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FStops on the Cropped Sensors Not Accurate?


johnortt
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Hi Everyone,
 
I am hoping for a little help with something that has been confusing me recently.  I have seen a lot of posts in the forums where people are saying things along the lines of the 56mm f1.2 Fuji lens should really be considered as a f1.8 when compared to lenses on a full frame camera?  Is this correct?
 
The thread I saw it in most recently was: http://prometheus.med.utah.edu/~bwjones/2014/08/fuji-56mm-f1-2/
specifically the following post:
 

 

Please don’t compare the 1.2 Canon FF-Lense with an Apsc-Lens regarding the size. Full-Frame needs more light and thus a bigger front-element to capture it. If you already say, 56mm is 85mm in FF, then also point out, that the aperture would be a F1.8 compared to a Full format. And the FF-Lenses with 85mm F1.8 are similar in size and weight. Or use the 85mm 1.2 from canon with a speedbooster on the fujifilm and you have got a real 1.2 Aperture, (or get a F. 0.8 for the apsc) not a cropped one.

Please don’t forget to add the crop-factor to the aperture too. Companies do it to fool people, but they do it on purpose.
 
Tobias K

 

 
I understand the theory, (ie bigger lenses mean more light) but surely this has all been factored into the Fuji lens calculations to ensure the comparisons are accurate.  There are also factors such as the sensors proximity to the back of the lens on mirrorless cameras that to some extent must cancel the size issue out.
 
I would really appreciate some help understanding this one.
 
Thanks John
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An f stop is an f stop is an f stop - well in photographic exposure terms it is where the infinitesimally small differences in light transmission due to the glass don't matter in the same way they do in the movie world where they have lenses marked in t stops which are corrected a little further.

 

Essentially the person you're quoting is wrong, or at least is interpreting things the wrong way.

It is very easy to test if you have a Nikon and a few lenses. Put a FX lens on a Nikon FX body and take a shot at (for example) f2.8 then swap it out for a DX lens and set that to f2.8 - the exposure will be the same. You can actually use some DX lenses on FX bodies and get a full frame exposed. The AF-S 35mm f1.8 G DX can just about do this once its stopped down a little and many zooms do the same as you alter the focal length- the Sigma 10-20mm from memory is full frame from about 16mm.

 

I think what the person in the quoted article is misunderstanding is the apparent difference in depth of field between the two systems for a given aperture. If you stood in one spot with a FX body and a 85mm f1.4 and took a picture at f1.4 then did the same with a crop sensor camera using a 56mm f1.2 set to f1.4 you would get about one stop extra depth of field - the exposure would be the same. By opening the 56mm up to f1.2 you would then get a picture with a similar DoF to the 85mm.

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Remember that f/stops are ratios between the focal length of the lens and the diameter of the lens opening.  Therefore the math doesn't take into account the "imagined" comparison of APS-C sensors to the old 35mm (full frame) size.  When film photographers try to compare what a certain focal length in APS-C might look like in their 35mm world, they multiply the focal length by the difference in sensor size.  For APS-C sensors (except for Canon), that means multiplying by 1.5 since the APS-C sensors are 2/3 the size of full frame (the inverse of 2/3 is 3/2 which is 1.5).  So, when you want to make this comparison, remember that this changes the math for the largest aperture on the APS-C lens.  An example would be a 35mm (physical size) f/1.8 lens for APS-C would be like shooting with a 52.5mm lens in the 35mm (full frame world).  Manufacturers often don't mention that this makes that lens equal to a (1.8x1.5) f/2.7 "equivalent" aperture in the full frame world.  It is not in their marketing's best interest to mention this fact as it makes it seem that their lens isn't quite as fast.  Your APS-C 35mm lens with a widest opening of f/1.8 has a diameter of 19.4 mm.  That's a physical fact.  I hope this clears it all up.

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Thanks very much guys,  

 

I really appreciate you taking the time to explain that to me.

 

So if I understand correctly...

 

Using a lens designed for a full frame 35mm camera on a crop sensor is effectively like reducing the lens opening size as you are utilising less of the glass at the front of the lens.

 

However that may not (should not) be the case for lenses designed for the smaller sensors as hopefully the lens has been optimised for the size of the sensor.

 

Please correct me if I have misunderstood and thanks again for your help.

 

Regards,

 

John

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Sort of...

 

A full frame lens throws a circle out the back onto the sensor with a diameter of around a 43mm. On a 35mm camera or a digital camera with a sensor the same size (referred to as Full Frame) you will be using all of that circle - see below - with a cropped sensor you simply use a smaller part of the circle. 

 

ADDSensorCrop.jpg

 

As you can see from the diagram below a FF lens produces a circle large enough for a FF sensor as well as working fine with smaller sensors which just crop off the edges - a lens designed for a cropped sensor produces a smaller circle that isn't large enough to cover the larger FF sensor. What you can also see is the subject - the mountains in this case - are always the same size as the focal length of the lenses in question are the same. The ONLY thing you change is the field of view due to how big (how much crop) the sensor is. You are not increasing the focal length of the lens by putting it on a crop camera and regardless of which camera you put it on a lens will ALWAYS be the focal length printed on the barrel - same goes for the aperture, it will always be the one you select.

 

1252021945_J22av-L.jpg

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Black Pearl's explanation is perfectly correct but may also confuse a little bit as it mentions "full frame lens".

All 56mm lenses have a deeper depth of field than 85mm lenses. It does not matter if the lens is made for full frame, APS-C or MF4/3 or any other format.

(Or more general longer focal length give less depth of field and vice versa.) And lower aperture numbers also give less depth of field.

 

For a typical portrait lens you would choose 85mm on a full frame but 56 on a APS-C. They will give the same field of view but at a given aperture the depth of field is different, shallower on the 85mm. You can compensate this by a lower aperture number on the 56mm i.e. 1.2 on the 56 instead of 1.8 on the 85. If you want to compete with with your Fuji with an 85/1.2 on a full frame you are screwed. You would need a 56.6/.80. To the best of my knowledge there is no  such lens.

 

But Bokeh is just one side of taking pictures. Sometimes I would prefer more depth of field for example often for documentary and street photography. In this case I can open my aperture by one half stop to get a faster shutter speed for less motion blurr but still get the some depth of field as on a full frame.

 

Exposure wise a 1.2 lens is a 1.2 lens regardless on what format it is used and for what format it is made.(Of course, some lenses have a better transition than others. This is the reason why cinematographers are using t-stop rather than f-stops, as Balck Perl has explained.

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Thanks for that explanation Black Pearl.

 

So would I be right in saying an 85mm f1.8 lens on a full-frame camera should get very close to the same result as a 56mm f1.8 cropped lens on a cropped sensor?

 

Apologies for labouring the issue but I want to make sure I have understood correctly.

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It would be close with only the difference between the way a 56mm and 85mm renders an image.

 

If you take in the perceived DoF between the two systems you want one stop larger aperture on the crop system to get as narrow a depth of field or one stop smaller on a full frame system to achieve as much DoF.

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Another perspective for you. The whole question of equivalency is a hangover from the days when 35mm film was dominant and focal lengths and DoF were relative to that size. We're long past those days now. Forget about equivalency. The best thing you can do (IMHO of course) is to take you camera (or cameras) and lenses and shoot with them at whatever focal length and multiple f stops until it becomes second nature to you to understand what kind of image a particular camera / lens / f-stop combination is going to give you.

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Another perspective for you. The whole question of equivalency is a hangover from the days when 35mm film was dominant and focal lengths and DoF were relative to that size. We're long past those days now. Forget about equivalency. The best thing you can do (IMHO of course) is to take you camera (or cameras) and lenses and shoot with them at whatever focal length and multiple f stops until it becomes second nature to you to understand what kind of image a particular camera / lens / f-stop combination is going to give you.

 

I'll second that superb advice!

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I will "third" that excellent advice. Forget about "equivalence." It's an illusion.

 

Use the format. Learn the format.

 

I laugh when people call 24x36 "full frame." I learned on Medium Format and I never really accepted 135 as the quality choice. Newspapers? Sure - convenience and portability win in that environment. But for quality film work - even MF is a compromise. "Full frame" is 8 inches by 10 inches. 300mm is a "normal lens." 

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FWIW I'll add one more practical point that can get lost in the shuffle: An 85mm f/1.8 on a FF sensor may be the closest FF comparison to what you get from the 56mm f/1.2 on Fuji, but that doesn't mean the Fuji 56mm f/1.2 and Canon 50mm f/1.2 lenses shouldn't be compared at all.

 

If you choose Canon but you want the size/cost benefits of their APS-C cameras (i.e. 7DII) then the FF lenses are your only choice for all but a couple of their cheapest primes. There's no way to get a small, cheap 35mm f/1.4 for Canon, you HAVE to buy the huge, expensive FF L lens even if it's going on your Rebel. This is because Canon treats their "crop sensor" lineup as a discount brand, rather than as a valid tradeoff of size/cost v. quality. 

 

Fuji on the other hand treats their APS-C cameras as top of the line, so their APS-C primes are smaller and lighter than what you'd have to carry to get the same effect on a Canon APS-C camera. 

 

The day Canon comes out with fast awesome primes in EF-S (crop sensor) or EF-M (their half-assed mirrorless format) this may change, but I doubt they'll ever come out with anything comparable to the 56mm f/1.2 or 16mm f/1.4 lenses designed specifically to take advantage of APS-C sensors. 

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I use a very simplified approach to this whole comparison question:

  1. For focal length comparison, find the crop multiplier and find the lenses that have the same field of view after applying that multiplier. For example, 50mm on a Canon 5D MkIII, 35mm on a Fuji X-T1, 25mm on an M4/3 camera. 
  2. For light gathering, take the f-stop and be done with it. Exposure setting should be the same when using f/1.4 on M4/3, APS-C or FF. 
  3. For depth of field, add one stop for each sensor size doubling, that would mean f/1.4 on M4/3 is similar to f/2 on APS-C which is similar to f/2.8 on FF. This isn't exact though, but it's close enough to be workable. 

So, it's true that for DoF and rendering comparison the 56 f/1.2 should be compared to an 85 f/1.8 on a FF body, but that doesn't apply to light gathering. But because a lot of FF sensors have better ISO performance than a lot of APS-C sensors, you could argue that you can just use one stop higher ISO on the FF camera and still get equivalent results. 

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The day Canon comes out with fast awesome primes in EF-S (crop sensor) or EF-M (their half-assed mirrorless format) this may change, but I doubt they'll ever come out with anything comparable to the 56mm f/1.2 or 16mm f/1.4 lenses designed specifically to take advantage of APS-C sensors. 

 

Not quite the same but the EF-S 60mm f/2.8 Macro USM is an outstanding lens with next to perfect resolution figures across the image frame.

 

I do agree that they protect their higher end cameras by not fully supporting those who use the APSC bodies and I'll also add I'm not a Canon fan - I've lost sympathy with Nikon too which I've used for 30+ years for the same reason hence the switch to Fuji. I honestly don't understand the reasoning behind the Big Two and their refusal to see there is a future without a mirror. We'll see how things pan out but we live in interesting times and I love to be shooting at the leading edge of what is possible and not what we are given.

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I use a very simplified approach to this whole comparison question:

  • For focal length comparison, find the crop multiplier and find the lenses that have the same field of view after applying that multiplier. For example, 50mm on a Canon 5D MkIII, 35mm on a Fuji X-T1, 25mm on an M4/3 camera.
  • For light gathering, take the f-stop and be done with it. Exposure setting should be the same when using f/1.4 on M4/3, APS-C or FF.
  • For depth of field, add one stop for each sensor size doubling, that would mean f/1.4 on M4/3 is similar to f/2 on APS-C which is similar to f/2.8 on FF. This isn't exact though, but it's close enough to be workable.
So, it's true that for DoF and rendering comparison the 56 f/1.2 should be compared to an 85 f/1.8 on a FF body, but that doesn't apply to light gathering. But because a lot of FF sensors have better ISO performance than a lot of APS-C sensors, you could argue that you can just use one stop higher ISO on the FF camera and still get equivalent results.

This nailed it. Could not have expressed it any better.

 

Also don't forget that modern aps-c sensors outclass older generation full frame sensors. The old arguments stem from a time when every aps-c system was partly crippled compared to the more semi-pro full frame dslr's. And while full frame will get you some advantages in iso and dof in very specific circumstances, I consider those benefits to be less important than the size and weight benefits aps-c offers.

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Not quite the same but the EF-S 60mm f/2.8 Macro USM is an outstanding lens with next to perfect resolution figures across the image frame.

 

Wow I never heard of that lens but it's a perfect example anyway! It's sublimely equivalent to Fuji's 60mm f/2.8 Macro, which also does extremely well in testing, but isn't a wonderful prime for non-macro because of it's slow aperture.

 

For Fuji the 60mm is one of it's cheapest and least compelling primes, there to offer affordable access to a specific genre of photography, and I think the same is true of Canon. The problem for Canon is that the 60mm is also maybe it's best APS-C prime lens, whereas Fuji has many other options that are better in many different ways (not least of which the upcoming high-end macro prime that will surely replace the 60mm as Fuji's macro flagship). 

 

Finally I'll note that while Canon themselves suck at EF-S primes, other companies aren't wasting the opportunity. That Sigma 18-35mm f/1.8 is a workhorse that challenges anything Canon OR Fuji have released for versatility and especially price. Would love to see Fuji or even Sigma releasing something like that for the X system. 

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Funny, I don't recall anyone referring to 24x36 as 'Full Frame' until APS-C digital sensors came along. But I'm old and my memory may be failing.

The whole "crop factor" thing was also practically heard of before DSLRs became popular, in my opinion.

In the past, the formats were just referred to by numbers (like 110 film for small consumer cameras).

 

Sent from my SM-G920F using Tapatalk

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Funny, I don't recall anyone referring to 24x36 as 'Full Frame' until APS-C digital sensors came along. But I'm old and my memory may be failing.

In fact I have. In 35mm days there were half-frame cameras and some very good ones. I have an Art Deco Olympus PenFT that was also had interchangeable lenses of high quality. Users most definitely distinguished half-frame from full frame. It went in the other way as well. My WideLuxe140 used a panoramic 24×59mm format, and theHasselblad XPan made by Fujifilm(TX1) and rebranded by Hasselblad used 24×65mm. The Robot Royal 24S had 24×24 mm frame size. So yes, when discussing them the term full frame or standard frame was certainly used.

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Maybe, but I surely would have gotten weird looks in the shop if I had ordered "full frame film".

 

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The film was the same 35mm (so yes you would have got wierd looks) :P , it was the camera that got 72 exposures instead of 36, hence half frame.  I certainly heard full frame and half frame mentioned back then,  as Larry says. I may be old but I`m not ready for the bin just yet. :)

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