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Archive 2012 · Does format size reduce lens brightness???
  
 
Alf Beharie
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p.1 #1 · Does format size reduce lens brightness???


I was reading a review for the Tamron SP 300mm f2.8 LD IF (60B) (Which I already have BTW) when I spotted this rather unsettling sentence:

"The big question is, how does it compare to a comparable ZD 4/3rds lens? Remember that all versions of this Tamron 300mm lens are designed for film cameras so its image circle is sized for a 35mm film frame. With a 4/3rds camera, only half that diameter is used and, as a result, you can expect the image to be slightly less bright compared to a lens that focuses all of its light onto a 4/3rds image sensor at the same f-stop."

Is this true..If I use a full frame lens on a crop format sensor camera do I actually lose f-stops of brightness at the sensor for the same f-stop on the lens?
I hope not as it would mean that when my 60B is wide open on my 1.7x crop camera it would in fact only be as bright as it is at about f4.5-f5 on a full frame sensor or film camera!
I also have a M645 80mm f2.8 N, made for much larger format sensors/film...If the above is true, how bright would that actually be when wide open on my crop format camera?
Before I read the above quote I thought that the brightness of the light in the lenses image circle stays constant per lens f-stop no matter how small the sensor format is, but now I'm confused...What do you guys think?



Mar 17, 2012 at 11:35 AM
borderlight
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p.1 #2 · Does format size reduce lens brightness???


You won't lose f-stops, but generally speaking the corners of a full-frame or crop sensor will be "slightly less bright" than on a MFT sensor. Lens performance is usually weakest in the corners, where the light has to be bent the most. The smaller sensor hides those weaknesses from you. Personally I wouldn't attempt to adapt a medium format lens or a large, heavy-duty telephoto intended for a 35mm camera on a MFT camera. It really doesn't make any sense. It would be like trying to attach a shotgun barrel to a .22. Buy lenses for the MFT camera.


Mar 17, 2012 at 01:43 PM
Monito
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p.1 #3 · Does format size reduce lens brightness???


It will be less bright in the viewfinder, but f/stops will be as accurate as they ever have been (about +/- 15 %, somewhat better on better lenses).


Mar 17, 2012 at 02:02 PM
Alf Beharie
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p.1 #4 · Does format size reduce lens brightness???


But if its less bright in the viewfinder does'nt that also mean its less bright at the sensor?


Mar 17, 2012 at 04:52 PM
Alf Beharie
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p.1 #5 · Does format size reduce lens brightness???


borderlight wrote:
You won't lose f-stops, but generally speaking the corners of a full-frame or crop sensor will be "slightly less bright" than on a MFT sensor. Lens performance is usually weakest in the corners, where the light has to be bent the most. The smaller sensor hides those weaknesses from you. Personally I wouldn't attempt to adapt a medium format lens or a large, heavy-duty telephoto intended for a 35mm camera on a MFT camera. It really doesn't make any sense. It would be like trying to attach a shotgun barrel to a .22. Buy lenses for the MFT camera.


What is an "MFT" camera?



Mar 17, 2012 at 04:53 PM
Monito
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p.1 #6 · Does format size reduce lens brightness???


Alf Beharie wrote:
What is an "MFT" camera?


It's a TLA* that means Medium Format. It is a symptom of over-indulgence in jargonism. Often MF is used instead, but that confounds with MF = Manual Focus. Ideally people would just type it out, or at least type it out once the first time they use the abbreviation in a thread.

* TLA = Three Letter Acronym. Not strictly true in this case because the T in MFT is at the end of the word. MFt would be better, but it is hopeless expecting people to type well, let alone write well.




Mar 17, 2012 at 05:38 PM
 

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Monito
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p.1 #7 · Does format size reduce lens brightness???


Alf Beharie wrote:
But if its less bright in the viewfinder does'nt that also mean its less bright at the sensor?


I misread the original post and the issue is subtly different from what I thought. Now I'm not sure of the reasons in detail for their explanation. I'd have to think about this some more.

However, until further notice I would use f/4 as if it really were f/4 and f/2.8 as if it is f/2.8, etc.



Mar 17, 2012 at 05:43 PM
tomrock
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p.1 #8 · Does format size reduce lens brightness???


I think when someone uses "MFT" and "smaller sensor" in the same sentence they're talking about micro four thirds.


Mar 18, 2012 at 11:54 AM
Alf Beharie
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p.1 #9 · Does format size reduce lens brightness???


tomrock wrote:
I think when someone uses "MFT" and "smaller sensor" in the same sentence they're talking about micro four thirds.


Good point!



Mar 18, 2012 at 12:55 PM
CVickery
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p.1 #10 · Does format size reduce lens brightness???


Alf Beharie wrote:
I was reading a review for the Tamron SP 300mm f2.8 LD IF (60B) (Which I already have BTW) when I spotted this rather unsettling sentence:

"The big question is, how does it compare to a comparable ZD 4/3rds lens? Remember that all versions of this Tamron 300mm lens are designed for film cameras so its image circle is sized for a 35mm film frame. With a 4/3rds camera, only half that diameter is used and, as a result, you can expect the image to be slightly less bright compared to a lens that focuses all of its light
...Show more

Doesn't make sense to me FWIW



Mar 19, 2012 at 03:45 AM
EB-1
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p.1 #11 · Does format size reduce lens brightness???


Alf Beharie wrote:
I was reading a review for the Tamron SP 300mm f2.8 LD IF (60B) (Which I already have BTW) when I spotted this rather unsettling sentence:

"The big question is, how does it compare to a comparable ZD 4/3rds lens? Remember that all versions of this Tamron 300mm lens are designed for film cameras so its image circle is sized for a 35mm film frame. With a 4/3rds camera, only half that diameter is used and, as a result, you can expect the image to be slightly less bright compared to a lens that focuses all of its light
...Show more

It is completely ridiculous. The circle of coverage is still the same when the lens is used on the camera with the smaller sensor; it is just projecting part of that onto the mirror box or elsewhere that does not capture an image. Wide open there will be more light falloff on the edges of the image on the camera that covers the full image circle, but there is a differEnt optical reason.

A supplementary lens (focal length reducer) in front of the sensor would be needed to in order for the lens to concentrate more light on the sensor. For example a 300/2.8 could be projected onto a half sized diagonal as equivalent to a 150/1.4, at least theoretically. IIRC some of the early Fuji-Nikon hybirds used such an optical reducing arrangement.

EBH



Mar 21, 2012 at 01:36 AM





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