[Lowell Goudge]

Looking through the lens when I press the shutter fully I can see the k-x closing the aperture to the ring setting in Av mode with a Pentax M 50mm 1:1.7. It would make more sense if it did not as the resulting picture does not reflect the ring position, that is, all positions result in a exposure with a fully open lens and a shutter time to match that (as long as the amount of light does not change).

very strange, I never looked into my camera before (I am afraid the self portrait would brake it ) but having said that, I tried with my K7, aperture stays open,. Are you sure you are not thinking about the mirror flipping up?

[jolepp]

very strange, I never looked into my camera before (I am afraid the self portrait would brake it ) but having said that, I tried with my K7, aperture stays open,. Are you sure you are not thinking about the mirror flipping up?

The mirror goes up too, but the aperture gets closed as well. It is really strange as the easy way of implementing Av as it is (open aperture exposure) would seem to be not closing it. Actually I assumed this would be the case and said (wrote) to that effect on a different thread where someone disagreed, and so I had to try it and see :-)

[blende8]

speed is calculated when you press the green button. Afterward, you can change the e-wheel f-stop if you want - that'll change what's written to exif, but doesn't affect actual exposure.

Of course, this is normal in M mode.
The question is what happens in Av mode when you change the aperture on the body?

[matam]

Of course, this is normal in M mode.
The question is what happens in Av mode when you change the aperture on the body?

The body calculate the shutter speed difference between the smallest aperture available and the one you've selected on the body.

EX. the smallest F-stop is f1.4 and you choose f4, the body will read the light coming from the lens wide open and adjust the shutter by lowering its speed by 4stop (hope i'm not wrong here with the calculation )

@newarts:
thanks you're too kind! but i'm not as good with english as you may think

[newarts]

Of course, this is normal in M mode.
The question is what happens in Av mode when you change the aperture on the body?

All I've ever talked about in this thread is M mode!

Perhaps I didn't mention that enough times.

I'll see what happens in Av mode & let you know.
====================================================
Ok I'm back.
K-x camera, In Av mode, M42 lens with K bayonet adapter (no levers of any kind), pin "A" shorted.
As e-wheel f-stop is increased, the image is progressively overexposed (as expected), regardless of actual f-stop on lens.
corollary: for my Macro-Takumar f:4, in Av mode, e-wheel set at f3.2 gives ok exposure for actual lens f-stop from f4 to f16
=====================================================

Dave

[blende8]

All I've ever talked about in this thread is M mode!
Perhaps I didn't mention that enough times.

Ok, I'm sorry.

[lguckert79]

No messing with the lens, just a small, reversible modification to the camera's mount.

This modification is easily reversible & increases the utility of my pre-set lenses (Macro-Tak 50:4, MIR 65:3.5 tilt/shift, bellows, Pentax 135:2.5, among others) a bit.

Dave



"so doing this will alow me to use my camera in other modes than av and m right?"

[jolepp]

so by doing this i will be able to use other modes on my camera besides av and m right

That would be the motivation, also it seems that there is something interesting going on with the AWB (and/or exposure?) that might make a faux A-lens to work better (with the k-x).

[lguckert79]

i use a lot of m42 lenses and a few m ones and would love to use them in tv or p mode so thank you

[jolepp]

"so doing this will alow me to use my camera in other modes than av and m right?"

OTOH, the aperture mechanics being different would negate most of the convenience advantage of using auto modes.

[lguckert79]

ok so what modes would i use then?i have a k100d

[jolepp]

i use a lot of m42 lenses and a few m ones and would love to use them in tv or p mode so thank you

It would really require the modification the OP (I think :-) had in mind, that is, making the aperture mechanics to match to really gain anything useful with M lenses. M42 lenses would be a different matter altogether as they don't even have an aperture lever, TTL metering might work with this, though.

[jolepp]

ok so what modes would i use then?i have a k100d

Without the modification to aperture mechanics one could use M lenses in the auto-modes with a table of exposure compensation for each aperture setting so this would defeat any convenience advantage. Also, the compensation range of the body might not be sufficient for even this.

In a nutshell: modifying the contact to make the body believe an M lens is an A lens is not really useful by itself.

[newarts]

Making the camera think all lenses are type A does little or nothing to correct problems with no aperture lever or an incorrectly calibrated aperture lever. Camera modes like Tv or P are not truly enabled because the aperture cannot be controlled properly by the camera.

The advantages of making the camera think any lens is type A include:
1) P-TTL flash works
2) Multi-point metering works (this may depend on the camera model)
3) EXIF may be correct (because e-wheel and lens apertures may be adjusted to agree).

I have found no significant downside to this modification. The only ones I can think of are:
1) wrong f-stop info may be written to EXIF (because e-wheel and lens apertures may not agree).
2) using an A lens in non-A mode may require making the e-wheel and lens aperture agree.

As I recall, this modification to my K100D helped me get more consistent exposures with my m42 and other pre-set lenses in both flash & non-flash modes (Av mode worked IIRC.)

The effect of allowing P-TTL to work with non-A lenses is perhaps the most significant advantage. For example, P-TTL is quite useful for macro work with a non-A type bellows or lens.

I have found it very difficult to do the same thing using a ball of foil one lens at a time & no compelling downside to doing it semi-permanently.

Dave

PS

In a nutshell: modifying the contact to make the body believe an M lens is an A lens is not really useful by itself.

I'm not quite sure what "not really useful by itself" means. I listed some advantages above; these advantages may be slight, but are real.

[jolepp]

...
I'm not quite sure what "not really useful by itself" means. I listed some advantages above; these advantages may be slight, but are real.

With a way to automatically deal with the difference in aperture mechanics (K/M vs. A) this would be "really" (= much more) useful.