Backwards of crop factor

[Major Eazy]

I know that a lens designed for a 35mm camera will have a 1.5 (ish) crop factor when using on a DSLR. (I know it vary with different companies and models, but let's use round up figures to make examples easier.)

Say a 50mm for a 35mm SLR on a DSLR will becomes 75mm because of 50 x 1.5 = 75.
A 200mm for a 35mm SLR on a DSLR will become a 300mm. 200 x 1.5 = 300.

But is there a way to work it out backwards? Say trying to find the lens designed for 35mm SLR on DSLR equivalent to 35mm on 35mm SLR?

For example: When using 35mm SLRs, we know it is common to use a 24mm for landscape, or say a 50mm as a normal general photography, or a 135mm is often considered to be a good one for taking head photos.

If i were to buy a 24mm lens designed for 35mm camera and put it on a DSLR, it would result in it being equivalent to a 36mm on a 35mm SLR. So how do I find a lens designed for 35mm SLR to be the equivalent of a 24mm on a DSLR?

Rather than struggling with guessing which one is the nearest equivalent by doing the multiply by 1.5 like..
28mm on DSLR is 28mm x 1.5 = 42mm, no not what I want, okay try....
24mm on DSLR is 24 x 1.5 - 36mm, no start again with a wider lens...
20mm on DSLR is 20 x 1.5 = 30mm, no!

So can I try the divide by maths to find the equivalent?

Can I try doing maths like thinking that a 35mm lens made for a 35mm SLR would be 35 divided by 1.5 = 23mm so a 35mm (made for 35mm SLR) would act like nearly a 24mm on a DSLR?

Can I say that a 200mm (made for 35mm SLR) is 200 divided by 1.5 = 133mm therefore a 200mm on a DSLR is the closes equivalennt of using a 135mm (made for 35mm SLR) on a DSLR.

Or is there going to be problems with using the divided by maths?

 

[arad85]

So can I try the divide by maths to find the equivalent?

Yes. But...

Lenses don't change focal length. All that changes is field of view. You get the equivalent field of view that a xxmm lens would give on full frame.

Also, a full frame DSLR and crop DSLR are both DSLRs. D just stands for DIGITAL. The sensor size is what makes it a crop or full frame DSLR.

 

[Nawty]

It doesn't work like that.

A 50mm lens is a 50mm lens no matter what, it is the Field of View that changes depending on sensor size.

However, lenses designed for smaller sensors need to project a smaller image as the sensor is smaller. Because of this when you put a lens designed for for a smaller sensor on a bigger sensor (assuming it would physically fit) the lens still projects the smaller image and so you effectively get a smaller image with a black circle (or vignetting) around the image.

What this means is that putting lenses designed for bigger sensors on a smaller sensor camera works as the lens designed for a larger sensor projects a large image, the sensor is smaller and fits in that image (and so the field of view changes), I imagine the sensor as a black shadow in a window of light. Conversely putting lenses designed for smaller sensors on a bigger sensor doesn't work (imagine the shadow is bigger than the projected image).

 

[odd jim]

You only have to worry about crop factors on a crop DSLR. You're assuming (or the post makes it sound as such) that all DSLRs are cropped?

A 24mm lens works as a 24mm lens on full frame DSLRs in terms of framing.

In answer to your question "So how do I find a lens designed for 35mm SLR to be the equivalent of a 24mm on a DSLR?" is, buy a 24mm lens

 

[boliston]

Can I try doing maths like thinking that a 35mm lens made for a 35mm SLR would be 35 divided by 1.5 = 23mm so a 35mm (made for 35mm SLR) would act like nearly a 24mm on a DSLR?

The "35mm" is really just a name for the type of film used (35mm wide) and has no direct link to the crop factor as a lot of the film width is just sprocket holes so not directly comparable to a sensor.

 

[Teflon-Mike]

The 'reverse' of a 1.5 crop factor is 0.66x..... but that's the right answer to the wrong question.

35mm film, 'Full-Frame' size is 24x36mm - film is 35mm wide, the square inside the image circle the lens projects is about 36m wide, but you only have about 24 mm of usable film between the sprocket holes, hence its about the biggest 'practical' image you a put on that size of film.

Worth noting that not ALL 35mm film cameras use 'Full-Frame'; the Olympus Pen, famously is a 'Half-Frame' camera (there are a number of others, including SLR's), exposing a portrait orientated rectangle, 24x18mm.. (as a 35mm movie camera, incidentally). Same size turned 90 degrees again to land-scape orientation, 18x24, was the 'frame area' of the short-lived Advanced-Photo-System film camera.

So there have been many different film sizes used over the years; 110 cartridge, 126 cartridge, 35mm, APS, 120 roll; and many different frame sizes have been used on those different films. (some 120 roll film cameras even have interchangeable film-traps; the film is aprox 60m wide, and with different traps fitted, you can expose 90x60mm, 70x60mm, 60x60mm or 60x45mm!)

Going Digital, the sensor size can be pretty much any size the camera maker likes, they don't have to make it compatible with 'standard' films; But, developing the early Digital SLR's, the existing 18x24 APS film cameras and 24x36mm 'full-frame' 35mm SLR's were a good starting point, just making a sensor to fit in the film trap of an existing film camera that could take existing lenses and accessories, leading to these being the 'usual' sensor sizes in contemporary DSLR's, of which, the generally cheaper, APS-Crop sensor cameras are by far the more common... but by far not the only DSLR's about.

So when you say 'DSLR'.. most likely you are referring to an 18x24mm 'APS-Crop sensor DSLR but could be a 24x36mm 'Full-Frame'SLR or even some other size, like a 36x48m 'Medium Format' DSLR, or the smaller 'mirrorless' micro-four-thirds pseudo DSLR, that has an 13x18mm sensor...

And ALL will have a different 'crop-factor' depending on what you want to work out an 'equivalence' for, and to what, depending on the actual capture frame size of the two cameras you wish to compare.

As has been said, focal length IS focal length, regardless of the size of the film or sensor it projects an image on.

If you have a smaller 'frame' behind the lens, then you only get a section from the middle of the image circle, and when you enlarge that smaller captured image, to the same viewing size, it's like 'zooming in' as if you had used a longer focal length lens exposing a larger area of film or sensor to the same magnification.

The Crop-Factor then, is a constant of proportionality, for a smaller frame or sensor size, that gives an 'equivalence' to the focal length you would need to have, to get the same angle of view on a larger sensor size; usually the 24x36 'Full-Frame' established by the 35mm film standard, but not always.

Now do Pythagoras on 24x36 and the diagonal hypotenuse is about 50, (actually closer to 44, but lets not quibble) and that is what sets the 'normal' focal length for a 24x36 'Full-Frame' as a 50mm focal length.

Do Pythagoras on 18x24 and the hypotenuse is about 35mm (again, actually closer to 30, but all close enough), setting the 'normal' focal length at 35mm for a 'half frame', APS or Crop-Sensor camera.

50mm divided by 35mm is aprox 1.5.. we are talking triangles and so by 'similar triangles' this ratio gives you the 1.5x 'Crop-Factor' that suggests that the 'effective' angle of view of a lens used on a half-frame/APS/Crop sensor camera will be approximately 'equivalent' to the angle of view of a lens with 1.5x that focal length, used on a camera with 24x36 'Full-Frame'.

Eg: 300mm lens used on a half-frame / APS / Crop camera, gives the 'equivalent' angle of view or 'zoom' as a 450mm lens used on a 24x36mm 'Full-Frame' film or digital camera.

35mm divided by 50mm is aprox 0.66, same triangles, going t'other way here; this ratio tells you, if you have a 210mm lens on a 24x36mm 'full-frame' film or digital camera, that will give the 'equivalent' angle of view as a 140mm focal length lens used on a half-frame / APS / Crop sensor camera.

The 'equivalence' can be worked either way, the two factors basically flipping the ratio upside down depending whether you want to compare a big frame camera to a little one, or a little frame camera to a big frame one.

So, untangling some of the muddle in your question... Crop factors provide equivalence for the effective angle of view of a lens, when used on cameras with different sized Sensors / film traps... not between cameras with different capture medium, digital vs film!

Crop factor between 18x24mm APS/Crop-Sensor and 24x36mm 'Full-Frame' (whether Digital or 35mm Film), is 1.5 going forwards or 0.6 going backwards.

There is NO crop factor between 24x36 Full-Frame 35mm Film, and 24x36mm Full-Frame Digital, as the captured frame sizes are the same, the crop factor is 1.

Other crop factors may apply if you are trying to work between other frame / sensor sizes.

 

[OldCarlos]

Easiest way to think about it is in effect......full frame Dslr's are 35mm.

 

[Simes123]

To the OP. I know what you are saying. You want to be able to work out equivalences in either direction for a crop frame DSLR to Full Frame 35mm SLR. It's simple and you are on the right lines. To work it out backwards, yes, divide by the crop factor, instead of multiply to find the equivalences to full frame 35mm film (or digital) cameras.

So if you want to know what lens you need on your crop factor DSLR to give you a similar field of view as a 35mm lens on a Full Frame 35mm film or digital camera, divide by your crop factor. 35mm/1.5 = 23.33mm - so yes, you are on the right lines. Thus a 24mm focal length lens on a crop factor sensor (where the crop factor is 1.5), will give you a similar field of view to a 35mm lens on a full frame camera (be that DSLR, or SLR). Canon use 1.6 as their crop factor. So a to get a 35mm lens field of view on a crop DSLR, you need to do 35mm/1.6 = 21.88mm (22mm is close enough for government work). Canon's 10-22mm lens, gives fields of view similar to a full frame camera's 16-35mm lens for example.

However, what has been pointed out is that focal length is focal length, no matter the camera. However, you see equivalent focal lengths used a lot, to represent similar field of views (which leads to the confusion). Also, you are using the terms for DSLR and SLR a little loosely, as DSLRs come in crop factor and Full frame versions, with Full Frame literally meaning Full as in the same size sensor as 35mm film.