RickSr Loc: St. Paul, MN

Rating the lens according to the field of view it will give on a full frame sensor gives us a standard to compare lenses with. It doesn't matter if you grew up with 35mm or not, the common denominator between lenses is expressed based on it's full frame sensor field of view which happens to be equal to 35mm film. When we factor in the crop factor based on smaller sensors the answer is still expressed in full frame terms and we have a standard of comparison. This is simply a fact of working in digital photography which should be learned and used.

burkphoto Loc: High Point, NC

4x5 film view camera crop factor is about .29, if my rough math is accurate. Divide the diagonal dimension of a full frame sensor into the diagonal of the larger format... That gives you the crop factor.

A NORMAL lens is based on the diagonal dimension of the film frame, and provides ROUGHLY a 45° angle of view. On a 4x5 it's about 150mm. On Full Frame, it is about 50mm. Those are rounded dimensions... On DX Nikon, and Sony, Pentax, and Fujifilm APS-C, it is about 35mm. On APS-C Canons, it is 28 to 35mm (take your pick, or set a zoom to something in the middle). On Micro 4/3, a normal lens is 25mm.

Lens makers have traditionally rounded their focal lengths to even numbers for clarity. Common full frame prime lenses are 20, 24, 28, 35, 50, 85, 100, 135, 180, 200, 300, 400, 500, 600... in mm.

For MOST sensor or film formats, there are lenses specifically designed to work with that size image. HOWEVER, because there were tens of millions of full frame 35mm film lenses around when digital cameras were developed, and the first sensors were smaller, those "full frame" lenses' MOUNTS were used on cameras with smaller DX and APS-C and APS-H sensors. The image circle projected is larger than the sensor. The smaller sensor "crops" the center out of the projected image.

Lenses designed specifically for a particular format are refracted to project an image circle just wider than the film or sensor. A lens of equivalent coverage for a larger format will be larger and heavier*, and when used on a smaller sensor, won't perform quite as well as a lens of equivalent quality that is designed specifically for the smaller format. (That statement may shock a few people, but if you look at lens test charts on dpreview.com, you can compare MTF performance of the same lens on full frame and APS-C cameras, and see the difference.)

This isn't usually much of an issue when full frame lenses are used on APS-C sensors, but it starts to become an issue when full frame lenses are used on Micro 4/3 sensors. That's one reason MetaBones developed the SpeedBooster... It reduces the size of the projected image circle to fit the smaller sensor better. This improves performance over what it would have been without the SpeedBooster, and concentrates the light over a smaller area, providing an extra stop or more of intensity.

*An 85mm f/1.2 Canon EF L ii weighs 2.2 pounds, but a 42.5mm f/1.2 DG Nocticron weighs 15 ounces. Both produce roughly the same angle of view, which is very useful for portraits. The full frame lenses have to gather 4X the light, to project a 4X larger area image circle for full frame than for the 4/3 sensor. You get more depth of field with the shorter lens — f/1.2 on the shorter one is equivalent to f/2.4 on the longer one.

It is important to note that there are FOUR *common* sensor sizes for interchangeable lens cameras: Full Frame (1x), APS-C Canon (1.6x), APS-C Nikon, Sony, Fujifilm, Pentax... (1.5x), and Micro 4/3 (2x for stills; video varies by camera). There are TWO types of digital cameras with interchangeable lenses — dSLR and mirrorless. The difference is that dSLRs have reflex viewing mirrors and optical viewfinders. Mirrorless cameras use electronic viewfinders and/or LCD or OLED screens to preview the image. dSLRs are available in full frame and APS-C formats. Mirrorless cameras are available in full frame, APS-C (1.5x), Micro 4/3 formats and Nikon's 1 system which uses a smaller CX sensor with a 2.7x crop factor.

burkphoto Loc: High Point, NC

You could say that, but the crop would be relative to some larger medium format film size, and there are numerous medium format film sizes!

6x4.5
6x6
6x7
6x8
6x9
6x17

Then you have Split 70mm full frame unperforated, 46mm unperforated in four formats, even 35mm unperforated in several formats...

So when there is nothing as a "medium format standard" to compare to, the "crop factor" reference remains full frame 35mm film (24x36mm). It just becomes a fraction.

G_Manos Loc: Bala Cynwyd, PA

FWIW I think it's OK to continue to talk about crop sensors because they are part of camera technology. It's the same as drawing the distinction between 120 film and 35mm film (in the old days).

In any case, the OP's question was about lenses, not crop sensors, and when choosing lenses it is valuable to understand the difference in image circles between the two. Otherwise the OP could purchase a crop frame lens for full frame camera and not realize he is not getting full value (or image) for the money.

oldtigger Loc: Roanoke Virginia-USA

Nothing like rousing the troops on a monday morning.......

fetzler Loc: North West PA

I posted this elsewhere but here is the reason.

The focal length of a so-called normal lens is determined by by the diagonal length of the sensor or film. You can calculate this using the formula f = SQRT( length^2 + width^2). SQRT = square root. n^2 = n squared.

For 35mm Film: f= SQRT[ 36 ^2 + 24^2) = 43mm usually rounded to 50mm for lens manufacture

For 6 x 4.5 cm film f = SQRT[60^2 + 45^2] = 75mm

For Nikon DX f = SQRT[23.6^2 + 15.7^2] = 28.3 mm usually rounded to 35mm for lens manufacture. Do the math an you will understand.

to calculate the crop factor between Nikon DX and 35mm film

43mm/28.3 = 1.52

For a DX camera with a 35mm lens the corresponding focal length for 35mm film would be 35 x 1.5 = 52.5mm A 35mm DX lens is approximately equal to a 50mm film lens. For a 6 x 4.5 cm the corresponding lens would be 75mm

amfoto1 Loc: San Jose, Calif. USA

Yes... and no.

Yes... Anyone who is new to photography and completely unfamiliar with film/sensor formats can simply forget about it. All they need to know is how any given focal length "behaves" on their particular camera. They'll learn that over time, probably using the zoom lens(es) that came in kit with their camera. There are more and more people who have never used film at all, so it's somewhat ironing that all the format terminology references film formats (full frame, medium format), some of which were short-lived and no longer exist (APS).

No... Someone who has used various camera formats in the past and has preconceived notions about what to expect from a certain focal length or is changing formats and looking to replicate the angle of view they got with lenses in the past will need to "do the math" with the crop factors.

In some respects, it makes some sense to use the 24x36mm image size that's very common with 35mm film as a reference point. It's what most film users were accustomed to and even a lot of modern "point n shoot" cameras' non-interchangeable lenses are still described in "35mm film equivalent" focal lengths. Whatever you want to call it... "full frame", 24x36mm or "35mm equivalent"... it's also somewhat "in the center of things", when you consider the wide range of film and digital formats... from tiny half-a-little-fingernail size at one extreme, to the size of a sheet of paper at the other.

Format comparisons and factors are really nothing new. Anyone who worked with various film sizes in the past also had to make calculations when choosing their lenses for each format.

RichardE Loc: California

HA! I have you beat! My Pentax is 1.50955414012739

RichardE Loc: California

Whoops, forgot the diagonals! Pentax diag crop factor: 1.5219

ORpilot Loc: Prineville, Or

For those that are new to the game or never really knew: APS-C started out in the late 1980s and became more popular in the 90s. Yes, I am talking about film. Most of the standard 35mm films were in APS. It was created to replace the aging 126 instamatic and to help those that were having trouble loading and unloading 35mm. It was essentially 35 mm in a special film can. The photographer never touched the film, ever. The photographer didn't have to set the ASA (iso) on the camera. You could even change film types in the middle of the roll and not lose a shot. Then put the partial used roll back in the camera and finish off the roll. The film was processed and returned in the same film can. Safe for storage and any damage. Prints usually came with an index print to located photos. The image size (negative) was your standard 8x10 . Which is exactly a full frame 35mm cropped to 8x10 format. So there was no wasted film. Very few people actually print a full frame 35mm image mostly it is in 8x10 and sometimes 5x7 format. So, the APS-C lives on in Digital Photography. Is Full frame Digital better than APS-C? That is like asking is Sony, Nikon, Pentax, Canon, Olympus, or Zeiss better? So you have a full frame 36mp camera , is it really better than the APS-C at 24mp..... For the final 8x10 print, They are exactly the same. You want the best final print.... Go with the largest format you can afford. The laws of physics are the same for film as it is for digital. Bigger is better. Happy shooting.

IBM

It's called normal for a reason , before dx cameras a 50mm lens was close to what the human eye sees , and we are the normal are we not that is why it's called normal . What did you have in mind for the boring name . Try this put a short zoom on any camera say 25-120 then look at straight edge ,the edge of your wall and a window or a tree trunk through the camera with one eye then open the other eye and look at the same spot, now start zooming your lens tell it matches what you see through the camera . OK now look and see where you zoomed to ,to match your bare eye , I bet you it's close to 50mm on the zoom lens , live and learn my boy .

romanticf16 Loc: Commerce Twp, MI

Lenses designed for a DSLR theoretically project an image to the sensor that is at 90 degrees from the plane of the sensor. Film SLR lenses were not designed to as tight a specification, because the demand of light hitting a film emulsion did not require such close design tolerance. Many high quality film lenses still perform well on digital sensor bodies!

BebuLamar

While I think all these crop factor thingy are silly they gave me the reason to only use FF camera because I don't have to do the math.

Longshadow Loc: Audubon, PA, United States

"Lenses designed for a DSLR theoretically project an image to the sensor that is at 90 degrees from the plane of the sensor."
Huh?

ORpilot Loc: Prineville, Or

The reply by romanticf16 is not all together true. The better lenses by Zeiss and Leitz as well as a few of the Canon and Nikon, were designed to focus the correct colors of light to it's 3 corresponding layers of emulsion on the color film. Note: that some Fuji films have 4 layers. Early lenses depended on an external color filter yellow, red green etc to only focus one color on the single layer of B&W emulsion. They did not do well with color films. Today's digital sensor does not have color layers only a flat tile pattern. The exception to that is the Fovon Sensor which has color layers like color film has. Computer technology has greatly leveled the playing field when it comes to brand of lenses. Note: also each color of light acts a little differently as is passes through the lens glass. A combination of glass formula, lens design and lens coatings help to compensate for those minor differences. The vast majority of lenses are zoom autofocus. They bye nature are not as sharp as prime lenses because of all the moving parts and it is designed to be a compromise of focal lengths. A prime lens has no moving parts except for the diaphragm and has the best sharpened of all. Then as now, the better the lens , the better the image. Happy shooting.