N90S aperture priority mode problems HELP!!

[sillago]

Hi all
I am hoping someone can help me with this problem I am facing with my setup

I am trying to take face portrait shots of fish using a n90s with a 105mm micro in a ikelite housing with 2 DS-125 strobes. Upon suggestion of most people I know, they recommended I use aperture priority mode on the n90s, ttl on the strobes and basically adjust the F-stop leaving the computer in the camera to decide the shutter speed.
The problem that I am facing is that I can't seem to adjust the F-stop on the camera while it's in the housing...is this 1) because I am missing some sort of adaptor that attaches to the lens that allows me to adjust the F-stop on the lens or 2) because my camera is actually not working properly because I can't adjust the F-stop using the command dial on the camera itself (LCD just shows F value of the lens and not surprisingly LO to indicate that it is underexposed)
Whatever it is, the end results is that when I take the shot, the strobes fire but there is a delay in the shutter speed because of the object being apparently underexposed...so the pictures just turn out being very blurred
Anyone have any clues what I am doing or what is wrong? Any advice would be greatly appreciated
Thanks
;-0

 

[bdombrowski]

Sillago:

I'm cross posting my reply here just incase others are interested or/and have similar problems. Hope you don't mind.

Brian


Sillago:

Sorry I haven't been to the Scuba board in quite some time.

The problem with apeture priority is...

You shouldn't use it! Especially with macro shots.

I know Ikelite says to use it in the manual, but they are completely wrong here. When you use apeture priority, you set the lens apeture manually using the apeture gear control knob, then the camera sets the shutter speed auto-magically to try to properly expose the film using the available light coming through the lens.

One problem with this is that the camera will almost always put the shutter speed to the slowest speed it can while in strobe sync (1/60th) unless your shooting up towards towards the sunlight. You'll never shoot towards the sunlight to create a balanced image (strobe lit foreground and available light background) using a 105mm macro lens. You only do this kind of stuff with a wide angle lens (ie. 20mm.) So, the shutter will almost always be set to 1/60th, which is too slow for most moving subjects. You'll get a blur.

I recommend you only use the N90s in manual mode underwater. In this mode you set both the apeture and shutter speed manually. You can still use TTL to control the duration of the strobes. I usually shoot all my macro at 1/250th on the shutter speed, which is the fastest strobe sync speed available. This keeps the images sharp, especially with moving subjects. The apeture needs to closed down as much as possible while still allowing the strobes to properly expose the film. This gives you the best possible depth of field, which is the challenge in macro work. This is generally between f/11 to f/22, depending on the reflectivity of the subject. I use f/16 a lot. At anything tighter than f/22 its likely the strobes will fire at full power and still under expose the image.

What you need to do to bracket your exposures underwater using TTL metering is this: Change the ISO speed setting to 1/2 and 1 full stop faster than the film's ISO. This technique is known as TTL bracketing. The pros say to take one attempt at the film's ISO, then another at 1/2 stop faster than a final at one stop faster. For example, if I'm using 100 ISO film, take one frame at ISO 100, one at ISO 160, and a final at ISO 200. I don't usually do this myself. I just shoot 2 frames, one at 1/2 and 1 full stop faster. Ikelite says when using dual strobes to set the film ISO one whole stop faster when using two strobes in TTL . I also found that this is a good idea even when using just one of their stobes in TTL. The Ikelite N90S housing allows you to changes the film's ISO underwater, so this is a great feature.

Now, about your apeture control. You should have Ikelite gears fitted to all your lenses that allow you to control the apeature underwater. If you don't have them, get them. If you're impatient, you might be able to get away setting the apeture to a fixed f/16 or f/22 before you put it in the housing.

Since you have the 105mm lens, you should also have a gear to allow you to do a manual focus underwater. The auto focus on the 105mm doesn't work well underwater. Most people use the manual focus on this lens. The Ikelite port for the 105mm AFD lens has a control to switch between auto and manual focus. When in manual focus, the zoom gear control on the housing works the focus gear on the 105mm lens. With the 60mm lens, the auto focus works fine.

If you want to see some of my results with the N90s, you're welcome to check out www.subsurfacemedia.com

Feel free to send questions or comments.

I hope this helps.

Best Regards,

Brian