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03-28-2016, 09:09 PM   #1
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please explain ff vs crop measurments

i know there are plenty of forums out there explaining just that.
i know there are several videos that display the difference between ff and crop, and i understand all that just fine.

however, i'm having a hard time figuring out measurements of certain lens in relation to one another.

for example, i have the FA 50mm 1.4 lens that is designed for full frame. on a crop, however, it acts more like a 75mm (crop of 1.5). and if you grab 35mm, it will act as a 52mm on a crop.
i also have a sigma 10-20mm 3.5 that, on a crop, should act as 15-30mm.

however, thats where my confusion lies. the 15-30mm pentax lens that is coming out states that on a full frame, this lens is 15-30, but on a crop it will be 22.5 to 46.

so does that mean that the sigma 10-20 has no equal in the full frame world without going fish eye?

or do lenses have 2 different measurments. one for full frame, and one for crop?

i'm sorry i'm a lil ignorant on this

03-28-2016, 09:19 PM - 1 Like   #2
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The focal length never changes. The FA 50mm has a focal length of 50mm on both full frame and APS-C. Since APS-C is a smaller sensor, it sees less of the frame and delivers the same angle of view as a (longer) 75mm lens would have delivered on full-frame.

QuoteOriginally posted by hadi Quote
however, thats where my confusion lies. the 15-30mm pentax lens that is coming out states that on a full frame, this lens is 15-30, but on a crop it will be 22.5 to 46.
This kind of advertising/labeling is both misleading and incorrect. It's 15-30mm on full frame and 15-30mm on crop Again, if you were to put a hypothetical 22.5-46 zoom on a full-frame camera, it would have the same field of view as the 15-30mm on crop. Bottom line: APS-C cameras need shorter focal lengths to deliver the same field of view as a full-frame cameras. Conversely, full-frame cameras can get away with longer focal lengths for ultra-wide lenses. You'd need a 10mm on APS-C to get the same field of view as a 15mm on full frame.

Give this article a read; hopefully it will clear things up

The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

IMO many people tend to over complicate crop factor, so if all else fails, just remember the two things in bold above

Adam
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03-28-2016, 09:26 PM - 2 Likes   #3
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The focal length is the focal length. This is irrespective of the sensor dimensions.

If you were to mount the Sigma DC (designed for crop sensor) lens on a camera with a sensor size of 24 x 36 mm (commonly called Full Frame) then it will likely vignette especially at the wider apertures because the size of the glass elements in the lens are designed to only cover the sensor of a APSC camera.

If you were to mount a Sigma DG (designed for FF) lens on a crop sensor camera, you only see part of the image circle (the center part) because some of the focused light falls outside of the area of the sensor. It is the same as taking a shot with a FF designed lens on a FF body and then cropping away the outer edges.

The angle of view does not change, you just see less of it, making it appear to have a different AOV.

The new 15 - 30 is designed to distribute focused light on a sensor with a size of approx 24 x 36mm. You could mount it on a FF body such as the K-1, and see all the lens can see, or you could mount it on a K-3 and see only part of what the lens can see. There is 'apparent' magnification with the crop sensor camera, but the exact same image is available on the FF sensor, cropped from the center portion of the image.

Last edited by wizofoz; 03-29-2016 at 02:39 AM. Reason: Cause I'm an idiot
03-29-2016, 04:13 AM   #4
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QuoteOriginally posted by hadi Quote
i know there are plenty of forums out there explaining just that.
i know there are several videos that display the difference between ff and crop, and i understand all that just fine.

however, i'm having a hard time figuring out measurements of certain lens in relation to one another.

for example, i have the FA 50mm 1.4 lens that is designed for full frame. on a crop, however, it acts more like a 75mm (crop of 1.5). and if you grab 35mm, it will act as a 52mm on a crop.
i also have a sigma 10-20mm 3.5 that, on a crop, should act as 15-30mm.

however, thats where my confusion lies. the 15-30mm pentax lens that is coming out states that on a full frame, this lens is 15-30, but on a crop it will be 22.5 to 46.

so does that mean that the sigma 10-20 has no equal in the full frame world without going fish eye?

or do lenses have 2 different measurments. one for full frame, and one for crop?

i'm sorry i'm a lil ignorant on this
the problem now, 15 years in to digital is that the are a large perceived number of people who have never known a full frame or film camera, so some genius in marketing has decided to reverse the crop factor example to explain to the uninformed what full frame means


Focal length is focal length. Full frame is a 24x36mm format, APS-C is a 16x24mm format.

The "crop factor" merely explains the difference in format, and since it takes a 1.5 X shorter lens to fill the smaller format with the same field of view, they used the "crop factor" as the explanation

03-29-2016, 04:46 AM   #5
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QuoteOriginally posted by Adam Quote
This kind of advertising/labeling is both misleading and incorrect. It's 15-30mm on full frame and 15-30mm on crop
The "crop factor" was only developed to help former film shooters relate to new lenses that were coming out for digital APS-C cameras. It was a way to describe the field of view in 35mm film terms. For those who got into photography after digital, it should have no real meaning. As Adam says in what I quoted, the focal length of the lens doesn't change. It's the format of the camera that changes things. This is nothing new, btw. Photographers had to do the same kind of head games back in the film days when moving from 35mm to medium format or large format. Visit the Q section in the mirrorless section of this board. The crop factor is 5X in 35mm terms. A 200mm lens becomes a super-telephoto. lol
03-29-2016, 07:47 AM   #6
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This is easily my most despised topic in the current photographic world.
As stated plainly above, the focal length is the focal length. It does not change, no matter what camera body it is mounted on. It does not "become" something else depending on the sensor.
I sympathize with the opening poster, there is so much confusing terminology used describing lens choice/application these days. And there is so much misinformation.
Particularly troubling is the suggestion that "digital" lenses don't have a crop factor. You may have met someone who realized that their 70-210mm "film lens" became a more potent zoom when used on their new digital body. But that same person will then assert the 55-200mm (DA, DX, DiII, EF-S... insert your choice) doesn't have that boost because it is intended for a digital (crop) body. Sigh!

I'll bet you there are still loads of crop sensor users who are convinced their 50mm "nifty-50" makes a great portrait lens because of its 1.5x crop factor, which their 18-55mm doesn't. Uggghh. Of course, if they only put the lenses on the camera, one after the other, and compared the perspective, they might just realize the 50mm lens has the same field of view as the 50mm setting on their "digital" kit lens.
They might realize it, except that all their internet research, and word of mouth advice (and sometimes misleading sales pitch from a mistaken salesperson) has told them they are two completely different things.
03-29-2016, 07:53 AM   #7
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Yep, it's not hard ... but there are Equivalence mystics out there making it seem like it is. ☺

03-29-2016, 07:59 AM   #8
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To be the contrarian, focal lengths are almost meaningless while the field of view (FOV) is incredibly important for our photos.
03-29-2016, 08:08 AM   #9
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Take a look at the nonsense that Panasonic is going on with:
http://www.panasonic.com/in/consumer/cameras-camcorders/camera/dmc-fz1000ga....irE2GdvHCjhtJQ
(Oh Oh.. you have to click on "See all Specs")

Not only that, but the Leica lens is marked with a fake focal length:
DMC-FZ1000GA Camera - Panasonic India
03-29-2016, 09:03 AM   #10
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QuoteOriginally posted by jk333 Quote
To be the contrarian, focal lengths are almost meaningless while the field of view (FOV) is incredibly important for our photos.
I though focal length equated to optical magnification. Such that 50mm was essentially the base of 1x, so 200mm would be 4x the optical magnification?

If that's truly the case, then focal still is extremely important.
03-29-2016, 09:06 AM   #11
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QuoteOriginally posted by jk333 Quote
To be the contrarian, focal lengths are almost meaningless while the field of view (FOV) is incredibly important for our photos.
I have to disagree, focal length is everything. It defines after all, magnification, and field of view, is after all defined by sensor size and magnification.


But any way, as long as we agree it is the "filling" of the image space (frame) with the subject , what we call it does not really matter
03-30-2016, 12:16 AM   #12
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QuoteOriginally posted by TaoMaas Quote
The "crop factor" was only developed to help former film shooters relate to new lenses that were coming out for digital APS-C cameras.
It existed long before, there were as many formats before as today. We now tend to use FF as the biggest format in digital with the exception of very few crop MF sensors, but back in time FF was the smallest format.

Crop factor is just the factor to apply to know what lens you have to use on your system to get the same framing and deph of field at same shooting distance than the picture taken by your friend on another system.

This may be APSC vs FF, but this is far from being limited to that. If one has 1" and the other m4/3 or one has APSC and the other 645z crop that again different.

People tend to compare to focal length in FF because it is the was the most popular system for photography for a long time and so it is more meaningfull to many to see 28-200 on a bridge than 7-50 on this model 5-40 on another and 9-66 on another one and fail to see they all cover the same framing capabilities but just differ in sensor size.
03-30-2016, 05:46 AM   #13
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QuoteOriginally posted by Nicolas06 Quote
It existed long before, there were as many formats before as today.
Right! Photographers have always had to do these kinds of mental calculations to determine the angle of view for a lens, but it was never called a "crop factor" in the past because most photographers were moving up in format size, not down. It was just something you had to take into consideration whether you were moving up from 35mm to a medium format camera or down to a Pen-F.
03-30-2016, 02:41 PM - 1 Like   #14
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QuoteOriginally posted by hadi Quote
i know there are plenty of forums out there explaining just that.
i know there are several videos that display the difference between ff and crop, and i understand all that just fine.

however, i'm having a hard time figuring out measurements of certain lens in relation to one another.

for example, i have the FA 50mm 1.4 lens that is designed for full frame. on a crop, however, it acts more like a 75mm (crop of 1.5). and if you grab 35mm, it will act as a 52mm on a crop.
i also have a sigma 10-20mm 3.5 that, on a crop, should act as 15-30mm.

however, thats where my confusion lies. the 15-30mm pentax lens that is coming out states that on a full frame, this lens is 15-30, but on a crop it will be 22.5 to 46.

so does that mean that the sigma 10-20 has no equal in the full frame world without going fish eye?

or do lenses have 2 different measurments. one for full frame, and one for crop?

i'm sorry i'm a lil ignorant on this
Hi Hadi,
Where you went wrong is that you need to divide by 1.5 when going from FF to crop while you times by 1.5 when going from crop to FF to get the same field of view.
10-20 on crop = 15-30 on FF.
Where you got confused is that if you were using a 15-30 on crop (rather than 10-20) you would only need a 22.5-46 on FF to replicate its field of view. Of course no one is going to buy the 15-30 to use on a crop camera since it is huge and expensive compared with zoom lenses made for crop cameras.
The correct way to compare the use of a lens is to state its field of view on each format. Ie FF = 90deg, APS-C = 60deg.
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