[JBox]

I am always interested in Pentax camera body and lens compatibility. Shortly after the announcement of the DFA* 50, I began to suspect that, it might work better on manual-aperture and aperture-priority-only film bodies than existing FAJ, DA, and DFA lenses that do not have aperture rings. This is due to reasons related to the new electronic aperture, which I will go into later. I rented a copy of the DFA* 50 to find-out how/if it would work on the LX specifically. I found that it actually works well, with the one major limitation that the only available aperture is the max of 1.4. Manual focus works as expected and aperture-priority auto-exposure also works. Again, it is aperture-priority in the sense that the aperture is fixed at 1.4. Below I will discuss why this works and why it works better than previous DFA lenses, as well as show some example pictures taken with the DFA* 50 and LX combo.

(I should note that I did this test quite some time ago, which is why it is summer in the pictures. I didn't post this at first because I was not thrilled with my scans. I recently looked at them again and thought they were not bad.)



Let's first discuss the technical aspects of the test, and try to understand if the situation might be better than previous DFA lenses. All previous K-mount lenses that are set to the A aperture setting or have no aperture ring will stop all the way down to the smallest aperture when the diaphragm actuator lever is let-out completely by the body. Aperture-priority-only film bodies can only let the diaphragm actuator lever out completely. Thus, a standard DA or DFA lens such as the DFA 28-105 (which has no aperture ring) will only be able to be used at the smallest aperture on such a body. In the case of the electronic aperture of the DFA* 50, there is no diaphragm actuator lever on the lens, so the attempts of the body to let-out the diaphragm actuator lever will do nothing. Further, no power is applied to the lens, and the default aperture with no power is open. The net result is that the aperture remains all the way open at all times. Thus, we are able to conclude that on an aperture-priority-only film body, the only available aperture on a standard DFA lens such as the 28-105 will be the smallest aperture, while the only available aperture on the DFA* 50 will be the largest aperture. While both are severe limitations, having only the largest aperture available is arguably a much more usable situation than having only the smallest aperture available. This is the key compatibility difference between the DFA* 50 and previous DFA lenses without aperture rings.

The remaining questions are: will the body know that the lens is wide-open and will auto-exposure work? This is where examining the mount of the DFA* 50 will tell us something:



On the left is the DFA* 50, and on the right is the FA* 28-70. The locations circled are approximately where the body expects the aperture ring coupler to be when the lens is wide-open. On the FA* there is an aperture range that the lens can indicate to the body (because it has an aperture ring). On the DFA* we see that there is an opening only at the wide-open end of the range. Thus, it seems that the DFA* will indicate to an aperture ring coupler enabled body that it is only wide-open all of the time. And this is what we want! I suspect that this opening's function is two-fold: to indicate that the aperture is wide-open, and to prevent damaging a body with an aperture ring coupler when such a lens is attached. If there were no opening in this location, attaching the DFA* 50 to the LX or other aperture ring coupler enabled body would bend the aperture ring coupler into the body.

Another picture of the aperture indication opening on the DFA* 50:




Finally as promised, here are some example pictures taken with the DFA* 50 on the LX. All are on Fujichrome Provia 100F. Exposure in general seemed reasonable with this combo. I used negative exposure bias for many of the pictures. This was for the look I was going for rather than correcting a technical issue. I punched-up the contrast on some of the scans, but I think it is still clear that some interesting results can be found even with the fixed aperture. The overcast day lent itself well to the wide-open aperture. If trying this on a sunny day, you would need an ND set and/or a polarizer.













I hope you found this investigation interesting!

[stevebrot]

Yep...it does stay full open and the LX body reads it as such. Why all the DA and D FA lenses provide an accurate coupling for wide open remains a mystery, akin to that of the pyramids and Spanish dolmens.


Steve

(...the truth is out there...)

[gofour3]

Well done! I only shoot film so this is very interesting to me. I wonder if the Z-1/Z-1p film bodies can control the DFA* 50 f1.4 lenses aperture via the body like A/F/FA lenses?

Thanks, Phil.

[stevebrot]

I wonder if the Z-1/Z-1p film bodies can control the DFA* 50 f1.4 lenses aperture via the body like A/F/FA lenses?

No...that is the rub. The aperture motor is in the lens and no body older than the K-50 is capable of doing the magic (no aperture actuator).


Steve

[Adam]

I wonder if the Z-1/Z-1p film bodies can control the DFA* 50 f1.4 lenses aperture via the body like A/F/FA lenses?

I don't believe so (regarding KAF4 lenses) because the cameras don't know how to tell the lens to stop down. KAF3 and older lenses will work fine though.

[gofour3]

No...that is the rub. The aperture motor is in the lens and no body older than the K-50 is capable of doing the magic (no aperture actuator).
Steve

I don't believe so (regarding KAF4 lenses) because the cameras don't know how to tell the lens to stop down. KAF3 and older lenses will work fine though.

Thanks I figured as much.

Phil.

[aslyfox]

Thanks I figured as much.

Phil.

you could take a look at this chart:

Pentax Lens Compatibility Chart
Copyright 2006-2018 Mosphotos.com. All rights reserved . . .

No Although the lens fits the body, it cannot be used: The shutter will not fire (manual focus lens) or the lens cannot be focused (DA focus-by-wire lens)..

The Pentax Camera Lens Compatibility Chart

it covers both Pentax DSLR and SLR camera bodies

[pid]

non of the bodys before K3 is providing the DFA 50/1.4. The reason is the electronic aperture only works if the bodies which support KAF4.

[gofour3]

you could take a look at this chart:


The Pentax Camera Lens Compatibility Chart

it covers both Pentax DSLR and SLR camera bodies

Right I did check and the charts indicates the shutter will not fire if the DFA 50/1.4 is used on a LX, which the OP has proven incorrect. The lens will manual focus, the shutter will fire and the camera meter works, though the aperture will be wide open.

Still using the DFA 50/1.4 wide open on a film camera is better than nothing. Not that I would ever consider buying it, it would be fun to rent over a weekend.

Phil.

[kiwi_jono]

Thanks for posting - it looks a surprisingly good combo! I particularly like the second and last shots. Certainly benefits from the films properties (not too much contrast / better exposure latitude).

P.S. the corners of the film frame had me doing a double take for a moment (was obviously not vignetting as that would not look like that at f1.4) - LOL.

[gylfimag]

Part of the beauty of the LX exposure system is that in auto mode (aperture priority) it does not matter if the camera can determine pre-exposure whether the lens is stopped down. The TTL system measures the light being reflected off the film during exposure and closes the shutter when the film is correctly exposed. This was state-of-the-art when the LX was introduced and basicallyl still is.

[pentageek]

I hope you found this investigation interesting!

Definitely - you made me think about similar experiments. Opposite combination (new camera/old lens) is quite common, but here we have something Pentax engineers probably never envisioned

[Adam]

@Ole looks like we need to separate out the DFA* 50mm from the 55-300mm PLM, since the former is not focus-by-wire.

[stevebrot]

non of the bodys before K3 is providing the DFA 50/1.4. The reason is the electronic aperture only works if the bodies which support KAF4.

As with the K-3, the K-50 supports KAF4 with firmware update.


Steve

[stevebrot]

Right I did check and the charts indicates the shutter will not fire if the DFA 50/1.4 is used on a LX

Why would the charts indicate that? None of those older cameras had shutter or even flash sync interlocks.


Steve