[Dave]

After seeing some of SPXXXX's recent wedding shots, I realised the F4 70-200 isn't giving the shallow DOF I would like. So this got me thinking. What would be the impact on DOF by using same lens on a FF camera instead of the 1.6 crop, vs going from F4 lens to F/2.8 on 1.6 crop camera

So using this great little calculator :- http://www.cambridgeincolour.com/tut...calculator.htm I have made the following conclusions.

To ensure the same framing of a shot , 100mm on a 1.6 crop camera would have to be measured as 160mm on FF camera. I used the calculator at max print of 16 inches , viewed at 25cm, with default eyesight

1.6 Crop vs FF

1.6 crop at 100mm @ F4 with subject at 7 Metres = 0.5M DOF
1.6 crop at 200mm @ F4 with subject at 7 Metres = 0.12M DOF

FF at 160mm @ F4 with subject at 7 Metres = 0.31M DOF
FF at 320mm @ F4 with subject at 7 Metres = 0.08M DOF

So FF gives reduction in DOF of approx 37% in this example. Simpler calc would have been 1-(FF/Crop factor) -> 1-(1/1.6) I think


F4 vs F/2.8

1.6 crop at 100mm @ F2.8 with subject at 7 Metres = 0.35M DOF
1.6 crop at 200mm @ F2.8 with subject at 7 Metres = 0.09M DOF

FF at 160mm @ F2.8 with subject at 7 Metres = 0.22M DOF
FF at 320mm @ F2.8 with subject at 7 Metres = 0.05M DOF

So F2.8 gives reduction in DOF of approx 30% in this example (using F4 figures from section above)


So keeping the F4 & moving to a FF camera would give a more dramatic reduction in DOF, than moving to F2.8. Now if narrow DOF is important, then for just 1 lens (eg. 70-200 F4L) then the extra stop of light for moving to F2.8 might make it more viable. But if you have more than 1 lens where the DOF is more important than speed of glass, then it could well be financially more astute to go for a FF body Of course you also need to take in to account change of focal range moving to FF.

Now of course if swapped the F4 for F/2.8 plus a FF you get a 66% reduction in DOF .... but that's just being greedy

I'm off to swap one of my 40D's for a 5D

[silkstone]

The DoF is a function of the Circle of Confusion which relates directly to sensor/film size.

Typical values for CoC are as follows:-

Full-frame 35mm: 0.0309
1.3x crop factor sensor - Canon 1DMkII, etc: 0.0246
1.5x crop factor sensor - Nikon D70, D80, D200, D300, etc: 0.0203
1.6x crop factor sensor - Canon 350D, 450D, 30D, 40D, etc: 0.0193
4/3rds system - Olympus, etc: 0.0155

Comparing full-frame and Canon APS-C (1.6x crop), 193/309 = 0.625 or 62.5%. So the reduction in DoF with full-frame is 100 - 62.5 = 37.5% which tallies with Dave's example.

If you want really shallow DoF, use medium format or a view camera. That's one reason why tripods were absolutely essential for landscape photographers of old - you needed very small apertures to get large DoF, and exposure times could be several seconds even in good light.

It's also why people making moving from compact cameras (with very small sensors) to DSLRs can have focusing problems - the small sensors give a huge DoF and are much more forgiving if the focus point isn't spot on.

[Ali]

Originally Posted by Dave

I'm off to swap one of my 40D's for a 5D

Ahem. If you have a 40D going spare...

[Dave]

I'm sure I will soon

[stepheno]

That is the beauty of having one of each

[Dave]

But I reckon replace the other 40D with a 1D MKII

[towershot]

Ey up!!!!!
Dave,s after spending again,

[stevegreen]

Originally Posted by Dave

I'm off to swap one of my 40D's for a 5D

Do it Dave, you won't regret a FF camera at all. Love my 5D and with fast L lenses

[Dave]

Well one 40D is sold to make way for the 5D. Just need to wait for 5D MKII to be launched so I can get a MKI 5D for decent money

[Dutchie]

Wahaha how not surprising I thought at first you went back to Nikon

[Markulous]

Originally Posted by Dutchie

I thought at first you went back to Nikon

He'll be there shortly!

[Dutchie]

Haha