What is the crop factor when a 35mm lens is mounted?

DavidNJ

DavidNJ

Carbonite Member
Since the lenses were designed to go behind a mirror system, which isn't there, does that reduce the crop factor by placing the lens closer to the sensor (the opposite of a tele-converter)? If so, how much? If it does, it would presumably be reduced or eliminated by needing stacked adapters.

On a similar note, since rangefinder 35mm lenses already are designed for cameras without a mirror mechanism, do they have the full 2x crop factor?

Many of the Cosina-built Voigtlander 35mm rangerfinder lenses have slow apertures of F4 and even F5.6. With the crop factor, will that make them too slow to be practical?
 
DavidNJ said:
Since the lenses were designed to go behind a mirror system, which isn't there, does that reduce the crop factor by placing the lens closer to the sensor (the opposite of a tele-converter)?
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The lens isn't closer to the sensor. The adapater is designed so that the lens distance to the sensor is the same as in the original mount.
 
the lens should have the same focal length as its supposed to have or it wont have infinity focus. and i dont think cropping affects the light transmission

another question would be whats the crop factor of DX and APS-C lenses? e.g. 18mm in dx lens is 27mm equivalent to 35mm
 
DavidNJ said:
If you move the sensor closer to the lens doesn't that effectively decrease the focal length? Is the inverse of that what a tele-converter does?
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No. A teleconverter has lenses. If you just used an empty tube to move the lens further away you would only affect the focusing distance. Many macro photographers use extension tubes to get closer focusing.
 
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Late said:
The lens isn't closer to the sensor. The adapter is designed so that the lens distance to the sensor is the same as in the original mount.
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Wouldn't that prevent stacked adapters, which apparently are common? Wouldn't using any SLR adapter on an SLR camera change the distance from the lens to the sensor?

dcloud said:
the lens should have the same focal length as its supposed to have or it wont have infinity focus. and i dont think cropping affects the light transmission
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A 2x cropping factor should reduce the light by 4x, 2 F-stops. That appears to be the case for both teleconverters and macro tubes.

Late said:
No. A teleconverter has lenses. If you just used an empty tube to move the lens further away you would only affect the focusing distance. Many macro photographers use extension tubes to get closer focusing.
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Aren't macro tubes doing the exact same thing as tele-converters: increasing the magnification by moving the lenses' optical center further away from the image plane? The difference seems to be that the macro tube reduces the minimal focal distance and depth of field.
 
DavidNJ said:
Wouldn't that prevent stacked adapters, which apparently are common? Wouldn't using any SLR adapter on an SLR camera change the distance from the lens to the sensor?
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You usually don't find adapters for mounts that are too close together. For example, the reason that the regular 4/3 has so many adapter available is because the mount distance is shorter than many other SLRs. This is even more true for micro 4/3.

DavidNJ said:
A 2x cropping factor should reduce the light by 4x, 2 F-stops. That appears to be the case for both teleconverters and macro tubes.
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I think you lose light with both yes.


DavidNJ said:
Aren't macro tubes doing the exact same thing as tele-converters: increasing the magnification by moving the lenses' optical center further away from the image plane? The difference seems to be that the macro tube reduces the minimal focal distance and depth of field.
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Magnification yes. Focal length no.
 
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No. The 2x crop factor is due to the size of the sensor, nothing more. An f3.5 lens on a Canon or Nikon is an f3.5 lens on the GH1. You don't lose any light because there's no difference between these lenses as adapted and in their original camera.

The stacked lenses result in the lens being the same distance from the GH1s sensor as it was from the original film plane or sensor in the original camera. A MFT to FT adapter and then an FT to Nikon combo works because the FT lens mount was between the MFT and Nikon mount distances.

An MFT to Nikon direct adapter would be the same ending thickness.

The rule of thumb crop factor is 2x to get 35mm equivalent. The GH1 sensor is slightly larger than the G1 so the actual factor in 16:9 mode may not be precisely 2x but in general terms just multiply by 2.
 
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DavidNJ said:
Wouldn't that prevent stacked adapters, which apparently are common? Wouldn't using any SLR adapter on an SLR camera change the distance from the lens to the sensor?
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The important thing is to keep the focal-flange distance(FFD) the same. For instance, the u43 mount is 20mm, if you want to mount an F-mount which is 46.6mm, you could use the u4/3->4/3 adapter, which increases the FFD to 38.67mm, and then use a 4/3-> F-mount that increases the FFD from 38.67->46.6mm. This way, the focal distance to the sensor is no further then it would be in the original Nikon.

Another example is the F-mount to EF-mount adapter. The EF-mount is 44mm, the F-mount 46.6mm. If you've ever seen a F-mount adapter for a Canon system they are very thin.

When stacking-adapters this is the issue, you don't want light leakage. Hence, good construction is important ( usually there are no problems). But the greater the FFD then easier mounts can be stacked and easier they are to be constructed.

DavidNJ said:
A 2x cropping factor should reduce the light by 4x, 2 F-stops. That appears to be the case for both teleconverters and macro tubes.
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If you use an adapter, it does not reduce light coming in from the lens- the amount of light hitting each pixel is the same. What is reduced is the spatial area that can collect light- you have a smaller sensor.

Its different from macro-tubes, etc. How extension-tubes work is that you increase the distance from the original focal distance of the lens from the sensor. A lenses aperture is derived from the focal length, hence an extension tube is reducing the aperture's efficacy relative to what the lens was originally designed; even though the sensor/film size you are collecting light on may be the same. Which is why you have reduced light using these products.
 
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