Focal length increase

When using non-digital Pentax AF lenses with the digi K SLR's...is there a 1.5 focal length increase?

Hi,

yes there is. The focal lengths on the DA range of lenses is still marked up at the 35mm equivalent, so you can just x1.5 for every lens you come across.

Cheers
Liam

The multiplier applies to "digital" lenses, too.

There is no magic about this. A digital sensor is small than a 35mm film frame, so the sensor only captures the central part of the image.

The "multiplier" applies to the angle of view that is recorded. For any given lens, more of the image will be captured on film, so the angle of view will be wider.

To compare the angle of view you will get on a Pentax DSLR to the angle that you would get with the same lens mounted on a 35mm film body, you multiply the focal length by 1.53. So a 50mm lens will give an ange of view equivalent to a 76.5mm lens when it is used on a digital camera.

G

just to complicate matters...
This is not a true magnification factor like you'd get with a teleconverter...more like the effect you'd get if you shot the image on filn then cropped in on the central area of the image, taking a little off each edge. f-stop, dof, etc won't change. but if your older lens, has light falloff at the corners, or gets a little soft at the edges, your digital will effectivly cut that junk off and you'll find yourself thinking your lens got a whole lot better!

George,
It's actually exactly 1.5, not 1.53. The angle of view calculations are always done on the diagonal across the sensor/film
Here's a diagram I did waaay back:
http://www.photon.me.uk/matt/cameras/pentax/istd_focal.htm

Matt

Matt,

I think you need to double check the sensor dimensions.

G

Horrible though it is to quibble over 0.03 of anything, I'm with George on this one, using the dimensions that Matt's got on his diagram. The diagonal of the 35mm frame is 1.531 times the diagonal of the digital sensor. Just as it's 1.531 times the length of the sides too.

Has anyone actually measured the dimensions of a 35mm frame though? And are the dimensions of the sensor for the effective pixel area, or the total pixel area?

Dan

Daniel,

May your intelligence and perspicacity never be doubted.

Matt, reach for your hat. I hope you're hungry.

G

amosstarz,

You may find this article interesting. As a rule of thumb it is safe to use 1.5x as the multiplier for the change in focal length. The alternative, if you want to be absolutely precise, is to add a calculator to your digital camera bag.

Since we're splitting hairs here, I thought I'd add my own piece...

Lengthwise, the sensor ratio is 36/23.5=1.53191.
Heightwise, the sensor ratio is 24/15.7=1.52866.
Diagonally, the sensor ratio is ((36^2+24^2)^0.5)/((23.5^2+15.7^2)^0.5)=1.53091.

So, chaps, however you wish to interpret it, there you have it. My tuppence worth.

You forgot to mention the ratio between the surface areas, which is 2.341780729, and the perimeter which is 1.530612245

Here's a ratio I'm sure you all need to know.
1.85

It's the ratio of my height to my weight.

At least it's a lot less than another make that I know of beginning with C... somewhere around 1.6-ish (not wishing to split any more hairs or embarass myself any further).

Serves me right for not actually working it out
(Grovel, grovel )
Matt

Hi Michael,

Just thought I'd clean up the equation for you..

(36²+24²)/(23.5²+15.7²)=1.53091

(Alt+0178 = ²) loads more ASCII characters can be used using the same method, and any figure repeated on both sides of an equation can be discarded

Cheers

Liam