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Archive 2014 · Crop lens aperture calculation

  
 
trueimage
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p.1 #1 · p.1 #1 · Crop lens aperture calculation


Hi,

I saw a video yesterday that described something I've never really thought about - how crop factor affects aperture. In the example he stated that a Fuji X-T1 (1.5x crop) with the Fuji XF 56mm f/1.2 lens wide open is equivalent to an 85mm f/1.8 lens on a FF body.

Is this really true or only partially true as with most conversions from FF to crop? Also if it is, why did I spend $999+tax on an 85/1.8?



Mar 28, 2014 at 02:03 PM
Jman13
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p.1 #2 · p.1 #2 · Crop lens aperture calculation


It's true from a depth of field standpoint. The lens is a 56mm f/1.2. You get the depth of field a 56mm f/1.2 achieves but you're standing at a distance that you would with an 85mm lens on full frame, so the DOF is like an 85/1.8. And no, you didn't spend $1000 on an 85/1.8, you spent it on a 56/1.2....one that is exceptional optically and made for your camera.


Mar 28, 2014 at 02:23 PM
DerKaiser
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p.1 #3 · p.1 #3 · Crop lens aperture calculation


The difference in DOF at a given f stop for crop sensors versus larger sensors has been talked about ad nauseum on many forums including this one. I'm sorry if it's news to you. However, f1.2 still gathers more light than f1.8 regardless of sensor size.

Only you can answer why you spent $999+ tax on the 56 f1.2. But other advantages include the aforementioned light advantage at f1.2, smaller size, cheaper camera body, etc. I probably won't buy the 56 until the next set of rebates comes along.



Mar 28, 2014 at 02:28 PM
CronoDL
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p.1 #4 · p.1 #4 · Crop lens aperture calculation


In terms of achieving the same depth of field, it's true. It's not quite exactly the same, but it's very close. You can go to dofmaster.com and use their DOF calculator. In terms of capturing light, 1.2 is 1.2, and is always faster than a 1.8.

You spent that much money because 1.2 lenses are expensive, and the fuji 56mm 1.2 is quite a bit smaller than Canon's 85mm 1.2, which can cover a full frame sensor.



Mar 28, 2014 at 02:44 PM
douglasf13
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p.1 #5 · p.1 #5 · Crop lens aperture calculation


DerKaiser wrote:
However, f1.2 still gathers more light than f1.8 regardless of sensor size.


A larger sensor still gathers more light. My X100 with the lens set to f4 gathers a lot more light than my iPhone 5 with its lens wide open at f2.4. Sure, I'll have to raise my ISO on my X100 a stop and a half more than the iPhone to reach the same exposure, but my X100 at ISO 600 is still MUCH cleaner than my iPhone at ISO 200.

In other words, you have to think of equivalencies in terms of both depth of field and light, not solely depth of field, because larger sensors are inherently cleaner than smaller sensors (assuming somewhat like sensor technologies.)

So, using a 56/1.2 at ISO 200 on Fuji X will result in similar depth of field and noise characteristics as using an 85/1.8 on something like a D600 at ISO 400, give or take.



Mar 28, 2014 at 08:16 PM
Steve Spencer
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p.1 #6 · p.1 #6 · Crop lens aperture calculation


I think the best way to think about it is that the 56 f/1.2 will have brighter light projected onto a smaller image circle or said the other way an 85 f/1.8 will project dimmer light across a larger area. It is about the same amount of light, but the light from the f/1.2 aperture is brighter. Just as two light bulbs that produce the same number of lumens (one a spot light and one a flood light) produce the same amount of light, the light has different properties. In every day language we say the spot light produces brighter light even though the amount of light produced by the two bulbs is the same. That has implications for shutter speed. At the same ISO the 56 f/1.2 will need a faster shutter speed for the same exposure as the 85 f/1.8. A little over twice as fast of a shutter speed.

Where things get controversial is that in general FF sensors have less noise at every ISO than crop sensors. So some say, why not turn up the ISO on the FF camera to get the same shutter speed as the crop camera. The trouble is that whether this makes sense depends on the specific cameras involved and how their sensors perform. It is also important to note that the Fuji camera have a different filter array on their sensors (X-trans vs. the typical Bayer) and the Fuji filter array helps performance at high ISOs, so comparing ISO between Fuji cameras and all the FF cameras (that have Bayer arrays) is not an easy thing to do. People have tended to use the short hand that adding a stop or so to the aperture on a 1.5 crop gets you the equivalent aperture on FF. That works for depth of field, but it is more complex and the light is not the same that the lens let in. The crop lens will let in brighter more tightly focussed light, whereas the FF lens will let in dimmer light spread out over a wider area. Even when you adjust aperture so that the same amount of light is let in, people should not think of the light as being the same.



Mar 28, 2014 at 08:51 PM
Steve Spencer
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p.1 #7 · p.1 #7 · Crop lens aperture calculation


douglasf13 wrote:
A larger sensor still gathers more light. My X100 with the lens set to f4 gathers a lot more light than my iPhone 5 with its lens wide open at f2.4. Sure, I'll have to raise my ISO on my X100 a stop and a half more than the iPhone to reach the same exposure, but my X100 at ISO 600 is still MUCH cleaner than my iPhone at ISO 200.

In other words, you have to think of equivalencies in terms of both depth of field and light, not solely depth of field, because larger sensors are inherently
...Show more

No it most certainly will not result in the same noise characteristics. When comparing the Fuji to the FF you will have to keep in mind that the Fuji has a different filter array that will reduce the noise at higher ISO (at the cost of some reduction in detail at low ISOs). So this sort of comparison goes completely out the window when comparing sensors that are quite a bit different. It of course also matter which FF camera we are talking about. I would not be surprised at all if the Fuji cameras have lower noise than an older FF camera like the Canon 1Ds. In fact, I am almost sure that they do. Large sensors are not inherently cleaner. They just tend to be cleaner. It does very much depend on the particular sensors you are comparing. Old FF sensors in fact can have more noise than many of the newer 1.5 crop sensors, and the Fuji sensors have a trick up their sleeve when light is lower that none of the FF sensors have. So, especially comparing with the Fuji the equivalence argument is broken. Or said another way your qualification, "assuming somewhat like sensor technologies," is an assumption we know we cannot make when comparing the Fuji sensors to any FF sensor. We know they are not similar technologies.



Mar 28, 2014 at 08:58 PM
rattymouse
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p.1 #8 · p.1 #8 · Crop lens aperture calculation


CronoDL wrote:
In terms of achieving the same depth of field, it's true. It's not quite exactly the same, but it's very close. You can go to dofmaster.com and use their DOF calculator. In terms of capturing light, 1.2 is 1.2, and is always faster than a 1.8.

You spent that much money because 1.2 lenses are expensive, and the fuji 56mm 1.2 is quite a bit smaller than Canon's 85mm 1.2, which can cover a full frame sensor.


The online DOF calculator at dofmaster.com is whacked out. I tried calculating the DOF of my 6 x 7 film camera, subject at 4 feet, lens 55mm, aperture at f/4.5. It came out to 3.2 meters. I kept everything the same, except switching to an APS-C sensor and found that I have LESS depth of field (0.9 meters), not more. WTF? A smaller sensor (in this case a MUCH smaller sensor) should have a lot MORE depth of field.





Mar 28, 2014 at 09:00 PM
Steve Spencer
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p.1 #9 · p.1 #9 · Crop lens aperture calculation


rattymouse wrote:
The online DOF calculator at dofmaster.com is whacked out. I tried calculating the DOF of my 6 x 7 film camera, subject at 4 feet, lens 55mm, aperture at f/4.5. It came out to 3.2 meters. I kept everything the same, except switching to an APS-C sensor and found that I have LESS depth of field (0.9 meters), not more. WTF? A smaller sensor (in this case a MUCH smaller sensor) should have a lot MORE depth of field.



Actually not. A 55mm lens is quite wide on 6 X 7 and a short tele on APS-C, so it is not surprising the 6 X 7 had more depth of field. If you use a focal length equivalent in terms of field of view, then the larger format will have less depth of field.



Mar 28, 2014 at 09:19 PM
rattymouse
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p.1 #10 · p.1 #10 · Crop lens aperture calculation


Steve Spencer wrote:
Actually not. A 55mm lens is quite wide on 6 X 7 and a short tele on APS-C, so it is not surprising the 6 X 7 had more depth of field. If you use a focal length equivalent in terms of field of view, then the larger format will have less depth of field.


Good god...I knew I was forgetting something....Thanks!!




Mar 28, 2014 at 09:23 PM





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