Focal Length...What is it??

[DarkLantern]

Hi all,

(newby here)

After doing alot of reading i have manage to decipher a lot of information about photography, one thing eludes me now....mystical art of focal length.

I realised (embarrassingly late ) that the number on a lense relates to the focal length. Now i own a Nikkor 18-200mm 3.5-5.6. (Good but not great allrounder). Can take nice landscapes as well as going through to Close ups.

Can anyone provide a non or limited technical description of focal length? Or point me to a great source. Especially on how (if at all) it affects the exposure triangle. Also the conversion of x1.5 from digital to film, why?

Secondly, what are the sweet focal ranges for the different 'genres' of photos. For example i have read that approx 100mm is a good spot for portraits. (Although on my lense i have read closer to 135mm). Yes i do realise this is a question somewhat like - how long is a piece of string, and it really depends on what you want and what lenses you have. Are ther some rule of thumbs, a guide to start?

This maybe a tutorial in the future, or maybe its so simple and i am a little slow.

All help appreciated.

Roger

[wulf]

If you have an 18-200mm lens the best way to grasp it is probably to go out and take some photos. Think of a few different locations easily available to you. For example, you could pick your kitchen (linking in with this week's photo assignment), a tree in a nearby park and an interesting local building.

Visit each one and keep you lens wide open at 18mm. What kind of things can you see through the lens? What options present themself? Try the same at some other settings - perhaps 50mm, 100mm and 200mm. I would probably do a circuit for each setting to deeply explore it but you could switch between them in each place.

Now look back at the photos and compare how each set comes out. That will give you some material to work with as you read further on the subject of focal length; you might even get some great photos out as an added bonus!

Wulf

[AnnaV]

Originally Posted by DarkLantern

Secondly, what are the sweet focal ranges for the different 'genres' of photos. For example i have read that approx 100mm is a good spot for portraits. (Although on my lense i have read closer to 135mm). Yes i do realise this is a question somewhat like - how long is a piece of string, and it really depends on what you want and what lenses you have. Are ther some rule of thumbs, a guide to start?

I'm quite interested in hearing the response to this question too Roger.

[captainkimo]

In landscape or macro photography, sometimes you give up resolution for depth of field. It may require f11 or f16 to produce deep DOF and as such you must be willing to let go of that softness for it.

However, the sweet focal range, or sometimes regarded to as sweet spot, may not be the most appropriate aperture available, although it may give you the best possible image.

Cheers!

Captain Kimo

[vandergus]

I've always like the tutorials at CambridgeInColour.com. Packed with info but stated clearly enough for just about anyone to unerstand. They also include lot's of visual examples to help illustrate the concepts, which is one area where I think most tutorials are lacking. Here's their entry on Understanding Camera Lenses, mostly about focal length and aperture.

Hope that helps.

[DarkLantern]

This is perfect, many hours of reading here.....(Cambridgecolour)

But a mixture of this technical info and Wulf's suggestion of try and see is the go.

Thanks All, much appreciated.

I hope Anna gets the info she wants out of this.

Regards

Roger

[inkista]

Originally Posted by DarkLantern

Can anyone provide a non or limited technical description of focal length? Or point me to a great source.

I like this one; it also points out the most typical focal length ranges for different types of photography. Also, the explanation there on max. aperture and why it's important in terms of lens choice is a good one.

...Especially on how (if at all) it affects the exposure triangle.

It does and it doesn't.

The main reason it doesn't is because aperture is described as a f-number ratio, rather than as a specific size. The f-number is the ratio of the lens opening's diameter to the len's focal length. So, essentially, apertures are "normalized" across focal lengths. If a lens is at f/2.8 at 100mm, that's the same f/2.8 as at 50mm, even though the opening diameters differ the ratio remains the same. So you don't have to adjust your exposure triangle for focal length.

You do, however, have to watch out for camera shake blur and the shutter speed in terms of focal length. The rule of thumb (without stabilization) for eliminating camera shake blur when handholding is that the shutter speed needs to be 1/focal_length or faster. So, if you're shooting with a 50mm lens, 1/50s or faster is your goal. If you're shooting with a 300mm lens, 1/300s or faster is your goal. But that should be the only effect that focal length plays on exposure settings.

Also the conversion of x1.5 from digital to film, why?

I second vandergus's recommendation on the Cambridge in Colour articles on sensor size and its effect on digital photography.

[OsmosisStudios]

Originally Posted by DarkLantern

Can anyone provide a non or limited technical description of focal length? Or point me to a great source. Especially on how (if at all) it affects the exposure triangle. Also the conversion of x1.5 from digital to film, why?

Secondly, what are the sweet focal ranges for the different 'genres' of photos. For example i have read that approx 100mm is a good spot for portraits. (Although on my lense i have read closer to 135mm). Yes i do realise this is a question somewhat like - how long is a piece of string, and it really depends on what you want and what lenses you have. Are ther some rule of thumbs, a guide to start?

To answer your first question: the x1.5 mutiplier from film to digital is because the sensor in your camera is smaller than a piece of film. A standard frame of 35mm film is 36mmx24mm. Your sensor is 24x18 (exactly half). What this means is that your sensor covers only the centre section of a frame of film

Essentially, imagine a soccer field. At each end there are the "boxes". There's the big one, and the smaller one. The big one is like film (or a full-frame digital) and the smaller one is like your digital sensor (it just gets moved to the centre in a camera).

What this means is that youre missing out on alot of the field of view of a lens. In this case, a 24mm lens on your digital will have the same field of view as a 36mm lens on a film camera. The same applies for the telephoto range too: a 200mm lens on your digital is equivalent to 300mm on film.

You dont really have to worry about it: just understand that, unless youre shooting full-frame digital (D700, D3 in Nikon, 5D and 1D in Canon), you have to multiply the length of a lens to get the 35mm equivalent. Its really only used when comparing crop-body cameras to full-frame, or digital to film.

As to your second question: The sweet spot on the Nikon 18-200mm is actually somewhere around 50-70. At 135mm, it's actually at it's WORST. 3rd party lenses (like Sigma, Tamron and Tokina) are actually inverted: theyre at their best in the middle (75-150) and at their worst at the ends.

As to your original subject: Focal length. If you want to get technical, it's the distance travelled by light from the front element to the focal point of your lens. For most lenses this is at the aperture blade plane, but now that we're getting into things like DIffractive Optics and the like, it can vary. Again, understanding this isnt necessary (or indeed useful!).

[Doug Pardee]

Originally Posted by DarkLantern

Can anyone provide a non or limited technical description of focal length?

Focal length is a measure of the magnification of a lens. It tells you how large the projected image is for an object of size "X" when viewed from a distance of "X" (the same "X"). For example, with a focal length of 135mm a building 683 feet tall when viewed from a distance of 683 feet will project an image 135mm tall onto the sensor or film (which probably is much smaller than 135mm so the image would be clipped off).

It's all proportional. Take the same 683 foot building and view it from a distance of 6830 feet (ten times as far) and with that 135mm lens the projected image is now 13.5mm (1/10th as large).

Object size / object distance = projected size / focal length.

(The above calculations are with the lens focused at infinity. Close focusing, as with macro lenses, will not be exactly according to the formula.)

how (if at all) it affects the exposure triangle

It doesn't. That's the point of measuring apertures in f-numbers: that cancels out the effect of focal length on exposure.

the conversion of x1.5 from digital to film, why?

Because way too many people treated focal length as measuring angle of view. Angle of view is the result of both magnification and the size of the sensor/film. The magnification determines how large the projected image is, and the size of the sensor/film determines how much of that projected image will be captured.

Anyway, an awful lot of people were of the belief that 35mm film was the only film size of any interest. So they used focal length as a shorthand to describe angle of view.

There have always been other sizes of films. But the 35mm world didn't care. Until now, that is. We now have popular DSLRs that have sensors of a different size than 35mm film.

But there are so many references to focal length as a measure of angle of view out there… it's too late to fix. So instead we talk about "equivalent focal length" where we apply a multiplier based on the size of the sensor relative to 35mm film. Equivalent focal length is technical nonsense but is reasonably useful in practice if we're careful to understand that the only way that it's "equivalent" is in terms of angle of view, and that it's not really a focal length at all.

[DarkLantern]

Reading through all the info out there, and the explanations given on thsi thread has helped me a great deal.

I believe wulf had given an exercise to go and try and that's my next stop.

I actually have the added benefit (?) of three borrowed lenses.Nikon 28mm 3.5 (non Converted) Prime, Nikkor 35-105 3.5-4.5, Nikon 18mm 4 - fisheye. None of them are digital and i have no metering. I might go and play with these to test some scenarios.

(Ofcourse i have the 18mm-200 still, by the way yes the 135mm is the worst (dpreview.com) i got confused) )

Last question; anymore suggestion on an exercise that will help the information sink in.

Thanks For you help and pointers.

Roger