Difference between digital

[cloudspirit]

I use a Pentax K10D which I bought because I already own Pentax lenses
that fit this camera. My question is what, if any, is the difference with film SLR lenses and lenses specifically built for digital. I feel the focus is lacking in some of my shots due to my lenses.
The lenses that I use now are 35-80mm, 100-300, 28-105.

Thanks for your help

[Matthias099]

The biggest difference that I am aware of is that DSLR sensors are smaller than a 35mm film frame. That results in greater focal length. For example a 100mm lens on a 35mm SLR has a focal length of ~140mm on a DSLR (that's for my Nikon D80, but others are similar I am sure).

Other differences can be in the features of lens/camera combinations. Some older SLR lenses might only work in manual AF mode etc. That depends on the manufacturer largely.

I am sure we have some people here that work with a Pentax outfit.

[cloudspirit]

Some of my lenses are older and do not work in all modes and must be used in manual, does this make a BIG diff?

[Matthias099]

It should not make a difference with the focus of your pics.

[MikeM]

The other thing is that the digital only lenses are smaller and lighter, but again, should not affect your focus.

If you haven't discovered this yet, the K10D will "beep" when you're in focus even with a manual lens if you have the shutter release half way depressed.

[mattdm]

Originally Posted by Matthias099

The biggest difference that I am aware of is that DSLR sensors are smaller than a 35mm film frame. That results in greater focal length. For example a 100mm lens on a 35mm SLR has a focal length of ~140mm on a DSLR (that's for my Nikon D80, but others are similar I am sure).

This isn't quite correct; the focal length is an inherent property of the lens and doesn't change when you plug it into different cameras.

However, one of the most noticeable effects of changing focal length is field of view — the amount of a scene which fills the frame. Field of view changes linearly as you change focal length, so if you double focal length, you something that was only half of the picture now fills it up. Zoom lenses, of course, do exactly that.

But the other way to fill up a frame with something half the size is to simply have a smaller frame — and for size and production cost reasons, most DSLRs do. In the case of Pentax dSLRs (and most Nikons and Sonys too) it's 1.5× smaller. So the effect on field of view is the same as increasing the focal length. (But everything else remains the same.)

This is important in the context of the original poster's question. Your original 35-80mm lens will be "shifted" towards the telephoto side and will have the field of view of a 52.5-120mm lens on a film camera. But if you buy the new designed-for-digital 50-135mm DA★, the same thing will apply, and you'll get framing like what you'd be accustomed to from a 75-202mm lens on a film camera.

There are other differences too — digital sensors are more reflective than film, so lens coatings are designed to better take that reflected light into account.I think there are other coating-related differences,too.

And, while film doesn't care what angle light comes from, digital sensors really need it to be straight on, so new lenses are designed with that in mind. (This is more of a problem near the edge of the image, so the crop factor helps with older lenses here.) The K20D's new sensor is supposed to have features that make this less of problem.

[Cuchulainn]

Originally Posted by cloudspirit

I feel the focus is lacking in some of my shots due to my lenses. The lenses that I use now are 35-80mm, 100-300, 28-105.
Thanks for your help

Digital only lenses usually project a smaller image circle than lenses designed for the 35mm format. In other words they are not designed to cover a 24X36mm frame, but are designed to cover the 16X24 or smaller sized sensors of (not full frame) DSLR cameras. Because they are made to project a smaller image circle they can be optimized to use less glass and are often smaller, lighter and cheaper than lenses designed for 35mm and full frame dSLR's.

While the old film lenses that project an image circle that is larger than the DSRL's sensor will work well other than the crop factor as explained by mattdm, the digital lenses usually do not work well with 35mm cameras as the full 35mm frame is not fully covered.

I use a few manual focus (circa early 1980's) lenses with my Nikon D300 and they work well. In some cases I prefer them.

Using your older Pentax lenses on your DSLR should not have any affect on your focus, but your depth of field and perspective may change from what you are used to. There is also an equivalent multiplier in the rule of thumb regarding shutter speed for hand held shots.

This wikipedia article explains this all in more depth...

http://en.wikipedia.org/wiki/Crop_factor