How do I convert focal length?

Hello

 

How do you convert a focal length on a DSLR to 35mm equivalent?

 

-Jiazi

try the search button

It's not a real conversion, just a way of thinking about the angle of view compared to what you would see on 35mm film. In any case it depends on the DSLR:

1.5x focal length for Nikon DSLRs

1.6x focal length for consumer Canon (Rebel models, 10D, 20D, 30D)

Just to elaborate a bit more:

 

When I use my Nikon 50mm lens on a Nikon DSLR, it has a field of view similar to what a 75mm lens would be on a film camera. Ditto my 20mm lens; it now has the field of view of a 30mm when I use it on a DSLR.

 

The focal length hasn't changed, just my expectations of the angle of view have.

to elaborate a little bit more...

 

If a 50 mm lens on full frame 35mm film camera is considered a "normalm" lens, (angle of view close to human vision perhaps), the same lens placed on a DSRL with crop factor of 1.5 (or 1.6), is considered a short "telephoto" or a "portrait" lens.

 

Just keep in mind that the same lens even if has angle of view of a telephoto lens on DSRL camera, it DOES NOT bring the photographed subjects any closer when comparing to full frame film camera, contrary to what some people tend to believe.

This question of 35mm equivalent is interesting and you should not just think of the focal length but also about the aperture and the ISO number.

 

Let us imagine that you have a DSLR APS-C (crop factor 1.5) with a 24mm, f/2.8, ISO 100 and 1/1000s.

 

Question: Which focal lens, which aperture and which ISO should you use on a 35mm camera to get the exactly the same picture?

 

Exactly the same picture means:

 

* the same stopping power,

 

* the same field of view,

 

* the same perspective,

 

* the same depth of field,

 

* the same background and foreground blur

 

* and the same image quality (assuming you are using 2 digital cameras with the same pixel number)

 

The answer is:

 

* 1/1000s shutter speed to get the same stopping power,

 

* 36mm ( 24 * 1.5) to get the same field of view,

 

* f/4.2 (2.8 *1.5 = f/4.2) to get the same depth of field and the same background and foreground blur,

 

* ISO 225 (actually 100*(1.5 * 1.5) =225) to get the same image quality. Indeed to get (approximately) the same image quality you need to collect the same amount of light on the sensor and therefore you have to multiply the ISO number of the APS-C camera by the surface ratio of the sensors.

 

* And take the picture from the same location to get the same perspective.

 

The 35mm camera has the advantage of a better image quality at high ISO and a shorter depth of field.

 

Cheers,

 

Nicolas

<i>

"The 35mm camera has the advantage of a better image quality at high ISO and a shorter depth of field."

</i>

<p>

At the risk of taking a radical left turn on this thread, Nicolas, I disagree with your 'high ISO' assertion. I can't debate it with you on a 'physics' level, but I can tell you that I have been producing better JPEGs out of my D2H and D2HS at 800 and 1600 than I ever got with Fuji Press 800 film.

<p>

High ISO available light shots at f/1.4 or f/2 are one of my favorite playgrounds, and digital has been a bit like discovering a 'miracle film' for me.

<p>

As I understand it, I could probably do even better in RAW with something like Noise Ninja, but so far I have avoided yet another 'glued to my computer screen' hobby.

<p>

My 'perfect camera' would probably be a Nikon D3H (same specs as a D2HS) with a full frame LBCAST sensor putting out 6MP, and it would be compatible (i.e. 'work well with') my 24/2.0 and 35/1.4 AIS lenses.

Hello Everybody

 

Thanks alot for all your replies on an amateur question :)

 

Does anyone know how to calculate the crop multiplier(This is a little more theoretical question, you don't have to answer)?

 

-Jiazi

It's just linear math. Nikon's sensors are all about 16x24mm. Standard 35mm film is 24x36mm. Take the film dimension devided by the sensor dimension and you get 1.5x.

 

Technically, you're supposed to use the diagonal, but the math works the same if the two rectangles have an identical aspect ratio.

In filming, one thinks in angular coverage. A scene might require a certain angular coverage. Then the format is chosen. One might be using 35mm , 4 sprocket holes for most of the film. But one might be using say a several cheapie 16mm cameras attached to a car for a wreck scene. One uses ones zoom finder, or American Cinematographers Handbook, or own cheat sheets to select the lens focal length, with inputs of angular coverage and film format. This whole "equalvalent focal length" confusion didnt happen until the dumbing down of marketing digital cameras. Its ok to use, but remember one is still using a 50mm lens whether its one a Nikon F, 16mm movie camera, or 8mm movie camera, or modern DSLR. The smaller formats just use less of the lenses actual usable coverage, and one gets less angle in the image. <BR><BR>In the home movie era, one often used a 25mm as a normal for 16mm film, and a 12 to 13 mmm as a normal for 8mm film. When one placed a 25mm lens on ones 8mm camera, we called it a telephoto lens for that camera.

Jiazi, here it is

 

* Nikon DLSR (D40, ?D2x) 1.5

 

* Canon 20D, 30D, 400D etc 1.6, Canon 5D, 1Ds 1.0 (full frame)

 

* Sigma DSLR (SD14) 1.7

 

* Olympus DSLR (E-330 E-500 E-400 etc) 2

 

* Leica (M8, digital back) 1.33

 

Todd,

 

I believe that you got better JPEGs with your D2H @ ISO 1600 than with Fuji Press 800 film.

 

My point was to compare a 35mm DSLR with an APS-C DSLR with the same pixel number (let say a Nikon D2xs and a Canon 5D). An APS-C DSLR (crop factor 1.5) with 24mm, f/2.8, 1/1000s, ISO 100 will give you the same picture (image quality included) than a 35 DSLR 36mm, f/4.2, 1/1000s, ISO 225. Indeed each pixel will get exactly the same amount of energy which means the same noise. One can (approximately) say that

 

APS-C DSLR ISO 100 = 35mm DSLR ISO 225 (with the same MP)

 

Which means that for the same ISO you should get a better image quality with the 35mm (digital) camera.

 

Cheers,

 

Nicolas.

Ah, I see your point now, Nicolas. It's the 'big pixels are less noisy than tiny pixels' thing. I assumed you were comparing film to APS-C digi sensors.

 

I too put some heed in 'big pixels' for low light work, but there is a danger in assuming too much between two different technologies. The Canon 5D is reputed to be very very good in low light. I'm not clear whether that's because the pixels are larger than other current generation cameras, or if maybe they just have a better sensor technology at this time.

<p>Thank you all for talking about this so-called 1.5 magnification factor. Here is what our website says in the Nikon forum introduction - "Most Nikon digital SLR bodies incorporate a "small sensor" or "APS-C" sized sensor. This is smaller than the standard 35mm film frame and effectively multiplies the magnification of any lens attached to the body."<br>

<br />Perhaps the webmaster will correct this, as it is quite confusing. I don't know how many times now I've put a non-AI 50mm onto the F2 and then onto the D40 to prove yet again to myself that only field of view changes and not focal length. I think it is not a misunderstanding of photography that has led to this but sloppy use of language. </p>