focal length and DOF equivalent question

Hi there.
I am a bit confused and would request a bit of a clarification please.

I am weighing on purchasing a FF (6d) in the near future
and I was also considering getting the 18-35 f1.8
which i know does not work on FF (i am not getting rid of my 7d)

so if I were to compare the 7d +18-35 vs the 6d
i would be getting 28.8-56 f2.8 EQUIVALENT, correct?
I am very confused about the equivalence part...

thanks for help in this matter

Bingo. The 7d has a 1.6x crop factor as you have obviously identified, so to get the same field of view from a full-frame camera, you need to multiply the focal length of the EF-S/DC/etc. lens by 1.6.

For clarity, the focal lengths are different however... 18mm on crop is not equal to 28.8mm on full-frame, but the field of view is the same. Similarly, sometimes people confuse the fact that a 50mm lens on full frame becomes an 80mm on crop... this isn't correct, it is still a 50mm lens no matter what camera it is mounted on, the field of view has just changed that's all.

I'm rambling now, but again... when you get the 6d and mount your 24-105 lens on it, you will notice that the wide end gives a much wider field of view on the 6d than it does on the 7d. Similarly, the 7d will give you more reach at 105mm because it is giving an equivalent full-frame focal length of 168mm.

http://www.tawbaware.c​om/maxlyons/calc.htm

You can enter different crop factors and compare say lens A on body B to lens A on body C.

ceriltheblade wrote in post #16800997
Hi there.
I am a bit confused and would request a bit of a clarification please.

I am weighing on purchasing a FF (6d) in the near future
and I was also considering getting the 18-35 f1.8
which i know does not work on FF (i am not getting rid of my 7d)

so if I were to compare the 7d +18-35 vs the 6d
i would be getting 28.8-56 f2.8 EQUIVALENT, correct?
I am very confused about the equivalence part...

thanks for help in this matter

Heya,

If you want thinner depth of field, the full frame will always allow for it. Equivalent fields of view, is what you're looking at. It's the angles that you compare. And that's the crop factor you calculated, the angle of view of the 18-35 on a 1.6 crop sensor, is equivalent to what the angle of view is on a full frame sensor with focal lengths similar to 28~56.
Aperture plays a role in depth of field. But so does distance. The closer you are to the subject, the thinner the depth of field. So a full frame will let you get closer with the same lens, than a 1.6 crop will, for the same composition frame up, hence, thinner depth of field from the full frame (it's not the sensor, it's merely the physical distance differences).

Very best,

ceriltheblade wrote in post #16800997
so if I were to compare the 7d +18-35 vs the 6d
i would be getting 28.8-56 f2.8 EQUIVALENT, correct?
I am very confused about the equivalence part...

thanks for help in this matter

Yes, that's the rough equiv in terms of FoV and DoF. If FoV is roughly the same, position relative to the subject will be roughly the same, but since you'd be using a longer FL on the FF, an aperture a little more than a stop slower will result in roughly the same DoF.

Focal length does not change between different sensors and the DOF difference at the same focal length is virtually the same.

I do not know about a difference in aperture equivalence but it seems reasonable since more light hits the larger sensor.

the DOF is different because you are either getting closer to your subject, or using a longer focal length to get the same framing as you did on a crop

thanks all for the help!
y'all are great!