SB-900 and gels — exposure question

[Kent Shafer]

<p>I've been experimenting with the SB-900 and the two orange gels that came with it: TN-A2 (light orange) and TN-A1 (dark orange). I've found that if you set the camera's white balance to Flash, as recommended in the SB-900 manual, the flash tells the camera what gel it's wearing and the camera automatically sets the correct white balance. Really amazing.</p>

<p>But where I'm having problems is with exposure. It's fine with naked flash, but images exposed with the light orange gel are underexposed by about 0.8 stops, and those exposed with the dark orange one are underexposed by about 2.3 stops. (These numbers are based on the exposure slider in ACR, which I assume is calibrated in f-stops. Anyway, the underexposure is very significant.)</p>

<p>I can't believe the camera/flash system wouldn't compensate for the light absorbed by the gels and am sure I must be doing something wrong but can't figure out what it is. Can someone point me in the right direction?</p>

<p>I did the tests by shooting white seamless from about four feet away so the answer can't be that the flash couldn't produce enough power. I used a D700 and 24-70 with the SB-900 mounted in the hotshoe and the following settings:</p>

 

<ul>

<li>Camera exposure mode: Manual (1/200 @ f/5.6)</li>

<li>ISO: 200</li>

<li>White Balance: Flash</li>

<li>Flash exposre compensation on camera: 0.0</li>

<li>SB-900: TTL Mode with 0.0 exposure compensation</li>

</ul>

[lornesunley]

<p>Apparently, the D700 will under expose if D-lighting is turned on in the camera and set to a value above the lowest one ... also, make sure the flash did not get into TTL-BL mode.</p>

<p>Shooting white seamless with TTL mode on the flash will expose the white as 18% grey. That would account for the 1+ stop under exposure. but that should happen with or without the gels. I'll try it out later tonight and see if I can duplicate the problem (I have a D700 and an SB-900)</p>

[Kent Shafer]

<p>Lorne, thanks.</p>

<p>Well, I'm an idiot. It turned out the ISO was actually set to 1600, not 200 as I thought, and the problem was that without the gel the flash couldn't reduce its power enough to expose properly. So the ungelled test shot was overexposed rather than the other way around. With the ISO set to 200, the exposures are very close with and without the gel.</p>

<p>Sorry to have wasted everyone's time. I will now have a dry martini as penance.</p>

[frank_skomial]

<p>Make sure that filter color confirmation indicator shows presence of the filter and the filter type, on your SB900 flash LCD display.<br />See SB900 Manual on page D35.</p>

[lornesunley]

<p>Good to hear that you figured it out ... less work for me ... now I'll finish my wine</p>

[rodeo_joe1]

<p>Lorne, actually an 18% grey exposure should underexpose a white backdrop by 2.5 stops, which ties in pretty closely with what Kent observed. And Kent, you're not the only one to have made that mistake. I did the exact same thing the other day and wondered why two different flashguns had apparently gone identically bananas and started overexposing. ISO was set to 3200 and not 200 - Doh!</p>

<p>Another recent idiot mistake was accidentally turning on redeye reduction instead of adjusting flash compensation. I never, ever use redeye reduction and wondered why my camera suddenly started having a long "think" before releasing the shutter - double doh!</p>