Underexposure problem on D70/SB800

[johnd]

Hi all, I hope someone can throw some light on an issue I am having with the D70 and SB800 combination. Using a Sigma 150mm f2.8 macro lens on the D70 with the SB800 in TTL mode, I am consistenly underexposing by about 2EV. I am shooting macro subjects indoors about 0.5metre away, f16, 1/250sec, ISO200, manual mode with the flash mounted in the D70 hotshoe and set to TTL mode (not TTL BL). When I try the same setup with the D200 instead of the D70, it still underexposes but only by about 1EV.

I'm really confused by this, because I though the SB800 would work out the required light intensity. I'm allowing enough time between shots for the SB800 to recycle. I did notice on the back of the SB800 it tells me the focal length of the lens is 105mm when in fact it's 150mm so I don't know if this has anything to do with it.

It seems a very consistent amount of underexposure so I guess I could force the SB800 up by +2EV or +1EV respectively, but before I do that I'm trying to work out if it's me, the cameras, lens, SB800 or what.

Any ideas?

Thanks
John

[gstark]

John,

Is the lens casting a shodow and causing the problem? With the flash in the hotshoe, and a subject fairly close to the camera, this can sometimes occur.

Try using the flash off camera, off to one side.

Also, at close distances, sometimes the flash has trouble metering correctgly, but I'd be willing to wager this is a lens shadow issue.

[phillipb]

John, you say that you are using the camera in manual mode. If this is the case then it is not up to the flash to determine the correct exposure, regardless of ttl or ttl-bl

[Justin]

I do know about:

focal length of the lens is 105mm when in fact it's 150mm

The SB800 zoom head will stretch to 105mm based on a 35mm frame. So based on that, it is zoomed to full stretch and still not reaching your 150mm - DX 225mm of power prime!!!

Some macro setups I have seen (on the net, no personal exp.) use a diffuser - have you tried perhaps an A4 sheet of matt white cardboard to help reflect the light?

I posted a thread a while ago with how to use the SB800 remotely (it's a bit of a pain to set up)

As Gary says 0.5m is real close and if the subject is 50cm from the lens - have you tried this with a shorter lens?

I just checked sigmaphoto.com - http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3283&navigator=5 - that is a beast of a lens! That very well may be a big factor.

[gstark]

As Gary says 0.5m is real close and if the subject is 50cm from the lens

In fact, the subject will be 50cm from the focal plane. If the lens is 15cm in length, then add a couple more cm for the thickness of the camera's body, and you're left with maybe a subject that's 31cm or so from the front of the lens. If the the lens is longer, the distance will be even less.

[Justin]

Hi Phillip just tried this on my D200 - camera in manual - TTL is fine, there is no TTL-bl. I tried multiple shots at different ISO and the pictures have come out substantially similar exposure in the viewfinder.


ah on the tokina 12-24- hang on no ttl-bl on the nikon 18-200 either.... hmmm

[Justin]

D'oh!!! matrix metering mode for ttl-bl - in that case it's all fine even in manual mode (I'm assuming manual mode on the camera body and ttl / ttl-bl on the flash)

[phillipb]

Justin, what I was trying to say is that if you use the camera in manual mode then the correct exposure will be determined by the combination of shutter speed and aperture that you choose. For example, say the correct exposure was f8 1/125 sec with ttl flash using aperture priority, if you then put the camera in manual mode and select f16 1/250sec with ttl flash, it is obviously going to be underexposed.

[Justin]

Hey phillip gotcha - I just set it to 1/30 and f3.5 (correct exposure) in mode A, fired, then 1/30 and f6.3 in mode M and fired, obvious differences in exposure.

So the question goes back to the metering

[johnd]

Justin, what I was trying to say is that if you use the camera in manual mode then the correct exposure will be determined by the combination of shutter speed and aperture that you choose. For example, say the correct exposure was f8 1/125 sec with ttl flash using aperture priority, if you then put the camera in manual mode and select f16 1/250sec with ttl flash, it is obviously going to be underexposed.

Thanks to everyone for the quick response. I think I have a lot to learn about the SB800.

I was assuming that by putting the camera into manual mode at f16 1/250 sec, then I was deliberatley underexposing with respect to the ambient light, but with the SB800 in TTL mode, the SB800 would know the correct light required from the camera's metering and would make up for the underexposure with it's own light. Have I got this wrong?

Maybe it's time to RTFM again.

Thanks for the help,
John

[johnd]

John,

Is the lens casting a shodow and causing the problem? With the flash in the hotshoe, and a subject fairly close to the camera, this can sometimes occur.

Try using the flash off camera, off to one side.

Also, at close distances, sometimes the flash has trouble metering correctgly, but I'd be willing to wager this is a lens shadow issue.

Thanks Gary, yes the 150 Sigma is a big mother, especially with the lens hood on. Casting a shadow is a very real possibility. I have an SC28 so I can try it off camera.

Cheers
John

[Justin]

Hey John you are not the only one!

[johnd]

I just checked out the angles to see if it was lens shadow. I don't think it is, but here's something I found. With the SB800 mounted on the hot shoe, you can't get the flash to aim below the horizontal. Looking along the barrel of the lens, I think the angle of light from the flash doesn't converge with the line of view of the lens until you get about 1 metre away. So maybe the light is going over the top of the subject when I'm shooting something a half metre from the camera.

Philip, on your point about the camera being in manual mode stopping the SB800 computing the correct light setting, if I put the camera into A, S or P mode, would the SB800 compute the correct light setting then?

Cheers
John

[phillipb]

Yes, that's when the camera and flash are talking to each other to work out what's right, but in manual mode all they're saying is bugger you, do what you want, you're the boss.

[johnd]

Thanks Philip. So it's probably me using the camera in M that's causing the issue. So it's A, S or P mode to get the camera and flash to talk to each other.

Gary, with the shadow issue and the need for off camera flash, I have a solution for that which I like better for macro work: R1C1

Some more intensive flash practice required.


Cheers
John

[phillipb]

Thanks Philip. So it's probably me using the camera in M that's causing the issue. So it's A, S or P mode to get the camera and flash to talk to each other.

John

Not necessarily, if you want to use manual exposure, then why not use manual flash as well. you just need to experiment. You can dial in 1/2 or 1/3 or even 1/4 power on the flash and see which one gives your desired effect with your chosen f stop and shutter speed.

[gstark]

Gary, with the shadow issue and the need for off camera flash, I have a solution for that which I like better for macro work: R1C1

That's certainly an <strike>expensive</strike> elegant solution.

I'd probably look for some sort of bracket that mounts the flash on the side and forward of the body. Easy to make if you want to, and then use wireless to trigger it.

[johnd]

Gary, with the shadow issue and the need for off camera flash, I have a solution for that which I like better for macro work: R1C1

That's certainly an <strike>expensive</strike> elegant solution.

I'd probably look for some sort of bracket that mounts the flash on the side and forward of the body. Easy to make if you want to, and then use wireless to trigger it.

Good idea Gary. Though perhaps I shouldn't have used the word "need". Maybe "want" is a better word. To tell the 100% truth, I've been wanting an R1C1 for quite a while now. The main macro photog in our house is my wife, Alison. If I get her an R1C1 then she will be set for her macro work and I get my 2nd SB800 back. So we both win. Sounds like just the sort of Christmas present a wife would want. Better than an iron or a chainsaw anyway.

Cheers
John

[johnd]

Thanks Philip. So it's probably me using the camera in M that's causing the issue. So it's A, S or P mode to get the camera and flash to talk to each other.

John

Not necessarily, if you want to use manual exposure, then why not use manual flash as well. you just need to experiment. You can dial in 1/2 or 1/3 or even 1/4 power on the flash and see which one gives your desired effect with your chosen f stop and shutter speed.

Spot on Phillip. I'm actually doing this already in some other work of mine. Using twin SB800s off camera controlled by pre flashes from my D200. I've gotten the hang of this by taking a shot, chimping, changing power of flash remotely from camera or even just moving the flash closer or further from the subject. This is all done M on D200 and Remote mode on SB800s.

I think I was trying to have it both ways with the D70 in manual mode.

Thanks for your help,
Cheers
John