[someguy42]

I have an odd problem. I snagged an SMC 50mm f/1.7 from Ebay, and I can adjust the aperture with the lens off of the camera throughout its range, but when I put it on the camera, it's stuck at 1.7. Anyone know why? Is there a setting I need to change in the camera somewhere?

[stevebrot]

I am assuming you have a SMC Pentax-M 50/1.7.

Are you saying you can't move the aperture ring? That would be very strange and an indication that something is not right (hard to say what) with your lens.

If, on the other hand, you are saying that you move the ring, but the aperture stays wide open, that is what you would expect. The lens should stay open until exposure time. That being said, your camera is not configured, out of the box, for use with vintage lenses lacking the "A" electric contacts on the mount face.

Detailed instructions may be found here:

https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...x-dslrs-f.html


Steve

[Cannikin]

All K-mount lenses are held wide open by the camera body at all times except exposure time. This is necessary for accurate metering, and to not have an overly dark viewfinder.

The way aperture works on K-mount lenses is that there is a small spring loaded lever on the back, which if unblocked would go straight to the smallest aperture (usually f/22). The aperture ring puts a small block somewhere along the path, corresponding to the desired aperture. When the lever is released, the lever goes to this point and is stopped, giving you the aperture desired. When the lens is off the body, the lever freely moves, so it is by default at whatever the aperture ring blocks it at. When on the camera, the lever is held all the way open. At exposure time, it releases the lever. If it has a manual aperture ring, it will hit whatever the aperture ring block is set to. If it is automatic, the camera directly controls the aperture lever.

Make sure to set the camera to allow aperture ring use in the custom settings, and use the green button to meter before taking the shot, because all Pentax DSLRs have a "crippled" K-mount in that they don't have the proper contacts to meter without automatic aperture.

[boriscleto]

https://www.pentaxforums.com/forums/pentax-lens-articles/216154-how-use-manua...slr-video.html

You Tube

You Tube

[someguy42]

All K-mount lenses are held wide open by the camera body at all times except exposure time. This is necessary for accurate metering, and to not have an overly dark viewfinder.

The way aperture works on K-mount lenses is that there is a small spring loaded lever on the back, which if unblocked would go straight to the smallest aperture (usually f/22). The aperture ring puts a small block somewhere along the path, corresponding to the desired aperture. When the lever is released, the lever goes to this point and is stopped, giving you the aperture desired. When the lens is off the body, the lever freely moves, so it is by default at whatever the aperture ring blocks it at. When on the camera, the lever is held all the way open. At exposure time, it releases the lever. If it has a manual aperture ring, it will hit whatever the aperture ring block is set to. If it is automatic, the camera directly controls the aperture lever.

Make sure to set the camera to allow aperture ring use in the custom settings, and use the green button to meter before taking the shot, because all Pentax DSLRs have a "crippled" K-mount in that they don't have the proper contacts to meter without automatic aperture.

Thank you for your responses.

Yes, I've found that lever, and I'm familiar with the rather cumbersome method of using the camera's metering capability via the green button. However, I want to do everything manually, without using the green button. I essentially do not want the camera to interact with the lens's aperture ring at all. Is it possible to do this?

[someguy42]

Okay, it seems that the reason that the lens is held wide open until the shutter is closed is to keep the image in the viewfinder bright. If that's the case, then I guess the green button is actually rather innovative. Thanks for the links, gents.

[Ole]

Thank you for your responses.

Yes, I've found that lever, and I'm familiar with the rather cumbersome method of using the camera's metering capability via the green button. However, I want to do everything manually, without using the green button. I essentially do not want the camera to interact with the lens's aperture ring at all. Is it possible to do this?

No, it is not possible with K_Mount lenses. They stay fully open until the moment of exposure.

It is possible with Asahi Pentax Takumar screwmount lenses, though. These attach with an M42 to K-Mount adapter (get the original from Pentax).

[troika]

I use my green button sometimes to meter, in other words, get the camera to tell me where it thinks my shutter speed needs to be based on the aperture and ISO that I have set, but then I stop up/down each of the 3 legs to where I want them to be and as long as I don't hit the button again, I'm operating fully manua. It's a useful tool, especially considering that you don't have the use it.

Alternately, you can use it for each shot as a quasi-auto exposure mode, but I think the only thing it will change is shutter speed, which is not always the right choice. For example, if I'm shooting in low light indoors, like bar, I'll have my ISO set conservatively, but will inch it up to get where I need for it to be. The camera via the green button on the other hand will slow my shutter speed down to something not useable while hand holding.

So, it takes a little getting used to, but it's a good tool if you don't let it take over. I've been saying that I didn't really care about A Lenses, because I wasn't missing anything. But lately, I've been shooting in TAv mode with my modern lenses and I'm getting kind of hooked on it. I can get to the same place with my K lens in M, but it takes a little longer. Something I'm sure experience would remedy as well.

[someguy42]

I use my green button sometimes to meter, in other words, get the camera to tell me where it thinks my shutter speed needs to be based on the aperture and ISO that I have set, but then I stop up/down each of the 3 legs to where I want them to be and as long as I don't hit the button again, I'm operating fully manua. It's a useful tool, especially considering that you don't have the use it.

Alternately, you can use it for each shot as a quasi-auto exposure mode, but I think the only thing it will change is shutter speed, which is not always the right choice. For example, if I'm shooting in low light indoors, like bar, I'll have my ISO set conservatively, but will inch it up to get where I need for it to be. The camera via the green button on the other hand will slow my shutter speed down to something not useable while hand holding.

So, it takes a little getting used to, but it's a good tool if you don't let it take over. I've been saying that I didn't really care about A Lenses, because I wasn't missing anything. But lately, I've been shooting in TAv mode with my modern lenses and I'm getting kind of hooked on it. I can get to the same place with my K lens in M, but it takes a little longer. Something I'm sure experience would remedy as well.

Yeah, the realization that you only have to hit the green button when you change the aperture dawned on me while I was reading through one of the links the other posters left above. Now I see what the green button is: a good way to get baseline settings when you change the aperture.

[troika]

... Now I see what the green button is: a good way to get baseline settings when you change the aperture.

Or when the light changes or you move or for whatever reason just want fresh baseline metering. I originally tried to use it as a "mode", which I grew very tired of.

[stevebrot]

Or when the light changes or you move or for whatever reason just want fresh baseline metering.

This is the key. In most circumstances, the light does not change much during the time spent with a particular subject. Realizing this is often the key to difficult lighting situations. A few years back I was shooting a stage presentation (spot-lit with dark backdrop as well as presentation screens in the background) and having terrible a terrible time with the getting appropriate exposure. During a break, I switched to M mode, took a gray card reading at the stage, and shot with the same settings for the remainder of the presentation. Worked great.

I also shoot quite a bit with meterless or stop-down metered film cameras. It is tedious and usually unnecessary to meter between each shot and strangely enough, I generally get somewhat better results than if I were using Av or P mode with my more capable cameras.


Steve