focal lengths K10D

[thomas5]

What I wanna know is are the DA lenses (and the FAs, for that matter) true in their focal lengths or are they

subject to the multiplier (1.5x) that I have to use to judge other lenses I buy? More to the point, is my kit

lens the 18-55 really that or is it a 27-70-something?

 

I wanna go wider than the lens I have, I just wanna know what that's gonna mean.

 

Thanks.

[ronald_moravec1]

Lenses are marked with true focal length. To estimate what thay will look like compared to film, multiply by about 1.5.

[personalphotos]

This is one of those questions that comes up a lot and the answer above is correct. So to add a little to that. The lens is still a 18-55 or whatever the focal length is. What changes with the crop factor of 1.5x is the Field of view (FOV). So the image is cropped with the smaller (than 35mm film) sensor to be the equivalent to 27-82mm lens. But the magnification doesn't change.

 

For example, if you wanted a FOV of what a 20mm lens gives you on 35mm film, you would need a 13mm lens.

 

If you want to go wider than the kit lens currently gives you there are several options. Notably the Sigma 10-20, Pentax or Sigma 12-24's or the Pentax 10-17 (fisheye) that all get good reviews here and elsewhere from owners. There are also a few wide angle lenses and fisheye's that you could consider like the 16mm Zenitar or 14mm Pentax.

 

I have the Sigma 12-24 and although a big lens, it is well built, works well and produces excellent, sharp results. Certainly wide enough for anything I would want to shoot.

 

Just an FYI to get faster and more complete answers some of the time. There is a search box on the top right side of any page on this forum. Type in a short request and you will get answers to this type of question that have already been posted before.

[thomas5]

Thanks, you two. I thought that might be the case, but I always get flummoxed when I

think about it too hard.

 

And for what it's worth, Peter, I did a preliminary search (the guys over in the MF forum are

ruthless if you don't :) and didn't find what I was looking for.

 

I'll have a look at your suggestions.

 

Cheers.

[davemoss]

focal length is a function of how far the lens' nodal point must be from the film plane to focus at infinity. it is the same for large format, medium format, 35mm, aps-c, etc.

 

an 80mm lens, is always an 80mm lens, no matter what camera you put it on. crop factors and fov have more to do with the size the image circle is being reduced to.

 

i'll bet i can guess who one of the mf rude boys was.

[andrewg_ny]

Thomas, just to add onto Peter's response--also consider the DA 16-45 f/4; that 2mm difference is significant when compared to the kit 18-55. 16mm has a field of view like a classic 24mm 135 format lens, and offers a higher quality level. Though if you already have the kit and want to go ultra-wide, you'll probably be shopping the lenses that Peter listed.

[paul_noble]

I think that this business of always expressing a lens' focal length in terms of how it relates to a 35mm camera is counterproductive.

 

When we first picked up a 35mm SLR, we had no frame of reference. Soon we learned that 45-55mm was "normal", 35mm or 28mm were "wide angle", and anything above 50mm was "telephoto".

 

Now, with conversion to digital and APS-C sensors, we simply have to learn a few new reference points. Now, 18mm is "wide angle", 30-35mm is "normal" and anything above that is "telephoto".

 

Just my $0.02.

[andrewg_ny]

Paul: Maybe it is kind of silly and unfortunate that these descriptions continue to be 135-format-centric but the issue here is that most people have no idea what the practical difference between a 35-degree angle-of-view vs. a 42-degree angle-of-view is so a sensor-size-insensitive unit of measure isn't very useful at this time.

 

Most of the people asking these questions are converting from either a 135 film background or they are converting from digicams that also expressed angle-of-view in terms of 135-equivalent focal lengths. The inexperienced will not intuitively know what a substantial difference 1 or 2mm will make in terms of angle of view using ultra-wide lenses so it helps to express it in terms where they have a more comfortable frame of reference.

 

I don't know that this confusion is going away anytime soon either as the industry has not settled on a standard sensor size--you have 135 conversion factors x1.3, x1.5, x1.6, x2.0 on different bodies, etc.