35mm Equivalent on Crop Sensor - DX vs FX Lenses

[Derrel]

If you want the "classic Nikkor lens medium telephoto look", the one made famous by the Nikkor 105mm f/2.5 lens models, and that focal length's angle of view on film-era Nikon cameras, then on a DX-sensor digital SLR, you want roughly a 68mm lens. The conversion factor is roughly 1.53 for Nikon d-slrs of the modern era. The short end of a 70-200 AF lens, or the short end of a 70-210mm manual focus lens, or the long end of a 28-70mm or 24-70mm zoom lens, will give a pretty close angle of view match to the classic 105mm lens that was so,so popular in the film era.

Now, if you happen to have a 105mm Nikkor lens, and use it on your DX-sensor model Nikon d-slr, multiply the actual focal length of 105mm by 1.53, and you come up with just a tad bit over 160mm...in other words, a pretty good, narrow-angle telephoto lens length, similar to the 150mm or 180mm lengths that we had in the 1980's on the 70-150mm zooms, or the 180mm prime lens models.

 

[ac12]

My approach is as mentioned, forget the crop ratio business and think based on your format.
As was mentioned, when I shoot 35mm film and 6x6 film, I don't think crop ratio. I simply think in terms of the lens for that format, based on the normal lens for that format.
Normal lenses for the various formats are:

• 35mm film = 50mm
  
• 6x6 = 80mm
• 4x5 sheet film = 150mm

If I want a tele on my 6x6, I go up based on the 80mm normal.
A 2x tele = 80mm x 2 = 160mm, then round to the nearest lens = 150mm.
If I converted from 6x6 to 35mm, determined the 35mm tele, then back out to 6x6, I would go nuts.

IMHO, going through the crop ratio conversion just muddies the water.
Today when I shoot

• micro 4/3 = 25mm normal lens
• DX = 35mm normal lens

IOW, just reset your brain for the format that you are shooting in, and ignore the crop ratio nonsense.

An easier method is to think in terms of magnification, and ignore the crop ratio.
If you get a 105mm lens and put it on a DX camera you have a 3x magnification lens. 105 / 35 = 3
A 24mm lens on your DX camera has a 0.7x magnification. 24/35 = 0.7

The only time I get into crop ratios is when I am changing systems and want a lens with a similar field of view.
So if I have a 24mm lens on my 35mm film/FX camera, and want a lens for my DX camera, with a similar field of view (FoV).

A 24mm lens on a 35mm film/FX camera has an 84 degree diagonal FoV.​

With a 1.5x DX crop. 24/1.5 = 16.
I need a 16mm lens on my DX camera to give me the same FoV as my 24mm lens on the 35mm film/FX camera.

A 16mm lens on a DX camera has an 83 degree diagonal FoV.​

So to maintain the same 84 degree FoV of my 24mm lens on a FX camera, I have to use a 16mm lens on my DX camera.


But if I put that 24mm lens on my DX camera, the 84 degree FoV of the 24mm lens on a FX camera, gets reduced by the 1.5x crop to 61 degrees on a DX camera.

The focal length of the lens has not and will not change, it is still 24mm.
The FoV had been reduced because of the smaller sensor size.​

Going backwards gets into that "equivalent FF focal length" stuff.
That reduced 61 degree field of view of the 24mm lens on a DX camera, will look like a 36mm lens on a FX camera. (24 x 1.5 = 36), of 62 degrees.

Here we maintain the FoV at 61 degrees, and changed the focal length to a lens with a 61 degree FoV on a FX camera.
So to get the same 61 degree FoV,

• on a DX camera you use a 24mm lens,
  
• on a FX camera you use a 36mm lens. This is the FF equivalent number.

If you want to play with this more, here is a site that lets you input the 2 variables (sensor size and lens focal length) to determine diagonal FoV.
Depth of Field (DoF), Angle of View, and Equivalent Lens Calculator | Points in Focus Photography{%22c%22:[{%22f%22:13,%22av%22:%228%22,%22fl%22:36,%22d%22:3048,%22cm%22:%220%22}],%22m%22:0}

 

[AlexGavillan]

Thank you to everyone who chimed in and offered such detailed information, I really appreciate it. Ok so...I think I got it and FoV is what I needed to hear/understand and what was confusing me.

Ok so here goes

A 35mm is a 35mm all day long, don't worry about the crop factor period. Got it. What I was trying to make sense of was the FX to DX, and like many of you said don't worry about that. I had that Ah Ha as I was sitting in traffic. I was trying to do an equation (that I didn't need to) to get what I thought was the focal length of an FX lens to be the same as a DX lens. But really what I was doing and didn't know it was a FoV conversion instead.

Example, when looking for a 105mm lens I was really looking for like an 80mm or something around there thinking "that gets me to 105 on my crop sensor, which is the FOCAL LENGTH I want" WRONG!!! I was doing a damn FoV conversion which isn't really relevant.

So is that to say (please don't slap me) if I bought a 105mm the focal length is the same, FX and DX but the FoV is what's multiplied by 1.5? So in reality, if it's the same exact lens on an FX camera and aaDX camera, I could achieve the same FoV if I backed up so to speak? I know there is probably more to it since the FX camera has a larger sensor and there are other factors to consider... But am I close to getting it?

 

[480sparky]

By jove, I think you've got it.

Another way to think of it is to take a photo with, say, a 50mm lens. Print the photo without cropping it.

Now, take a pair of scissors and trim off the outer 25% of the image, cutting equally on all four sides. The image was still taken with a 50mm lens.

But the visual FoV makes it appear it was taken with a longer focal length.

 

[Designer]

But am I close to getting it?

Yes.

Now I'm curious as to why all that is a concern of yours. I shall "project" into your situation, and try to imagine what is going through you mind.

Say your have a limited space in a studio where you can't expand the space, and you still want to minimize distortion, so you would like to calculate what focal length of lens to use in order to produce optimum results.

You wish to make portraits with an apparent focal length of around 150mm, so you purchase a lens with a focal length of 100mm to get that look.

This line of thinking will probably give you the results you want, or close to it.

 

[AlexGavillan]

Success haha!! Well I thought

But am I close to getting it?

Yes.

Now I'm curious as to why all that is a concern of yours. I shall "project" into your situation, and try to imagine what is going through you mind.

Say your have a limited space in a studio where you can't expand the space, and you still want to minimize distortion, so you would like to calculate what focal length of lens to use in order to produce optimum results.

You wish to make portraits with an apparent focal length of around 150mm, so you purchase a lens with a focal length of 100mm to get that look.

This line of thinking will probably give you the results you want, or close to it.

Well now there is no concern haha. I was just trying to wrap my head around all the DX, equivalent, etc stuff. Once for education and 2 so I can be informed when I go to purchase my next lens. This all cropped up because I was looking at picking up a wide angle lens to shoot some interior scenes for a family members rental property. I had been reading about ideal focal lengths, and once again thought, "I need X so that its X on my camera." but now that doesn't matter (as much) I should say.


BOOM:

 

[ac12]

re: the video
Today, focal length is not physical, but usually optical.

• Optics are used to shorten a long focal length lens. This makes the lens easier to handle, than the old "stove pipes" of yesteryear.
  
• Optics is used so that a SHORT focal length lens can be used on your DSLR camera. If it were physical, the mirror would have to be locked UP, because the lens would be sticking so far into the camera that it would break the mirror.

I agree with one of his comments. Many of the new generation of photographers were brought up on DX/crop cameras and have no idea what a FF/FX image looks like. So to say "105mm FF equivalent" means nothing, because they don't have that FF reference point. Going in reverse, a DX/APS-C equivalent would make more sense for them.

Personally I feel it less confusing to convert to magnification.
Like degrees of angle in a "Field of View," magnification is a number that is independent of film/sensor size. But unlike having to look up the FoV for a particular lens on a particular film/sensor size, magnification is easy to determine.

Lens focal length / normal lens focal length = magnification
A 24mm lens on a FF camera has 0.48x magnification (24 / 50 = 0.48).
So you would need a 0.48x magnification lens to get the picture.​

Once you have the magnification, all you need to know is the normal lens of the target format(s).

Normal lens x magnification = desired focal length
On a DX camera, you need a 17mm lens(35 x 0.48 = 16.7)
On a m4/3 camera, you need a 12mm lens (25 x 0.48 = 12)
On a 6x6 camera, you need a 39mm lens (80 x 0.48 = 38.4)
On a 4x5 camera, you need a 72mm lens (150 x 0.48 = 72)
​

Note that the magnification method is "good enough" for most uses but not exact. This is because the "normal lens" is based on the generally used normal lens for that format, not the computed normal lens based on the diagonal measure of the film image or active area of the image sensor.
Example. The computed normal lens for 35mm film/FF sensor of 24 x 36mm is 43mm, not the generally used 50mm.
For the curious, the equation for the film diagonal (normal lens) is from high school geometry for a right triangle

A^2 + B^2 = C^2 => 24^2 + 36^2 = C^2 where C=43​

There is also some minor adjustments that have to be made because of the different formats are different, and have different horizontal and vertical ratios.

 

[Ysarex]

And this is why I wish the 'crop factor conversion' idea would disappear from the face of the earth. Erase it. Delete it. Send it to the Trash Bin. Eradicate it. As if it never existed.

What I find amazing is back in my film days, there was no such thing as a 'conversion factor' to compare lenses between 135, 120 and 4x5 formats. I never heard of a number to multiply (or divide by) when changing from 35mm to 6x4.5 format, or to 6x7 format, or to 4x5 format. I never had to 'convert' the 80mm lens of my Mamiya 645 to 'the equivelant of __mm on my 35mm camera". Nor convert the 150mm on my 4x5 to 'the equivelant of __mm on my RB67 camera".

So Sparky back in the day I used to shoot a modular Arca system that took interchangeable backs. I could leave the lens on the front standard and just swap out backs. One of my favorite lenses from back then was a 90mm Fujinon SW. So if I put the 90mm on the camera and started out with the 4x5 back the 90mm would have a crop factor of 1? And then when I swapped the 4x5 back for the 6x9 back without changing the lens that same lens would get a new crop factor which would change it's focal length from 90mm to 162mm? And then when I swapped out the 6x9 back for the Nikon adapter the lens would get another new crop factor and change focal length again? Oh I'm so confused now and I didn't even know I was confused then!!

Joe

 

[ceemac]

Aw you guys, just when Alex was starting to figure this out... I'm sure it can be explained by simple calculus or something.

 

[ac12]

Aw you guys, just when Alex was starting to figure this out... I'm sure it can be explained by simple calculus or something.

Boo hiss
I hated and still hate high math.

 

[ceemac]

Boo hiss
I hated and still hate high math.

C'mon admit it you feel the need to finally use calculus for something. That's up there with fitting Frederick the Great of Prussia into a conversation.

 

[AlexGavillan]

Hahaha.

See the below is why I've been so confused. Reading articles like the one sited, take a look at what they are saying.

Link: Buying Your First Prime Lens - 35mm or 50mm?

"Another thing to consider when choosing between the 35mm and 50mm focal lengths is crop factor. When I bought my first prime lens, I was shooting with a Canon 7D as my primary body. I was actually deciding between a Canon 24mm f/1.4L and a 35mm f/1.4L. With my 1.6 crop sensor body, I wanted to achieve a 35mm look so I opted for the 24mm f/1.4L. With the crop factor, that gave me about a 38mm lens."

 

[Derrel]

Hahaha.

See the below is why I've been so confused. Reading articles like the one sited, take a look at what they are saying.

Link: Buying Your First Prime Lens - 35mm or 50mm?

"Another thing to consider when choosing between the 35mm and 50mm focal lengths is crop factor. When I bought my first prime lens, I was shooting with a Canon 7D as my primary body. I was actually deciding between a Canon 24mm f/1.4L and a 35mm f/1.4L. With my 1.6 crop sensor body, I wanted to achieve a 35mm look so I opted for the 24mm f/1.4L. With the crop factor, that gave me about a 38mm lens."

The above makes total sense. It's good information.

 

[AlexGavillan]

Hahaha.

See the below is why I've been so confused. Reading articles like the one sited, take a look at what they are saying.

Link: Buying Your First Prime Lens - 35mm or 50mm?

"Another thing to consider when choosing between the 35mm and 50mm focal lengths is crop factor. When I bought my first prime lens, I was shooting with a Canon 7D as my primary body. I was actually deciding between a Canon 24mm f/1.4L and a 35mm f/1.4L. With my 1.6 crop sensor body, I wanted to achieve a 35mm look so I opted for the 24mm f/1.4L. With the crop factor, that gave me about a 38mm lens."

The above makes total sense. It's good information.

I think I get what he's saying, now but when reading that I guess the writer is assuming the reader knows a thing or two about photography.

So when he says "35mm look" what's he reffering to, what he sees on the viewfinder or picture itself?

 

[ac12]

IMHO it is the wider FoV. If your back is up against the wall with a 50mm lens, you cannot backup any more to get a wider FoV. Hence the 35.
This is why when I shot 35mm film, I used a 24mm lens. I did not always need that wide a coverage, but when I did, I did. And the wider the better. But because the 24 is so wide, I did not use it as my only lens. I always used it in conjunction with another narrower lens. Whereas a 35 could be used as the only lens.

He is also considering how the background and out of focus blur looks.