In the move from film to digital ...

Has my 50mm f1:1.4 effectively become a 75mm f1:1.4 because of the crop factor?

Yes, it's still a 50mm but has the angle of view of a 75mm when attached to a D or K digital body.

Hi,

Yes, you now have effectively a 75mm lens, but it now also still has the 'larger' depth of field of the 50mm lens it still is...so if you like selective focus effects with wide apertures one of the downsides of digital is about to hit home..

cheers Steve.

It's more a 75/1.7 in terms of DOF.

It's impossible to change DOF using a crop. If you have 10cm of DOF you WILL have the same depth in crop. The only change is you see only a part of full frame area with full resolution.

yes but you will have the same depth of field as a 50mm lens. This is more DOF than you would have had if you were using the 75mm lens which you would need to get the same field of view on film.

That's not quite correct - you will have more DOF than a 75mm/1.4 on film, but less than the 50/1.4 had on film (technically, this is because digital requires a smaller "Circle of Confusion".

If you play with an online DOF calculator (eg www.dofmaster.com), you will see what I mean

Cheers,
Reuben0

To imagine the crop factor you could take a print and a scissors. Just cut four stripes from sides of the print off.

yes, that is exactly it, and is the reason why I'm right, though I prefer to think of cutting down a 35mm negative to APS size.

Reuben, thanks for the link, I knew that but couldn't prove it so decided to leave it out for the sake of clarity.

A 50mm lens is always a 50mm lens and always has the same DOF.

If you change the parameters by deciding on a different circle of confusion, that's one thing, but you haven't changed the lens you've changed your parameters for assessing the lens.

It's well established that in practical terms, for a given field of view, smaller formats have more DOF than larger formats. This is because the standard lens for a given format changes. So a 24mm lens for 110, a 50mm lens for 35mm, a 80mm lens for 6x6cm, etc.

In the case of the digital comparison you are using a 50mm lens and getting a telephoto effect. This will give more DOF than using a 75mm lens on a 35mm body.

This is why small digital compacts have so much DOF that we can forget out of focus backgrounds.

John, I'm afraid that's not mathematically correct

There is no such thing as an absolute DOF value for any lens/format. It all depends on what "circle of Confusion" value you apply (which is a measure of how sharp the OOF parts have to be to be regarded as in focus):

The relevant equations are here:
http://www.dofmaster.com/equations.html

Cheers,
Reuben0

My understandfing is that the DOF of a lens depends on the lens and image magnification.

If you take a photo using a 50mm lens on a 35mm film body, then crop that photo to show the same view as a 50mm lens on a DSLR, and view both images at the same size, they will show the same DoF characteristics, ie the 50mm lens will have apparently less DOF, similar to a 75mm lens on a 35mm film body.

We've been through all of this before....Yes, magnification is the key, but in a practical sense what we observe about DOF is much simpler, as I mentioned above.

I stand by the statement that a 50mm lens remains a 50mm lens, as it clearly does. The value of circle of confusion is, although not totally arbitary, nit relevant to that observation as it is a value that we select for ourselves. All I am saying is that a 50mm lens is a 50mm lens and it's characteristics do not change because of what we put behind it.

That's true John, but DOF is a matter of perception rather than an intrinsic property of a lens. Let me explain why

When a lens is focussed at a particular distance, detail at that distance is rendered sharply. As you examine details further from the focus point (in front or behind), they become gradually less sharp until we think of them as "out of focus". There isn't a sharp cutoff between in and out of focus that is somehow an intrinsic property of the lens.

Now what we regard as sharp/in focus depends on the degree of magnification that you inspect the details with. Large formats/small prints require much less critical sharpness to appear in focus than large prints/small formats. This sharpness criterion is what the "Circle of Confusion" species. I'm sure we've all had the experience of looking at an image in print or on the screen and it looks in focus until we've zoomed in and realised the focus point is slightly different and the parts we thought were in sharp focus are now less so.

Note that as CoC is subjective, There isn't a "correct" value. Thus, some people use 0.025 mm as the circle of confusion for 35mm format, while others will use 0.030 mm. Digital is given a smaller CoC value than film, as the format is smaller. Similarly, you might want to use a smaller value than normal for large prints from film.

It's easy to verify all of this for yourself, rather than just taking my word for it, by trying out a DOF calculator like the one mentioned earlier. Those mathematically inclined can inspect the previously cited DOF equations.

Cheers,
Reuben0

I think the original thread that covers all these points will be found in the Lenses section, should anyone wish to view it.