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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 21 May 2009 (Thursday) 15:10
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Exposure change for different lens's?

 
USER876
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May 21, 2009 15:10 |  #1

I was just shooting the same scene with 3 different lenses, and I noticed that with one of the lens's to obtain the same exposure the shutter speed was about cut in half (same ISO and aperture). No Filters

Is that strange?




  
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bohdank
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May 21, 2009 15:13 |  #2

Not if you were capturing different FOV's.

If you shot a 50mm and a 24-70, set to 50mm with exactly the same framing, then, yes, it would be weird.


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May 21, 2009 15:17 as a reply to  @ bohdank's post |  #3

I haven't experienced that, but I have wondered the same thing.

Can the quality of the glass affect the light transmission, therefore causing exposure to be off?

I guess if one lens contains clearer glass, it would be brighter. I guess this is why film lenses use T stops instead of f stops.


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bohdank
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May 21, 2009 15:23 |  #4

EF lenses work on both Canon film (newer bodies) and digital. They're the same lenses.


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phigment
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May 21, 2009 15:27 |  #5

bohdank wrote in post #7964846 (external link)
EF lenses work on both Canon film (newer bodies) and digital. They're the same lenses.

Ah, sorry for the confusion. By 'film', I meant lenses used one many motion picture cameras.


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nureality
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May 21, 2009 15:31 |  #6

phigment wrote in post #7964813 (external link)
I haven't experienced that, but I have wondered the same thing.

Can the quality of the glass affect the light transmission, therefore causing exposure to be off?

I guess if one lens contains clearer glass, it would be brighter. I guess this is why film lenses use T stops instead of f stops.

by film lenses you mean Motion Picture cameras... and then you are correct. By saying film lenses as you did, you confuse some people. Movie cameras use T-stops, still cameras use f-stops.

But I've found the same phenomenon as you have. The bottom line is, if you are comparing one focal length to one that is twice as wide, the wider one is allowing light from many more angles of incident (about 2x as much), so more lights rays = more light = different exposure.

I think its funny how some people will take this opportunity to point out that handheld light meters don't take focal length into consideration, and stick anchovies in their ears so as not to hear your response. But if you were to try shooting 12.5mm f/4 @ 1/125, 25mm f/4 @ 1/125, 50mm f/4 @ 1/125, 100mm f/4 @ 1/125, and 200mm f/4 @ 1/125 all ISO 100, from the SAME POSITION, in the SAME LIGHT (read controlled light setup, with same lights - not the sun), you would find 5 differently exposed shots, even though they all have the same exposure. And the 12.5mm would be the brightest, and the 200mm would be the darkest.


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CyberDyneSystems
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May 21, 2009 15:33 |  #7

Most, (especially zooms) if not all lenses when shooting at there max apertures, are in fact only "close to" the rated aperture.

Getting the correct numbers from all those elements added up in a tube is not an exact science,. all manufacturers use 1/2 or 1/3 stop increments for the ratings, regardless of where the actual measured number ends up.
On the Bright side ( ;) ) which side of the rating they end up on seems to be about 50/50%..
ie: you are just as likely to get an f/2.4 rated as f/2.8 as you are likely to get an f/2.9...

Check out your Pop Photography lens reviews, and notice that in all cases they provide the actual measured numbers, and almost always you will see that the Max aperture is off, and actually measures as some odd number not quite in keeping with our 1/2 or 1/3 stop mindset.

For our own calculations, using the standard full, or half etc. stops works best, but the TTL meter in your camera sees it differently, it cares not for any of those prepackaged numbers and uses what it feels are the actual values, then it tries to use the shutter speeds it can actually set,.. ( back to those 1/3 stops, as shutter speed is not infinitely variable .. ) so as to get as close to what the camera thinks is correct exposure, and suddenly two lenses with the same rating are getting differing shutter speeds.

This may be where your discrepancy arises...

This is also true of focal length, rare is the zoom lens who's described focal range is exactly accurate.


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bohdank
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May 21, 2009 15:43 |  #8

nureality wrote in post #7964886 (external link)
by film lenses you mean Motion Picture cameras... and then you are correct. By saying film lenses as you did, you confuse some people. Movie cameras use T-stops, still cameras use f-stops.

But I've found the same phenomenon as you have. The bottom line is, if you are comparing one focal length to one that is twice as wide, the wider one is allowing light from many more angles of incident (about 2x as much), so more lights rays = more light = different exposure.

I think its funny how some people will take this opportunity to point out that handheld light meters don't take focal length into consideration, and stick anchovies in their ears so as not to hear your response. But if you were to try shooting 12.5mm f/4 @ 1/125, 25mm f/4 @ 1/125, 50mm f/4 @ 1/125, 100mm f/4 @ 1/125, and 200mm f/4 @ 1/125 all ISO 100, from the SAME POSITION, in the SAME LIGHT (read controlled light setup, with same lights - not the sun), you would find 5 differently exposed shots, even though they all have the same exposure. And the 12.5mm would be the brightest, and the 200mm would be the darkest.

It has nothing to do with light gathering ability of wider FOV's. If you stood in the same place and was able to take a shot of big grey wall, so that in each case it filled the frame and was evenly lit, you would get the same exposure readings for all the setups.

The problem when you change FOV is that now there are things in the frame that weren't there with the tighter FOV. They may be darker or lighter affecting the meter reading, unless you are using spot.

If you add what CyberDyneSystems said in his post, and he is right, it complicated it even more.


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May 21, 2009 15:58 |  #9

I was just shooting the same scene with 3 different lenses, and I noticed that with one of the lens's to obtain the same exposure the shutter speed was about cut in half (same ISO and aperture). No Filters

Is that strange?

If you are really confirming that the lenses require one full stop different shutter speed to render the exact same final tones (not camere meter suggestions as different FOV can screw that up) then yes - one full stop difference is excessive.

I guess if one lens contains clearer glass, it would be brighter. I guess this is why film lenses use T stops instead of f stops.

Modern optical coatings allow lenses (the whole thing) to transmit over 95% of the light. This makes T-stops irrelevant which is why nobody reports them anymore. The differences due to transmission loss are going to be something like 1/24th stop. One full stop is totally out of the question.

Most, (especially zooms) if not all lenses when shooting at there max apertures, are in fact only "close to" the rated aperture.

Getting the correct numbers from all those elements added up in a tube is not an exact science,. all manufacturers use 1/2 or 1/3 stop increments for the ratings, regardless of where the actual measured number ends

1/3 stop error in quoted aperture is reasonable. 1 full stop is not.

IMO either the lens in question has a rather large error in quoted aperture or (more likely) the OP looked at the scene with two different focal lengths mounted and mistranslated a metering error due to different FOV with a reflective meter.


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Madweasel
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May 21, 2009 16:30 |  #10

nureality wrote in post #7964886 (external link)
...The bottom line is, if you are comparing one focal length to one that is twice as wide, the wider one is allowing light from many more angles of incident (about 2x as much), so more lights rays = more light = different exposure...

But that is why we use the focal ratio. The physical size of the effective aperture is larger on the longer lenses, exactly compensating for that effect.


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May 21, 2009 17:17 |  #11

The f/stop shown in the engraving may NOT be the true f/stop! An f/2.8 rated lens might be truly f/2.7 or f/2.9, for example. Tests published in magazines, when done well, have reflected exactly this difference between engraving and reality! And that would explain why there might be an observed 1/2 EV difference between two lenses from two manufacturers.


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May 21, 2009 19:44 |  #12

nureality wrote in post #7964886 (external link)
But I've found the same phenomenon as you have. The bottom line is, if you are comparing one focal length to one that is twice as wide, the wider one is allowing light from many more angles of incident (about 2x as much), so more lights rays = more light = different exposure.

I think its funny how some people will take this opportunity to point out that handheld light meters don't take focal length into consideration, and stick anchovies in their ears so as not to hear your response. But if you were to try shooting 12.5mm f/4 @ 1/125, 25mm f/4 @ 1/125, 50mm f/4 @ 1/125, 100mm f/4 @ 1/125, and 200mm f/4 @ 1/125 all ISO 100, from the SAME POSITION, in the SAME LIGHT (read controlled light setup, with same lights - not the sun), you would find 5 differently exposed shots, even though they all have the same exposure. And the 12.5mm would be the brightest, and the 200mm would be the darkest.

AGAIN with this nonsense!
Checked your ears for anchovies recently?




  
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Wilt
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May 21, 2009 21:29 |  #13

nureality wrote in post #7964886 (external link)
I think its funny how some people will take this opportunity to point out that handheld light meters don't take focal length into consideration, and stick anchovies in their ears so as not to hear your response. But if you were to try shooting 12.5mm f/4 @ 1/125, 25mm f/4 @ 1/125, 50mm f/4 @ 1/125, 100mm f/4 @ 1/125, and 200mm f/4 @ 1/125 all ISO 100, from the SAME POSITION, in the SAME LIGHT (read controlled light setup, with same lights - not the sun), you would find 5 differently exposed shots, even though they all have the same exposure. And the 12.5mm would be the brightest, and the 200mm would be the darkest.

Incident meters read the light falling on the scene...that does not change with FL in use. Any difference you think you associate with FL change is merely the inclusion or exclusion of more bright areas (sky) or dark areas (shadow) which is within the Field of View...but is NOT focal length related.

Put a camera on a tripod and aim it at a uniformly lit wall. Any lens you put on the camera will register the same reading because the uniformly lit wall is always filling the frame, regardless of the FOV and the FL of the lens.


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USER876
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May 21, 2009 22:36 as a reply to  @ Wilt's post |  #14

I used a 10-22, 24-105, and 55-250.

Photographed the same scene. The 10-22 and 55-250 seemed to follow the sunny 14 rule using the camera meter in ap. priority mode. The 24-105 had shutter speed of half the other lens's to obtain the same exposure ( the pics all looked the same). Just wondering why the 24-105 needed one stop more light




  
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Wilt
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May 21, 2009 22:40 |  #15

USER876 wrote in post #7966921 (external link)
I used a 10-22, 24-105, and 55-250.

Photographed the same scene. The 10-22 and 55-250 seemed to follow the sunny 14 rule using the camera meter in ap. priority mode. The 24-105 had shutter speed of half the other lens's to obtain the same exposure ( the pics all looked the same). Just wondering why the 24-105 needed one stop more light

Because the scene content's relative mix of light and dark areas in the scene resulted in a different brightness reading.


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