580EX II Zoom Range

[Jose]

Canon makes an issue of telling the user that this flash "automatically recognizes EOS Digital camera's image size and automatically sets the flash coverage for lens focal lengths from 24 to 105 mm."

When I use the flash with Canon EOS 1Ds MKII with a 16-35 mm L II it reads from 35 to 24 and if I go beyond 24 still reads 24. Fair enough, they tell us about this.

When I use the flash with a Canon 40 D and the same lens it still reads from 35 to 24. But then this lens is actually approximately a 25 to 50 mm when used with the 40D. Why would not the flash read from 24 (close enough to 25) to 50 mm?

[grsphoto]

When you change bodies, it doesn't change the laws of physics.

A 35 is a 35 no matter which body it is on... the sensor just crops in tighter on the image.

[Jose]

Originally Posted by grsphoto

When you change bodies, it doesn't change the laws of physics.

A 35 is a 35 no matter which body it is on... the sensor just crops in tighter on the image.

That is correct but the sensor's cropping reduces the area of coverage to about a 50 mm so should not the flash concentrate its illumination to an area of about 50 mm?

For example with the 40D and the 16-35 mm if let the flash in automatic, the range of the flash at 35 mm is about 7 meters but if I go manual, and set it to 50 mm the range becomes 12 meters. I did test the lens and flash in automatic so that the flash was at the focal lens of the lens (35 mm), then I set the flash to 50 mm. The results with the flash at 35 mm was that the illumination resulted in an unexposed image while when set at 50 mm the illumination was better distributed; I used the side of a barn as a subjet.

[gryphonslair99]

The flash is adjusting to the FOV of the lens, not the FOV of the sensor. That guarantees that FOV of the lens is covered, and not clipped just a bit. Standard flash, lens compatabiliy since zoom flash heads have been made.

Now if you want to pay around $1000 for the same flash that will calculate for a FF, 1.3 crop, 1.6 crop, 1/1.7, 1/2.5 or any other sensor size that Canon cameras use that will accept an external flash. And you can guarantee sufficent sales of the units at that kind of price that Canon will make a profit, then I'm pretty sure that they would build it.

It's a matter of economics. As grsphoto said, a 35mm is a 35mm no matter what body you put it on. Makes it easy for the flash to sync with the lens.

[Jose]

"The flash is adjusting to the FOV of the lens, not the FOV of the sensor."

That is exactly what I am addressing; the flash does not adjust to the FOV of the lens based in the size of the sensor of the camera in use as Canon claims.

Page 21 of the 580EX II manual states that "the Speedlight recognizes the EOS DIGITAL camera's image size and automatically sets the flash coverage for lens focal lenghts from 24 to 105." I am not experiencing this feature with the Canon 40D and 16-35 mm L II when set at 35 mm.

[swampler]

Originally Posted by Jose

Page 21 of the 580EX II manual states that "the Speedlight recognizes the EOS DIGITAL camera's image size and automatically sets the flash coverage for lens focal lenghts from 24 to 105." I am not experiencing this feature with the Canon 40D and 16-35 mm L II when set at 35 mm.

Yes you are. Note the part I bolded..."lens focal lengths." It does NOT say 35mm equivalent focal length.

[jfrancho]

I don't understand the issue? Are you not getting the coverage from your flash unit?

[Jose]

What I am trying to say is that the 580EX II claims that it adjust automatically to the digital camera sensor in use but this not appear to be correct. As the size of the sensor in the camera changes so does the focal lens of the camera in use. As explained before, using the 16-35 mm with a Canon 40D it becomes a 25.6-56 mm. When using this combination and zooming to 35 mm, the flash zoom adjust to 35 mm; I maintain that it should adjust the zoom to the 50 mm. Using the 40D in P I shot the images below as set by the camera (both with identical f stop, shutter speed and ISO) but shot one as set by the flash zoom at 35 mm and then manually change the zoom to 50 mm. And there is a difference (disregard that both images were out of focus, underexposed and the presence of a hot spot--other topics of discussion). The first image was shot with the flash zoom at 35 mm and the second at 50 mm. It is obvious to me that with the flash zoom in the 50 mm the image exposure was better. I did not manipulate this images in any manner when converting to jpeg format from RAW.

[swampler]

Originally Posted by Jose

What I am trying to say is that the 580EX II claims that it adjust automatically to the digital camera sensor in use but this not appear to be correct.

I understood what you were trying to say, but the flash does not claim that it adjusts to the camera sensor in use. It claims to adjust to the lens focal length, and it does.

As for your first example of underexposure, that is what I would expect. You're shooting a white wall, which the camera will try to make middle grey, as it should, thus underexposing the image. You have to use a positive exposure compensation (+1 or so) when shooting a scene that is mostly white to get it looking correct. And on top of that, you have a window in the image which will reflect the flash. Canon camera's try to protect from blown highlights. This means that if one shot had more flash reflecting from the window than the other, that image will be darker because the camera tries to prevent the reflection from getting blown out.

[grsphoto]

Originally Posted by Jose

What I am trying to say is that the 580EX II claims that it adjust automatically to the digital camera sensor in use but this not appear to be correct. As the size of the sensor in the camera changes so does the focal lens of the camera in use. <snip>

NO it does not! Changing the sensor size does not change the laws of physics. A 35 is a 35 no matter which body you put it on. The sensor size just adjusts how much of the image that the lens produces, gets used. It is a crop factor..ie it is the similar to cropping in tighter on the image.

( note for the techno geeks ( like me) I said similar, not the same, there are some differences in DOF and edge distortion but for 99% of shooters these don't matter, and I don't think they are important in this conversation)

Glenn

[Jose]

So if I put a 2x extender in a 200 mm lens it does not become a 400mm focal length lens? The image is 2x larger? Does not a 1.6X sensor does the same other than preserving the original f stop of the lens?

And yes, I do concur tha the original lens focal lens remains the same but the use of the 1.4X extender or the 1.6X sensor alters its original focal lenght.

[grsphoto]

Putting an extender on a lens does alter it's focal length, you are changing changing the physic of the lens ( glass and distance)

By using a different sensor you do not change the distance or the glass, all you are doing is using a smaller sensor that gets blown up to a larger size...cropping the image.

A 1.4x entender changes the physical properties of the lens, while changing the cropping factor ( sensor size) does not change anything about how the lens moves light.

Glenn

[jfrancho]

Originally Posted by Jose

So if I put a 2x extender in a 200 mm lens it does not become a 400mm focal length lens? The image is 2x larger? Does not a 1.6X sensor does the same other than preserving the original f stop of the lens?

And yes, I do concur tha the original lens focal lens remains the same but the use of the 1.4X extender or the 1.6X sensor alters its original focal lenght.

A teleconverter does actually affect the focal length, since there are lens elements involved. An extension tube does not, but merely allows closer minimum focusing distances. Sensor format does absolutely nothing to the focal length of a lens. All Canon flashes provide full coverage at the FL specified when zooming, regardless of format. It does this be being calibrated for a full frame FOV at those FL increments. Since an APS-C sensor is a field of view crop camera, the area is smaller, and therefore no adjustment is necessary.

Again, I don't see the issue. If you want to make a comparison of flash coverage between two different sensors, then I suggest setting the camera on a tripod, use manual flash only, and do not move the camera when switching bodies, and you'll see that there is zero difference in the exposure. ETTL-II along with differences between camera distance to subject between formats introduces to many variables to be considered anything viable.

[Jose]

Originally Posted by grsphoto

Putting an extender on a lens does alter it's focal length, you are changing changing the physic of the lens ( glass and distance)

By using a different sensor you do not change the distance or the glass, all you are doing is using a smaller sensor that gets blown up to a larger size...cropping the image.

A 1.4x entender changes the physical properties of the lens, while changing the cropping factor ( sensor size) does not change anything about how the lens moves light.

Glenn

No argument here but the cropping magnifies the images; same effect as the use of an extender (optical v.s. digital). And the issue if not one of coverage; it is one of exposure...as shown in the images presented, when the lens is set at 35 mm and the flash head as 35 mm, the image is underexposed compared as to when the lens is at 35 mm and the flash head is at 50mm (note that illumnation across the image is even at both the 35 and 50 mm flash zoom settings). My point is that the Flash should automatically set the zoom to 50 mm rather than 35 mm since in the Canon 40D the 1.6X sensor makes a 35 mm a virtual 50 mm.

[jfrancho]

Originally Posted by Jose

No argument here but the cropping magnifies the images; same effect as the use of an extender (optical v.s. digital). And the issue if not one of coverage; it is one of exposure...as shown in the images presented, when the lens is set at 35 mm and the flash head as 35 mm, the image is underexposed compared as to when the lens is at 35 mm and the flash head is at 50mm (note that illumnation across the image is even at both the 35 and 50 mm flash zoom settings). My point is that the Flash should automatically set the zoom to 50 mm rather than 35 mm since in the Canon 40D the 1.6X sensor makes a 35 mm a virtual 50 mm.

You are confused. Cropping magnifies nothing, and is nothing similar to a teleconverter. The reason the image is underexposed in the first example has nothing to do with the zoom range selected, but is a byproduct of ETTL metering. It is well known that Canon's flash system underexposes consistently by about 2/3 stop. If you look closer at the second image, the center is slightly brighter, due to the narrower beam produced by selecting a longer zoom setting. The image is still underexposed.

[taygull]

I think what he is trying to say is....the flash does not compensate for the crop and it should...(I kinda agree).

What would be interesting look at is set a 40D next to a 5D. Put a 50mm Lens and a 580EX on the 5D and then put your 16-35 on the 40D along with a 580 EX, shoot the same subject with everything being the same.

Set the 40D to a focal length of 31.25 which should be very close to capturing the same as the 5D as far as amount of subject in the image. I'd be curious to see how the lighting would be different since the 40D would adjust the spread to a 31.25 but it would tell the 5D to spread the light to 50mm.

[grsphoto]

A 35 is wider then the 50. The spread from a flash at 35 should be wider then the flash at 50.

[grsphoto]

Originally Posted by jfrancho

<snip> It is well known that Canon's flash system underexposes consistently by about 2/3 stop. <snip>

I have to disagree here... in my very unscientific testing ( experience with many systems at my workshops) the more expensive the body, the less under exposed the exposure system is, for both Nikon and Canon. This seems to be in all modes, including manual.

So I agree with the underexposure being built in, just not the amount.

Glenn

[jfrancho]

I, and many other xxD series users, find this to be true. I have no experience with 1 series cameras.

[grsphoto]

From my experience, you are right for the xxD series, the rebels and the g series underexpose more, and the 1 series are close to being right on... If I don't get any test shots, I usually start my compensation about 1/3 to 2/3 of a stop down on my MkII, .

Sorry for the threadjack.

Glenn