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canon 70-200 f2.8 is 11 vs canon 400mm f2.8 is 11
Oct 18, 2012 12:31:55   #
psurg Loc: illinois
 
when using the same object and same amount of light in the
same environment,the image is 3x darker with 400mm lens.
I would be interested in your view.Is this to be expected?The
tv and aperture being the same the iso had to be increased
3x to achieve the same exposure with the 400mm f2.8 v11
as compared to 70-200mm f2.8 is v11.Your response would
be appreciated.This is a great forum and i have recommended
it to others.

Reply
Oct 18, 2012 13:51:42   #
robert-photos Loc: Chicago
 
psurg wrote:
when using the same object and same amount of light in the
same environment,the image is 3x darker with 400mm lens.
I would be interested in your view.Is this to be expected?The
tv and aperture being the same the iso had to be increased
3x to achieve the same exposure with the 400mm f2.8 v11
as compared to 70-200mm f2.8 is v11.Your response would
be appreciated.This is a great forum and i have recommended
it to others.


I don't have the spare time to dig deep into this but you have two different lenses and are comparing f/stops which are only geometric equivalents. You need to compare t/stops because f-numbers do not accurately correlate with light transmitted.

It is surprising to me that there is a 1.5 stop difference in light transmitted with these two excellent lenses at the same f/stop. I'm assuming 200mm at f/2.8 and 400mm at f/2.8 (with each field of view having the same exposure such as an evenly illuminated background) and ISO 200 vs ISO 600 or some multiple thereof.

I'm thinking it has something to do with the lens lengths and will look into it when I have time. I do know that my main two lenses, a 24-70mm f/2.8 and a 70-200mm f/2.8 are not equivalent at the same f/stop but they are not 1.5 stops off. I just deal with the differences w/o much thought.

Try: http://en.wikipedia.org/wiki/F-number

Good luck :thumbup:

Reply
Oct 18, 2012 22:42:08   #
psurg Loc: illinois
 
Thank you robert for responding to my enquiry.I also went to the web site and tried to grasp some information.Thanks for
introducing me to that site.I did talk to canon service and
the gentleman told me that since 400mm f 2.8 lens has so many more elements in it,therefore,there is much more light fall off compared to 70-200mm lens.However my concern is that the
light fall off makes the 400mm f2.8 equivalent to 5.6.This would mean that I would have to jack up the iso from 800 to
3200 to get the same amount of exposure.This would likely add more noise in the picture.400mm 2.8 is like 200mm 5.6 with
respect to exposure although the image is magnified due to longer focal length of 400mm lens.Does that make the 400mm 2.8 really a 2.8 lens or 400mm 5.6?

Reply
 
 
Oct 19, 2012 00:08:32   #
robert-photos Loc: Chicago
 
psurg wrote:
Thank you robert for responding to my enquiry.I also went to the web site and tried to grasp some information.Thanks for
introducing me to that site.I did talk to canon service and
the gentleman told me that since 400mm f 2.8 lens has so many more elements in it,therefore,there is much more light fall off compared to 70-200mm lens.However my concern is that the
light fall off makes the 400mm f2.8 equivalent to 5.6.This would mean that I would have to jack up the iso from 800 to
3200 to get the same amount of exposure.This would likely add more noise in the picture.400mm 2.8 is like 200mm 5.6 with
respect to exposure although the image is magnified due to longer focal length of 400mm lens.Does that make the 400mm 2.8 really a 2.8 lens or 400mm 5.6?
Thank you robert for responding to my enquiry.I al... (show quote)


You didn't really understand the difference between f/stop and t/stop or t-number.

F/stop is a physical measurement of the ratio of aperture to focal length, regardless of actual light transmitted. Mathematically it is the lens focal length divided by the diameter of the entrance pupil (effective aperture).

e.g. A 100 mm focal length lens with an aperture setting of f/4 will have a pupil (iris) diameter of 25 mm (100/25=4.0). A 200 mm focal length lens with a setting of f/4 will have a pupil (iris)diameter of 50 mm (200/50= 4.0). (note to sticklers: for simplification I have equated pupil diameter to diaphragm [ iris ] diameter which for many lenses is not exact because of the magnifying effect of lens elements in front of the aperture [iris] )


T/stop is the f/stop corrected to to measure light transmission rather than aperture ratio.

T/stop is equal to f/stop only if the transmission of light through all the glass elements of a lens is 100% (practically...an impossible situation).

Since different lenses have different numbers of lens elements and those elements have different light light absorbing qualities (transmission rates) f/stop comparisons cannot be made accurately as you have discovered.

In other words since all lenses absorb some light, the T-number of any given aperture on a lens will always be greater numerically (less light transmission) than the f-number. The T-stop corrects exposure for absorption of light.

Now in your case, you posted that you had to increase ISO by a factor of three. This is 1.5 stops not 2 stops. ISO 100 x 3 = ISO 300. ISO 100 to ISO 200 = 1 stop, ISO 200 to ISO 300 = 1/2 stop.

That 400mm lens is still an f/2.8 lens because the maximum iris diameter is ~143mm (400/~143mm=2.8).

Unfortunately, it is difficult if not impossible to obtain t/stop or light transmission info on lenses. MTF lens graphs, used to compare lenses of different manufacturers, do not contain that data.

I apologize for building you a watch when all you asked was "What time is it?"

:roll:

Reply
Oct 19, 2012 08:39:13   #
xesko Loc: London
 
Hi Robert! I don't know about psurg, But I definitely appreciated you built that watch. I learned something today, and that's why I like this little UHH community every day a bit better. Thanks a lot for taking the time to explain the difference between T/stop and f/stop!

Reply
Oct 19, 2012 12:15:12   #
psurg Loc: illinois
 
Thank you Roberts for explaining to me the factors involved
in the light transmission through a lens.I have realised
that it is not as straight forwards as i had thought.With the
information that you gave me and the explanation was much
more satisfying than what i got from the canon lens corporation.I am however disappointed that this lens which
i bought for indoor sports and other activities [where flash
is not allowed]may not give the expected exposure without
tweaking some other less desirable adjustments.Thank you
so much for you time and explanation.I would also like to thank UHH for having this forum.

Reply
Oct 19, 2012 12:35:34   #
robert-photos Loc: Chicago
 
psurg wrote:
Thank you Roberts for explaining to me the factors involved
in the light transmission through a lens.I have realised
that it is not as straight forwards as i had thought.With the
information that you gave me and the explanation was much
more satisfying than what i got from the canon lens corporation.I am however disappointed that this lens which
i bought for indoor sports and other activities [where flash
is not allowed]may not give the expected exposure without
tweaking some other less desirable adjustments.Thank you
so much for you time and explanation.I would also like to thank UHH for having this forum.
Thank you Roberts for explaining to me the factors... (show quote)


That Canon 400mm f/2.8 is a supurb lens. Take a look at the MTF graphs to compare it to other lenses of the same aperture/focal length and I'm sure you'll agree with me.

If you are truly dissatisfied with that lens you can send it to me and I will even pay your shipping costs. PM me for my address. :lol:

Kidding aside, a prime 400mm for indoor sports, other than pro football or soccer, is a bit long for me. I find that my 70-200mm f/2.8 and occassionally my 120-300mm f/2.8 Sigma are adequate.

Good luck and happy shooting :thumbup:

Reply
 
 
Oct 19, 2012 12:36:57   #
robert-photos Loc: Chicago
 
xesko wrote:
Hi Robert! I don't know about psurg, But I definitely appreciated you built that watch. I learned something today, and that's why I like this little UHH community every day a bit better. Thanks a lot for taking the time to explain the difference between T/stop and f/stop!


Thanks but I have one question: What is "psurg"?

Reply
Oct 19, 2012 12:42:16   #
xesko Loc: London
 
Sorry, I was referring to the user that asked the question. His nickname in the forum is "psurg". :)

Reply
Oct 19, 2012 13:13:23   #
jdventer Loc: Wallingford, CT, USA
 
robert-photos wrote:
psurg wrote:
Thank you robert for responding to my enquiry.I also went to the web site and tried to grasp some information.Thanks for
introducing me to that site.I did talk to canon service and
the gentleman told me that since 400mm f 2.8 lens has so many more elements in it,therefore,there is much more light fall off compared to 70-200mm lens.However my concern is that the
light fall off makes the 400mm f2.8 equivalent to 5.6.This would mean that I would have to jack up the iso from 800 to
3200 to get the same amount of exposure.This would likely add more noise in the picture.400mm 2.8 is like 200mm 5.6 with
respect to exposure although the image is magnified due to longer focal length of 400mm lens.Does that make the 400mm 2.8 really a 2.8 lens or 400mm 5.6?
Thank you robert for responding to my enquiry.I al... (show quote)


You didn't really understand the difference between f/stop and t/stop or t-number.

F/stop is a physical measurement of the ratio of aperture to focal length, regardless of actual light transmitted. Mathematically it is the lens focal length divided by the diameter of the entrance pupil (effective aperture).

e.g. A 100 mm focal length lens with an aperture setting of f/4 will have a pupil (iris) diameter of 25 mm (100/25=4.0). A 200 mm focal length lens with a setting of f/4 will have a pupil (iris)diameter of 50 mm (200/50= 4.0). (note to sticklers: for simplification I have equated pupil diameter to diaphragm [ iris ] diameter which for many lenses is not exact because of the magnifying effect of lens elements in front of the aperture [iris] )


T/stop is the f/stop corrected to to measure light transmission rather than aperture ratio.

T/stop is equal to f/stop only if the transmission of light through all the glass elements of a lens is 100% (practically...an impossible situation).

Since different lenses have different numbers of lens elements and those elements have different light light absorbing qualities (transmission rates) f/stop comparisons cannot be made accurately as you have discovered.

In other words since all lenses absorb some light, the T-number of any given aperture on a lens will always be greater numerically (less light transmission) than the f-number. The T-stop corrects exposure for absorption of light.

Now in your case, you posted that you had to increase ISO by a factor of three. This is 1.5 stops not 2 stops. ISO 100 x 3 = ISO 300. ISO 100 to ISO 200 = 1 stop, ISO 200 to ISO 300 = 1/2 stop.

That 400mm lens is still an f/2.8 lens because the maximum iris diameter is ~143mm (400/~143mm=2.8).

Unfortunately, it is difficult if not impossible to obtain t/stop or light transmission info on lenses. MTF lens graphs, used to compare lenses of different manufacturers, do not contain that data.

I apologize for building you a watch when all you asked was "What time is it?"

:roll:
quote=psurg Thank you robert for responding to my... (show quote)

Thanks for the explanation of f vs t/stop.

Meaning that f/stop is most useful for DOF and we have to depend on in-camera meters for exposure?

Reply
Oct 19, 2012 14:14:12   #
robert-photos Loc: Chicago
 
xesko wrote:
Sorry, I was referring to the user that asked the question. His nickname in the forum is "psurg". :)


Brain F#rt! Sorry, I'm multi-tasking PP'ing wedding photos.

Just didn't remember the OP's handle :oops:

Reply
 
 
Oct 19, 2012 14:21:08   #
robert-photos Loc: Chicago
 
jdventer wrote:
Meaning that f/stop is most useful for DOF and we have to depend on in-camera meters for exposure?


Sort of....all the rules (sunny 16, etc.) still apply and you get used to the idiosyncrasies of your tools. I myself don't pay much attention to f/stop except for DOF and that it is one part of the exposure triangle.

Reply
Oct 19, 2012 14:45:30   #
jdventer Loc: Wallingford, CT, USA
 
robert-photos wrote:
jdventer wrote:
Meaning that f/stop is most useful for DOF and we have to depend on in-camera meters for exposure?


Sort of....all the rules (sunny 16, etc.) still apply and you get used to the idiosyncrasies of your tools. I myself don't pay much attention to f/stop except for DOF and that it is one part of the exposure triangle.


How can sunny 16 be applied if 16 does not allow the same amount of light to reach the sensor depending on the lens or is it just that sunny 16 is an approximation based average historical lens transmission?

Reply
Oct 19, 2012 14:51:13   #
robert-photos Loc: Chicago
 
jdventer wrote:
robert-photos wrote:
jdventer wrote:
Meaning that f/stop is most useful for DOF and we have to depend on in-camera meters for exposure?


Sort of....all the rules (sunny 16, etc.) still apply and you get used to the idiosyncrasies of your tools. I myself don't pay much attention to f/stop except for DOF and that it is one part of the exposure triangle.


How can sunny 16 be applied if 16 does not allow the same amount of light to reach the sensor depending on the lens or is it just that sunny 16 is an approximation based average historical lens transmission?
quote=robert-photos quote=jdventer Meaning that ... (show quote)


You missed "you get used to the idiosyncrasies of your tools". You adjust. :thumbup:

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