[lotech]

Hi there,

I used some m42 lenses mainly Russian made on my K3 via "non-flush" adapter, and I enabled the "use aperture ring" setting so that the camera will trigger with or without coupling. When I shoot I focus wide open and then stop down and press the green button to meter before release, that worked ok, but I find I always get about 1 stop over than wide open doing so. As far as I can see the metering mode is fixed when using non coupled lenses, I don't know if this is the reason for the over exposure, any comment ?

[BigMackCam]

When you press the green button to meter, the camera stops the lens diaphragm down then takes it's meter reading... but unlike most K-mount lenses, M42 lenses don't have an aperture lever (most will have a pin, but not one that can be actuated by the camera); hence, green button metering doesn't work. You need to manually set the aperture on the lens, and the camera will meter correctly.

Some M42 lenses are auto-only, requiring the aperture pin to be depressed in order to stop the diaphragm down. That's a problem, as the camera has no way of pressing the pin - however, most of these can be quite easily modified to keep the aperture pin held in, so that the lens works manually.

[Canada_Rockies]

If it is a 1 stop correction, I would simply set -1 stop before using the green button with these lenses. The +/- works with the green metering button just fine.

[D1N0]

You can shoot m42 lenses in AV mode. Generally the camera will underexpose wide open, but it will improve as you stop down. The camera cannot stop down m42 lenses. You need to do that manually. If the lens has a auto-manual switch(or A/M), put it to manual or it won't stop down. Older lenses are preset. They have two aperture rings. The front one presets the aperture you want to use but leaves the lens wide open. The second one closes down the aperture. So you can focus wide open. close the aperture to the preset value without looking at the lens and shoot. I generally use a 0.7 correction wide open and correct the rest in post.

[lotech]

Russian m42 lenses so far I used has no coupling and automatic aperture of any kind, the aperture is completely independent to the camera, so the only thing the green button can do is to meter, and the camera always treated the lens wide open. Since the exposure error is not much, I believe the metering mode and full frame lenses in crop sensor might have something to do with that.

[Dartmoor Dave]

The inaccuracy of stop-down metering with M42 lenses on DSLRs is caused by the focusing screen. The camera's built-in meter reads the amount of light hitting the screen, but the focusing screens used in modern DSLRs don't have linear light transmission as you stop down. There's no reason why they should, since they are designed only with open aperture metering in mind. A bright viewfinder image with the lens wide open is considered much more important than accurate stop-down metering, because stop-down metering is not how a DSLR is intended to work.

So metering inaccuracy is inevitable with M42 lenses on a DSLR. Many people use the camera's built-in meter and apply exposure compensation, but personally I use a combination of Sunny 16 and an incident meter.

[lotech]

Oh that's new to me, thanks !

[johnyates]

You'll get a more accurate meter reading if, after stopping down, you shift into Live View and hit the green button.

[Canada_Rockies]

You'll get a more accurate meter reading if, after stopping down, you shift into Live View and hit the green button.

that's something I will have to try. Simple.

[BigMackCam]

The inaccuracy of stop-down metering with M42 lenses on DSLRs is caused by the focusing screen. The camera's built-in meter reads the amount of light hitting the screen, but the focusing screens used in modern DSLRs don't have linear light transmission as you stop down. There's no reason why they should, since they are designed only with open aperture metering in mind.

Wow. I've been shooting with old M42 lenses for a couple of years or more and didn't know this. Makes complete sense. Thanks for educating us (me!), Dave!

[psoo]

I had a similar problem using a Takumar 200 mm, f/5.6, lens on my K100D. It is an M42 lens that I use with a Pentax screw/bayonet adapter. I hadn't used this combination for a while and had to fiddle around to get it to work. When setting the selection dial to A (aperture) the shots were all underexposed. I switched to the M mode, focussed at full aperture, and then stopped down to my desired aperture. I held down the AE-L button, heard a nice "click", and then pressed the shutter button. I think the AE-L button is equivalent to the Green Button on your camera. Exposures were fine.

[Lowell Goudge]

This problem has been around for years, and ultimately linked to the fact that today's focusing screens ( which the camera meters from) are not linear relative to the incident light, and that the light incident to the focusing screen that the meters are reacting to has a huge dependence on the lens aperture.

Lenses with aperture contacts work fine in open aperture metering because the camera is told/programmed for their aperture, but move off open aperture metering (green button on non A lenses for example) and the metering is not accurate,

See the post linked here,

The graph began development in2007 (that's right 10 years ago)

Strange exposure problem but only w/teleconvertor - PentaxForums.com