35mm (film) lenses on digital camera, do I lose lens speed as well?

Okay, I have to multiply focal length by 1.5 when I put a lens for film
on my digital camera. Does this mean I have to do the same with F-numbers
like a teleconverter?

Thanks,

Steve

 

Actually, it is more of a FOV divider.

No.

 

No. The multiplier is meant to mean "_effective_" focal length vs. the
35mm standard 36x24mm frame size.

The sensor is smaller on your DSLR (~ 24x16mm). The image circle formed
by the lens is still the same size, but the sensor is that much smaller
than the std. 35mm frame. So it is like you're "zoomed in" by 1.5x the
FL of the lens.

Since the true FL has not really changed and the diameter of the
aperture has not changed (for a given setting), the aperure number
(ratio of FL over aperture diameter) has not changed either.
So, exposure is not affected*.

The DOF _has_ changed as that is purely related to print size. So since
you would need to blow up the smaller sensor image to a greater factor
than with the larger film image to get to a given print size, the DOF is
shallower than the markings on the standard 35mm format lens suggest.

Cheers,
Alan.

*As the different camera/sensor manufacturers do not seem to be
perfectly lined up with respect to ISO on digital cameras, there is a
variance between cameras in results. If you meter with a seperate meter
then different cameras will give different saturation for the same
scene. This difference is about 2/3 of a stop among the various
brands/models. Which could fairly be said for the wide variety of film
types in any case.

 

So, if I put a 200 mm f/4 lens with a 2x teleconverter (effectively a
400 mm f/8) on my Nikon digital SLR, I'm getting the equivalent of a
600 mm f/8? That's useful to know...

 

Equivalent what?

 

Correct, in effect.

 

Whoa, I followed you right up to the DOF changing. Does the DOF become
shallower because the circle of confusion would be blown up along with the
sensor image to get a given print size?

 

Yep.

The CoC is constant in size v. the constant in size aperture, but now
smaller sensor; DOF is related to intended print size. The lens
manufacturer makes an assumption about print size (depending on what you
read and when and where it was written the "standard" CoC print size is
8x10 (from sheet film cameras)) when putting the DOF scale on the lens.
(and it could be that lenses for cropped sensor cameras (Nikon, Canon,
Pentax, Sigma, etc.) take this into account, but I don't know for sure.

To get an 8x12 from a 24x36mm negative, the blowup is 8.47 times.

To get an 8x12 from a 16x24mm sensor, the blowup is 12.7 times.

So the CoC's are "spread" out that much more so you get shallower
"critical focus" DOF on the print.

Of course to get the same (almost) image from a 1.5 cropped sensor at
the same focal length you will have to step back 1.5 times further
resulting in deeper DOF and somewhat restoring balance to the DOF
universe. Or at the same distance use a shorter FL. But this will have
a different outcome as the hf distance is related to the square of the
FL so it's not linear like the focal plane distance from the film plane.

I'm too lazy to bother with the math, I'll leave it for you...

the main point to remember is that the DOF markings on a full frame lens
are wrong when used with a cropped sensor.

Cheers,
Alan.

 

It becomes more shallow if you use the same actual focal length, but
deeper if you adjust focal length to preserve field of view (FOV).

The following web-pages demonstrates what happes to FOV, DOF and
perspective when you use a cropped image circle:

- http://hannemyr.com/photo/crop.html
- http://hannemyr.com/photo/crop2.html