iTTL flash issues

[ATJ]

I use my D70 in an Ikelite housing for underwater use. I also have two Ikelite strobes. I have been having occasional problems with overexposure and thought I'd do a bit of testing.

For some reason, I'm getting different results with different lenses. I mostly use the D70 kit lens (Nikkor AF-S DX 18-70mm f/3.5-4.5G) or a Nikkor AF Micro 60mm f/2.8D. I rigged up the camera on a copy stand and mounted one of the Ikelite strobes (DS-125) so that everything was in the same position between shots. For some reason the shots with the 18-70mm seem well exposed but those with the 60mm are over exposed.

18-70mm


60mm


I also tried using the internal flash and while the results with the 60mm are better, it is still more exposed than the 18-70mm.

18-70mm


60mm


I guess it could simply be that there is more background with the 18-70mm and so the average picture is different. However, I get similar results with just the background.

18-70mm with DS-125


60mm with DS-125


One final question... In the photos above, the 18-70mm was set to 70mm and yet, I get more magnification with the 60mm even though the camera was exactly the same distance from the subject. Why is that?

[digitor]

Hi ATJ,

Interesting. Can you please tell me what is the distance from the carpet to the "film" plane? I have both of these lenses, (but not a carpet of exactly the same shade ) so I could try an independent test.

Cheers

[ATJ]

Note that if I adjust the camera to -1.0 EV or the housing (which controls the strobes) to -1.0 EV when using the 60mm, I get very similar results to the 18-70mm.

First photo below is 18-70mm at 0.0 EV, the second is 60mm at -1.0 EV on the camera and the third is -1.0 EV on the housing.

[ATJ]

Can you please tell me what is the distance from the carpet to the "film" plane?

Digitor,

It is around 365mm from the film plane to the towel. It is not actually carpet but I did think to myself that it looked like carpet in the photos.

[digitor]

Can you please tell me what is the distance from the carpet to the "film" plane?

It is not actually carpet ...

Thank goodness for that! You should always know where your towel is... *

* Gratuitous HHGTTG reference

Cheers

[Alpha_7]

THat towel looks in very good nic, where are the stains ?

[gstark]

AT 365mm film plane to subject, you're very close to the subject. What is the light source top subject distance? My guess is that it's less ...

You're looking at about a foot ... probably less ... I suspect that you're simply too close and as a result finding it difficult to get even lighting, especially with strobes, which means that any attempt at metering is doomed.

[ATJ]

AT 365mm film plane to subject, you're very close to the subject. What is the light source top subject distance? My guess is that it's less ...

Around 310mm, so slightly less.

You're looking at about a foot ... probably less ... I suspect that you're simply too close and as a result finding it difficult to get even lighting, especially with strobes, which means that any attempt at metering is doomed.

Since when has there been "too close" for a strobe? I have been using strobes for macro work for years both with and without TTL and never had a real issue (although I generally prefer manual, but that's a real pain undewater).

The results above (and all my other tests) are consistent and the lighting certainly appears even to me. Each one of the photos posted above is simply resized to 640x425 nothing has been cropped and the lighting on the towel looks nice and even. Yes, the sultana box does show a bit of flaring from the flash, but other than that it is even.

The only thing that changes between the two lenses is that the 60mm results in about 1 stop of overexposure.

Well... maybe the strobe to subject distance is having an effect, although I don't understand why. I just moved the strobe to be 620mm from the subject (twice the distance, one quarter the light) and I now get around about the same exposure with both lenses.

Why is the effect of the closer strobe to subject distance different for the two lenses?

[ATJ]

18-70mm with DS-125 at 620mm

60mm with DS-125 at 620mm

[gstark]

Since when has there been "too close" for a strobe? I have been using strobes for macro work for years both with and without TTL and never had a real issue (although I generally prefer manual, but that's a real pain undewater).

Yes, but strobes for macro work are often lower powered, and/or placed in relation to the subject so that the camera and les doesn't get in the way.

In these shots, shooting the towel on its own will be different than shooting the box; that redusces the subject to flash distance even more, plus it adds a depth of subject issue: while the package may not be all that thick, relative to the amount of light being sent and the subject to light distance, it's a pretty reasonable percentage.

Let's go back to basics for a moment: what is your goal here? Let's look to solve the problem by addressing your goals, rather than by trying to correct what may or may not be wrong here.

[ATJ]

Let's go back to basics for a moment: what is your goal here? Let's look to solve the problem by addressing your goals, rather than by trying to correct what may or may not be wrong here.

My goal is to take underwater photographs of very small subjects such as nudibranchs and to have them well exposed.

For example, these two photographs was taken with the 60mm lens and as you can see they are overexposed.





It seems the simple solution may be to just set the camera or the housing to -1.0 EV.

[ATJ]

In these shots, shooting the towel on its own will be different than shooting the box; that redusces the subject to flash distance even more, plus it adds a depth of subject issue: while the package may not be all that thick, relative to the amount of light being sent and the subject to light distance, it's a pretty reasonable percentage.

Note that in these shots, you can even forget the package. Just look at what happens with the towel. With the exact same camera to subject and strobe to subject distances, the shots taken with the 60mm lens are overexposed by 1 stop relative to the 18-70mm lens (@70mm).