The Focal Length Multiplier Confusion

[tsdh]

Originally posted by Kho King

I think u misunderstood what CK wrote, read again...

If you use a 50mm lens on a DSLR, the same 50mm lens on a 35mm SLR, shoot the same subject at the same subject distance, the 50mm lens on 1.5x DSLR will give the same effect as a 75mm lens on the 35mm camera, including the DoF.

Note the "same subject distance..."

CK's statement is correct, the DOF is the same for the same focal length with the same subject distance (as long as the subject is also printed to the same size at final print).
I didn't talk about that statement, but regarding this one:

Originally posted by Ckiang
But if you do that, it's different. The 1.5x FLM DSLR has inherent 1.5x multiplication.

If you just use scissors to cut, you end up with a smaller print.

When I tried a 28-70 on a D100, and a 28-70 on a F100, the 70 end on both doesn't quite look the same in terms of background blur (and DoF).

There you may notice that there's enlargement factor involved which make the DOF different.

 

[denizenx]

Originally posted by reflecx


That's exactly the point! The DOF would be very small in that instance if you viewed the 22m x 33m print from the same distance as you would a 8R print.

When you talk about DOF, it is always in relation to printing a photo a certain size (say 8x10) and viewing from a certain distance (say 2-3 feet). Given this size and viewing distance, you are then judging whether a certain given point in the photo is in focus or not. Objects within the DOF range are considered to be in acceptable focus. Objects outside are not considered to be in acceptable focus and will therefore not appear sharp in the photograph.

no sense at all leh? meaning that I shoot a boy and enlarge to 22m then there exists a boy that is 22m? shouldn't it be relative to subject? otherwise I can advertise all digicams as true macros (which is true) because the ccd is so small and I have a 19" monitor? mmmm then there's never to be a consensus because each person is measuring by his own scale...

 

[ckiang]

Originally posted by reflecx

The f number, focal length and distance is the same for a 1.5x FLM DSLR and a 35mm SLR.

But.... the circle of confusion is NOT the same for the DSLR and the SLR because of the difference in film image size. This is because you have to magnify the DSLR image more to get to same printed size compared to the SLR. So the DOF for the DSLR is actually less than that of the SLR.

Use this DOF calculator if you still don't believe.

http://www.silverlight.co.uk/resources/dof_calc.html

CoC is different for different film/sensor sizes. So a HF SLR has a different CoC from a FF SLR. You cannot use the same CoC for both systems, so there.

Regards
CK

 

[ckiang]

Originally posted by tsdh

CK's statement is correct, the DOF is the same for the same focal length with the same subject distance (as long as the subject is also printed to the same size at final print).
I didn't talk about that statement, but regarding this one:


There you may notice that there's enlargement factor involved which make the DOF different.

That's right. Take an image from a HF SLR and a FF SLR, and display them on a common monitor. Let's just assume that we get 3072 x 2048 images from both. Let's also assume that say, a 50mm lens is used for both.

Now, when we then compare the 2 images, the output from the HF SLR is bigger than the one shot with the FF SLR. This is the inherent magnification. If you crop from the FF SLR, there is no magnification. Also, if you then interpolate this crop to 3072 x 2048, it's not quite the same either, coz this will be kind of doing "Digital Zoom".

And since HF and FF are really 2 different systems (the "35mm equivalent" is only there as a guide), we should really be using a different CoC when calculating DoF. Right? :dunno:

Regards
CK