Canon 17-40mm loses light as you zoom in?

H

HansSteinert

Well-known member
Those of you who have video mode or live view mode, let me know if this happens to you:

Go to f8-f13 (it happened to me when I was filming within that range).

Zoom in and out while recording.

See if the image is much darker when zoomed in. Looks like I'm almost losing a half stop at 40mm compared to 17mm.

My experience:

40mm image:

http://img15.imageshack.us/img15/2958/40mmq.jpg

17mm image:

http://img6.imageshack.us/img6/7382/17mmf.jpg

17mm 100 crop:

http://img263.imageshack.us/img263/2596/17mm100crop.jpg

All I did was stand in the same place with the same settings and zoom in/out.

Here's a video of it:

http://vimeo.com/27173912

I bought this lens from Canon refurbished a few weeks ago. Not sure if this is normal or not.
 
This is pretty normal. I'm not exactly sure why this is the case but here's my guess; If you imagine the lens capturing photons which are coming toward it, when you zoom in, you're rejecting a large proportion of the light presented by the scene. In addition to this, you're also spreading the light which you're not rejecting over a larger number of pixels, each pixel therefore will receive less light. Even with a constant aperture it makes sense that the exposure lowers when the focal length is increased.
 
Something is funky about this. When I zoom the lens on my HMC-150 ( or any normal ENG style video camera ) with a specific F-Stop set ( say f/4.0 ), the image does NOT get brighter or darker. A fixed F-Stop should be a fixed F-Stop, and will expose like prime lenses do. ( provided vignetting is not an issue )
 
TheDingo said:
Something is funky about this. When I zoom the lens on my HMC-150 ( or any normal ENG style video camera ) with a specific F-Stop set ( say f/4.0 ), the image does NOT get brighter or darker. A fixed F-Stop should be a fixed F-Stop, and will expose like prime lenses do. ( provided vignetting is not an issue )
Click to expand...

Cinema or video zoom lenses are much different than still zooms and often much more costly. With a still camera zoom you can have changing f-stop as you are not zooming on one frame. The opposite is true for a lens designed to work with moving pictures.
 
phudsontx said:
With a still camera zoom you can have changing f-stop as you are not zooming on one frame. The opposite is true for a lens designed to work with moving pictures.
Click to expand...

My understanding has always been that still photo zooms that are designed to have one constant F stop ( not so common these days ), will maintain that one constant F-Stop over the entire zoom range. I shot stills professionally for 10 years, and when I shot models with fixed F-stop zoom lens using a narrow exposure range slide film like Kodachrome 64, I could use the entire zoom range during the shoot without having to adjust my strobe-packs, and every frame I shot had the identical exposure, despite zooming in for a tight head-shot or zooming out for a full body pose.

re: T-Stops vs F-Stops

T-stops represent the amount of light transmitted by a lens, as the optics of a lens can absorb a portion of the light, thus a lens with many optical elements may absorb enough light to show a significant difference between the F-stop of the lens and the T-stop of the lens. A still photo zoom lens with a fixed F-stop design will not vary it's light absorption depending on the zoom setting, it will normally transmit the same amount of light over it's zoom range.

It seems to me that today's still photo lens designers are not always being honest with their F-stop designations if a lens is supposed to be a fixed F-stop zoom design but actually varies it's F-stop when actually used.
 
the aperture may stay constant at 17mm and 40mm but that does not mean the exposure does

the exposure is also affected by the lengthening of the zoom chasis/body. Hence the need for T stop - a way to measure constant exposure
 
This looks more like electronic compensation by the camera for the highlight overexposure as you zoom in than something the lens is doing on its own. Does it still do this if you dial your exposure down a notch or two?
 
here's a thread about people testing the canon 24-70mm f2.8 and them finding similar results, half stop difference along zoom length
http://reduser.net/forum/showthread.php?39611-Canon-24-70mm-not-constant-2.8

also you might find this wikipedia entry interesting

http://en.wikipedia.org/wiki/F-number#T-stops

[h=3]T-stops[/h]Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The T in T-stop stands for transmission.[SUP][5][/SUP] The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness. Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly. Modern cinematographic lenses now usually tend to be factory-calibrated in T-stops. T-stops measure the amount of light transmitted through the lens in practice, and are equivalent in light transmission to the f-stop of an ideal lens with 100% transmission. Since all lenses absorb some quantity of light, the T-number of any given aperture on a lens will always be greater than the f-number. Consequently, the depth of field for a given T-number will be slightly less than that when the f-number is set to the same value, since the aperture diameter is slightly greater. In recent years, advances in lens technology and in film and sensor exposure latitude have reduced the need for T-stop values, although T-stops are still considered industry standard for cinematographic lenses[SUP][who?][/SUP].
Click to expand...
 
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Hmmm... This is interesting. I am now wondering if current fixed F-stop still photo zoom lens designs have sacrificed a true constant aperture compared to the lenses made in the 80's and 90's. I know that new lens design is a lot sharper than the old lenses, which I attribute to better glass and a lot more computing power available to lens designers. I will have to see if I can dig up some old constant F-Stop glass from 15 -20 years ago, and test this for myself. ( When I used to shoot pro stills, one quarter F-stop difference was quite noticeable on a roll of K64 )
 
J Davis said:
here's a thread about people testing the canon 24-70mm f2.8 and them finding similar results, half stop difference along zoom length
Click to expand...

I read through the whole thread and not everyone came to this conclusion. One member that owned this lens stated that he thought people were commenting on vignetting and not the exposure performance of this lens.
 
I am guessing that the longer the the focal lengths, and the broader the range of zoom the greater the change in exposure.

For example a Tokina 11-16 still zoom has very short focal lengths and a very short range. Duclos tested it at 100% transmission, its f stop equals its t-stop
 
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