using k mounted lenses on DSLR: worthwhile?

hey everyone, im new to photography and am interested in buying a new DSLR camera. Recently iv inherited some old school camera gear off my now blind grandfather including :
-pentax k1000 asahi camera
-SMC pentax-m 1:2 50mm lens
-makinon 1:4.5 f=75-250mm lens
-2x teleconverter

I think that both lenses and the teleconverted are k-mounted. Im interested in buying a k100s for my first dslr because i figured it would be handy to be able to use the lenses i already have (particularly the 75-250mm).

However I read on the pentax website that k mounted lens are usable with some limitations on the k100s. Further research revieled that the lenses are only useful at full apeture. i know what this means, but i dont understand the its repercusions. how will that effect my photos? will it mean that the lens will only be usefull under certain lighting conditions? is it worthwhile to choose pentax over another brand just to be able to use k mounted lens, even though the lenses need to be at full apeture (aparently)?

any help would be really appreciated
thanks
Matt

incase u were wondering im actually considering the k100d rather than the non existant k100s lol however im sure you all already knew that

Manual lenses have to be used in Manual mode (after all, they're manual lenses) but due to the lack of the cam that traces the aperture controller, you need to set a menu item to allow lenses that are not on A setting to fire the shutter, and you use the green button to take a light reading.

To do this properly, you need to set the green button to set the shutter speed in Manual mode.

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You certainly don't need to use the lenses at open aperture if you don't want to - in manual mode you can stop the lens down to any aperture you like (using the aperture ring) and take a meter reading. The open aperture thing is only relevant in Av mode, when the camera does not close down the aperture on these lenses.

So the lenses you have will work fine with the K100D, just in manual mode.

Wherever you found the contrary information after 'further research' was wrong, you'll get accurate info here!

If you had a manual focus lens which had an 'A' setting on the aperture ring, you would be able to use all the exposure modes on the camera.

Dan

All Pentax lenses for 35mm and digital SLRs since the mid 1970s have used the K mount. The difference is that more recent ones (~1980s, often called A series or KA mount lenses) have electrical contacts and very recent ones (~1990s, the F and FA series, often called KAF mount) have autofocus and yet more electrics.

Since the DSLRs lack the necessary cam to detect the aperture ring position on M series and earlier lenses, they can only be used in one of two ways.

1. With the mode dial in M, set the shutter speed on the camera and the aperture on the lens. If you hit either the green button or exposure lock (deppending on your exact model of DSLR) the camera briefly stops down the lens and picks you a shutter speed. If you stop down the lens yourself using the DOF preview then you get a meter reading as normal.

2. With the mode dial anywhere but M, the camera operates in Aperture priority mode with the lens locked wide open. It doesn't matter where the aperture ring actually is, as the stop down actuator is not activated when the shutter is fired in this mode.

Note that the K10D has issues with its stop down metering system which are causing headaches all over the place. I don't know if this effects the K100D but I haven't heard it mentioned in relation to that camera. Certainly the stop down metering works perfectly on the *ist series and since the K100D seems to be quite closely related to the *istDS2/DL2 it is probably fine.

I use several M series lenses on my *istDL2 with great success. Less so on the K10D because of its exposure problems.

I think the lenses you have are probably not a killer reason to stick with Pentax if another system has something you really want. I think the Pentax system as a whole combined with the ability to mount those old lenses is a winning combination, but if I didn't think that I probably wouldn't be posting on this forum!

thanks for all your feedback, youv really cleared that up for me!

Let me ask a follow up or two if I may:

I understand the old M series lenses can be used in manual mode on the K100, using Depth of Field preview to check the exposure.

My query is about the optical performance; take the standard 50mm M series lens mounted on a K100, would that correspond (in field of view, perspective terms) to a 75mm lens used on a 35mm film camera? Do the 'f' numbers on the old lenses mean the same in the DSLR format - i.e. would you set 1/ISO equiv speed at f/8 for a cloudy day outdoors? What about depth of field markings on the lenses, do they carry over?

Regds, mechanic.

In terms of quality all the pentax bayonet mount lenses come from an era of computer aided design and are top quality. Some film lenses are however not at their best on digital because of chromatic abberation effects (colour fringing) but most are fine. The 50mm lenses are all superb, and do indeed have a field of view similar to a 75mm lens on a 35mm film camera. In other words, an ideal portrait lens.

The f number is a ratio and means exactly the same on any lens and any format.

As regards depth of field, in practice and for a given magnification (please note these caveats) don't forget that a 50mm lens remains a 50mm lens and hence there would be a little more DOF on a reduced size digital format. There are not many lenses with DOF scales these days, but older ones will have. I would use them on the cautious side anyway. If you use say f11 aperture then use the DOF for f8 and you won't go far wrong. It's all very approximate.

The easy way to get maximum DOF is to use a small aperture and focus on a point about a third of the way into your scene.

Old M series lenses can be used as you describe on the DSLRs, but it is more convenient and reliable (exposure-wise) to use A series lenses or later.

Hope that helps!

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Best regards, John