Whether shutter speed or lens speed, metadata offers a number of useful background data. In addition, information about the lens used are normally included. However, confusions can arise, due to the so-called LensID.
Let us have a look behind the scenes. In order for the camera to know which lens is connected, it requires two factors: the LensID and a related, predefined table. The LensID is a special identification number and is deposited with further information on an integrated chip in current lenses that passes the data to the body. The camera can then detect which lens is behind the transferred LensID, by using a stored internal table. These predefined tables contain the LensIDs including the associated lens description that will be saved in the metadata. Thus, a connected lens can be clearly identified.
However, the reality looks different. In some cases, the metadata description of the mounted lens seems to be ambiguous. Why does this happen? Every camera manufacturer assigns unique IDs to their own lenses. If a third-party lens is used, it is possible that its ID number coincides with a number of the camera manufacturer’s lenses. In this case, the camera will identify the lens within the deposited table as one of its own lenses.
Although LensIDs are allocated to other lens manufacturers, many other lenses use a free or already existing LensID number. Because there is no standardization, it is possible that several lenses have the same LensID number. Therefore, it is not surprising that discrepancies can occur in relation to the metadata.
Composite Tag LensID
Let us take a look at the metadata information that is given by the so-called EXIF-Tool. Here you can find the values for LensType and LensID. We will use the Canon EF 50mm f/1.4 USM as an example. Reading out the metadata by using the Exif-Tool, the lens is defined by the LensType as "Canon EF 50mm f/1.4 USM or Zeiss Lens". However, with the Composite Tag LensID it is assigned as "Canon EF 50mm f/1.4 USM". The information under "LensID" is more reliable. But why? If we take a look in the metadata lists from Phil Harvey’s Exif-Tool, we will notice that several lenses can have the same integral LensType value. This is because several lenses use the same manufacturer LensID. Thus, they cannot be clearly identified.
What does that mean for third-party lenses?
The correct assignment plays a significant role for camera internal processes as well as for later picture editing: autofocus fine adjusting or RAW development, for example. Suppose we have two lenses and we use lens A to make an individual autofocus fine adjustment in the camera. Later, we switch to a third-party lens B. If lens B has the same LensID as lens A, the camera assumes that lens A is still connected. Thus, it is possible that the fine adjustment is still active, which in turn may lead to slight focusing errors while shooting images with lens B. In the case of third-party lenses, it is recommended to turn off the AF fine adjustment. Also during RAW development, the multiple assignments in the metadata could lead to discrepancies while developing.
There are a number of metadata tools for reading out or adapting data. A well-known program is the above-mentioned Exif-Tool from Phil Harvey. In order to differentiate between the lenses, it uses basic metadata from the file itself. First, the metadata is read out. From the so-called Composite tag information, further conclusions about the lens description or about the LensID tag can be made, for example. Here, metadata information like the LensType, the FocalLength, the MaxAperture and other data is pulled up. The LensType information is especially needed for differentiating between lenses, because it includes information about the LensID number. According to camera manufactures, the Exif-Tool now uses an advanced LensType or LensID value list (lookup tables) to define a lens correctly. The information gathered in time-consuming work and continuously updated lists contain the associated lens declarations.
The program assigns the ID number of a lens to an associated lens name. If there are several lenses that have the same integer ID number, the lens is determined with the help of other metadata (FocalLength, the MaxAperture, etc) and is listed by decimal values. Nevertheless, there is a danger that if the lenses have the same LensID and the same basic metadata, duplications can occur. However, the reason for this is not the program, it is the lack standardization, due to which duplicate LensIDs occur. So, without clear definitions, it happens that several lenses can have the same ID number.
Importance of differentiation
The declaration LensID appears from the manufacturers and in relation to the Exif-Tool. However, be careful. The Composite tag’s described LensID in the program should not be mixed up with the lens’ LensID. Whereas the manufacturer’s ID clearly assigns one number, the Composite tag’s LensID contains the lens descriptions that were filtered out based on several basic metadata. It is important to always consider that the camera only knows the LensIDs included in its allocated table. The numbers that can be found in the LensID or LensType value lists are based on manual supplement and cannot be decrypted by the camera. While recording photos or developing RAW images, it is advisable to keep this in mind and, if necessary, to adjust the settings manually.
Manufacturer’s LensIDs → clearly assigned numbers for their own lenses
Composite Tag LensIDs → Lens names inferred using metadata