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Set Me Straight: Math For GH4 Crop Factor & Lenses


WideEyedPictures
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...and a Speedbooster!

 

I'm trying to wrap my head around what the mathematical formula must be to gauge how wide/zoomed lenses will be on a GH4 with a Speedbooster. What I know so far:

 

- Shooting 4K, the crop is 2.3

- Shooting 1080, crop is 2.0

- Metabones Speed Booster lessens crop by 0.71

- Generic Speed Boosters lessen crop by 0.72

 

Is the math: " FF LENS x CROP x BOOSTER ="?

 

or for a 18mm lens:

 

18mm [Lens] x 2.3 [4K Crop] x 0.72 [Metabones] = A 29.808mm lens equivalent?

 

To make matters more confusing to me, that only covers full frame lenses, so how do we determine the results on lenses for APS-C lenses? That I have NO idea on how the math may work.

 

Thanks so much!

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Is that so?

 

As I understood it, APS-C lenses actually direct the light into a more narrow area. This is why an APS-C lens on a full frame camera shows so much vignetting.

 

Thus, if the amount of area the APS-C lens's light is producing, that seems like it's helping to decrease the crop factor some by putting more of the entire lenses image onto the sensor than that of a full frame lens.

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Focal lengths are focal lengths. The sensor size they cover is independent of that.

 

EDIT: To be clear - always calculate based on the focal length. To base FoV equivalent calculation on the projected image circle is like saying a pound of feathers is lighter than a pound of iron.

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As I understood it, APS-C lenses actually direct the light into a more narrow area. This is why an APS-C lens on a full frame camera shows so much vignetting.

 

This part is right. To get the APS-C lens fill the full frame sensor, you'd need a focal expander, the opposite of a Speedbooster, with loss of light.

 

 

Thus, if the amount of area the APS-C lens's light is producing, that seems like it's helping to decrease the crop factor some by putting more of the entire lenses image onto the sensor than that of a full frame lens.

 

I see your point. You say a pound of iron will slay your neighbour beneath your window, but the feathers won't. You mean, a 50mm f 2.0 that's calculated for full frame will appear darker on APS-C than a 50mm f2.0 that's calculated for APS-C? 

 

No, because these values are invariable, only the FoV changes (not because of the lens, but because of the sensor size, the APS-C lens just might have a vignette). 

 

It's more puzzling with a focal reducer. The FoV changes, the amount of light changes, but actually the DoF doesn't change. But then again it does, because you always think about DoF in conjunction with FoV. With narrowFoV you could always have shallowDoF.

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I may have fallen victim to the mistake several others have made. I certainly understand the difference between focal length and field of view. I said the former but meant the latter.

So my curiosity lays in what the field of view equivalent is of a normal lens when placed on the GH4 with not only it's 2.3x crop field of view inherent of that sensor, but the effect of the focal reducer which opens the crop up a bit more.

And my curiosity and confusion increase with lenses made specifically for APS-C lenses. I understand a lens like that will "see" just as far as any lens of the same resume but it's field of view is narrowed thus lessening the crop factor - I just don't know what math to do to figure that out.

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The OP is asking a simple question. Why are you guys bringing all this complicated semantics into it?

 

To answer your question, your calculation is wrong.

It should be Lens focal length * Speedbooster * Crop factor.

 

So a 35mm lens * 0.71 speedbooster * 2x crop factor = 50mm (or 49.7mm to be exact). This basically means that a lens with a speedbooster attached to a Micro Four Thirds camera will become the same focal length as if the lens was just mounted on an APS-C camera.

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To simplify,

GH4 in 4K: 2.3 crop factor.
GH4 in 4K with SB: 1.65x crop factor.

GH4 in 1080p: 2x crop factor
GH4 in 1080p with SB: 1.44x cropfactor

That's for full frame equivalence of course.

18mm ff lens on GH4 in 4K with SB: 18 x 1.65 = 29mm on full frame FOV

Don't think too much about it! :D

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The OP is asking a simple question. Why are you guys bringing all this complicated semantics into it?

 

To answer your question, your calculation is wrong.

It should be Lens focal length * Speedbooster * Crop factor.

 

 

To simplify,

GH4 in 4K: 2.3 crop factor.
GH4 in 4K with SB: 1.65x crop factor.

GH4 in 1080p: 2x crop factor
GH4 in 1080p with SB: 1.44x cropfactor

That's for full frame equivalence of course.

18mm ff lens on GH4 in 4K with SB: 18 x 1.65 = 29mm on full frame FOV

Don't think too much about it! :D

 

 

I appreciate your help (and defense) very much!

 

It would appear that the result is the same regardless of the order of multiplication (listen... I'm an artist who's not good at math - sorry!) Lens*Crop*SB is the same as Lens*SB*Crop

 

Either way, it looks like that makes total sense to me now. I know the formula and I can apply that for any future lenses I may check out.

 

But the last mystery remains - how to do this with an ASP-C lens who's FoV would be different on a MFT lens as opposed to a full frame equivalent. But I THINK I know how to figure this out and will report back.

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I appreciate your help (and defense) very much!

 

It would appear that the result is the same regardless of the order of multiplication (listen... I'm an artist who's not good at math - sorry!) Lens*Crop*SB is the same as Lens*SB*Crop

 

Either way, it looks like that makes total sense to me now. I know the formula and I can apply that for any future lenses I may check out.

 

But the last mystery remains - how to do this with an ASP-C lens who's FoV would be different on a MFT lens as opposed to a full frame equivalent. But I THINK I know how to figure this out and will report back.

My bad :) When I did your formula the first time, I got a completely different result.

 

APS-C lenses (like Canon EF-S or Nikon DX) still use the standard focal length numbers. The only difference is that those lenses are not big enough to fill a full frame camera's sensor, hence the vignette/circle. This is also why you can use APS-C lenses on speed booster on MFT but you cant do the same on APS-C cameras like the Sony NEX.

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just don't worry about the math of APS-C lenses.  They are still fullframe lenses, but will have vignetting if placed on a fullframe camera.

 

Take for instace the Sigma 18-35 mm.  If the speedbooster takes the crop factor of the GH4, down past 1.5x or below you will have vignettting @ 18mm. keep it at 1.6 and above, and you're good.

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Ye gods I thought I was pretty clear. Focal lengths are focal lengths. Calculate your FoV crop based on the focal length of the lens. Doesn't matter what image circle it covers. That only affects whether or not you can actually use the lens, but not your math.

 

So just do this:

 

Focal Length * Crop Factor * Focal Reducer Factor

 

 

just don't worry about the math of APS-C lenses.  They are still fullframe lenses, but will have vignetting if placed on a fullframe camera.

Which means they are not full-frame lenses, because they project an image circle that does not cover an area of 36mm x 24mm.

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  • 2 years later...

So Canon send you a unit, purchases a lot of sponsored content on DPR to praise the video capabilities and now considers a major firmware to unlock this body ? The sales must be very bad. Seems to be little and too late for Canon.

In my case the 5d4 was the last drop of non sense from this company. I'm done, I bought a GH5. They would have to come up with more than band aid fix to convince me to drop another $3500. A bit of improvement on the crop and codec is not going to cut it. Does the mark 4 even have the hardware to fix all the issues ? Let's see what they announce at NAB.

 

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