorless cameras do not have a mirror box to house a mirror as found in 35 mm format single-lens reflex cameras, the flange back of mirrorless cameras is about half the thickness of 35 mm format single-lens reflex cameras. For this reason, an adapter for a mirrorless camera can ensure sufficient thickness, so a wealth of combinations can be manufactured.

Recently, mirrorless cameras equipped with a 35 mm format full-frame sensor have become widespread, so the number of people who use mirrorless cameras in combination with adapters has been increasing significantly.

 

4. Accuracy of the adapter (thickness / parallelism)

An adapter must meet the mount standards of the combined lens and the camera body with a high degree of accuracy. If the thickness or parallelism of the adapter is incorrect, the original performance (depiction) of the lens cannot be brought out.

In particular, when the focus ring is rotated from the shortest to infinity, the smaller the change in the total length of lens (wide-angle lens), the greater the effects of the adapter thickness (error) on the infinity accuracy and distance scale accuracy, so attention needs to be paid.

 

[Thickness of the adapter]

When an adapter with a high-precision thickness is used, the ∞ mark on the lens and the infinity of live-action match with a high degree of accuracy.

Also, the higher the accuracy of the thickness, the more the lens distance scale of the lens and the distance to the actual focus position match.

When the adapter thickness is equal to the specified value

Equipment: Contax Planar 50 mm F1.4 (aperture f2), Sony α7II
※ When the infinity of the lens is misaligned (individual lens difference), the shooting result may differ from the following.

 

・ Shot with the distance scale at the ∞ mark

It is in focus precisely at infinity.





・ A chart 3 m from the camera was shot with the distance scale at 3 m
The chart at a distance of 3 m is in focus precisely.





● When the adapter thickness is incorrect (specified value - 0.2 mm)
Equipment: Contax Planar 50 mm F1.4 (aperture f2), Sony α7II

・ Shot with the distance scale at the ∞ mark
Since the distance scale passed over the infinity of the lens, it was out of focus at infinity.
(It is in focus farther than infinity.)





 

・ Shot while focusing at infinity
The distance scale was brought back from infinity and was at a position a little over 10m in this combination.



 

・ A chart 3 m from the camera was shot with the distance scale at 3 m
Even if the distance scale was set to 3 m, a distance where the chart was located, the chart was out of focus.

 

・Shot while focusing on the chart and with the distance scale at 3 m
The distance scale was brought back to the short distance side

 

 

[Parallelism of the adapter]

If the mount surface on the lens side and the mount surface on the camera body side of the adapter are not parallel, the optical axes of the lens and the sensor surface (film surface) are misaligned.

If the optical axes are not aligned, it may cause a one-sided out-of-focus photo (the degree of blur differ at the upper limit of the screen or between the left and right).

 

・ When the adapter is not tilted
Equipment: Contax Planar 50 mm F1.4 (aperture f2), Sony α7III
※ When the infinity of the lens is misaligned (individual lens difference), the shooting result may differ from the following

・ Shot while the center area is in focus

 

・ When the adapter is tilted (for an adapter with a 0.2 mm tilt)
Equipment: Contax Planar 50 mm F1.4 (aperture f2), Sony α7III

・ Shot while the center area is in focus
The image at the center area of the image is equivalent to the photo shot using an adapter without tilt. However, the blur on the left side of the image tends to be large.

 

 

 

5. Quality of the adapter
(internal reflection, feel of attachment, robustness)

Internal reflection

It is ideal that the inner walls of a camera mirror box have as little internal reflection as possible.
As with the mirror box, measures for internal reflection are also important for an adapter through which the light from the lens passes.
Careful matting treatment, light shielding processing, etc. can further bring out the original lens performance.

Feel of attachment

When attaching a lens or camera body to an adapter, it is ideal that it can be attached and detached with a feel close to that of the combination of genuine products.
If the feel is too soft, there is a possibility that looseness occurs at the mount part and the lens optical axis shifts downward, affecting the depiction, or the mount is disengaged from the lens or camera body.
If the feel is too hard, there is a possibility that the mount cannot be removed from the lens or camera body.

Robustness
An adapter is a critical part to attach and use an important lens and camera body.
You can use it with confidence because the material and the surface treatment such as plating are reliable and each part has a margin in the strength.
In the case of an adapter with a tripod mount, it is important that there is no looseness during use (that the adapter body and the tripod mount are fixed firmly).

 


As the number of pixels of digital cameras increases, the accuracy of the adapter tends to affect the shooting result increasingly. Therefore, we recommend high-precision adapters that can be used with confidence.

 

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