C100: canon 24-105 F4....not an F4?

Darren Levine

Darren Levine

Veteran
i've been using the lens for a bit now, and i noticed right off the bat that it's definitely not a F4 constant. it seems at least a half stop darker at the long end. was a bit disappointing considering the praise of L lenses. i do love it, but seems it should be correctly labeled as non constant. anyone else had this experience or otherwise?
 
When I switch from the 70-200 f4 IS to the 24-105 f4 IS, without changing any setting on the camera, everything is underexposed by at least one stop. I agree, it is a bit deceiving, although the lens is nice and useful.
I haven't noticed exposure not being constant throughout the zoom range, but could also possibly be.
 
same here, I have the 28-300 F3.5-5.6 , when I put my aperture at F8, I still see 2 stop light loss when zooming from 28-300

this is cheat
 
As far as my understanding goes, the aperture is constant throughout the zoom range, but the transmission efficiency (or T-stop) may not be.
 
This cant be right. Naturally when you zoom in and less light is present then yes but the only way to really know if this is true is by shooting a perfectly even scene like a white seamless edge to edge perfectly flat lighting. Step back and zoom in to recompose the exact same shot and check waveform.
 
This is true. The 24-105, like most still photography zoom lenses with constant aperture stops, does not have a constant t-stop. It loses about half or 2/3 stop of light at the long end compared to the wide end. Most still photo zooms have the same issue, and it's always worse the longer the zoom range. Still a great and versatile lens(I love mine), but if you use still photo zooms, you're going to have compromises.
 
cowpunk52 said:
This is true. The 24-105, like most still photography zoom lenses with constant aperture stops, does not have a constant t-stop. It loses about half or 2/3 stop of light at the long end compared to the wide end. Most still photo zooms have the same issue, and it's always worse the longer the zoom range. Still a great and versatile lens(I love mine), but if you use still photo zooms, you're going to have compromises.
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Yep. Most photo zooms are like that. on the 24-105 Brian is right, it's about 2/3 a stop. Zoom in and open up 2/3s and vice versa. The lens still rocks though on the C series.
 
I don't have a C100 to test this out, but I've generally found that when zooming in on my 24-105, I'll notice some light loss, but I'll gain it back by closing down then opening back up. This may be due to the fact that, as a stills lens, it's not meant to keep its constant stop while actively zooming, but the true f4 would be activated when actuating the shutter.
 
Couldn't Canon fix this with software? Perhaps a mode that adjusts the ISO slightly while zooming certain lenses?
Should definitely be something you could disable if needed. <--had to say that in case Canon's actually reading this.
 
Here is some myth busting for you:
1. the only difference between f stop and T- stop is the internal glass reflection. due to multi coating this has been brought to near zero so the difference between f and T stoops in modern lenses is negligible- less than 1/4 stop.
2 all lenses would be more likely to slow down towards the long end because the f stop is a ratio between the FL and the aperture size so the longer the lens the harder it is to keep the (apparent- not actual) aperture size big enough to maintain the f stop ratio.
Hence the lens designers tend to put "optimistic" f stop numbers towards the long end.
3 the last post lost me- lenses are kept wide open for focusing- why would Canon stop it down for focusing just to open up for the shot. That makes no sense
 
Interesting, thank you for letting this know! I think of going to Tamron 24 70 2.8 IS lens, hear good things about it.
 
It's like Canons 100/2.8 L macro lens. It looses about two stops of light when you go to 1:1 macro. The lens is a F2.8 lens but looses light to achieve macro. All macro lenses works the same way.
 
OlegKalyan said:
Interesting, thank you for letting this know! I think of going to Tamron 24 70 2.8 IS lens, hear good things about it.
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Just FYI I was testing this in store today using my C100 and the autofocus would not work correctly with it. I tried two of them on the C100 and the autofocus wouldn't correctly work. I threw the lens on a 5D3 and it was working properly...
 
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