Focal length and camera shake

Having used nothing but completely manual film SLRs for years, I am quite familiar with the equation of focal length of lens not exceeding shutter speed with hand held shots, i.e. don't use anything slower than 1/60th sec with a 50mm lens, and so on.

How does this affect digital cameras, whose lens focal length differs from that of 35mm film cameras?

Am I going to have to do some mental arithmetic any time I use an old non-digital lens?

This is important to me as I frequently don't have my tripod with me and I like to shoot landscapes at f11 or f16 with a polarizer, or small plants in poor light with as much depth of field as possible.

I would just do a little bit of mental arithmetic (or trial and error!)

A small point (I may be wrong) but I think even the digital lenses are marked up with a 35mm scale - so you may have to do some sums even if you are using a 'digital' lens

hope thats of use

You'll have to multiply the focal length by one and a half.

And you'll have to do it for every lens, digital or not.

So for a 100mm lens, you'll have to use a shutter speed of 1/150th of a second.

Not impossibly difficult, I should have thought.

G

the K10D manual recommends 1/1.5x the focal length unless you are using shake reduction. 1/focal length or better is still pretty safe though if you have reasonably steady hands.

Worth re-iterating at this point that a 50mm lens on a Film and DSLR has the same focal length - 50mm. On the DSLR the field of view is equivalent to that of a 75mm lens.

Trust Tim to throw a spanner in the works!

TIM....YOURE HURTING MY BRAIN!!!!! (sadly my brain has the equivelent of dyslexia when it comes to math)

God only knows how I manage the occasional decent shot.

It gets even worse than that

You would expect that a 17mm pre digital lens would give a digital focal length of about 26mm; it doesn't in fact it gives about 19-20mm

As per this thread

https://www.pentaxuser.com/forum/viewtopic.php?t=3640&highlight=

Some of the preceding posts have really thrown me now!

Based on something I read somewhere, I was thinking my 18-55 digital kit lens was the equivalent of 28-90 in a 35mm film lens.

Meanwhile, George advises I'll have to multiply the focal length by one and a half, which he thinks is not impossibly difficult. George, you don't know how bad my mental arithmetic is!

Looks like I'll have to go back to school.....

Itsme,
For rectilinear lenses (ie, not fisheye) then you do multiply by about 1.5x.
So the 18-55 behaves much like a 28-80mm (ish) on a film camera.

With fisheye lenses the result is a bit different (Yes, I said I'd look up the maths and work it out... it's definitely not mental arithmetic )

EDIT: Just checked on Wikipedia and there are several types of fisheye all with their own equations... not sure what the Pentax 17-28mm fisheye is ( http://en.wikipedia.org/wiki/Fisheye_lens )

EDIT: Equations are here: http://www.bobatkins.com/photography/technical/field_of_view.html

Matt

Hi,

I knocked this up a few months ago.. thought I'd pos it again, if it helps..

Hope that helps..

Cheers

Liam

Just worked out 17mm in an equisolid angle projection fisheye and I get 158 degree AoV on film and 97 degree on digital.
So the 1.5x seems to stay true...

But fisheyes can be funny little things
Looking at the chart on Bob Atkin's site, stereographic and orthogonal fisheye lenses won't follow the 1.5x, but equidistance and equisolid angle will

Matt

Thanks for the Diagram Liam

Really helps when my head stopped spinning

Fun part is I just pointed the K10D out my bedroom window and took a 90mm shot with a digital lens. The replaced the lens with a 90mm SMC FA lens and took the same shot.

Guess what - they both looked exactly the same size wise

So I've given up worrying about it now and spend the time concentrating on the shot rather than the focal length the lens either is or isn't giving me

Looks like the most foolproof solution for me is going to be to make sure I hump my Benbo Trekker around with me every time I go out with my camera, learn how to use Mirror Lock Up and Self-Timer or buy a Remote Control.

I'm discouraged from taking my tripod every time because it's a bit long and top-heavy to put in a rucksack, and if I use the the carry bag the damn strap continually slides off my shoulder.

Seriously, though, thanks for the 1.5-times formula, it'll come in handy when I'm out with friends and getting grab shots, or on holiday taking memory shots.

A so-called "digital" lens has exactly the same angle of view as a non-"digital" lens of the same focal length.

All that has chnaged is that the lens designer has tried to make the light path from the lens to the sensor as close to a 90 degree angle (to the sensor) as possible. This is because when light hits the sensor at an angle, some of it is not recorded. This leads to the vignetting which is particulary problematic on Canon 35mm-sized sensors.

Much better to have APS-C sized sensors, and wide lenses optimised for them. This is something Pentax offers, but Canon hasn't really got round to yet.

But all this is getting very far from the original purpose of the thread. To keep a picture sharp on a Pentax DSLR, shoot at one speed higher than you would on film. Unless, of course, you have shake reduction switched on, in which case you can shoot at one speed (or 2 with the K10D) slower.

G