Question about shallow depth of field and long lenses

[Lasers_pew_pew_pew]

Hey,

 

Need to try and work something out...

 

If you get a really nice shallow depth of field on a canon 2.8 70-200 (at the 200mm end), on a c300, would you get the same shallow depth of field on a 50 - 150mm lens on a cropped sensor?

 

For example, if I buy a 50 - 150mm 2.8 sigma zoom for my camera, on my gh4 speedbooster with a crop factor of 1.6, at the long end it becomes a 240mm.

 

At this end at 240mm, will it have a very similar look and a shallow depth of field similar to the 200mm end of a canon 70-200mm 2.8?

 

I'm wondering basically is the lovely shallow depth of field from those long lenses due to the actual lenses inside being made to the bigger focal lengths, what causes that lovely look? Or will I achieve the same look with a cropped sensor and smaller lens?

[Nikkor]

The lovely look comes from the longer focal length.

 

The focal lenth of a lens doesn't change by cropping, neither does the aperture ratio. But you can use equivalent numbers for orientation.

 

To have the same sort of look as with a FF sensor and 200@2.8 you would need a (200,2-8)/1.6 lens which would be 125@1,75. Obviously the FF will always look better because the longer the actual focal length the better it behaves optically. I also think speedboosters aren't that great with longer focal length, at least Metabone's White Paper says so.

[Guest Ebrahim Saadawi]

The GH4 with speedbooster is roughly similar to the C300 in terms of sensor size. So yes 200mm f2.8 on the C300 will give the same field of view & dof on a GH4 + SB.

Without the SB however, you'd need a 130mm f/1.8 to match 200mm f/2.8 on C300.

[Nikkor]

Oh, I thought the c300 was FullFrame.

[Inazuma]

http://www.dofmaster.com/dofjs.html

[Lasers_pew_pew_pew]

The GH4 with speedbooster is roughly similar to the C300 in terms of sensor size. So yes 200mm f2.8 on the C300 will give the same field of view & dof on a GH4 + SB.

Without the SB however, you'd need a 130mm f/1.8 to match 200mm f/2.8 on C300.

 

Do you mean sorry that a 150mm on my gh4 with speed booster (still a slight crop compared to c300), will give the same shallow depth of field as a 200 on c300?

 

Or is everyone saying it's the actual lens size that gives the look, and cropping won't give you the same look?

 

Basically it looks like if I want that look I will have to get a big ass lens, I can't just use a smaller lens on a crop?

[Guest Ebrahim Saadawi]

Do you mean sorry that a 150mm on my gh4 with speed booster (still a slight crop compared to c300), will give the same shallow depth of field as a 200 on c300?

Or is everyone saying it's the actual lens size that gives the look, and cropping won't give you the same look?

Basically it looks like if I want that look I will have to get a big ass lens, I can't just use a smaller lens on a crop?

Just consider them (GH4+SB & C300) to be equal to avoid confusing yourself.

GH4 + SB in 1080p = 1.4x crop
GH4 + SB in 4K = 1.6x crop
C300 = 1.55x crop

There is a difference but very slight and negligble in terms of aesthetics. So now let's consider them equal.

Then, a 200mm F/2.8 on a C300 will give you the same dof as a 200mm F/2.8 on a GH4. A 150mm F2.8 on a GH4 will give you the same as a 150mm f2.8 on the C300.

Regarding the question on lens size, no you can't use a smaller lens to get the same asthetic on a smaller sensor, You'll need to use:

1-The same s35 big lens (same size of course) + SB
2-A m43s lens with a bigger aperture (therefore same size) if without a SB.

For example:

-A 200mm F2.8 lens designed for S35 will be similar in size to a 130mm F1.8 designed for m43s, because of the larger aperture. Both will give the same depth of field and field of view.

-A 50mm 1.8 for s35 will be similar in size to a 35mm f/1.2 for m43s, again because it's a 1.2, therefore larger, matching the larger s35 1.8 lens.

[Lasers_pew_pew_pew]

Such a helpful answer thank you!

[richg101]

depth of field is solely a lens characteristic and sensors don't affect the depth of field.  the sensor size determines the field of view you get from a particular lens. 

 

 

what you are wanting is the same ratio between the dof and the field of view.  200mm f2.8 is lovely - particularly on full frame

 

in order to match the dof and fov ratio of a 200mm f2.8 on the c300  you will need around a 135-150mm lens at f2 attached to your gh4. Your 0.7x focal reducer in combination with your lens actually equals this.

 

 

What you do need to factor in is that though the numbers add up, your 200mm lens is now no longer a 200mm lens - it's a 140mm lens due to the 'focal reduction' of 0.7x.  

I have done quite a bit of experimentation on the matter and find that the effect of a longer slower lens on a larger sensor is still impossible to match with a wider, faster alternative on a smaller sensor.  the 'step' between the in focus/out of focus material in frame is not as crisp when using a wider lens.  Focal reducers mounted between the camera and lens are far from optimal.  You will find that although the marketing says that focal reducers improve image quality, they actually boost the prominence of fringing drastically when used wide open.  Fringing is probably the least pleasing aberration and one which is very hard to rectify effectively in post.