Weird Vignetting on Pentax 645n with flash?

Hi all,

 

I've recently had some problems with a 45-85mm FA lens that I just purchased off of ebay to be used with my Pentax 645n.

I was doing a test with Portra 400, my pentax body, this new lens as well as my contax tla 200 flash unit (the little one).

 

I was doing a few tests on AE metering mode, at various focal lengths (45mm, 65mm, 85 mm etc.), at -1, 0, +1 exposure stop settings to see how the new lens would react with the flash.

 

The results kind of have me freaked out, as when I shoot with the wide end 45 or 55 mm there is extreme vignetting as you will see:

 

45mm

 

http://i.imgur.com/vHTkKWc.png

 

65 mm minor vignetting

 

http://imgur.com/2sGLuh9.png

 

55 mm, +1

 

http://i.imgur.com/vR6zsNK.png

 

45 mm

 

http://i.imgur.com/wBa00SF.png

 

 

My question is: what is causing this vignetting?

Granted I have no tested this lens with just ambient light no flash.

 

Another question for those familiar with metering with the Pentax 645n, when I place the flash on top of the camera, does it meter taking into consideration there is a flash unit or does it meter as normal i.e. nothing on top of the flash?

 

Of course I am using the contax TLA 200 in manual mode as there is not TTL communication between the Pentax and the contax. however, do the dials on top of the contax flash unit have any impact on the flash itself? I.e. the 45 mm, 90 mm etc. settings? Or it has no functionality?

 

Any help would be great!!

 

 

I don't know Contax flashes. Usually a focal length dial reshaapes the flash's built in reflector since flashes tend to barely ever be powerful enough. Focusing their little bit of light for telephoto lenses might gain up to one fstop extra reach. And yes 90mm on 35mm should be enough light for 135mm on 6x4.5cm.

Maybe read out the effects of your flashe's zoom reflector with a flash meter?

 

Looking at hotshoes of RTS II & K10D the Contax communications pins seem 1mm further towards the VF than Pentax' so hopefully no misleading communication.

Pentax TTL metering needs to know there is a charged Pentax system flash switched on via the before mentioned pins. Otherwise metering will only read ambient light.

I'm thinking this is a flash coverage issue. Looking at the specs on the TLA200 I see it covers from 28 - 90mm, and that's referring to coverage on a 35mm camera. Your shooting with a 45-85mm zoom on a 6x4.5 camera. That lens on that film size should give you an angle of view roughly equivalent to a 28-53mm lens on a 35mm camera. Should work, but your question about the flash units controls has me wondering where you have the flash set. I've never used a TLA200 but there is a good chance that the zoom control will work even when not on a Contax camera. You might be able to see this as you switch the selector. As Jochen said, flash units usually reshape their reflectors when you change the zoom settings. If you look at the flash lens as you change the setting switch you may be able to see movement inside the flash telling you that it is adjusting. You should be able to hear something happening too. That's a pretty low power flash and if not set wide enough I'd expect sharp vignetting. I've also noticed over the years that lower powered flash units tend to be a bit optimistic in their coverage claims. You may find that you need to have the flash set wider than you'd think would be needed.

Hi Jochen and Ed, thanks for your feedback! I just did another test with and without flash. Without flash there is almost zero vignetting and playing around with the flash unit setting it at the widest setting 28mm, I experienced no vignetting either even at the widest end of the zoom! So problem solved.... I didn't think it would make a difference seeing how small the flash unit is but wow what difference it makes