Steve Cushing Impresionist Fine Art Photography

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Flange Distance



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Flange Focal Distance refers to the distance the lens needs to be mounted from the film plane. To be more specific the flange distance is the the distance (usually expressed in millimetres) from the front of the flange on a particular lens body mount to the plane of focus, i.e. the film or sensor surface (see images below). Original film cameras had a very short flange distance. A shorter flange distance allows for thinner cameras, thus making them smaller and lighter compared to cameras with longer flange distances. As a result of this short flange distance original film cameras had to use a separate viewfinder and to get focus you needed to guess the distance to the object being photographed as you could not focus through the lens. This is why the lenses had distance markings on them.
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With the advent of the SLR where a mirror was needed so that it was possible see through the lens via the viewfinder flange distances had to increase to allow for the mirror (see first image above). The throat diameter (size of the hole on the front of the camera) combined with flange distance determines the maximum possible angle of incidence of the rays from the lens, which is important in designing lenses – generally, the larger the angle of incidence, the easier it is to make high-performance lenses. While lens mount adapters might look like extension tubes – they are not. There is no light light loss as they are just ensuring that the correct flange distance is achieved – in other words they simply place the lens at the proper distance from the film plane or sensor.


This distance varies between cameras and lens mounts which explains the depth (or thickness) of different lens mount adapters.


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Many of the lenses from old film cameras became redundant when the DSLR came around as the flange distance was too short. You can't put a short flange distance lens onto a longer flange distance camera and get a full range focus.


Moving a lens forward beyond its flange distance makes it a macro lens with no infinity focus but some light loss. But with an adapter you can put a lens with a longer flange distance onto a camera with a shorter flange distance, you just need a spacer .

Personally I like to mount lenses on a helicoid so that I can change the flange distance. This is essential with strange lenses such as projector lenses as the flange distance is unknown and you cannot buy ready made adapters. It also allows me to use macro by extending the flange distance whilst still allowing for infinity focus.


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So by moving the lens forward from its flange distance you magnify the image BUT you also lose the far limit capabilities of the lens as you change its DEPTH OF FIELD. As an example the register distance (flange length) of the M42 mount is 45.46mm. Let us assume you have a DSLR that is designed for this lens. In order to allow focus to infinity with a 50mm enlarger lens, the rear nodal point of the enlarger lens needs to be (surprise!) 50mm from the centre of the sensor. If you move it further away than this you will magnify the image BUT longer have infinity focus as you will change what is called the hyperlocal distance and thus DEPTH OF FIELD.


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With the advent of modern mirrorless cameras the flange length has reduced to a similar distance to pre-war cameras. Shorter flange distances on modern camera, also allows lens designers to place a more powerful actuator on lenses for faster autofocusing.

Most pre war old lenses do not have autofocus so must be used manually. But even when the lens does have autofocus as it was made for a later DSLR it is important to note that when adapting lenses from other mounts, due to the proprietary nature of autofocus systems, differences in exchange of information between camera body and lens via electronic contacts (the number of which also vary from system to system) and other issues, most adapters end up being “dumb” adapters with manual controls. Camera manufacturers themselves often provide adapters with the release of shorter flange distance systems to be able to mount lenses from other camera mounts that they developed in the past, but they almost never provide adapters for competing systems.

Summary

When adapting lenses from other systems, it is important to make sure that the target lens mount has a longer flange distance in order to be able to achieve infinity focus. Furthermore, the difference in flange distance between the source and the target system has to be big enough to have enough room for an adapter to sit between the lens and the camera for a dumb adapter.

For digital, designs for lenses needed to take into account the increased reflectance of sensor material over film. This means that there's more stray light bounced back into the lens than there was before. Additionally, most sensors are less forgiving of light that isn't coming from straight-on, making telecentric design more important.

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Download full pff chart HERE or visit HERE for additional list showing wider range of 35mm cameras rather than mounts.


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