zoom flash incompatibility with all Nikon D-SLR's!

I just posted a similar message to rec.photo.digital, but I figure the

people here are more Nikon savvy. I hope one of you can find a flaw

in my argument, and prove to me that this really isn't as bad as I

think... I've been a Nikon fan for a long time - and noticing this

problem and hearing the tech support guys lame response to it has been

a bit disheartening.

 

The digital SLR's have a sensor that is smaller than film, so we get a

cropped image, and have to talk of things like "equivalent focal

length" when comparing to film cameras. Zooming to 24mm on a digital

SLR is like 36mm on film.

 

High end flashes have a zoom feature. For telphoto, they concentrate

the light to increase the guide number, for wide angle, they spread it

out for full coverage.

 

The problem is that in auto-zoom mode, the flash will track the ACTUAL

focal length, not the EFFECTIVE focal length. At a lens zoom of 24mm,

the flash zoom is also at 24mm. But it should be at 35mm! It's

illuminating a wider area than the camera is capturing!

 

The Nikon rep said they "consider this a minor issue, having no impact

on pictures, guide numbers, or overall performance of our equipment."

But I don't buy it. Let's do some math, shall we?

 

A frame of 35mm film is actually 24mm by 36mm, for an area of 864 sq

mm. This is the area that the flash will illuminate in it's auto zoom

setting.

The digital SLR sensor is only 16mm by 24mm, for an area of 384 sq mm.

384 divided by 864 is .44, or 44%

 

Thus, 56% of the light coming off the flash is illuminating an area

that is not being recorded by the camera.

 

So, if the flash fires at maximum, it's only giving you 1/2 the guide

number as advertised. When it doesn't fire at maximum, it's using

twice the battery power as was actually necessary.

 

The Nikon rep said this focal length discrepancy is common to all the

Digital SLR's, and all the flashes, not just the D100 / SB-50DX

combination I've got. I asked if they could update the camera

firmware so it corrects the focal length data sent to the hot shoe,

and he flat out said no.

 

So it seems the only fix to get the advertised power of the flash is

to keep it on manual zoom. And that is hardly convenient! They

advertise these flashes as being fully compatible with the digital

models, but it seems that's not so.

 

Anyone know if the Metz 54MZ compensates for the wrong focal length

being sent to it? I've been told that it's the only 3rd party flash

fully compatible with all the Nikon digital SLR features.

 

Thanks.

 

Darren Cokin

"<i>It's illuminating a wider area than the camera is capturing!</i>" - Darren Cokin<P>This isnt a HUGE problem. When your flash auto-zooms, it is covering an area still wider than your lens does. This could actually be a good thing, because you can be assured that the flash will cover your picture area plus a little more.<P>On the other hand, like you said, you may get less power efficiency, but I doubt you are losing half the power.<P>In regards to a solution to this issue, I don't think it should be so hard to fix. When I was looking at the EXIF data from my D100, it also wrote the 35mm focal length equivalent. If the camera knows what the 35mm FL equivalent is, then the camera could use that to set the autozoom instead of the lens' actual FL on 35mm. This could probably even be corrected with firmware updates, if Nikon wanted to.......

I was actually wondering about that tonight as I was testing a sb28 (non dx) on D100. I was <i>hoping</i> that the DX flashes would compensate for the crop factor and zoom in more. It would sure be nice! It's really too bad it's not the case! On the other hand, the flashes ARE giving us the guide number exactly as rated. They're just wasting a lot of battery power when mounted on a D1 series camera (or D100). That being said, I usually use some sort of reflector or sto-fen, so the issue is partially moot for me.

<p>

Hey, now you've got me thinking...I wonder if some sort of fresnel lens could be mounted that would focus the beam more, to match the crop factor. You know, a less extreme version of what wildlife photogs use to get catchlight in bird and big game eyes. If so, it'd fix everything you're talking about. The reach would be longer at max power, and in D-TTL the flash would quench sooner...The trick would be to find somone who could make one that would condense the flash just the right amount. It'd be the opposite of the w-a thingy that's built into the flashes now.

<p>Does anyone know if there is such a thing already?

Darren,<br><br>

theoretically you're right. From the practical point of view, there's not really a power loss. Check your manual: for the SB-28 in the range between 18mm-28mm the difference in GN between two focal lenghts are 2 meters; the most "critical" range is between 35mm and 50mm with a delta of 6 meters. That means that in worst case you "loose" max. 15% of the available power. Nevertheless, if the flash fires at maximum power, you're still getting the advertised guide number. For saving batteries the most effective way (without changing to an higher-speed film) is to open-up the aperture.<br><br>

BTW, I seriously doubt that 3rd party manufactures know the Nikon flash system better than Nikon itself.

I think this is a "tempest in a teapot" issue. The difference in terms of "wasted power" is trivial. Hey, most of us remember when there were no "zoom features" on flashes at all.

Having just bought a new non-DX flash prior to buying a D100 I don't want to outlay or trade in against a DX variant. Are there any aftermarket alternatives that D-TTL yet?

think about this , the image being projected onto the film/sensor is the same size. the sensor of the digital only captures about 75% of that image. there is no difference between the focal length of the lens, to the plane of the sensor or the film. so why shouldn't the flash work properly with either? it would be like using smaller film, the multiplier isn't a pysical distance multiplier, only a cropping factor. i think your rep was wrong, there is no issue i can see....

By simply leaving out the flash illumination from your calculations you can deduce that all digital photography is underexposed. Right?

 

Of course not. The "compensation" you want is by necessity implicit in the determination of the sensitivity of the sensor. Changing the firmware according to your request would give overexposed flash shots.

 

In fact, by your logic, even with normal 35mm film you would need a stronger flash if you intended to crop the image later.

Yeah, Douglas is right in saying your rep was wrong. This isn't a minor issue, it's not an issue at all.

 

To expand on my previous post, With a given lens on a DSLR vs a FSLR, the amount of flash light being reflected from the subject is the same if the flash is at the same zoom setting. This corresponds exactly to what you'd get with any other light source. The Sun doesn't suddenly shine twice as bright (or get concentrated into half the area) just because you use a D100 rather than a F80.

 

With a change in firmware somewhere, it should be possible to illuminate further out, something like a 40% higher GN (at the loss of close distances). But in the current state, you haven't lost anything, you merely haven't gained what you never expected to get.

"But in the current state, you haven't lost anything, you merely haven't gained what you never expected to get"

 

i am going to have to find a place and time to use that sentence...

thanks for the affirmation Ivar...