SLR Lenses on DSLR Bodies

[slh]

Hi everyone. I'm pretty new to photography and learning everything I can. I have a couple of questions about using film lenses on my digital camera, in peticular, I have an old Pentax with a couple of lenses, 50mm 1.2, 28mm 3.5 and a Tamron 80-210 3.8-4. I would like to use these on my Sony A230. I know I have to get an adapter and I will lose the auto functions.. but thats ok. From what I have read, the the SLR lens when adapted to a digital camera will change the focal length of the 35mm format, ie the 50mm will be the equivlent of a 75mm on my camera (50 x 1.5), how will this affect the pictures taken? The other example is with the Tamron. The focal length will change from 80-210 to 120-315, right? Then add in the 2x teleconverter and you end up with 630mm! Is this right? Also, they say that you have to use stop down metering in cases like this. What does this mean? For example, I would'nt be able to use the 50mm at f1.2, I would have to use it at f2 or more and if this is true, then why? Is there anything else I should know about before trying this?

Thanks for all your help, this is a great site!!

Steve

[i speak in math]

You could just get a K-x and use them like normal lenses instead.

[slh]

Originally Posted by i speak in math

You could just get a K-x and use them like normal lenses instead.

lol, ok. wanna buy a slighty used a230?

[inkista]

I know more about adapting lenses to Canon EOS, so I'm hesitant to say anything, but I believe that the Sony Alpha/Minolta AF mount is problematic to adapt most manual focus lenses to because of the size of the mount opening being too small. If you know differently, then feel free to ignore this statement.

But, I do believe that Pentax threadmount (M42) is adaptable with a simple ring. So the first thing to figure out is whether your old Pentax is M42 (threadmount--the mount is a simple screw thread) or K-mount (bayonet mount).

If you have K-mount lenses, you may or may not be able to adapt with a simple ring. You may have to have an adapter with glass in it, so that you can reach focus at infinity. This may not bother you, but for some folks, it will be compromising optical quality to an unacceptable degree.

When you adapt a manual focus lens for a dSLR, there are more than a few shortcomings. You lose autofocus. And you lose aperture control from the camera body. The practical upshot of which are you have to manually focus, you can only shoot in full Manual or aperture-control, you have to use stop-down metering which may or may not be compatible with your camera body's metering system, AND your EXIF isn't going to be complete.

And then, there's the focal length thing...

Originally Posted by slh

...From what I have read, the the SLR lens when adapted to a digital camera will change the focal length of the 35mm format, ie the 50mm will be the equivlent of a 75mm on my camera (50 x 1.5), how will this affect the pictures taken?

The focal length of the lens won't change, but the field of view that you'll see on the sensor will be smaller. So, it's similar/equivalent to what you'd see using a longer lens on a film/full-frame body. The main upshot of this is that it's much harder to go wide angle on a crop body than on a full-frame.

The other example is with the Tamron. The focal length will change from 80-210 to 120-315, right? Then add in the 2x teleconverter and you end up with 630mm! Is this right?

Well, yes, and no. As I said, the focal length doesn't actually change from the crop. It does change with a teleconverter, but adding a teleconverter will a) be blurrier, and b) make your max. aperture a lot smaller. A 2x TC reduces the max. aperture of your lens by two stops. I.e., if you put an f/2.8 lens on a 2x tc, it effectively becomes an f/5.6 lens.

The problem is your Tamron is probably an f/5.6 lens. And putting a 2x tc on it makes it an f/11 lens. And most dSLR bodies stop autofocusing if the max. aperture gets to f/8. So you'll lose autofocus. Using a tc is really not very equivalent to getting a longer lens.

Also, they say that you have to use stop down metering in cases like this. What does this mean?

When you use a dSLR lens on your Alpha, the metering and autofocusing are all performed with the lens wide open--at its maximum aperture. This gives you (and the camera) the most light possible by which to "see"/compose/focus/meter. The lens's aperture is only closed down to the aperture you set right before the picture is taken and the shutter is released. Then, it goes back to being wide open.

Stop-down metering means that the aperture is actually closed down to the aperture setting you want to use while you meter/focus/compose. You're doing this on a manual focus lens with the aperture ring on the lens. This lowers the amount of life coming in, and will make the viewfinder go darker, just like using a DoF preview button--in fact, the DoF preview button is doing exactly the same thing: closing down the lens physically to the aperture setting you've chosen.

For example, I would'nt be able to use the 50mm at f1.2, I would have to use it at f2 or more and if this is true, then why?

It can be true, if the adapter you use has a glass element in it. In that case, it's essentially like a teleconverter, and does that same thing. The basic math (sorry!) goes like this:

Your aperture is a ratio.

f-number = focal_length/aperture diameter.

When you put a tc on a lens, the aperture diameter in the lens doesn't change. But your focal length just got bigger when you added a tc.

So, let's say that you're using a 50mm f/1.2 lens at f/1.2.

1.2 =~ 50mm/41.6mm

If you add a 2x tc, then

f-number = 100mm/41.6mm =~ 2.4.

That's why you lose max. aperture whenever you use a tc. And a glass-element adapter will act like one, in order for you to focus at infinity.

[slh]

Wow Inkista, thank you very much for the informative explanation. That helped to explain a lot, things make a little more sense now. The Pentax I have are k mounts, the only adapters I can find for the Alpha have glass in them (for infinity focus). For $30 ( adapter), I guess it might be worth it to have a few extra lenses available. The whole reason for even going this way is for the 50mm f/1.2. I love that lens on the Pentax, but I hardly ever use it anymore. I'm using the Sony much more. The fact that everything will be manuel is ok with me, gives me more flexibility and I'm still learning, lol. Thanks again for the info!

Steve

[candleman]

question....
why buy a Sony when you have Pentax lenses?

The K-x is a pretty good little body..

[slh]

Sony was a gift from... yup the wife. She knew I wanted to go digital, didnt realize (I guess) that Pentax also make digital. I might end up selling the Pentax and lenses for it.. its an old ME. The camera itself isnt worth anything, but I might get a few $$ for the 50mm.

[inkista]

I'd say try the $30 adapter and see if it works for you. It's certainly worth giving it a try for the 50/1.2.

And you will definitely have a buyer for that lens. It can be adapted to Canon dSLR bodies, so you'll have both Pentax AND Canon shooters after it, if you put it up for sale.

[Zulu109]

Originally Posted by inkista

I know more about adapting lenses to Canon EOS, so I'm hesitant to say anything,

Glad you said this because this was brought up yesterday at work

We have an older EOS Rebel (film model) and there's been some discussion about getting a DSLR and swapping the lense over. What questions do we need to answer before we can consider this idea?

[BCampbell]

Originally Posted by Zulu109

We have an older EOS Rebel (film model) and there's been some discussion about getting a DSLR and swapping the lense over. What questions do we need to answer before we can consider this idea?

As Inkista mentioned, some lens to body adaptations require intermediate optics to reach infinity focus. The quick explanation is that this has to do with "backspacing" -- the distance from the lens mount to the film/sensor plane. If the backspacing of the camera body mount you are using is shorter than the backspacing of the adapted lens mount you want to use, you're fine and just need something engineered to mate the two. If it's longer, you need an optical element which usually also acts as a mild teleconverter and introduces one more piece of glass and one more possible source of image quality issues.

Personally, I would not use a lens/camera combo that required an optical adapter unless you're talking about some really exceptional lens and you can't reasonably find a camera body that works otherwise.

As for your situation, if the lenses you want to use are EF mount, any EOS digital Canon will accept them. But I'm guessing you want to use old FD mount lenses. Bad news is, the EOS digital backspacing is 44mm, and the FD backspacing is 42mm. That means you need an optical adapter.

Lenses that are adaptable to EOS digital Canons without optics include Contax, Leica, Pentax K-mount, M42, Nikon, Olympus, and a few others.