07-22-2010, 03:44 PM
Hi Jeroen,
[quote name='jkeijmel' date='22 July 2010 - 05:13 PM' timestamp='1279811629' post='1236']
Hello Yakim,
I have not had this situation before, neither am I a macro-photographer. Howerver I think there is a simple explanation for the Mamiya lens.
Since the FL changes from 120 to 205, it acts like it has, "a 205/120= 1,7 teleconverter" buildt in, on 1:1. ln((1,7)^2)/ln(2)=1,5 stops (just like a 1,4 TC has 1 stop light loss and a 2,0TC has 2 stops light loss). So 1,5 plus the 2 stops from the Cambridgeincolor.com for macro makes it 3,5 stops.
Why the Canon only has 1,5 stops loss and not 2 stops? I do not know. Maybe something with the IF mechnism. Does anybody know?
Greetings,
Jeroen[/quote]
The FL of the Mamiya doesn't change, the tube length and distance of the optical rear node to the sensor does. Furthermore, at 1:1 a 120 mm lens will have an extension of 240 mm to the rear optical node and 240 mm from the frontal node to the subject, under normal circumstances. You have to add the register distance of mount to sensor to the extension of the lens, IOW.
Neither does it have a teleconverter built in.
Any lens at 1:1 loses two stops of light, no matter what the FL is. That is a given, BTW. It is exactly that, no more, no less.
So how the differences between these two lenses?
First of all, the Canon has IF and shortens the FL at 1:1 to approximately 70 mm, no longer 100 mm IOW, and this is a contimous change from infinity to 1:1. This means it captures a lot more of the environmental light in the background than the Mamiya does, and that light may include a larger chunk of bright(er) areas. Spot metering with the 7D is still 4 percent or thereabouts.
Secondly, the Mamiya must be closed down manually for metering, while the Canon meters wide open. Here alone you will likely get a difference.
Thirdly metering with a manual lens, especially when stopping down, is not as accurate as with a EF lens. This may not be a large influence, if at all in case of using the lens wide open, however.
Then there is the issue of daylight and testign in an essentially non-controlled environemnt. This needs to be tested a bit more rigorously IMO, with a fixed amount of light rather than with day light, IOW, indoors with an evenly illuminated wall either with flash or some fixed lighting.
Lastly, I assume the Mamiya is an older lens, with coating not up to digital standards, and with more transmission losses due to lesser coating than the Canon. It may just have a rear element that is not coated at the back, the camera side, which will cause reflections. Furthermore, it may not be as transparant as the Canon lens due to lesser coatings, and suffer from some more reflections internally. These reflections may mislead the metering system. By focusing close, the reflections seen by the metering system may actually get considerably less, because the lens tube gets a lot longer, whiel at normal focusing distances this lens may well underexpose. I reckon the Canon macro and body combination is spot when metering.
All in all I think it is a combination of things at play.
Kind regards, Wim
[quote name='jkeijmel' date='22 July 2010 - 05:13 PM' timestamp='1279811629' post='1236']
Hello Yakim,
I have not had this situation before, neither am I a macro-photographer. Howerver I think there is a simple explanation for the Mamiya lens.
Since the FL changes from 120 to 205, it acts like it has, "a 205/120= 1,7 teleconverter" buildt in, on 1:1. ln((1,7)^2)/ln(2)=1,5 stops (just like a 1,4 TC has 1 stop light loss and a 2,0TC has 2 stops light loss). So 1,5 plus the 2 stops from the Cambridgeincolor.com for macro makes it 3,5 stops.
Why the Canon only has 1,5 stops loss and not 2 stops? I do not know. Maybe something with the IF mechnism. Does anybody know?
Greetings,
Jeroen[/quote]
The FL of the Mamiya doesn't change, the tube length and distance of the optical rear node to the sensor does. Furthermore, at 1:1 a 120 mm lens will have an extension of 240 mm to the rear optical node and 240 mm from the frontal node to the subject, under normal circumstances. You have to add the register distance of mount to sensor to the extension of the lens, IOW.
Neither does it have a teleconverter built in.
Any lens at 1:1 loses two stops of light, no matter what the FL is. That is a given, BTW. It is exactly that, no more, no less.
So how the differences between these two lenses?
First of all, the Canon has IF and shortens the FL at 1:1 to approximately 70 mm, no longer 100 mm IOW, and this is a contimous change from infinity to 1:1. This means it captures a lot more of the environmental light in the background than the Mamiya does, and that light may include a larger chunk of bright(er) areas. Spot metering with the 7D is still 4 percent or thereabouts.
Secondly, the Mamiya must be closed down manually for metering, while the Canon meters wide open. Here alone you will likely get a difference.
Thirdly metering with a manual lens, especially when stopping down, is not as accurate as with a EF lens. This may not be a large influence, if at all in case of using the lens wide open, however.
Then there is the issue of daylight and testign in an essentially non-controlled environemnt. This needs to be tested a bit more rigorously IMO, with a fixed amount of light rather than with day light, IOW, indoors with an evenly illuminated wall either with flash or some fixed lighting.
Lastly, I assume the Mamiya is an older lens, with coating not up to digital standards, and with more transmission losses due to lesser coating than the Canon. It may just have a rear element that is not coated at the back, the camera side, which will cause reflections. Furthermore, it may not be as transparant as the Canon lens due to lesser coatings, and suffer from some more reflections internally. These reflections may mislead the metering system. By focusing close, the reflections seen by the metering system may actually get considerably less, because the lens tube gets a lot longer, whiel at normal focusing distances this lens may well underexpose. I reckon the Canon macro and body combination is spot when metering.
All in all I think it is a combination of things at play.
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....