Does K-7 overexpose with manual lenses?

alehel · 07-05-2009, 02:39 AM

I was just wondering if Pentax has fixed the overexposure problem with manual lenses on the K-7. Have any of those who have had a pre-production model tried it out with a manual lens?

eagle · 07-05-2009, 04:03 AM

I didn't have a problem with my K200D, and also not with the K7. Though, haven't used it much on the K7 (having it for only 1 day so far)

Took the K7 out for a run today with my Sigma 50-150 (Auto) and I won't say it 'overexposed' with this lens, as maybe it would underexpose on the K200D?

Will take some more time shooting and adjusting to the new body I think is all.

brkl · 07-05-2009, 06:16 AM

Well, that's a lens that would work well with the K10D and K20D too. But old manual focus lenses, such as M42 lenses and older K-mount lenses will either over-expose or under-expose quite a bit on my K10D depending on the aperture and lens.

kittykat46 · 07-05-2009, 09:15 AM

Pentax is almost alone among the major dSLR manufacturers in providing fairly good functionality with the older generation of lenses.
The old manual lenses basically communicate little or no electronic information to the camera body, so I imagine its going to be hard for them to make any improvements in this department.
My K-7 only arrives next week, will try it out on manual focus and Non-A lenses - I only have 1 of each, the rest are DA and autofocus FA.

Rickster · 07-05-2009, 10:35 AM

I don't think that's an issue that can be solved easily in the camera. The basic problem is exceeding the camera's metering spec by attempting to use stopped down metering. Modern digital cameras need a minimum amount of light for their metering systems to work properly.

The K7 appears to be spec'd at ...

Metering range: EV 0.0 - 22 EV (ISO 100, 50 mm F1.4)

The K20D is spec'd at ...

Metering range: EV 0.0 - 21 EV (ISO 100, 50 mm F1.4)

the K10D is spec'd at ...

Metering range: EV 0.0 - 21 EV (ISO 100, 50 mm F1.4)

From this we can see that the K7 has added 1 EV of metering range on the high side.

frank · 07-05-2009, 10:35 AM

I did a quick test here a while ago:
Experiences with the new K-7 Camera (lots of imgs)

Seems K-7 works better w/ full manual lenses than a K10D.

stevebrot · 07-07-2009, 11:45 PM

Originally Posted by Rickster

I don't think that's an issue that can be solved easily in the camera. The basic problem is exceeding the camera's metering spec by attempting to use stopped down metering. Modern digital cameras need a minimum amount of light for their metering systems to work properly.

The K7 appears to be spec'd at ...

Metering range: EV 0.0 - 22 EV (ISO 100, 50 mm F1.4)

The K20D is spec'd at ...

Metering range: EV 0.0 - 21 EV (ISO 100, 50 mm F1.4)

the K10D is spec'd at ...

Metering range: EV 0.0 - 21 EV (ISO 100, 50 mm F1.4)

From this we can see that the K7 has added 1 EV of metering range on the high side.

EV 0.0 - 21 (ISO 100) is a pretty incredible range considering that EV 0.0 is 1 second at f/1. That is very, very dim. Interestingly, it is not too difficult to attain that dimness at the sensor when stopped down in moderately low light. I have had the display blinking with manual aperture lenses in Av mode at f/11. This type of limitation is present for all cameras capable of stop-down metering. In fact, it was not unusual for the manufacture to publish a graph indicating supported ranges for aperture/shutter speed/ISO back in the ancient days of stop-down cameras.

However, from all available evidence, the more metering issues are not directly related to meter sensitivity range except at narrow apertures in dim light. Increase the light level and underexposure at the narrow end disappears. The pattern of severe underexposures at apertures f/4 and wider (up to 2.5 stops) is related to focus screen design and mathematical correction factors applied by the camera to reconcile the meter readings with an acceptable exposure. (I.e. The camera's meter is not linear for stop-down metering with most screens.)

Steve

(Currently own two stop-down meter film cameras...both with linear meter response...)

*arpaagent* · 07-08-2009, 12:01 AM

Just a quick note that I have noticed with my K-7 that arrived today. In order for an M42 (SMC Takumar for example) to meter the same in Av mode as in M green button mode ("stop-down metering"...sort of), you need to dial in +0.7 EV compensation. I still can't understand why they can't make Av mode work the same as green button metering for M42 lenses since you are manually stopping the aperture down and the green button isn't doing anything...

Well, this is at least a big improvement from the K200D. It would require about +2 EV for anything less than f/2.8 in Av mode, and closwer +1 EV for anything over that. I can deal with dialing in +0.7 compensation on the K-7 but I wish it just wasn't necessary at all, and I really see no reason why it isn't possible, and easily...