Canon 24-70mm not constant 2.8?

[Kim Frank]

I just shot a Musicvideo with a lot of fast Zooms.
In post I recognized a brightness loss about a half stop
on the longer end of my new 24-70mm Zoom.
Made CC really complicated.
Is that normal (I don't hope so!)

[Scott Crawley]

The 24-70L is consistant from end to end. I don't know of any reason why the lens would loose light on the tele end, and it doesn't stop down on the wide end.

[Kim Frank]

Okay thanks...
Than I picked a bad one.

[Roberto Lequeux]

Others here have reported the similar findings.

[Kim Frank]

Called the shop where I bought it and he tried to explain to me, that this is normal.
He said that this would have to do with the film modus of the 5DII.
What to believe.
I'm going to give it a test picture vs. film,
but maybe someone knows.

[Kim Frank]

Others here have reported the similar findings.

Thanks Roberto, but does "the same" mean it's normal or its a faulty lense`?

[Roberto Lequeux]

the film modus of the 5DII.

The Whha?!

Smells like :icon_bs:


Thanks Roberto, but does "the same" mean it's normal or its a faulty lense`?

It would seem that it is "normal". I'll try to post a comparison for you.

[Roberto Lequeux]

These are 20% horizontal but full vertical crops with the same region roughly in the same area of the image circle. Same distance and lighting of course. RGB measurements from the PS eyedropper tool.

[Kim Frank]

Thank you very much!
That's exactly what I mean.
And I always thoght it would be constant.

[Roberto Lequeux]

Might be worth pointing out the obvious, one measurement won't be an very accurate quantification of the light loss.

[Stephen Williams]

If you stop down to T4 does the same problem happen? Most 2/3" ENG lenses suffer wide open. Motion picture lenses cost a lot of money, this is one of the issues they have sort out.

[Roberto Lequeux]

That comparison I posted was at f/3.5 which is probably around T4.

[Stephen Williams]

That comparison I posted was at f/3.5 which is probably around T4.

Try 1 stop down from wide open, I would expect it to be fine.

[Kim Frank]

Well the lense has a F 2.8.
The video I shot was at a 5.6
and the shift was there.
But I'll give it another shot

[Michael Hastings]

These are 20% horizontal but full vertical crops with the same region roughly in the same area of the image circle. Same distance and lighting of course. RGB measurements from the PS eyedropper tool.

What do those numbers translate to in F stops?

[Jannard]

That's just one of the reasons you pay more for a cine zoom.

Jim

[Matthew Duclos]

That's just one of the reasons you pay more for a cine zoom.

Jim

EXACTLY!!!!
Thank you Jim.

[Kim Frank]

What would be a full frame cine zoom with a T3 and how much would it cost.
I'd say correcting half a stop difference in post is more in my budget range.

[Scott Crawley]

He said that this would have to do with the film modus of the 5DII.


I presume he was referring to the shooting mode and/or metering mode.

[Kim Frank]

Don't know if I understand?
Always talking manual settings.
No auto metering. But he was wrong.
It's a lens issue, not camera or shooting mode

[Scott Crawley]

I see.

I've never used mine to shoot cine, so I never gave it any thought until you asked. So I shot a couple test frames and I do see it with my gear.

It seems counter intuitive. At the tele end the elements are closer together... Mr. Duclos, do you care to offer a brief primer on the subject?

[Alexander Ibrahim]

Mounted on a RED, I see very little shift in exposure. Ditto on my 7D.

What I am seeing here is too much for the lens alone to be at fault.

I don't actually own that lens though, so maybe I am mistaken.

If you are shooting with a Canon 7D or other recent HD DSLR check that "Auto Lighting Optimizer" is turned off. That can cause a shift like this.

I wrote elsewhere on REDuser about that setting, which causes a lot of trouble with lighting levels.

http://reduser.net/forum/showpost.php?p=533872&postcount=4

[Charles Angus]

I see.

I've never used mine to shoot cine, so I never gave it any thought until you asked. So I shot a couple test frames and I do see it with my gear.

It seems counter intuitive. At the tele end the elements are closer together... Mr. Duclos, do you care to offer a brief primer on the subject?

I'm not 100% sure as to why this is, but many lenses are slower at the long end. I have never seen one that is slower at the wide end.

I always figured it's because the physical aperture is must be larger at the long end for the same f/ stop, and that that wasn't possible.

[Roberto Lequeux]

And let's not ignore that there are no hard stops on the focus ring. Also that the focus throw is about 120 degrees, IMO too short for comfort at the long end.

Here is the same comparison at f/5.6.

[KETCH ROSSi]

I just shot a Musicvideo with a lot of fast Zooms.
In post I recognized a brightness loss about a half stop
on the longer end of my new 24-70mm Zoom.
Made CC really complicated.
Is that normal (I don't hope so!)

I'm afraid so, we have been testing all kind of lenses, for my S3D set up to shoot "Rape of a Beauty" and off course considered the possibility to have a tight budget we look at Still lenses, both Canon and Nikon, as we know both Epic and Scarlet will offer this mounts with full controls, but the results were the same, all zooms tested showed differences form end to end, especially considerable was some of the light fall off, and Pincushion and Barrel distortions during their zooming, making them almost impossible for a S3D set up, unless we really want to spend a tone of money and time in post to fix the extreme Disparities, or give our viewers huge headaches and never get the financing.

On the other hand the RPZ 18-85 shoed incredible quality and well controlled movement with surprisingly almost non existing Barrel and Pincushion distortion, making it ideal for S3D set up, but only when Parallax settings are wide, do to the size of the lens, and or when set up on a Images splitter rig.

Any way no still zoom lenses for me for shooting Movies, unless you use them as Vary Primes, and deal with the light fall off accordingly, which can still be a pain, having to re-meter the scene every time you zoom in and or out, but then again, my self I would not shoot with different focal using the same lights, unless they were okay off course which some times they are.

Also unfortunately this is some what more so an issue in some copies more then others, so if yours has lots of fall off, sale it and get a new one.

I have tested so many lenses before I got to find the kippers, that I wan't even tell you.

[Scott Crawley]

I'm not 100% sure as to why this is, but many lenses are slower at the long end. I have never seen one that is slower at the wide end.

I always figured it's because the physical aperture is must be larger at the long end for the same f/ stop, and that that wasn't possible.

Yes, I mis-spoke and stated it backward. Clearly I'm not as intimate with the particulars of how zooms are assembled or I wouldn't have made such an error of memory. Guess I should take this up with a book or two. :-) I have some Canon video lenses that close down up to 1 1/2 stops at the long end making them slower.

To the best of my rather poor recollection I had some much older zooms years ago that required special care at the long end. It was a mechanical thing I think... A danger of physical damage to the iris blades by mashing them into the mount or mirror... But I was a kid then, and now I'm a rambling, older guy. :blush2: Sorry Kim.

[Kim Frank]

So I tried to do a little test by my own.
It's my first time doing this, so please be patient, I'm open for advice.
I shot my neatvideo chart, had nothing else.
The pictures are just Photoshop screenshots, to get the RGB values.

The first two pictures shot at 2.8 show a brightness loss at 75mm of about half a stop. The RBG values change about 8-12 points

The third and the forth picture shot at 5.6 just show a RGB change of about 3-5 points.

In the fifth picture I opened up a to a 5.0 what gives a RGB change of about 12-16.

I don't know how to do such a test the correct way. Please tell me.
I want to know if the brightness loss really gets less by closing down the appeture.

4341

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[Stephen Williams]

Always stop the lens down from wide open, I would expect the lens to be worst wide open & get better as stopped down, that's basically happening.

[Kim Frank]

Yes I knew that in terms of image quality but not about appeture consistance on a zoom lens.

[Stephen Williams]

I am talking about image brightness, which is what this thread is about. Stopping the lens down, so backlash is taken up in the iris.......

[Brandon Ruiz]

It's the lens.

Over the last few months, more than one Canon dealer has discouraged me from buying the 24-70L, saying it doesn't deserve the "L" designation due to softness, distortion, and vignetting - which has been echoed by others. They've all told me to wait for the rumored IS replacement.

[Roberto Lequeux]

Just 0.02, common sense thoughts which you probably already took into account:
_ It seems you lit from overhead, so the higher you measure the higher the values you'll get.
_ Noise will give you slightly different values, so if you want such accurate measurements you'd have to get several measurements and average them out, quite a bit of work.
_ Make sure you measure off the originals or you'll also have to worry about compression artifacts which could give you wildly different readings close to high contrast areas.

[Justin McAleece]

It's an okay lens. I've been using it for a year on the Birger and it definitely ramps on the long end. I would say at least half a stop when at 2.8 and 70mm. It telescopes which sucks the most and overall is a little soft. It's decent but the 16-35 is better for those reasons. It would be pretty awesome if Canon brought out a 16ish-85ish 2.8 that solved some of those problems. I would sell 2 lenses and pay $2K.

[Daniel Browning]

In post I recognized a brightness loss about a half stop
on the longer end of my new 24-70mm Zoom. Is that normal?

No, it is definitely not normal. I traded my 24-70 for a 24mm f/1.4, but I'm certain that the 24-70 I had did not do this. I shot it in manual mode all the time, and I think I would have noticed a half-stop change every time I zoomed.

I think veiling glare is a more likely explanation. Music videos like to have a lot of flaring lights, and at 24mm you'll get more glare/flare from them then at 70mm. The flare is what increases the brightness and gave you the impression of ramping.

If you take an image of an even-toned subject with no sources of glare, I think you'll find that the f-number is indeed constant with zoom.

[Kim Frank]

But Daniel, did you take a look at the quick tests Roberto and me shot?

[Scott Crawley]

It's the lens.

Over the last few months, more than one Canon dealer has discouraged me from buying the 24-70L, saying it doesn't deserve the "L" designation due to softness, distortion, and vignetting - which has been echoed by others. They've all told me to wait for the rumored IS replacement.

For the kinds of still work that I frequently do, it is one of only two lenses that I carry. The other is my favorite, the venerable 70-200L. The 24-70 isn't as sharp, but then I know of no other zoom that is. OK, it has a fall off of brightness at the very end, but we are talking about a half stop. It's still a fast lens, and for still work it is perfectly adequate.

Dan makes a good point. I'll admit that my own test was quick and dirty. I'll give it a more careful go later.

The fact that it does telescope was going to preclude it from getting any Cine use on my future Scarlet, so this fall off news isn't all that upsetting to me. It's good to know though. Thanks for pointing it out.

[Roberto Lequeux]

FYI I didn't have the hood on for those, but there was a lamp shade on that light.

[Daniel Browning]

But Daniel, did you take a look at the quick tests Roberto and me shot?

Yeah, that doesn't look like flare. I must be wrong.

Another possibility is that the f-number is indeed constant, but this is a difference in vignetting. On full frame, there is a conspicuous bright center spot and rapid falloff at 24mm, but since you're using S35, most of the frame is taken up by the bright spot. At 70mm, the bright spot evens out and the falloff is smoother.

[Scott Crawley]

I reshot with a more careful approach and will leave you to your own conclusions. This was for brightness, so critical attention was not paid to focus.

I shot a medium grey paper backdrop and placed 20 targets diagonally across the center. I spot metered just above each target, and entered the average reading on the target for each of the 2 exposures. Camera distance remained unchanged.

I repeated x4:
24mm metered at 1/60 f4.0 8 shot at 1/60 f4.0 ISO 100
http://farm3.static.flickr.com/2761/4265930343_bf77b73bce_o.jpg

70mm 1/60 f4.0
http://farm5.static.flickr.com/4022/4265930319_60e79701da_o.jpg

70mm metered at 1/60 f2.8 5, shot at 1/60 f2.8 ISO 100
http://farm3.static.flickr.com/2762/4265930277_6766692561_o.jpg

24mm 1/60 f2.8
http://farm5.static.flickr.com/4021/4266676052_1815a3dd87_o.jpg

[Stephen Williams]

That's a really horrible lens IMHO, OK 70mm F4 but not much good anywhere else.


I reshot with a more careful approach and will leave you to your own conclusions. This was for brightness, so critical attention was not paid to focus.

I shot a medium grey paper backdrop and placed 20 targets diagonally across the center. I spot metered just above each target, and entered the average reading on the target for each of the 2 exposures. Camera distance remained unchanged.

I repeated x4:
24mm metered at 1/60 f4.0 8 shot at 1/60 f4.0 ISO 100
http://farm3.static.flickr.com/2761/4265930343_bf77b73bce_o.jpg

70mm 1/60 f4.0
http://farm5.static.flickr.com/4022/4265930319_60e79701da_o.jpg

70mm metered at 1/60 f2.8 5, shot at 1/60 f2.8 ISO 100
http://farm3.static.flickr.com/2762/4265930277_6766692561_o.jpg

24mm 1/60 f2.8
http://farm5.static.flickr.com/4021/4266676052_1815a3dd87_o.jpg

[Scott Crawley]

That's a really horrible lens IMHO, OK 70mm F4 but not much good anywhere else.

I appreciate your opinion, but I hope you didn't come to that conclusion based soley on my test shots, since they only represent 2 focal lengths and 2 individual stops. "Anywhere else" is simply not represented here.

What I see is vignette at the short end that is fairly significant wide open. It isn't uncommon in still zooms. I don't know which others are better in this regard. Does anyone? I'm curious now to k ow how it compares to others. Sure one might expect better from an L series lens, but I can deal with it myself. Heck I'm so used to it that it didn't even occur to me when I first read the OP.

It appears more pronounced when the iris is open. Would that indicate that it is related to the curvature of the elements?

Cheers

[HansSteinert]

I have the exact same problem with my 17-40 f4 L lens... except it's arguably even more extreme than your findings.

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

[Jeff Kilgroe]

This "ramping" effect is very common on zooms, especially SLR zooms and other lenses rated in F-Stops, not designed to maintain constant light transmission across the entire zoom range. An F-Stop or F-Number represents the ratio between the diameter of the aperture and the focal length of the lens. Lots of things happen inside a lens as focal lengths change and elements move. Just because a lens may maintain a constant aperture ratio (F-Stop) across an entire zoom range, it does not mean that it maintains consistent exposure or light transmission, which are measured in T-Stops.

As Jim said earlier in this thread, over a year ago, this is one very good reason why people spend the extra money on a cine-lens that can maintain constant T-Stop or light transmission at all focal lengths. Keep in mind that the F-Number ratio can change over the zoom range as the lens adapts to maintain the desired T-Stop. Or what that means is a lens could be T2.8 across the entire zoom range, but could be F3.0 at one focal length and F3.3 at another. Likewise, if you take a lens that is constant F2.8 across the entire zoom range, it could be T2.9 at one end and T3.6 at the other end of the zoom range.