Designed for Digital or not

As a newcomer, when i read about lenses, they always seem to say that for digital camera equivalent multiply by 1.5 in the case of the K10D, i.e a 70mm to 300mm is the same on a K10D as a 105mm to 400mm. So if the DA range is specifically designed for digital does this remove this multiplication factor, and if so is this lens equal to divide by 1.5 to convert to 35mm.

No.

All lenses are stated in 35mm format terms, regardless of whether they are designed for digital or not. So a DA* 16-50 fitted to a K10D is effectively 24-75 in 35mm terms.

Even on point & shoot or bridge cameras, you will see the zoom ratio referenced back to the 35mm standard.

I am sure that others will be able to explain better than I can the problems of using digital only lenses on a 35mm camera.

Actually pentax lenses, corectly, are marked in the actual focal length. This remains the same for a lens regardless of what format you put behind it to receive the image.

Now imagine that the image circle projected by the lens is as big as we need for anything, just for a moent. Let's say that's enough for a 5"x4" negative, quite huge. Now substitute ibstead a tiny APS-C sensor and it only sees the very centre of the image, the bit that falls on it. So effectively it looks like a telephoto lens. Now we don't need all that extra image circle for the smaller format, so we can design a much smaller lens that only covers APS-C and it's no longer much use on our larger format.

But the focal length is still the same.

"35mm-equivalent" figures are only quoted to give us an idea as to what the lens will look like compared to the 35mm format we were used to. Frankly, that's becoming forgotten now and I'm pretty sure we are getting used to lnowing what a 18-55mm lens will do without thinking, "That would need a 28-80mm lens to get a similar angle of view on my 35mm SLR."

Hope that helps!

I knew that I should have left the answering of this question to someone else

I do think that many people moving to digital from 35mm SLR are not aware of this crop factor issue.

With the Olympus four thirds system, you appear to be buying a camera with a kit lens that goes from extreme wide angle to wide angle (due to the 2x crop factor).

Maybe not directly in relation to your question... but you also have to remember that there is more to it than just field of view. When you say, for example, 50mm you are defining a set of attributes of physics that do not change because of the sensor you have behind it.

One such attribute is your Depth Of Focus (Depth of field??). Putting a cropped sensor behind a 50mm lens does not change the DOF characteristics of the lens to that of a 75mm lens.
Obviously, because the smaller sensor has cropped/zoomed in to the image, the area covered by the "area in focus" seems larger. (Still, it is not equivalent to DOF of a 75mm lens).

The too often used term "35mm equivalent" really is no such thing -- unless perhaps when further qualified with "field of view".

I never had much film experience or experience with full sized sensors, so for me 50mm is only familiar in the DSLR cropped sensor world.
Saying 35mm equivalent has no real meaning to me in any sense.

As time goes on, this will be increasingly true for more and more photographers.

The problem is we need some form of 'standard' by which to compare the FOV of cameras with different size sensors. Maybe it should be the diagonal angle of view in degrees or (if it was desirable that longer focal lengths give bigger numbers) 180/diagonal angle of view. Both of these would be logical but would need the industry to decide on a standard which about as likely as flying penguins...

So we are probably stuck with 35mm equivalent for a while yet. Having used 35mm since 1984 that is fine by me...

It's actually got nothing to do with 35mm. A 300mm lens is a 300mm lens, no matter what size the image sensor is, be it 645, 6x7, 35mm or APS-C, or any other.

The confusion, if you can call it that, is simply that most people who have used a camera in the 30 or so years before digital, used a 35mm camera, and got used to the field of view of a particular focal length.

When you put an old lens on a K10D and are prompted to tell the camera what the focal length of the lens is, you put the actual focal length of the lens, 300mm or whatever, and completely disregard the crop factor.

It's not actually anything I think about when I use a lens. If the one I have on the camera doesn't deal with the composition I'm after, I choose another one that does.

Previously when a new format for film has emerged, such as 110 or 126, then no-one has batted an eyelid over the new focal lengths needed.

The reason it's become an issue with digital is that there are so many different sensor sizes, so the standard lens for each one will be different. Quoting field of view has been mooted many times in the past, but nobody has ever liked it. Far too difficult to deal with, much worse than focal length.

However, we'll soon get used to the new figures and I don't think about "35mm equivalents" at all. In fact, I don't really think about the focal length much, just am aware of which lens to pick up for which purpose! Then you look through the viewfinder until you see what you want.

Thanks for everybody's replies, i'll take them all into consideration, but i think the idea of just putting on the lens and seeing if the view i get through the viewfinder is ok is probably the one to go for'

I'd really like to see an end to this '35mm equivalency' stuff, some of us move between formats several times a day and can know which lens is wide and which is telephoto in any given format. Its just patronising.

I agree, since the focal length is an actual measure within the lens from the optical middle (not physical middle) to the film/sensor plane. so in fact all lenses are marked correctly!

I've read this thread with interest because I'm thinking of buying a Sigma 100-300 . However I'm none the wiser because I still think in 35mm terms. So is my proposed lens purchase from my K10d going to give me in 35mm terms 150-450mm. I ask because I'm after a new telephoto and I want it to give me at least 400mm, Thats the 'reach' I need. So is this lens long enough or not. (maybe I should post this in the lens section)

Yes, a 100-300mm lens on a Pentax digital SLR provides the equivalent of 150-450mm on a 35mm camera.

Thanks Pete,
Another question answered, something new learnt.