GH4: Full Frame Field of View? (Sigma 18-35mm f1.8 + Speedbooster)

P

panther0271

Active member
So, I decided it as time to invest in a new camera system and purchased a GH4 + Sigma 18-35mm f1.8 + Nikon > M43 Speedbooster. The last camera I bought was the original HVX200 .... so my understanding of interchangeable lenses, crop sensors, focal lengths, FOV, etc is... severely limited.

Anyway, I think this lens combination gives me a FF FOV of 18mm-35mm at f1.3.



1- The Sigma lens is made for APS-C sized sensors so 18-35mm = 27mm-52.5mm on FF. http://www.sigmaphoto.com/product/18-35mm-f18-dc-hsm-a

2.- The M43 SB makes lens 0.71x wider so... (18mm*.71) + (35mm*.71) = 12.78mm-24.85mm . http://metabones.com/products/details/MB_SPNFG-m43-BM1

3.- The SB adds a 1.42x crop when put on a M43 camera so... (12.78*1.42) + (24.85*1.42) = 18-35mm

4.- Adding a stop = f1.8 divided by f1.4 = f1.3 http://www.dpreview.com/forums/thread/615648


Could someone please confirm my math or correct me where I went wrong.

Does the speedbooster boost the lens a stop, regardless of whether it's a full frame or APS-C?
 
I have also been considering the Sigma 18-35mm lens with Nikon Speedbooster and I was just trying to figure these calculations in my head yesterday. I'm glad I'm not the only one who is unsure. You are following a logical calculation path that make sense to me.

I am also anticipating the release of the Olympus 7-14mm f/2.8. The Olympus won't be as fast and therefore won't have as shallow a depth of field as the Sigma but as someone on this forum pointed out, a shallow DoF is not as desired the wider your lens gets. The plus sides to the Olympus is that it is wider than the Sigma and it is a native M43 lens with AutoFocus which can be useful in some situations. The Olympus should also be smaller and lighter.

Out of curiosity, why did you include the APS-C to full frame calculations?
 
You might be overcomplicating things for your thoughts on future lenses...

Consider the SpeedBooster (or any focal reducer, or teleconverter) to modify the lens, not the sensor. MFT mount especially has several different cameras with different sized sensor crops, from Black Magic's 1" sensor to JVC's forthcoming Super35 sensor. The GH4 has at least two different crop modes, 2x @ 1080 and 2.3x @ 4K

So...

The SB is a 0.71 reducer. That converts the Sigma 18-35 to a ~12.8-24.9mm (18*.71, 35*.71). Its maximum aperture (assumign it works fully as advertised) would be 1.8*.71=1.28. These values hold for that lens + converter combination regardless of what camera body you use with it.

FF equiv Field of view on the GH4 @ 1080 would then be 25.6-49.8 (x2 crop), and @4K, 29.4-57.3 (x2.3 crop).
 
True North said:
Out of curiosity, why did you include the APS-C to full frame calculations?
Click to expand...

I'm trying to get an apple-to-apples comparison.

I chose the Sigma as an affordable way to replace primes in its field of view. Nearly every lens is designed for full frame cameras, but the Sigma 18-35mm lens is DIFFERENT. It is designed for APS-C and so according to Sigma's website it's full frame FOV is 27mm-52.5mm.

I think this information is significant... but I don't see people using 27mm-52.5mm in their calculations. I see everyone constantly using 18-35. Am I misunderstanding something?

@mcbob Wow.. I forgot about the different crops for 1080 and 4K. Okay now I'm really confused. Should I ignore #3 on my post where BlackMagic states on its website that the SB will give a 1.42 crop on a M43 camera... and instead use this 2x and 2.3x crop information?

AAAAAARRGGGh! That's the sound of me pulling my hair out trying to understand this!
 
Yes, pitch #3 from your list and you're otherwise good. Ignore the "1.42 crop" bit, because that crop will be different with different m43 bodies. That crop only applies when you combine the SpeedBooster (x0.71) and standard MFT crop (x2)... 2 x 0.71 = 1.42. As we see, that can confuzzle things when the sensor crop changes.

The Speedbooster modifies the lens (x0.71), no matter what sensor it was designed for. As they say, "a 50 is a 50 is a 50." The FOV from an APS Sigma 35mm is the same as any "full frame" 35mm lens is the same as a 35mm c-mount lens designed for 16mm film, no matter what camera mounts them. The sensor for which a lens is designed only affects the area their imaging circle will cover.

Once you have the lens math all worked out, then you apply the sensor crop.

Very rough estimation: the Sigma 18-35 on 2x crop m43 sensors yields approximately 25-50mm FFEFOV, exposes as a f1.3, and has DOF similar to f2.6... so it'll look similar to a standard FF 24-70/2.8... but without the reach.

Without the Speedbooster, it still works darned well for interviews. As a note, I use the Sigma 18-35 along with a Sigma 50-150/2.8, (generally boosting the 50-150 to a 35-105/2 and using the 18-35 straight), as just about my only interview lenses these days. Having two sharp fast parfocal zooms does ease a lot of lens-changing worries in my workday and (despite their heft) clears up a good deal of space in a bag.
 
panther0271 said:
I'm trying to get an apple-to-apples comparison.

I chose the Sigma as an affordable way to replace primes in its field of view. Nearly every lens is designed for full frame cameras, but the Sigma 18-35mm lens is DIFFERENT. It is designed for APS-C and so according to Sigma's website it's full frame FOV is 27mm-52.5mm.

I think this information is significant... but I don't see people using 27mm-52.5mm in their calculations. I see everyone constantly using 18-35. Am I misunderstanding something?

AAAAAARRGGGh! That's the sound of me pulling my hair out trying to understand this!
Click to expand...

Hopefully this will make it simpler: All lenses are advertised in terms of a 35mm frame (full frame) size.

If you mounted the Sigma 18-35 on a FF camera, it would have the same FOV as a 18-35 lens made for FF. When a lens is made for APS-C, it means that the image circle the lens produces is optimized for APS-C sensors (i.e. smaller than a FF-optimized lens). If it were mounted on a FF camera, it would likely vignette heavily (especially at the wide end).

Here's a good example of that in this review of the Sigma 18-35.

Where your 27-57.5mm number comes in is when you want to know the equivalent FOV it will have on an APS-C camera. APS-C's 1.5x crop factor gives an 18-35 lens a FOV of 27-57.5.

mcbob is spot on with the other calculations, but hopefully this explanation will help you as you evaluate other lenses.
 
Personally, I find it easier to remember/calculate the crop factor(s) and then multiply the lens' focal length. For example:

When shooting FHD (which uses the full width of the sensor), the GH4 has a crop factor (relative to a full 3:2 photography frame) of 2.18X (which I usually round off to 2.2, YMMV). When you add a SpeedBooster to the equation, it becomes 2.18 X 0.71 = 1.5478 (which I would round off to 1.5, again YMMV). So 18mm X 1.5 = 27mm on the short end, and 35mm X 1.5 = 53mm on the long end. And f1.8 X 1.5 = f2.8

Now just substitute an original (unSpeedBoosted) crop factor of either 2.6X (or 2.61X if you prefer two decimal places) for (regular) 4K or 2.5X (or 2.48X if you prefer two decimal places) for Cinema. e.g. 2.61 X 0.71 = 1.8531 and 18mm X 1.8531 = 34mm on the short end and 35mm X 1.85 = 65mm on the long end for "regular" 4K.

HTH - Mark

P.S. if anyone really wants the crop factor calculation (for Cinema):

43.2 / ( ( ( (4096 / 4608) * 17.3 )^2 + ( (2160 / 4608) * 13)^2 )^0.5)

Substitute 3840 for 4096 into the above for "regular" 4K.
 
Last edited:
First thing to do is forget equivalencies, and just deal with the actual numbers. The lens is 18-35. That's not "how it's marked", that's the actual measurement of the focal length of the lens.

The SpeedBooster gives you a .71x multiplication factor to the effective focal length. It changes the optical image coming through the lens, so you'd apply that multiplication factor to the lens focal length, to come up with a modified focal length of 12.78 - 24.85 (or, let's just round it off, 13-25mm).

So the lens with the SpeedBooster attached is delivering an m43-sized image and a modified focal length of 13-25mm. It should be a little wider than the 14mm fixed lenses, and at full telephoto it should be delivering the field of view of a 25mm lens.

If you want to equate that to what the image would be like on a full-frame camera, that's where you apply the crop factors. In regular HD the GH4 has a 2.0x crop factor, so it means the SpeedBooster-equipped 18-35 lens will deliver the same field of view as a 26-50mm zoom on a full-frame camera. (18x.71x2.0 = 26, 35x.71x2.0=50).

However, in 4K, the GH4 has a 2.3x crop factor, so you'd have to factor that in. That means the SpeedBooster-equipped 18-35 lens would deliver the same field of view as a 30-57mm lens on a full-frame camera.
 
You have way more experience than me with lots of different cameras, and it may be easy for you to know how a 25mm lens at f/2.8 will look on a 1.8x crop factor camera... For me, I prefer to think always in terms of equivalent focal length. At least until I have the camera and the lenses: then, it's my gear, and I want to know it in-and-out, so yes, if I own a 25mm lens and a 1.8x crop factor camera, I will know how the image looks at f/2.8.



There are two ways to think about a speedbooster:

* "it makes my sensor bigger": this is the easy way to think about it, and the one I use all the time; my APS-C camera becomes FF, and ISO is doubled, end of story. Now I know how a 35mm will look when set it at f/2.8, both in terms of FoV and DoF. Easy.

* "it changes the focal length and aperture of the lens": this route will mess with your brain pretty badly, at least until you get used to it... but it's the correct one, I'm afraid. The sensor is not getting bigger when you put a loupe in front of it. What's happening is that your Samyang 35mm f/1.4 (12 elements in 10 groups) is turning into a 25mm f/1.0 (16 elements in 14 groups). The combination of the lens+SB is indeed 25mm, and it is indeed f/1.0 (the physical size of the aperture hasn't changed, the focal length has, so the ratio becomes 1.0). Exactly the same as when you zoom with your 18-55 f/3.5-5.6 kit lens: glass elements move and this changes focal length, the aperture hole changes size but not much, and f/ is altered; the sensor doesn't get bigger or smaller.

The second one is the correct option... but the first one is way simpler to think about.



So, given how I like to think about it:
* the sensor is 17.3mm wide, which means a 2.08x crop factor (I always use the horizontal measure only; it doesn't make sense to recalculate the crop factor if I crop the image vertically, say, for a 2.35:1 finish)
* the sensor is 4608 pixels wide, which means 2.34x crop factor shooting 4096 and 2.50x crop factor shooting 3840
* the standard speedbooster is 0.71x which makes those crop factors 1.66x and 1.77x
* the BMCC speedbooster is 0.64x which makes those crop factors 1.50x and 1.60x

The GH4 with a BMCC speedbooster plus Tokina 11-16, Sigma 18-35, Sigma 50mm Art, and Samyang 85mm, is a very compelling <$5K set.
In FF equivalent for FoV and DoF, that would be a 18-26mm f/4.5, 29-56mm f/2.8, 80mm f/2.2, and 135mm f/2.2.
 
You must log in or register to reply here.
Back
Top