Is it true the 24-105 loses a stop of light on long end?

Just read some other people saying it loses a t stop at the long end.. Anyone have experience with this? If so will the exposure meter realize this and compensate?

i don't see why it would...

DreDaze wrote in post #17186909
i don't see why it would...

transmission issues with many lenses apparently.

No. This is easy enough to debunk.

No. I actually checked a number of Canon lenses for differences in transmission, and the best of all that I owned was the 85L II which was less than 1/6th of a stop and the worst was the 70-200/2.8 IS which was close to 1/3 stop. The 24-105L is in the same 1/3 stop range.

If you want to talk about light loss, the 24-105L does actually have very heavy vignette at the wide end. So at least a part of the frame is not so bright as you would expect I guess.

If you believe the DxOMark website, http://www.dxomark.com …-4L-IS-USM---Measurements, the lens actually gets slightly faster at the long end. The lens is, however, closer to f/5 than f/4.

After opening this link, click on the "Transmission" link to see the results.

An odd question perhaps, but I wonder if I have an inkling where this is coming from.

Any chance you are zooming while shooting video or in LV and somehow keeping the aperture 'locked'?

If you do this you can also 'gain' a stop by zooming out.

I'm not sure I can explain this phenomenon, so please don't ask any more. If you have the lens, try zooming while shooting video staring with the exposure you want at the wide or tele end and you should see what I mean.

Larger FOV captures more light in general. Smaller FOV captures a tiny scene with less light. It's not the lens. It's the light upon the scene.

I first noticed this when shooting video. It does get darker when you zoom from 24 to 105 but I can't say exactly how much. I would estimate 1/2 to 2/3 EV.

I don't notice it if I shoot wide open at f/4 but it is very noticeable at f/8

Here is a video sample:
http://youtu.be/52qMoI​3K1RE

msowsun wrote in post #17188268
I first noticed this when shooting video. It does get darker when you zoom from 24 to 105 but I can't say exactly how much. I would estimate 1/2 to 2/3 EV.

I don't notice it if I shoot wide open at f/4 but it is very noticeable at f/8

Here is a video sample:
http://youtu.be/52qMoI​3K1RE

again every lens will do that. You are capturing less light from the scene. an area the size of a stamp only reflects so much light.... an area the size of a paper reflects more light even though the ambient light has not changed.

same way a FF sensor is better at low light, it has larger pixel size that can capture more light then an equal crop sensor with smaller pixels.

I am not convinced.....

Why doesn't it do it at f/4? And why doesn't my 70-200 2.8 II do it?

(I only notice this during Video and not with Stills)

Here is another Video shooter who notices the same thing: http://briancweed.com …e-canon-24-105-f4-is-lens

Talley wrote in post #17188352
again every lens will do that. You are capturing less light from the scene. an area the size of a stamp only reflects so much light.... an area the size of a paper reflects more light even though the ambient light has not changed.

This is why f shop is a function of focal length -- ie the apeture is 6mm wide @ f4 @ 24mm and 26.25mm wide @ f4 @ 105mm -- as you rightly say, you've chucked away much of the scene, but then the apeture is physically bigger to make up for that.

That's why we use f stops not apeture diameters - f4 is the same amount of light through the lens, regardless of focal length.

I've not watched the videos but i'd guess what's more likely happening is that the cameras meter is guessing a different exposure for the zoomed in scene, which is understandable -- cameras meters are not perfect. Try doing the same thing with the camera on manual.

I'm sure you can meter a white wall or something. All I know is Aperture is only the technical opening. Transmission can differ based on glass elements and coatings. My 50 1.2 L didnt transmit 1.2 in light, maybe 1.4 in light levels.

with a zoom, there has to be some corrections done to maintain the same light levels, and it wouldnt be surprising if levels dropped due to the corrections.

RMH wrote in post #17188454
This is why f shop is a function of focal length -- if the apeture is 6mm wide @ f4 @ 24mm and 26.25mm wide @ f4 @ 105mm -- as you rightly say, you've chucked away much of the scene, but then the apeture is physically bigger to make up for that.

That's why we use f stops not apeture diameters - f4 is the same amount of light, regardless of focal length.

I've not watched the videos but i'd guess what's more likely happening is that the cameras meter is guessing a different exposure for the zoomed in scene, which is understandable -- cameras meters are not perfect. Try doing the same thing with the camera on manual.

Right metering.

I forgot the metering plays a big part in it. I just checked my 24-105 and had manual settings in place during video and did not notice any light changing.

My 70-200 F4 IS however did... I believe it was the vignetting though. It darkened up from 70 to 200 again with manual controls dialed in.