Jump to content

Metering issue, DA on film body


ricardo_tomasi

Recommended Posts

Hi,

 

Just went through a dozen shots with the DA 18-55 and SA 50-200 on a K1000 body,

using ISO 100 slide. After some shots I noticed that something was off. Having

no aperture ring, the lens is always wide open (3.5-5.6), but despite this, I

had to use shutter speeds as low as 1/30 in bright sunlight to get the camera

metering right.

 

After I got home, I checked with a M lens at f/4. When using the digital lenses,

at the equivalent aperture, the in-camera meter acts like there is up to 6 stops

less light.

 

Any thoughts on what's going on? Should I aim for the correct exposure as the

meter is telling me, or should I use exposure settings based on a manual lens?

Which will be the correct exposure?

 

I don't know anything about the inner workings of the K1000, but maybe the light

sensor is located outside of the image circle, where the DA lens can't reach?

 

I'd appreciate any suggestions, thanks.

Link to comment
Share on other sites

Pentax lenses set to the "A" setting are, by default, at their smaller lens opening on bodies

like the K1000 that do not support the aperture regulation from the body and depend on

the lens mechanisms to regulate the aperture.

 

DA lenses are akin to Pentax lenses with an "A" setting permanently fixed to "A". The

automatic iris function works, even on bodies without "A" function support, but at

exposure time the lens will stop down to f/22 or whatever it's minimum aperture is. Thus

the long exposure times required for proper exposure.

 

Godfrey

Link to comment
Share on other sites

I know the DA lens are not designed for use on a film body, but after reading dozens of comments of "I wonder how..." I decided to try it myself. Having an ultra-cheap 18mm is very appealing, and as I thought I'd be shooting at f3.5-5.6 it would be very useable. Guess I skipped some basic lessons...

 

A question keeps hammering on my head: how does the poor K1000 predicts the aperture!? If the aperture lever running distance is related to the aperture values, could I block it half-way to use a wider aperture? :)

 

Anyway, even at f22 it might be useful, as I don't have any really wide lenses for film. The downside is that the distortion is really horrible, and the perspective changes are really weird as you point the lens around. Same with the 50-200, where you can actually see the pincushion effect bending everything at the long end.

 

Thanks for helping!

Link to comment
Share on other sites

<center>

<img src="http://www.gdgphoto.com/grp-walking/pannyL1_21mm.jpg"><br>

Panasonic L1 + Pentax DA21mm f/3.2 Limited<br>

</center><br>

 

The K1000 and other Pentax bodies designed for use with pre-A series lenses have a

follow-finger that fits into a slot in the lens mount. This slot, on other series of Pentax

lenses, has a mechanical position stop so that the body knows what aperture the lens is

set to independent of the iris actuating lever and can meter at full aperture. <br>

<br>

The default position of this mechanical stop in the K/M/A/F/FA/D-FA lenses is at the lens'

minimum aperture, which is also where the mechanical aperture ring is when you set the

"A" position on the ring control; as you open up the lens, the mechanical stop pushes the

follow finger to inform the body of other aperture settings. When you use the aperture ring

to set the lens opening with these lenses, at exposure time the iris actuator is simply

pushed by the body's actuating lever to the limit that the aperture ring has set<br>

<br>

A-series lenses and the bodies designed to work with them on the "A" position

communicate the aperture setting electrically, the body ignores the follow finger signal.

On these lenses, at the time of exposure the body's iris actuating lever moves the lens' iris

lever the correct distance to achieve the desired aperture. <br>

<br>

The DA lenses do not have the mechanical regulation mechanism and follow stop, they are

designed to communicate with the body electronically and the lens opening is set by the

body's iris actuating lever. However, since the slot is there and the default position is

honored (as if the lens were simply permanently set at the "A" position"), the K1000 body

knows the exposure time lens opening will be minimum aperture and achieves correct

exposure as a result, assuming that the metering range can accommodate f/22 with the

amount of light that's in the scene and the sensitivity setting. <br>

<br>

The trick that I use to achieve larger lens openings with a 4/3 System body and a DA

series Pentax lens, as seen in the picture above, would not work with the Pentax bodies

because 4/3 System bodies don't have a mechanical lever to work the iris actuator at all:

these bits are just hanging about back there, unconnected with anything. On those bodies,

fitting the DA lens with an adapter, I can use a little bit of museum putty and "glue" the iris

actuator to a useful lens opening like f/5.6-f/8. <br>

<br>

This kind of solution could jam the mechanical actuator on the K1000 and other Pentax

bodies, or worse fall off and get caught up into the shutter mechanism. I don't recommend

trying anything like it. <br>

<br>

Godfrey<br>

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...